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A spatiotemporal analysis of hydrological trends and variability in the Athabasca River region, Canada
Journal of Hydrology 509 (2014) 333–342
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A spatiotemporal analysis of hydrological trends and variability in the Athabasca River region, Canada Allison J. Bawden a, Hayley C. Linton b, Donald H. Burn a,⇑, Terry D. Prowse b a b
Department of Civil and Environmental Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada Water and Climate Impacts Research Centre, Environment Canada and University of Victoria, Victoria, BC V8W 3R4, Canada
a r t i c l e
i n f o
Article history: Received 26 June 2013 Received in revised form 19 November 2013 Accepted 25 November 2013 Available online 1 December 2013 This manuscript was handled by Andras Bardossy, Editor-in-Chief, with the assistance of Ashish Sharma, Associate Editor Keywords: Trend analysis Streamflow Climate change RHBN Athabasca
s u m m a r y Trends and variability in the hydrologic regime of the Athabasca River region were analyzed. Twenty hydrologic variables were selected for flow analysis within both the Athabasca River Basin (ARB) and, for comparison, surrounding watersheds. Intra- and inter-basin scale analyses were performed, including a comparison of changes in streamflow at stations forming part of the Reference Hydrometric Basin Network (RHBN) and non-designated gauges. Streamflow trends were also compared with trends in air temperature and precipitation over the entire Athabasca and surroundings study region. Noteworthy results include strong decreasing trends in annual, warm season (March to October) and summer month flows over the majority of the study region, in addition to a greater number of decreasing trends in Athabasca watershed flows compared to the surrounding basins. The timing of the spring freshet was found to have not shifted toward an earlier onset, contrary to results from previous studies. Lastly, trends in streamflow were similar to those for precipitation over the ARB and surrounding region, but did not relate strongly to trends in air temperature. The results of this study should be of assistance to water- resources managers and policy makers in making decisions about water use in this rapidly changing watershed. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Climate change/variability can have profound impacts on the hydrologic regime of a watershed. These impacts are likely to be especially severe when a watershed is located in an area that is particularly susceptible to the effects of a changing climate, such as the Canadian North, or when there are other stresses on the hydrologic regime, as may occur when there are large withdrawals from the watershed. Changes in regional water availability can affect many aspects of human society, from agricultural productivity and energy use to flood control, municipal and industrial water supply, and fisheries and wildlife management (Xu and Singh, 2004). Water is extremely important to both society and nature, thus understanding how a change in climate could affect regional water supply is essential for future water-resources planning (Kienzle et al., 2012). Streamflow is affected by changes in both evapotranspiration, which is dependent on air temperature, as well as precipitation. To quantify the effects of climate change/variability, a number of recent studies have conducted trend analyses on Canadian hydrometric and climatic time series. In a comparison of two different decades of Canadian hydrologic and meteorological data, for exam⇑ Corresponding author. Tel.: +1 (519) 888 4567x33338. E-mail address: [email protected] (D.H. Burn). 0022-1694/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhydrol.2013.11.051
ple, Whitfield and Cannon (2000) found the more recent decade to be generally warmer and characterized by both increases and decreases in precipitation and streamflow; these patterns were found to be spatially consistent in different areas of Canada. Zhang et al. (2001) found generally decreasing trends in Canadian river flow for 11 hydrometric variables, particularly in August and September, while significant increases in March and April streamflow were also observed. In an analysis of hydrologic variables from a network of 248 catchments across Canada, Burn and Hag Elnur (2002) found there to be pronounced spatial heterogeneity in climate-related effects on the hydrologic response. Centering on the high latitudes, Déry and Wood (2004, 2005) investigated trends in streamflow in the Canadian Arctic and found generally decreasing discharge that they related to various large-scale atmospheric phenomena. In northern British Columbia and the Yukon, Fleming and Clarke (2003) noted different trend responses to climatic warming in annual streamflow volume for catchments with and without a glacial cover. Although broad scale assessments of hydroclimatic trends provide useful indications of overall change/variability, more regionalized studies are typically needed to evaluate localized impact and potential adaptation measures. Given the importance of the western cordillera as a major water source in Canada, a number of studies have been conducted of rivers that flow to the western prairies or northern Canada from this alpine region. Rood et al.
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(2008), for example, identified earlier timing of spring peak flows and decreased summer flows for Rocky Mountain rivers. For the alpine-fed Athabasca and Liard River basins, Burn et al. (2004a,b) identified a general trend to earlier onset of spring runoff, but contrasting monthly trends for the two watersheds. A strong trend to increasing winter flow and the occurrence of an earlier spring freshet was similarly identified for the larger Mackenzie River Basin (MRB) by Abdul Aziz and Burn (2006); a trend to the latter effect in most recent years was also noted by Burn (2008). In an examination of flow for rivers feeding the western prairies, Schindler and Donahue (2006) found drastic reductions in summer discharge since the early 20th century, ranging from 20% in basins where human impact has been relatively minimal, such as the ARB, to 84% in regions where damming and large water withdrawals have occurred, such as the Saskatchewan River Basin (SRB). Most recently, Peters et al. (2013) identified different responses in runoff, temperature and precipitation for locations along the main stem of the Athabasca River, including a decline in runoff in the lower reaches of the ARB between 1958 and 2009 contrasted with a general lack of trend in the upper reaches. A fuller assessment of how hydrologic trends in the ARB compare with those of other catchments in the broader hydroclimatic region of the leeward slopes of the Canadian western cordillera can help to determine the degree of influence that changing climate has on streamflow in this watershed. A focus is placed on the ARB because of its ecological significance and rapidly transitioning industrial water uses. In the case of the former, the Peace-Athabasca Delta (PAD), formed by the confluence of the Peace and Athabasca rivers and the western end of Lake Athabasca, provides breeding habitat for a variety of wildlife (Wolfe et al., 2005), in addition to food and recreational opportunities for a number of First Nations communities residing in the surrounding municipalities. Any changes in flow to the PAD (e.g., by withdrawal, diversion or natural drying) could affect this ecologically sensitive wetland ecosystem. The ARB also contains a significant bitumen deposit, and a related extraction industry that accounted for 75% of total water allocations from the Athabasca River in 2010 – a volume that has been growing since 2000 (Alberta Environment, 2013a). Twenty variables, reflecting a variety of different characteristics of the hydrologic regime of the ARB, were examined. Data from 38 Water Survey of Canada (WSC) hydrometric gauging stations were obtained: 19 stations from within the ARB and 19 stations from nearby, surrounding watersheds. All gauges are considered to reflect unregulated flow conditions. Hydrologic variables were inter-compared among basins and with specific climatic variables to explore the extent of their effect on streamflow.
2. Data and methodology 2.1. Trend detection Trend detection in hydroclimatic research is frequently conducted by means of the Mann–Kendall (MK) non-parametric test (Mann, 1945; Kendall, 1975); this method was therefore adopted for this analysis. The MK test is rank-based and hence is robust to non-normality of the underlying data. The MK test is based on the null hypothesis that the data in a series are independently and identically distributed (i.e., that there is no trend present in the data); rejection of this hypothesis at a local significance level a indicates a significant trend in the data set. Following Burn et al. (2004a), the data used in this analysis were corrected for serial correlation through a modified version of the Trend Free Pre-Whitening (TFPW) approach developed by Yue et al. (2002). The TFPW approach attempts to separate the serial correlation that
arises from a (linear) trend from the remaining serial correlation, and then only removes the latter portion of the serial correlation. Although the TFPW procedure involves fitting and removing a linear trend, the MK trend detection procedure does not make any assumptions about the nature of the trend in a data set. Field significance was evaluated to assess if the trend results from the collection of stations were unusual; the field significance of the MK results was evaluated using Walker’s test (Wilks, 2006). Walker’s test considers the magnitude of the p-value of each of the K individual (local) trend tests to determine if the global null hypothesis – that all K local null hypotheses are true – can be rejected at global significance level aglobal. For field significance to be sustained, the smallest local p-value must be no larger than some critical p-value derived based on aglobal and the number of local tests K. If the smallest p-value is small enough, it can be concluded with high confidence that the collection of K local p-values did not result from independent draws from a uniform distribution. Evaluation of field significance using the minimum local p-value as the global test statistic allows for better identification of locations with significant differences, because fewer than aglobal 100% (on average) of apparently significant local tests will have resulted from local null hypotheses that are actually true (Wilks, 2006). In addition to MK test statistics, trend slopes were estimated for each variable using a robust non-parametric slope estimator (Sen, 1968). The trend slope indicates the magnitude and direction of the trend in runoff depth (or days, depending on the hydrologic variable) per time period. To verify homogeneity in the data, a regime shift detection method was employed (Bering Climate, 2006). In hydrology, regimes shifts are generally symptoms of natural climate change/ variability, and are often linked to decadal fluctuations in atmospheric circulation (Stewart et al., 2004). Continuity, or a lack of regime shifts, is often necessary to draw accurate conclusions from trend analysis. The method developed by Rodionov (2004) was used to detect whether a common regime shift at each station could have contributed to trends detected by the MK test. 2.2. Variables analyzed A number of hydroclimatic studies have hypothesized that climate change could cause a variety of impacts on the hydrologic regime of the ARB region; hence, twenty variables that reflect different aspects of the hydrologic response were selected. These included: annual mean flow, monthly mean flow for each of the 12 months, mean flow for the warm season (an average of the flows from March to October), annual maximum daily flow and its date of occurrence, annual minimum daily flow and its date of occurrence, and a measure of the onset of the spring freshet and the delay (number of days) from the start of the freshet to the date of occurrence of annual peak flow. The date of the start of the spring freshet was estimated as the Julian day by which ten percent of the annual streamflow volume had occurred (hereafter referred to as the ten percentile or ‘‘10P’’ date). The 10P date was calculated based on the calendar year. Burn (2008) concluded that the 10P date is a more effective measure of the onset of the spring freshet than the pulse date (Cayan et al., 2001), a measure that has also been used to define the beginning of the spring freshet. Watershed size does not affect the efficacy of the 10P date to capture the onset of the spring freshet, even when a large range of basin areas is considered (Burn, 2008). The data used in this analysis were acquired via Environment Canada’s WSC HYDAT database (WSC, 2010). To further understand the effects of climate change on the hydrology of the ARB, hydrologic trend results were compared with climatic trends calculated for meteorological data spanning
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the entire study region. Daily mean air temperature and total precipitation data were acquired through a newly available, high resolution (10 km 10 km) gridded dataset produced by Natural Resources Canada based on the methods described in Hutchinson et al. (2009) and McKenney et al. (2011). The MK test was used to calculate trend at each grid point, and pixel trajectory analysis was applied to map trend slope values over the study region (Gralewicz et al., 2010). 3. Application 3.1. Athabasca River Basin The ARB spans the provinces of Alberta and Saskatchewan, forming the southern-most component of the MRB. At 1538 km in length, the Athabasca River is Alberta’s largest undammed river, with a total drainage area in excess of 150,000 km2. The river’s headwaters originate in the Columbia Icefield near Jasper and extend northeast to Lake Athabasca before eventually emptying into the Arctic Ocean via the Mackenzie River. Basin elevations range from 185 m.a.s.l. near the Lake Athabasca lowlands to nearly 3,700 m.a.s.l. in the mountainous headwaters. Tributary rivers flow over four of Alberta’s six natural regions: the Rocky Mountains, the Foothills, the Boreal forest and the Canadian Shield (ARBRI, 2013). The ARB experiences a mean annual temperature of 2 °C and an average yearly precipitation of 460 mm, roughly 75% of which occurs as rain. The hydrologic regime of the basin is characterized by low winter flows, with a snowmelt-driven rising hydrograph beginning in late April or May. Peak flows generally occur in June or July and are fuelled by snowpack runoff from the Rocky Mountains. The remaining months experience gradually declining flows until winter low flow conditions are resumed in December (Burn et al., 2004b; Toth et al., 2006). Maximum flows in the ARB are sensitive to variability in winter/spring precipitation and temperature, as snow accumulation/ablation and the rate and timing of snowmelt are highly influenced by these climatic drivers. Peters et al. (2013) also demonstrated a strong relationship between ARB runoff and the Pacific North American mode of atmospheric variability, in addition to a weaker teleconnection with the Pacific Decadal Oscillation (see also St. Jacques et al., 2010). The ARB is rich in natural resources, including forestry, mineral deposits, peat deposits, fish and game, and most notably, oil and
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gas. The Athabasca oil sands, in combination with two similar deposits in the nearby Peace River and Cold Lake regions, comprise the world’s third largest crude oil reserve after Saudi Arabia and Venezuela (Alberta Energy, 2013). The Athabasca River and its tributaries flow through the Athabasca oil sands deposits resulting in erosion of oil-containing sands. Most current and proposed oil sands developments are located within the lower portions of the Athabasca River, primarily downstream of Fort McMurray, Alberta. The development of the oil sands formations can have potential environmental impacts on the air, land and water, including increased water use (both surface and groundwater), production of waste streams, and changes in hydrologic connectivity resulting from land disturbance (Wrona and di Cenzo, 2011). The locations of the gauging stations analyzed are shown in Fig. 1. The attributes of these stations are summarized in Table 1. Stations that are part of the RHBN are also indicated in Fig. 1 and Table 1. The RHBN (Harvey et al., 1999) is a sub-set of stations in the national hydrometric network and contains over 200 gauges that have been identified for use in the detection, monitoring and assessment of climate change. There are only four RHBN stations in the ARB, while a further five RHBN stations were selected from watersheds surrounding the ARB.
4. Results 4.1. Trend analysis The results of MK trend detection are summarized in Table 2. The hydrometric time-series data were analyzed for three different periods: 1976–2010 (35 years), 1971–2010 (40 years) and 1966– 2010 (45 years). The three periods reflect a trade-off between greater spatial coverage with a shorter period versus greater power for the statistical tests for a longer period. For each case, a data set was only used for analysis if there were no more than four years of missing data within the period. As a result, the nominal number of stations for the longer periods was less than the total number of stations overall due to missing data for some variables and stations. This was particularly problematic for stations that operated on a seasonal basis only (generally March to October), for which flows in the winter period were not available. To account for this, the mean warm-season flow variable was included in the analysis.
Fig. 1. Map of the ARB and surrounding watersheds showing locations of hydrometric gauging stations analyzed. RHBN stations are identified by white crosses. Top-right map shows location of the ARB within Canada.
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Table 1 Characteristics of stations analyzed. WSC Station
River
Period of record
Drainage area (km2)
Elevationa (m.a.s.l.)
Athabasca River Basin 07AA001b 07AA002b 07AD002 07AF002 07AG003 07BB002 07BC002 07BE001 07BF002 07BJ001 07BK007 07CA005 07CA006 07CD001b 07DA001 07DA006 07DA008 07DC001 07DD002b
Miette River near Jasper Athabasca River near Jasper Athabasca River at Hinton McLeod River above Embarras River Wolf Creek at Highway No. 16A Pembina River near Entwistle Pembina River at Jarvie Athabasca River at Athabasca West Prairie River near High Prairie Swan River near Kinuso Driftwood River near the Mouth Pine Creek near Grassland Wandering River near Wandering River Clearwater River at Draper Athabasca River below McMurray Steepbank River near Fort McMurray Muskeg River near Fort MacKay Firebag River near the Mouth Richardson River near the Mouth
1974–2010 1970–2010 1961–2010 1954–2010 1955–2010 1955–2010 1961–2010 1952–2010 1959–1970c, 1971–2010 1961–1970c,1971–2010 1968–1970c,1971–2010 1966–2010c 1970–1997, 1998–2010c 1957–2010 1957–2010 1974–1986, 1987–2010c 1974–1986, 1987–2010c 1972–1986, 1987–2010c 1970–1986, 1987–2010c
629 3870 9760 2560 826 4400 13,100 74,600 1150 1900 2100 1460 1120 30,800 133,000 1320 1460 5990 2730
1062 1059 954 920 862 718 609 516 588 588 550 556 549 261 237 275 253 227 258
Peace River Basin 07EE007 07FB001b 07FC001 07GE001 07GH002 07GJ001 07JD002
Parsnip River above Misinchinka River Pine River at East Pine Beatton River near Fort St. John Wapiti River near Grande Prairie Little Smoky River near Guy Smoky River at Watino Wabasca River at Wadlin Lake Road
1968–2010 1961–2010 1961–2010 1960–2010 1959–2010 1955–2010 1970–2010
4930 12,100 15,600 11,300 11,100 50,300 35,800
712 549 465 514 477 378 307
Lake Athabasca Drainage 07LB002 Waterfound River below Theriau Lake 07LE002b Fond du Lac River at outlet of Black Lake 07MA003 Douglas River near Cluff Lake 07MB001 Macfarlane River at outlet of Davy Lake
1974–2010 1963–2010 1975–2010 1967–2010
3160 50,700 1690 9120
420 221 297 335
Churchill River Basin 06AD006 06AG002 06BB005 06BD001b 06CD002b 06DA004b
1955–2010 1971–2010 1973–2010 1967–2010 1963–2010 1966–2010
14,500 2960 4730 3680 119,000 7730
511 427 423 430 353 415
1971–2010 1969–2010
876 1890
930 665
Beaver River at Cold Lake Reserve Dore River near the Mouth Canoe River near Beauval Haultain River near Norbert River Churchill River above Otter Rapids Geikie River below Wheeler River
North Saskatchewan River Basin 05DD009 Nordegg River at Sunchild Road 05EA005 Sturgeon River at Villeneuve a b c
Elevations are given for each gauging station. Stations forming part of the RHBN. Seasonal records containing data from March to October only.
Application of a regime shift detection method indicated that streamflow data from the ARB and surrounding watersheds did not appear to be overall affected by any common shift(s) in the climatic system over any of the three periods; hence, trend results are assumed to reflect trends in the data as a result of continuous changes in this region. Table 2 presents results separately for gauging stations in the ARB and from surrounding watersheds. This was done to allow a determination of any unique trend patterns for the ARB. Percentages indicate the fraction of stations within each grouping (i.e., Athabasca or surrounding stations) that displayed significant increasing (") and/or decreasing (;) trends. The total number of stations for each variable was dependent on the lengths of data records; the longer time periods and winter variables tended to include results from fewer stations in the calculation of percentage. Table 2 also shows the variables that demonstrated field significance, indicating that the collection of K local results was globally significant. Field significance was evaluated separately for the ARB and for the surrounding watersheds. As this analysis was exploratory in nature, it was desirable to look at both established as well
as emerging trends in the data; thus, all statistical tests (i.e., MK and field significance) are reported at the 10% significance level. Table 2 reveals that many of the variables exhibited results that are field significant. Most streamflow variables displayed primarily decreasing trends and tendencies, where all significant trends were in the negative direction, while a minority of variables exhibited both significant decreasing and increasing trends; very few variables exhibited more significant increasing than decreasing trends. Other noteworthy results included the general tendency for decreasing mean annual flow, decreasing mean warm-season flow and corresponding decreasing flows in the months of June, July and August, decreasing annual maximum flows, and both decreasing and increasing annual minimum flows. The general pattern of decreasing flow, particularly for the warm season and on an annual basis, is a potential cause for concern, especially considering the level of proposed development within the ARB (see also Schindler and Donahue, 2006). Interestingly, there were comparatively few trends identified for the timing variables. Earlier work (Burn, 1994, 2008) showed decreasing trends for both the date of the annual maximum flow and the start of the spring freshet, implying
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A.J. Bawden et al. / Journal of Hydrology 509 (2014) 333–342 Table 2 Percentage of stations showing a significant decreasing and/or increasing trend in streamflowa. Hydrologic Variable
Annual mean Warm season mean January February March April May June July August September October November December Annual maximum Date of maximum Annual minimum Date of minimum Date of 10th percentile 10P Date to max date
ARB
Surrounding watersheds
1976–2010
1971–2010
1966–2010
1976–2010
1971–2010
1966–2010
;46/"0 ;37/"5 ;21/"0 ;21/"7 ;26/"5 ;11/"0 ;5/"0 ;16/"0 ;21/"0 ;53/"0 ;21/"5 ;26/"0 ;23/"8 ;23/"0 ;13/"0 ;13/"0 ;15/"15 ;8/"0 ;0/"8 ;8/"0
;67/"0 ;67/"0 ;33/"0 ;42/"8 ;25/"6 ;38/"0 ;25/"0 ;44/"0 ;50/"0 ;50/"0 ;38/"6 ;53/"0 ;33/"8 ;42/"0 ;54/"0 ;0/"0 ;42/"8 ;0/"0 ;9/"0 ;0/"0
;38/"0 ;55/"0 ;25/"0 ;25/"0 ;9/"9 ;36/"0 ;27/"0 ;36/"0 ;18/"0 ;55/"0 ;36/"0 ;45/"0 ;40/"10 ;30/"0 ;40/"0 ;10/"0 ;25/"13 ;13/"0 ;0/"0 ;0/"0
;21/"0 ;26/"0 ;16/"5 ;16/"5 ;16/"5 ;5/"11 ;5/"0 ;16/"0 ;16/"0 ;21/"0 ;11/"11 ;11/"5 ;11/"0 ;5/"11 ;21/"0 ;5/"5 ;28/"17 ;6/"0 ;6/"0 ;0/"6
;47/"0 ;47/"0 ;29/"12 ;24/"12 ;24/"6 ;18/"18 ;29/"0 ;24/"0 ;41/"0 ;41/"0 ;12/"0 ;29/"0 ;24/"6 ;29/"0 ;35/"0 ;0/"0 ;29/"7 ;7/"7 ;15/"0 ;0/"0
;40/"0 ;36/"0 ;9/"9 ;18/"9 ;18/"9 ;9/"18 ;18/"0 ;18/"0 ;9/"0 ;18/"0 ;9/"9 ;18/"0 ;9/"0 ;9/"9 ;10/"0 ;0/"0 ;22/"11 ;0/"11 ;0/"0 ;11/"0
a Downward (upward) pointing arrows represent percentage of stations exhibiting decreasing (increasing) trends significant at the 10% level. Entries in bold indicate results that are field significant at the 10% level. The ARB and surrounding watersheds each contain 19 stations with 35 years of data for all variables, however fewer stations were considered for the longer periods due to shorter data records.
that these events have occurred earlier in more recent years. While Table 2 shows some decreasing trends for the dates of both the annual maximum and minimum flows (more prevalent for stations in the ARB), none of these results were field significant, implying the trends were not strong. No consistent decreasing or increasing trends were detected for all three periods for either of the freshet timing variables. Table 2 provides a comparison of the results for the stations within the ARB with those from the surrounding watersheds. There were two potential complicating factors in comparing these results. First, the stations from the Athabasca tended to have smaller drainage areas than the surrounding stations. This can be an issue as others have found that trend response can be different for watersheds of different sizes. However, this was not likely to be a major issue for this data set, as the range of watershed areas for the two collections of stations was comparable. Second, there were nine stations from the ARB that were seasonal for at least part of the available period of record (see Table 1). There were no seasonal stations selected for the watersheds surrounding the ARB. Furthermore, all of the seasonal stations were from smaller watersheds (drainage areas of less than 6000 km2). For these stations, many of the hydrologic variables described above could not be defined and hence there were fewer stations for which trends could be calculated. Particularly problematic in this regard were the annual mean flow measure and the monthly mean flow for the coldseason months of November to February. In spite of the limitations outlined above, there were strong similarities in both the overall trend patterns and in the hydrologic variables that exhibited field significance when comparing the two collections of stations. It is noteworthy that the Athabasca stations tended to exhibit more decreasing trends and slightly fewer increasing trends than were observed for the stations from the surrounding watersheds. Similar results were noted for all three periods, indicating that this was a general effect, and not particular to one period. 4.2. Warm-season trends Fig. 2 displays warm-season flow trend results for the 1976– 2010 period. This period was selected for display because it
provides the best spatial coverage of stations of all the periods considered. Results for the warm-season variable are shown since this measure provides a better indication of the spatial change patterns within the ARB (compared to the mean annual flow variable) as data are available for all gauging stations, including seasonal stations. Fig. 2 indicates the locations of gauging stations that exhibited increasing (or decreasing) trends significant at the 10% significance level, as well as the trend tendencies for the stations that did not exhibit statistically significant trends. As can be seen in Fig. 2, the stations that exhibited significant decreasing trends and tendencies were spatially concentrated in the central and upper portions of the ARB as well as in adjacent portions of the Peace River basin and, to a lesser extent, the North SRB. This observation was also noted for the mean annual flow trend results. Interestingly, the seasonal stations located in the lower portions of the ARB exhibited primarily (non-significant) increasing tendencies in the warm-season flow, and one station in this region (the Firebag River near the mouth) exhibited a significant increasing trend. A dependence on elevation is evident for streamflow trends in the ARB, with mainly decreasing trends and tendencies observed in the higher elevation headwaters, and primarily increasing tendencies (or a lack of trend) noted at lower elevations (i.e., at stations located at elevations lower than 300 m.a.s.l., with the exception of the Fort McMurray station). This relationship was not as pronounced in the surrounding watersheds, though the number of high-elevation stations was also lower in this region. The magnitude of the trend slope provides a measure of the effect of a trend for a particular variable. Trend slopes were calculated for all variables and periods, and the results for the flow variables were converted to a runoff depth (reported in units of mm per hydrologic variable time frame) to remove the effects of watershed size on the slope magnitude. Fig. 3 summarizes the trend slope results in the form of box plots for the warm-season flow variable for each of the three periods. The first box plot in each pair shows results for stations from the ARB, while the second box plot shows results for the stations from surrounding watersheds. The results shown in Fig. 3 reveal considerably more negative slopes for the stations from the ARB in comparison to the surrounding stations. Almost all of the stations from the ARB
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Fig. 2. Warm-season (March–October) streamflow trends at each station for the 1976–2010 period.
Fig. 3. Box plots of trend slopes for the warm-season (March–October) runoff. Box defines 25th, 50th and 75th percentile trend slopes, and whiskers extend to the most extreme data points not considered outliers. Outliers are shown as crosses.
displayed negative slopes, indicating a decreasing trend in warm-season flow. While the majority of the surrounding stations also showed negative slopes, there were also a fair number of positive slopes for all three periods. This general pattern of more negative slopes in the ARB than in surrounding watersheds was also noted for the mean annual flow variable as well as for the months of April, May and July to December, again implying that there were differences between the trend responses for the ARB and the corresponding responses for the surrounding areas. Box plots of runoff trends by percent change per year showed similar results, indicating that differences in the relative amount of overall runoff at each station was not a factor in these results.
4.3. RHBN stations Since RHBN stations are intended to reflect watersheds that have experienced minimal changes in land-use, withdrawals and regulation, these stations can be expected to provide a good representation of the effects of climate change over the watershed. It was therefore useful to compare the trend response for RHBN stations with the trend responses of other stations. Since the climate
signal at all stations should be similar, noticeable differences between RHBN and non-RHBN stations can be interpreted as likely arising from non-climatic factors, such as land-use changes. Warm-season trends for the RHBN stations analyzed are presented in Fig. 4. As noted, there are four RHBN stations located within the ARB. Two of these stations, the Miette River near Jasper and the Athabasca River near Jasper, are located in the headwaters of the ARB. Both of these stations have fairly short data records, but do provide a contrast with the non-RHBN stations within the ARB. Both stations exhibited a significant decreasing trend in August, as was also observed for the ARB as a whole, but beyond this, there were very few significant trends for the two RHBN stations. There were also more variables that displayed increasing tendencies than was noted for the other stations in the ARB. The third RHBN station, the Clearwater River at Draper, exhibited very few significant trends, although in contrast to the two headwater RHBN stations, displayed generally decreasing tendencies, more in line with the decreasing trends and tendencies noted for the other stations in the ARB. The final RHBN station within the ARB is the Richardson River near the mouth, which is a seasonal station. A significant increasing trend was observed for September for this station, and many other variables demonstrated increasing tendencies. Of the five RHBN stations from outside the ARB, one is located in the Peace River Basin, three are from the Churchill River system, and one is part of the Lake Athabasca drainage area. The station from the Peace River Basin, the Pine River at East Pine, exhibited very few significant trends. This station displayed a generally increasing tendency in the cold season and a decreasing tendency during other parts of the year. Two of the stations from the Churchill River system, the Haultain River near Norbert River and the Churchill River above Otter Rapids, both exhibited increasing trends in the cold season with mixed results for the other seasons (generally increasing tendencies for the Haultain and decreasing tendencies for the Churchill). The third RHBN station from the Churchill River system, the Geikie River below Wheeler River, showed results that were generally consistent with the non-RHBN stations from the ARB (i.e., generally decreasing trends on both an annual basis and during the warm season). The RHBN station from the Lake Athabasca drainage area, the Fond du Lac River at outlet of Black Lake, exhibited very few significant trends. The trend tendency for this station was generally increasing, with the exception of the most recent period (1976–2010), where the tendency was generally decreasing.
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Fig. 4. Warm-season (March–October) streamflow trends at RHBN stations for the 1976–2010 period.
There were, therefore, observed differences in trend behavior between RHBN and non-RHBN stations both within and around the ARB. The RHBN stations were more likely to exhibit increasing trends in the cold season (November to February), while non-RHBN stations in this region tended to show decreasing trends or tendencies during these months. Similarly, there were more increasing trends on an annual basis and in the warm season for the RHBN stations than what was observed for the majority of the other stations in the ARB and surrounding watersheds. These results imply that at least some of the trends observed in the ARB were not solely related to changes in climate, but more likely reflected changes in land use and/or withdrawals from the system. 4.4. Temperature and precipitation results Warm-season temperature and precipitation data for the ARB and surrounding region were analyzed for the period of 1976– 2010. Mean temperature and total precipitation trends were calculated at each (10 km 10 km) grid point for the 35-year period warm season, and slope magnitudes were mapped over the entire study region. Significant trends are shown through p-value contour
lines and are reported at the 10% significance level. Results are displayed in Figs. 5 and 6, respectively. Warm-season temperatures both increased and decreased in no consistent pattern over the study region between 1976 and 2010, as shown in Fig. 5. This result is in agreement with the findings of Peters et al. (2013), who observed no noteworthy temperature trend over the ARB since 1977. Western Alberta, including the area just north of the ARB headwaters, experienced the strongest decreases in warm-season temperatures (significant at the 10% level) over the 35-year period. Several smaller regions in the nearby North SRB also exhibited significant decreasing temperature trends, while significant increasing trends were noted in the lower ARB as well as in the south-eastern portion of the study region. Trends in warm season streamflow in the ARB and surrounding region did not display a strong spatial relationship with temperature trends, although the cluster of significant decreasing streamflow trends in the mid-ARB tended to associate with nonsignificant decreasing tendencies in temperature. This observation, however, stands in contrast to the expected relationship between temperature and flow, where typically warmer temperatures result in drier conditions (reduced flow) and vice versa. Temperature
Fig. 5. Trend slopes (°C/yr) indicating mean temperature trends over the study area for the 1976–2010 warm season (March–October). Significant trends are indicated by 10% p-value contours. Warm-season streamflow trends at each gauge (indicated by upward- or downward-pointing triangles) are presented for comparison.
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Fig. 6. Trend slopes (mm/yr) indicating total precipitation trends over the study area for the 1976–2010 warm season (March–October). Significant trends are indicated by 10% p-value contours. Warm-season streamflow trends (indicated by upward- or downward-pointing triangles) are presented for comparison.
trends did not appear to be stronger or more significant in the ARB versus the surrounding watersheds, as was found with streamflow. Cold-season temperatures in the ARB exhibited no significant trend overall, but a non-significant increasing tendency was noted over much of the basin. Fig. 6 reveals that the majority of the study region experienced decreasing trends and tendencies in warm-season precipitation between 1976 and 2010. Similar observations were noted for the cold months of November to February. The upper and mid-ARB showed the strongest decreasing precipitation trends, significant at the 10% level. Decreasing warm-season streamflow patterns clearly reflected decreases in precipitation, as the majority of significant decreasing trends for both variables were concentrated in the region between the mountainous headwaters and the mid-ARB. In addition, increasing streamflow tendencies in the lower ARB corresponded to non-significant increasing tendencies in precipitation detected in this area. Trend slopes tended to be stronger for streamflow than for precipitation, as shown in Fig. 7. Unlike temperature trend results, precipitation trends did appear to be stronger and more negative in the ARB compared to the surrounding watersheds, in agreement with streamflow observations. Figs. 5 and 6 suggest that, for the 1976–2010 warm season, precipitation was a much stronger driver of streamflow in the ARB and surrounding regions than was temperature. Further analysis of the relationship between climate and streamflow was accomplished through trend analysis of temperature and precipitation data spatially averaged over each WSC gauged watershed. The localized effects of changing temperature and precipitation on runoff trends in this region are illustrated in Fig. 7, which shows that warm season precipitation trends accounted for more than twice the amount of variability in warm season runoff than did warm-season temperature trends (R2 of 62% versus 30%, respectively). Following the approach of McCabe and Wolock (2011), a multiple linear regression (MLR) model was developed to demonstrate the sensitivity or ‘‘elasticity’’ of streamflow in the ARB and surrounding regions to temperature and precipitation over the 1976–2010 warm season. The model expressed trend in runoff (mm/yr) as a function of trend in temperature (°C/yr) and trend in precipitation (mm/yr). The result of MLR over the entire study region defined runoff trend as:
Y runoff ¼ 0:46 13:38xtemp þ 5:39xprecip
Fig. 7. Scatterplots of trend slopes for temperature (°C/yr) and precipitation (mm/ yr) against runoff (mm/yr) for each WSC gauged watershed for the 1976–2010 warm season (March–October). Percent variance of runoff explained by each climate variable is given by the R2 value.
The R2 value of the MLR was 0.62. The MLR model temperature trend parameter was negative, indicating an overall negative relationship between temperature and runoff, as would be expected. As eluded to by the results of Figs. 5 and 7, however, this parameter was not significant at the 10% significance level, thus warm season runoff was not significantly affected by changes in temperature in this region for this period. The precipitation trend parameter, on the other hand, was positively and significantly (10% significance level) related to runoff over the entire study region, indicating a strong connection between trend in precipitation and trend in runoff. Analogous models with 10% significant precipitation trend parameters and non-significant temperature trend parameters were established for both the ARB (R2 = 0.69) and surrounding watersheds (R2 = 0.48). The higher R2 value for the ARB MLR model reflects the tendency of precipitation and streamflow trends to be more similar in the ARB than in the surrounding watersheds.
5. Discussion of results The overall decreasing trends observed for mean annual and warm season flow, flow during the summer months and maximum flow for both the ARB and surrounding watersheds may have even more critical implications than would at first appear. Schindler and
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Donahue (2006) indicated that while streamflow in major rivers feeding Canada’s Western Prairie Provinces (WPP) has significantly decreased over the past century, this period also appears to have been one of the wettest centuries of the past two millennia. Despite an average air temperature warming of 1–4 °C in the WPP over the past 80–118 years, 18th and 19th century droughts are reported to have been more frequent and more severe than any dry spell experienced during the 20th century. Several additional studies (e.g., Johnston et al., 2009; Wolfe et al., 2011) have confirmed that climate during the 20th century was unusually stable and moist. Wolfe et al. (2008) observed that freshwater in western Canada was only in a similar plentiful supply during one other multi-centennial interval (2000–1500 B.P.) of the past 5200 years, and that the transition from water abundance to scarcity can occur as fast as within one human lifespan. The Athabasca River has seen summer flows in its lower reaches decline by 30% since 1970 (Schindler and Donahue, 2006). Most large glaciers feeding the ARB headwaters have experienced shrinkage by approximately 25% during the last century and, if climate change continues as per projections, could lose an additional 80–90% of their volume by 2100 (Marshall et al., 2011); glacial sources to the Athabasca River may therefore eventually cease to exist. Numerous reports have indicated that glacial retreat in southwestern Canada has already advanced to the point where we may start to see significant decreases in runoff from the headwaters (Demuth et al., 2008; Moore et al., 2009; Peters et al., 2013). The ARB may already be showing signs of this; the three headwater gauges examined in this study (WSC stations 07AA001, 07AA002 and 07AD002) displayed decreasing tendencies annually and during the warm season for all periods with available data (see Fig. 2 for warm season variable trend results), resulting in significant decreases in mid-ARB runoff. Although the purpose of trend analysis is not to predict future occurrences, the direction and behavior of streamflow in the ARB and surrounding regions since at least 1976 suggest that declining water availability could very well continue into the future. Kerkhoven and Gan (2011) indicated that 21st century warming will result in further reduced mean annual flows due to declines in the spring snowpack, and that the hydrologic response of the ARB is more sensitive to projected temperature than precipitation changes. Here, however, we have shown that historical changes in Athabasca River region streamflow and runoff depend much more strongly on precipitation than on temperature. Unfortunately, the quality and certainty of climate-model projections of precipitation are less reliable than those of air temperature, thereby restricting the ability to project future streamflow. The impacts of these projections are critical when considering the growth rate of communities and industry in northeastern Alberta. The petroleum sector is the largest water user in the ARB, using primarily surface water for bitumen extraction via mining processes and in situ steam assisted gravity drainage (SAGD) systems. Current operations typically require two to four barrels of freshwater per barrel of oil produced through surface mining, and approximately 0.5 barrels of freshwater per barrel of oil produced by SAGD operations, although a great deal more water is often used during the earlier stages of these developments (Alberta Environment, 2013b). The petroleum industry accounts for 92% of licensed surface water use in the ARB, approximately 35.5% of which was actually used in 2005. The sector’s use of surface water has, however, been forecasted to increase by between 120% and 165% of its 2005 usage by 2025 (Mannix et al., 2010). Increases in surface water use could be of particular importance during the winter low flow months given the small capacity for storage in the ARB upstream of Lake Athabasca. As the oil industry continues to expand, the number of workers needed to run these operations also continues to grow, resulting in
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further water demands in the ARB for municipal needs. According to the provincial agency responsible for its management, water allocations in the ARB are growing at a rate nine times faster than the provincial average, having increased by 88% since 2000 (Mannix et al., 2010). These findings have important implications for water resources managers in western Canada and particularly in Alberta, as historical hydrologic and climatic data indicate that we may be heading into a period unprecedented over the past millennium. Recent tree-ring reconstructions of drought history in the eastern Rockies and Canadian Prairies have indicated that while the 20th century was representative of drought frequency over the long term, there are droughts of greater severity and especially duration in the proxy record (Axelson et al., 2009; Sauchyn et al., 2011). If similar droughts were to occur in the future, it is not clear what impacts they would have on activities that rely heavily on inputs from summer moisture (i.e., agriculture, hydropower, consumptive supplies) (St. George et al., 2009), especially given that at least some of the pre-instrumental periods of stable water deficits sustained droughts that current water use and management could not endure (Fleming and Sauchyn, 2013). Moreover, the hydrograph of the 21st century may become more extreme than that of the ‘‘Medieval Megadrought’’ (900–1300 A.D.) – a period described by flashy spring freshet and extremely low summer flows – when reduced glacial meltwater contributions were partly compensated by abundant snowmelt runoff (Wolfe et al., 2008). Wolfe et al. (2011) suggest that water allocations not be based solely on the short instrumental record (see also Sauchyn et al., 2011; Fleming and Sauchyn, 2013) as this will likely lead to an over-estimation of water availability in the Athabasca River region, particularly as the effects of climate warming are felt. 6. Conclusions An examination of trends in hydrologic variables for gauging stations in and around the ARB revealed generally decreasing trends over three time periods that were more prominent in the ARB than in the surrounding watersheds. Results indicated that at least some of the observed reductions in streamflow were likely a result of non-climatic factors, based on a comparison of results for RHBN and non-RHBN stations, while an analysis of trends in meteorological data over the study region revealed a strong relationship between precipitation and streamflow. A review of some of the relevant literature suggests that the recent hydroclimatic regime in the ARB has been unusually wet in comparison to conditions that have occurred in the past, and could occur again in the future. It is therefore reasonable for water managers to plan based on the possibility of substantially less runoff in the years to come. Moreover, climate-model projections for precipitation still require a great deal of improvement, thus future runoff in this region remains difficult to predict. Acknowledgements The authors gratefully acknowledge the reviewers for their helpful comments and feedback on this work. This work was supported by funds from Environment Canada and the Natural Sciences and Engineering Research Council of Canada (NSERC). References Abdul Aziz, O.I., Burn, D.H., 2006. Trends and variability in the hydrological regime of the Mackenzie River Basin. J. Hydrol. 319, 282–294. Alberta Energy, 2013. Facts and Statistics., (accessed June 2013). Alberta Environment and Sustainable Resource Development, 2013a. Athabasca River Basin. , (accessed June 2013).
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Marshall, S.J., White, E.C., Demuth, M.N., Bolch, T., Wheate, R., Menounos, B., Beedle, M.J., Shea, J.M., 2011. Glacier water resources on the eastern slopes of the Canadian Rocky Moutains. Can. Water Resour. J. 36 (2), 109–133. McCabe, G.J., Wolock, D.M., 2011. Independent effects of temperature and precipitation on modeled runoff in the conterminous United States. Water Resour. Res. 47 (W11522), 1–11. McKenney, D.W., Hutchinson, M.F., Papadopol, P., Lawrence, K., Pedlar, J., Campbell, K., Milewska, E., Hopkinson, R.F., Price, D., Owen, T., 2011. Customized spatial climate models for North America. Bull. Amer. Meteor. Soc., 1611–1622. Moore, R.D., Fleming, S.E., Menounos, B., Wheate, R., Fountain, A., Stahl, K., Holm, K., 2009. Glacier change in western North America: influences on hydrology, geomorphic hazards and water quality. Hydrol. Process. 23, 42–61. Peters, D.L., Atkinson, D., Monk, W.A., Tenenbaum, D.E., Baird, D.J., 2013. A multiscale hydroclimatic analysis of runoff generation in the Athabasca River, Western Canada. Hydrol. Process. 27 (13), 1915–1934. Rodionov, S.N., 2004. A sequential algorithm for testing climate regime shifts. Geophys. Res. Lett. 31 (L09204), 1–4. Rood, S.B., Pan, J., Gill, K.M., Franks, C.G., Samuelson, G.M., Shepherd, A., 2008. Declining summer flows of Rocky Mountain rivers: changing seasonal hydrology and probable impacts on floodplain forests. J. Hydrol. 349, 397–410. Sauchyn, D.J., Vanstone, J., Perez-Valdivia, C., 2011. Modes and forcing of hydroclimatic variability in the upper North Saskatchewan River basin since 1063. Can. Water Resour. J. 36 (3), 205–218. Schindler, D.W., Donahue, W.F., 2006. An impending water crisis in Canada’s western prairie provinces. Proc. Natl. Acad. Sci. 103 (19), 7210–7216. Sen, P.K., 1968. Estimates of the regression coefficient based on Kendall’s tau. J. Am. Stat. Assoc. 63, 1379–1389. St. George, S., Meko, D.M., Girardin, M.-P., MacDonald, G.M., Nielsen, E., Pederson, G.T., Sauchyn, D.J., Tardif, J.C., Watson, E., 2009. The tree-ring record of drought on the Canadian Prairies. J. Clim. 22 (3), 689–710. St. Jacques, J.M., Sauchyn, D.J., Zhao, Y., 2010. Northern Rocky Mountain streamflow records: global warming trends, human impacts or natural variability? Geophys. Res. Lett. 37 (L06407), 1–5. Stewart, I.T., Cayan, D.R., Dettinger, M.D., 2004. Changes toward earlier streamflow timing across Western North America. J. Clim. 18, 1136–1155. Toth, B., Pietroniro, A., Conly, F.M., Kouwen, N., 2006. Modelling climate change impacts in the Peace and Athabasca catchment and delta: I – Hydrological model application. Hydrol. Process. 20, 4197–4214. Whitfield, P.H., Cannon, A.J., 2000. Recent variations in climate and hydrology in Canada. Can. Water Resour. J. 25 (1), 19–65. Wilks, D.S., 2006. On ‘‘Field Significance’’ and the False Discovery Rate. J. Appl. Meteorol. Clim. 45, 1181–1189. Wolfe, B.B., Karst-Riddoch, T.L., Vardy, S.R., Falcone, M.D., Hall, R.I., Edwards, T.W.D., 2005. Impacts of climate and river flooding on the hydro-ecology of a floodplain basin, Peace-Athabasca Delta, Canada since A.D. 1700. Quaternary Res. 64 (2), 147–162. Wolfe, B.B., Hall, R.I., Edwards, T.W.D., Jarvis, S.R., Sinnatamby, R.N., Yi, Y., Johnston, J.W., 2008. Climate-driven shifts in quantity and seasonality of river discharge over the past 1000 years from the hydrographic apex of North America. Geophys. Res. Lett. 35 (L24402), 1–5. Wolfe, B.B., Edwards, T.W.D., Hall, R.I., Johnston, J.W., 2011. A 5200-year record of freshwater availability for regions in western North America fed by highelevation runoff. Geophys. Res. Lett. 38 (L11404), 1–5. Wrona, F.J., di Cenzo, P., 2011. Lower Athabasca Water Quality Monitoring Program, Phase 1, Athabasca River Mainstem and Major Tributaries, Environment Canada, ISBN 978-1-100-18471-5. Water Survey of Canada (WSC), 2010. HYDAT Database., (accessed November 2012). Xu, C.Y., Singh, V.P., 2004. Review on regional water resources assessment models under stationary and changing climate. Water Resour. Manage. 18, 591–612. Yue, S., Pilon, P.J., Phinney, B., Cavadias, G., 2002. The influence of autocorrelation on the ability to detect trend in hydrological series. Hydrol. Process. 16 (9), 1807– 1829. Zhang, X., Harvey, K.D., Hogg, W.D., Yuzyk, T.R., 2001. Trends in Canadian streamflow. Water Resour. Res. 37 (4), 987–998.
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A spatiotemporal analysis of hydrological trends and variability in the Athabasca River region, Canada Allison J. Bawden a, Hayley C. Linton b, Donald H. Burn a,⇑, Terry D. Prowse b a b
Department of Civil and Environmental Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada Water and Climate Impacts Research Centre, Environment Canada and University of Victoria, Victoria, BC V8W 3R4, Canada
a r t i c l e
i n f o
Article history: Received 26 June 2013 Received in revised form 19 November 2013 Accepted 25 November 2013 Available online 1 December 2013 This manuscript was handled by Andras Bardossy, Editor-in-Chief, with the assistance of Ashish Sharma, Associate Editor Keywords: Trend analysis Streamflow Climate change RHBN Athabasca
s u m m a r y Trends and variability in the hydrologic regime of the Athabasca River region were analyzed. Twenty hydrologic variables were selected for flow analysis within both the Athabasca River Basin (ARB) and, for comparison, surrounding watersheds. Intra- and inter-basin scale analyses were performed, including a comparison of changes in streamflow at stations forming part of the Reference Hydrometric Basin Network (RHBN) and non-designated gauges. Streamflow trends were also compared with trends in air temperature and precipitation over the entire Athabasca and surroundings study region. Noteworthy results include strong decreasing trends in annual, warm season (March to October) and summer month flows over the majority of the study region, in addition to a greater number of decreasing trends in Athabasca watershed flows compared to the surrounding basins. The timing of the spring freshet was found to have not shifted toward an earlier onset, contrary to results from previous studies. Lastly, trends in streamflow were similar to those for precipitation over the ARB and surrounding region, but did not relate strongly to trends in air temperature. The results of this study should be of assistance to water- resources managers and policy makers in making decisions about water use in this rapidly changing watershed. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Climate change/variability can have profound impacts on the hydrologic regime of a watershed. These impacts are likely to be especially severe when a watershed is located in an area that is particularly susceptible to the effects of a changing climate, such as the Canadian North, or when there are other stresses on the hydrologic regime, as may occur when there are large withdrawals from the watershed. Changes in regional water availability can affect many aspects of human society, from agricultural productivity and energy use to flood control, municipal and industrial water supply, and fisheries and wildlife management (Xu and Singh, 2004). Water is extremely important to both society and nature, thus understanding how a change in climate could affect regional water supply is essential for future water-resources planning (Kienzle et al., 2012). Streamflow is affected by changes in both evapotranspiration, which is dependent on air temperature, as well as precipitation. To quantify the effects of climate change/variability, a number of recent studies have conducted trend analyses on Canadian hydrometric and climatic time series. In a comparison of two different decades of Canadian hydrologic and meteorological data, for exam⇑ Corresponding author. Tel.: +1 (519) 888 4567x33338. E-mail address: [email protected] (D.H. Burn). 0022-1694/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhydrol.2013.11.051
ple, Whitfield and Cannon (2000) found the more recent decade to be generally warmer and characterized by both increases and decreases in precipitation and streamflow; these patterns were found to be spatially consistent in different areas of Canada. Zhang et al. (2001) found generally decreasing trends in Canadian river flow for 11 hydrometric variables, particularly in August and September, while significant increases in March and April streamflow were also observed. In an analysis of hydrologic variables from a network of 248 catchments across Canada, Burn and Hag Elnur (2002) found there to be pronounced spatial heterogeneity in climate-related effects on the hydrologic response. Centering on the high latitudes, Déry and Wood (2004, 2005) investigated trends in streamflow in the Canadian Arctic and found generally decreasing discharge that they related to various large-scale atmospheric phenomena. In northern British Columbia and the Yukon, Fleming and Clarke (2003) noted different trend responses to climatic warming in annual streamflow volume for catchments with and without a glacial cover. Although broad scale assessments of hydroclimatic trends provide useful indications of overall change/variability, more regionalized studies are typically needed to evaluate localized impact and potential adaptation measures. Given the importance of the western cordillera as a major water source in Canada, a number of studies have been conducted of rivers that flow to the western prairies or northern Canada from this alpine region. Rood et al.
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(2008), for example, identified earlier timing of spring peak flows and decreased summer flows for Rocky Mountain rivers. For the alpine-fed Athabasca and Liard River basins, Burn et al. (2004a,b) identified a general trend to earlier onset of spring runoff, but contrasting monthly trends for the two watersheds. A strong trend to increasing winter flow and the occurrence of an earlier spring freshet was similarly identified for the larger Mackenzie River Basin (MRB) by Abdul Aziz and Burn (2006); a trend to the latter effect in most recent years was also noted by Burn (2008). In an examination of flow for rivers feeding the western prairies, Schindler and Donahue (2006) found drastic reductions in summer discharge since the early 20th century, ranging from 20% in basins where human impact has been relatively minimal, such as the ARB, to 84% in regions where damming and large water withdrawals have occurred, such as the Saskatchewan River Basin (SRB). Most recently, Peters et al. (2013) identified different responses in runoff, temperature and precipitation for locations along the main stem of the Athabasca River, including a decline in runoff in the lower reaches of the ARB between 1958 and 2009 contrasted with a general lack of trend in the upper reaches. A fuller assessment of how hydrologic trends in the ARB compare with those of other catchments in the broader hydroclimatic region of the leeward slopes of the Canadian western cordillera can help to determine the degree of influence that changing climate has on streamflow in this watershed. A focus is placed on the ARB because of its ecological significance and rapidly transitioning industrial water uses. In the case of the former, the Peace-Athabasca Delta (PAD), formed by the confluence of the Peace and Athabasca rivers and the western end of Lake Athabasca, provides breeding habitat for a variety of wildlife (Wolfe et al., 2005), in addition to food and recreational opportunities for a number of First Nations communities residing in the surrounding municipalities. Any changes in flow to the PAD (e.g., by withdrawal, diversion or natural drying) could affect this ecologically sensitive wetland ecosystem. The ARB also contains a significant bitumen deposit, and a related extraction industry that accounted for 75% of total water allocations from the Athabasca River in 2010 – a volume that has been growing since 2000 (Alberta Environment, 2013a). Twenty variables, reflecting a variety of different characteristics of the hydrologic regime of the ARB, were examined. Data from 38 Water Survey of Canada (WSC) hydrometric gauging stations were obtained: 19 stations from within the ARB and 19 stations from nearby, surrounding watersheds. All gauges are considered to reflect unregulated flow conditions. Hydrologic variables were inter-compared among basins and with specific climatic variables to explore the extent of their effect on streamflow.
2. Data and methodology 2.1. Trend detection Trend detection in hydroclimatic research is frequently conducted by means of the Mann–Kendall (MK) non-parametric test (Mann, 1945; Kendall, 1975); this method was therefore adopted for this analysis. The MK test is rank-based and hence is robust to non-normality of the underlying data. The MK test is based on the null hypothesis that the data in a series are independently and identically distributed (i.e., that there is no trend present in the data); rejection of this hypothesis at a local significance level a indicates a significant trend in the data set. Following Burn et al. (2004a), the data used in this analysis were corrected for serial correlation through a modified version of the Trend Free Pre-Whitening (TFPW) approach developed by Yue et al. (2002). The TFPW approach attempts to separate the serial correlation that
arises from a (linear) trend from the remaining serial correlation, and then only removes the latter portion of the serial correlation. Although the TFPW procedure involves fitting and removing a linear trend, the MK trend detection procedure does not make any assumptions about the nature of the trend in a data set. Field significance was evaluated to assess if the trend results from the collection of stations were unusual; the field significance of the MK results was evaluated using Walker’s test (Wilks, 2006). Walker’s test considers the magnitude of the p-value of each of the K individual (local) trend tests to determine if the global null hypothesis – that all K local null hypotheses are true – can be rejected at global significance level aglobal. For field significance to be sustained, the smallest local p-value must be no larger than some critical p-value derived based on aglobal and the number of local tests K. If the smallest p-value is small enough, it can be concluded with high confidence that the collection of K local p-values did not result from independent draws from a uniform distribution. Evaluation of field significance using the minimum local p-value as the global test statistic allows for better identification of locations with significant differences, because fewer than aglobal 100% (on average) of apparently significant local tests will have resulted from local null hypotheses that are actually true (Wilks, 2006). In addition to MK test statistics, trend slopes were estimated for each variable using a robust non-parametric slope estimator (Sen, 1968). The trend slope indicates the magnitude and direction of the trend in runoff depth (or days, depending on the hydrologic variable) per time period. To verify homogeneity in the data, a regime shift detection method was employed (Bering Climate, 2006). In hydrology, regimes shifts are generally symptoms of natural climate change/ variability, and are often linked to decadal fluctuations in atmospheric circulation (Stewart et al., 2004). Continuity, or a lack of regime shifts, is often necessary to draw accurate conclusions from trend analysis. The method developed by Rodionov (2004) was used to detect whether a common regime shift at each station could have contributed to trends detected by the MK test. 2.2. Variables analyzed A number of hydroclimatic studies have hypothesized that climate change could cause a variety of impacts on the hydrologic regime of the ARB region; hence, twenty variables that reflect different aspects of the hydrologic response were selected. These included: annual mean flow, monthly mean flow for each of the 12 months, mean flow for the warm season (an average of the flows from March to October), annual maximum daily flow and its date of occurrence, annual minimum daily flow and its date of occurrence, and a measure of the onset of the spring freshet and the delay (number of days) from the start of the freshet to the date of occurrence of annual peak flow. The date of the start of the spring freshet was estimated as the Julian day by which ten percent of the annual streamflow volume had occurred (hereafter referred to as the ten percentile or ‘‘10P’’ date). The 10P date was calculated based on the calendar year. Burn (2008) concluded that the 10P date is a more effective measure of the onset of the spring freshet than the pulse date (Cayan et al., 2001), a measure that has also been used to define the beginning of the spring freshet. Watershed size does not affect the efficacy of the 10P date to capture the onset of the spring freshet, even when a large range of basin areas is considered (Burn, 2008). The data used in this analysis were acquired via Environment Canada’s WSC HYDAT database (WSC, 2010). To further understand the effects of climate change on the hydrology of the ARB, hydrologic trend results were compared with climatic trends calculated for meteorological data spanning
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the entire study region. Daily mean air temperature and total precipitation data were acquired through a newly available, high resolution (10 km 10 km) gridded dataset produced by Natural Resources Canada based on the methods described in Hutchinson et al. (2009) and McKenney et al. (2011). The MK test was used to calculate trend at each grid point, and pixel trajectory analysis was applied to map trend slope values over the study region (Gralewicz et al., 2010). 3. Application 3.1. Athabasca River Basin The ARB spans the provinces of Alberta and Saskatchewan, forming the southern-most component of the MRB. At 1538 km in length, the Athabasca River is Alberta’s largest undammed river, with a total drainage area in excess of 150,000 km2. The river’s headwaters originate in the Columbia Icefield near Jasper and extend northeast to Lake Athabasca before eventually emptying into the Arctic Ocean via the Mackenzie River. Basin elevations range from 185 m.a.s.l. near the Lake Athabasca lowlands to nearly 3,700 m.a.s.l. in the mountainous headwaters. Tributary rivers flow over four of Alberta’s six natural regions: the Rocky Mountains, the Foothills, the Boreal forest and the Canadian Shield (ARBRI, 2013). The ARB experiences a mean annual temperature of 2 °C and an average yearly precipitation of 460 mm, roughly 75% of which occurs as rain. The hydrologic regime of the basin is characterized by low winter flows, with a snowmelt-driven rising hydrograph beginning in late April or May. Peak flows generally occur in June or July and are fuelled by snowpack runoff from the Rocky Mountains. The remaining months experience gradually declining flows until winter low flow conditions are resumed in December (Burn et al., 2004b; Toth et al., 2006). Maximum flows in the ARB are sensitive to variability in winter/spring precipitation and temperature, as snow accumulation/ablation and the rate and timing of snowmelt are highly influenced by these climatic drivers. Peters et al. (2013) also demonstrated a strong relationship between ARB runoff and the Pacific North American mode of atmospheric variability, in addition to a weaker teleconnection with the Pacific Decadal Oscillation (see also St. Jacques et al., 2010). The ARB is rich in natural resources, including forestry, mineral deposits, peat deposits, fish and game, and most notably, oil and
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gas. The Athabasca oil sands, in combination with two similar deposits in the nearby Peace River and Cold Lake regions, comprise the world’s third largest crude oil reserve after Saudi Arabia and Venezuela (Alberta Energy, 2013). The Athabasca River and its tributaries flow through the Athabasca oil sands deposits resulting in erosion of oil-containing sands. Most current and proposed oil sands developments are located within the lower portions of the Athabasca River, primarily downstream of Fort McMurray, Alberta. The development of the oil sands formations can have potential environmental impacts on the air, land and water, including increased water use (both surface and groundwater), production of waste streams, and changes in hydrologic connectivity resulting from land disturbance (Wrona and di Cenzo, 2011). The locations of the gauging stations analyzed are shown in Fig. 1. The attributes of these stations are summarized in Table 1. Stations that are part of the RHBN are also indicated in Fig. 1 and Table 1. The RHBN (Harvey et al., 1999) is a sub-set of stations in the national hydrometric network and contains over 200 gauges that have been identified for use in the detection, monitoring and assessment of climate change. There are only four RHBN stations in the ARB, while a further five RHBN stations were selected from watersheds surrounding the ARB.
4. Results 4.1. Trend analysis The results of MK trend detection are summarized in Table 2. The hydrometric time-series data were analyzed for three different periods: 1976–2010 (35 years), 1971–2010 (40 years) and 1966– 2010 (45 years). The three periods reflect a trade-off between greater spatial coverage with a shorter period versus greater power for the statistical tests for a longer period. For each case, a data set was only used for analysis if there were no more than four years of missing data within the period. As a result, the nominal number of stations for the longer periods was less than the total number of stations overall due to missing data for some variables and stations. This was particularly problematic for stations that operated on a seasonal basis only (generally March to October), for which flows in the winter period were not available. To account for this, the mean warm-season flow variable was included in the analysis.
Fig. 1. Map of the ARB and surrounding watersheds showing locations of hydrometric gauging stations analyzed. RHBN stations are identified by white crosses. Top-right map shows location of the ARB within Canada.
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Table 1 Characteristics of stations analyzed. WSC Station
River
Period of record
Drainage area (km2)
Elevationa (m.a.s.l.)
Athabasca River Basin 07AA001b 07AA002b 07AD002 07AF002 07AG003 07BB002 07BC002 07BE001 07BF002 07BJ001 07BK007 07CA005 07CA006 07CD001b 07DA001 07DA006 07DA008 07DC001 07DD002b
Miette River near Jasper Athabasca River near Jasper Athabasca River at Hinton McLeod River above Embarras River Wolf Creek at Highway No. 16A Pembina River near Entwistle Pembina River at Jarvie Athabasca River at Athabasca West Prairie River near High Prairie Swan River near Kinuso Driftwood River near the Mouth Pine Creek near Grassland Wandering River near Wandering River Clearwater River at Draper Athabasca River below McMurray Steepbank River near Fort McMurray Muskeg River near Fort MacKay Firebag River near the Mouth Richardson River near the Mouth
1974–2010 1970–2010 1961–2010 1954–2010 1955–2010 1955–2010 1961–2010 1952–2010 1959–1970c, 1971–2010 1961–1970c,1971–2010 1968–1970c,1971–2010 1966–2010c 1970–1997, 1998–2010c 1957–2010 1957–2010 1974–1986, 1987–2010c 1974–1986, 1987–2010c 1972–1986, 1987–2010c 1970–1986, 1987–2010c
629 3870 9760 2560 826 4400 13,100 74,600 1150 1900 2100 1460 1120 30,800 133,000 1320 1460 5990 2730
1062 1059 954 920 862 718 609 516 588 588 550 556 549 261 237 275 253 227 258
Peace River Basin 07EE007 07FB001b 07FC001 07GE001 07GH002 07GJ001 07JD002
Parsnip River above Misinchinka River Pine River at East Pine Beatton River near Fort St. John Wapiti River near Grande Prairie Little Smoky River near Guy Smoky River at Watino Wabasca River at Wadlin Lake Road
1968–2010 1961–2010 1961–2010 1960–2010 1959–2010 1955–2010 1970–2010
4930 12,100 15,600 11,300 11,100 50,300 35,800
712 549 465 514 477 378 307
Lake Athabasca Drainage 07LB002 Waterfound River below Theriau Lake 07LE002b Fond du Lac River at outlet of Black Lake 07MA003 Douglas River near Cluff Lake 07MB001 Macfarlane River at outlet of Davy Lake
1974–2010 1963–2010 1975–2010 1967–2010
3160 50,700 1690 9120
420 221 297 335
Churchill River Basin 06AD006 06AG002 06BB005 06BD001b 06CD002b 06DA004b
1955–2010 1971–2010 1973–2010 1967–2010 1963–2010 1966–2010
14,500 2960 4730 3680 119,000 7730
511 427 423 430 353 415
1971–2010 1969–2010
876 1890
930 665
Beaver River at Cold Lake Reserve Dore River near the Mouth Canoe River near Beauval Haultain River near Norbert River Churchill River above Otter Rapids Geikie River below Wheeler River
North Saskatchewan River Basin 05DD009 Nordegg River at Sunchild Road 05EA005 Sturgeon River at Villeneuve a b c
Elevations are given for each gauging station. Stations forming part of the RHBN. Seasonal records containing data from March to October only.
Application of a regime shift detection method indicated that streamflow data from the ARB and surrounding watersheds did not appear to be overall affected by any common shift(s) in the climatic system over any of the three periods; hence, trend results are assumed to reflect trends in the data as a result of continuous changes in this region. Table 2 presents results separately for gauging stations in the ARB and from surrounding watersheds. This was done to allow a determination of any unique trend patterns for the ARB. Percentages indicate the fraction of stations within each grouping (i.e., Athabasca or surrounding stations) that displayed significant increasing (") and/or decreasing (;) trends. The total number of stations for each variable was dependent on the lengths of data records; the longer time periods and winter variables tended to include results from fewer stations in the calculation of percentage. Table 2 also shows the variables that demonstrated field significance, indicating that the collection of K local results was globally significant. Field significance was evaluated separately for the ARB and for the surrounding watersheds. As this analysis was exploratory in nature, it was desirable to look at both established as well
as emerging trends in the data; thus, all statistical tests (i.e., MK and field significance) are reported at the 10% significance level. Table 2 reveals that many of the variables exhibited results that are field significant. Most streamflow variables displayed primarily decreasing trends and tendencies, where all significant trends were in the negative direction, while a minority of variables exhibited both significant decreasing and increasing trends; very few variables exhibited more significant increasing than decreasing trends. Other noteworthy results included the general tendency for decreasing mean annual flow, decreasing mean warm-season flow and corresponding decreasing flows in the months of June, July and August, decreasing annual maximum flows, and both decreasing and increasing annual minimum flows. The general pattern of decreasing flow, particularly for the warm season and on an annual basis, is a potential cause for concern, especially considering the level of proposed development within the ARB (see also Schindler and Donahue, 2006). Interestingly, there were comparatively few trends identified for the timing variables. Earlier work (Burn, 1994, 2008) showed decreasing trends for both the date of the annual maximum flow and the start of the spring freshet, implying
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A.J. Bawden et al. / Journal of Hydrology 509 (2014) 333–342 Table 2 Percentage of stations showing a significant decreasing and/or increasing trend in streamflowa. Hydrologic Variable
Annual mean Warm season mean January February March April May June July August September October November December Annual maximum Date of maximum Annual minimum Date of minimum Date of 10th percentile 10P Date to max date
ARB
Surrounding watersheds
1976–2010
1971–2010
1966–2010
1976–2010
1971–2010
1966–2010
;46/"0 ;37/"5 ;21/"0 ;21/"7 ;26/"5 ;11/"0 ;5/"0 ;16/"0 ;21/"0 ;53/"0 ;21/"5 ;26/"0 ;23/"8 ;23/"0 ;13/"0 ;13/"0 ;15/"15 ;8/"0 ;0/"8 ;8/"0
;67/"0 ;67/"0 ;33/"0 ;42/"8 ;25/"6 ;38/"0 ;25/"0 ;44/"0 ;50/"0 ;50/"0 ;38/"6 ;53/"0 ;33/"8 ;42/"0 ;54/"0 ;0/"0 ;42/"8 ;0/"0 ;9/"0 ;0/"0
;38/"0 ;55/"0 ;25/"0 ;25/"0 ;9/"9 ;36/"0 ;27/"0 ;36/"0 ;18/"0 ;55/"0 ;36/"0 ;45/"0 ;40/"10 ;30/"0 ;40/"0 ;10/"0 ;25/"13 ;13/"0 ;0/"0 ;0/"0
;21/"0 ;26/"0 ;16/"5 ;16/"5 ;16/"5 ;5/"11 ;5/"0 ;16/"0 ;16/"0 ;21/"0 ;11/"11 ;11/"5 ;11/"0 ;5/"11 ;21/"0 ;5/"5 ;28/"17 ;6/"0 ;6/"0 ;0/"6
;47/"0 ;47/"0 ;29/"12 ;24/"12 ;24/"6 ;18/"18 ;29/"0 ;24/"0 ;41/"0 ;41/"0 ;12/"0 ;29/"0 ;24/"6 ;29/"0 ;35/"0 ;0/"0 ;29/"7 ;7/"7 ;15/"0 ;0/"0
;40/"0 ;36/"0 ;9/"9 ;18/"9 ;18/"9 ;9/"18 ;18/"0 ;18/"0 ;9/"0 ;18/"0 ;9/"9 ;18/"0 ;9/"0 ;9/"9 ;10/"0 ;0/"0 ;22/"11 ;0/"11 ;0/"0 ;11/"0
a Downward (upward) pointing arrows represent percentage of stations exhibiting decreasing (increasing) trends significant at the 10% level. Entries in bold indicate results that are field significant at the 10% level. The ARB and surrounding watersheds each contain 19 stations with 35 years of data for all variables, however fewer stations were considered for the longer periods due to shorter data records.
that these events have occurred earlier in more recent years. While Table 2 shows some decreasing trends for the dates of both the annual maximum and minimum flows (more prevalent for stations in the ARB), none of these results were field significant, implying the trends were not strong. No consistent decreasing or increasing trends were detected for all three periods for either of the freshet timing variables. Table 2 provides a comparison of the results for the stations within the ARB with those from the surrounding watersheds. There were two potential complicating factors in comparing these results. First, the stations from the Athabasca tended to have smaller drainage areas than the surrounding stations. This can be an issue as others have found that trend response can be different for watersheds of different sizes. However, this was not likely to be a major issue for this data set, as the range of watershed areas for the two collections of stations was comparable. Second, there were nine stations from the ARB that were seasonal for at least part of the available period of record (see Table 1). There were no seasonal stations selected for the watersheds surrounding the ARB. Furthermore, all of the seasonal stations were from smaller watersheds (drainage areas of less than 6000 km2). For these stations, many of the hydrologic variables described above could not be defined and hence there were fewer stations for which trends could be calculated. Particularly problematic in this regard were the annual mean flow measure and the monthly mean flow for the coldseason months of November to February. In spite of the limitations outlined above, there were strong similarities in both the overall trend patterns and in the hydrologic variables that exhibited field significance when comparing the two collections of stations. It is noteworthy that the Athabasca stations tended to exhibit more decreasing trends and slightly fewer increasing trends than were observed for the stations from the surrounding watersheds. Similar results were noted for all three periods, indicating that this was a general effect, and not particular to one period. 4.2. Warm-season trends Fig. 2 displays warm-season flow trend results for the 1976– 2010 period. This period was selected for display because it
provides the best spatial coverage of stations of all the periods considered. Results for the warm-season variable are shown since this measure provides a better indication of the spatial change patterns within the ARB (compared to the mean annual flow variable) as data are available for all gauging stations, including seasonal stations. Fig. 2 indicates the locations of gauging stations that exhibited increasing (or decreasing) trends significant at the 10% significance level, as well as the trend tendencies for the stations that did not exhibit statistically significant trends. As can be seen in Fig. 2, the stations that exhibited significant decreasing trends and tendencies were spatially concentrated in the central and upper portions of the ARB as well as in adjacent portions of the Peace River basin and, to a lesser extent, the North SRB. This observation was also noted for the mean annual flow trend results. Interestingly, the seasonal stations located in the lower portions of the ARB exhibited primarily (non-significant) increasing tendencies in the warm-season flow, and one station in this region (the Firebag River near the mouth) exhibited a significant increasing trend. A dependence on elevation is evident for streamflow trends in the ARB, with mainly decreasing trends and tendencies observed in the higher elevation headwaters, and primarily increasing tendencies (or a lack of trend) noted at lower elevations (i.e., at stations located at elevations lower than 300 m.a.s.l., with the exception of the Fort McMurray station). This relationship was not as pronounced in the surrounding watersheds, though the number of high-elevation stations was also lower in this region. The magnitude of the trend slope provides a measure of the effect of a trend for a particular variable. Trend slopes were calculated for all variables and periods, and the results for the flow variables were converted to a runoff depth (reported in units of mm per hydrologic variable time frame) to remove the effects of watershed size on the slope magnitude. Fig. 3 summarizes the trend slope results in the form of box plots for the warm-season flow variable for each of the three periods. The first box plot in each pair shows results for stations from the ARB, while the second box plot shows results for the stations from surrounding watersheds. The results shown in Fig. 3 reveal considerably more negative slopes for the stations from the ARB in comparison to the surrounding stations. Almost all of the stations from the ARB
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Fig. 2. Warm-season (March–October) streamflow trends at each station for the 1976–2010 period.
Fig. 3. Box plots of trend slopes for the warm-season (March–October) runoff. Box defines 25th, 50th and 75th percentile trend slopes, and whiskers extend to the most extreme data points not considered outliers. Outliers are shown as crosses.
displayed negative slopes, indicating a decreasing trend in warm-season flow. While the majority of the surrounding stations also showed negative slopes, there were also a fair number of positive slopes for all three periods. This general pattern of more negative slopes in the ARB than in surrounding watersheds was also noted for the mean annual flow variable as well as for the months of April, May and July to December, again implying that there were differences between the trend responses for the ARB and the corresponding responses for the surrounding areas. Box plots of runoff trends by percent change per year showed similar results, indicating that differences in the relative amount of overall runoff at each station was not a factor in these results.
4.3. RHBN stations Since RHBN stations are intended to reflect watersheds that have experienced minimal changes in land-use, withdrawals and regulation, these stations can be expected to provide a good representation of the effects of climate change over the watershed. It was therefore useful to compare the trend response for RHBN stations with the trend responses of other stations. Since the climate
signal at all stations should be similar, noticeable differences between RHBN and non-RHBN stations can be interpreted as likely arising from non-climatic factors, such as land-use changes. Warm-season trends for the RHBN stations analyzed are presented in Fig. 4. As noted, there are four RHBN stations located within the ARB. Two of these stations, the Miette River near Jasper and the Athabasca River near Jasper, are located in the headwaters of the ARB. Both of these stations have fairly short data records, but do provide a contrast with the non-RHBN stations within the ARB. Both stations exhibited a significant decreasing trend in August, as was also observed for the ARB as a whole, but beyond this, there were very few significant trends for the two RHBN stations. There were also more variables that displayed increasing tendencies than was noted for the other stations in the ARB. The third RHBN station, the Clearwater River at Draper, exhibited very few significant trends, although in contrast to the two headwater RHBN stations, displayed generally decreasing tendencies, more in line with the decreasing trends and tendencies noted for the other stations in the ARB. The final RHBN station within the ARB is the Richardson River near the mouth, which is a seasonal station. A significant increasing trend was observed for September for this station, and many other variables demonstrated increasing tendencies. Of the five RHBN stations from outside the ARB, one is located in the Peace River Basin, three are from the Churchill River system, and one is part of the Lake Athabasca drainage area. The station from the Peace River Basin, the Pine River at East Pine, exhibited very few significant trends. This station displayed a generally increasing tendency in the cold season and a decreasing tendency during other parts of the year. Two of the stations from the Churchill River system, the Haultain River near Norbert River and the Churchill River above Otter Rapids, both exhibited increasing trends in the cold season with mixed results for the other seasons (generally increasing tendencies for the Haultain and decreasing tendencies for the Churchill). The third RHBN station from the Churchill River system, the Geikie River below Wheeler River, showed results that were generally consistent with the non-RHBN stations from the ARB (i.e., generally decreasing trends on both an annual basis and during the warm season). The RHBN station from the Lake Athabasca drainage area, the Fond du Lac River at outlet of Black Lake, exhibited very few significant trends. The trend tendency for this station was generally increasing, with the exception of the most recent period (1976–2010), where the tendency was generally decreasing.
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Fig. 4. Warm-season (March–October) streamflow trends at RHBN stations for the 1976–2010 period.
There were, therefore, observed differences in trend behavior between RHBN and non-RHBN stations both within and around the ARB. The RHBN stations were more likely to exhibit increasing trends in the cold season (November to February), while non-RHBN stations in this region tended to show decreasing trends or tendencies during these months. Similarly, there were more increasing trends on an annual basis and in the warm season for the RHBN stations than what was observed for the majority of the other stations in the ARB and surrounding watersheds. These results imply that at least some of the trends observed in the ARB were not solely related to changes in climate, but more likely reflected changes in land use and/or withdrawals from the system. 4.4. Temperature and precipitation results Warm-season temperature and precipitation data for the ARB and surrounding region were analyzed for the period of 1976– 2010. Mean temperature and total precipitation trends were calculated at each (10 km 10 km) grid point for the 35-year period warm season, and slope magnitudes were mapped over the entire study region. Significant trends are shown through p-value contour
lines and are reported at the 10% significance level. Results are displayed in Figs. 5 and 6, respectively. Warm-season temperatures both increased and decreased in no consistent pattern over the study region between 1976 and 2010, as shown in Fig. 5. This result is in agreement with the findings of Peters et al. (2013), who observed no noteworthy temperature trend over the ARB since 1977. Western Alberta, including the area just north of the ARB headwaters, experienced the strongest decreases in warm-season temperatures (significant at the 10% level) over the 35-year period. Several smaller regions in the nearby North SRB also exhibited significant decreasing temperature trends, while significant increasing trends were noted in the lower ARB as well as in the south-eastern portion of the study region. Trends in warm season streamflow in the ARB and surrounding region did not display a strong spatial relationship with temperature trends, although the cluster of significant decreasing streamflow trends in the mid-ARB tended to associate with nonsignificant decreasing tendencies in temperature. This observation, however, stands in contrast to the expected relationship between temperature and flow, where typically warmer temperatures result in drier conditions (reduced flow) and vice versa. Temperature
Fig. 5. Trend slopes (°C/yr) indicating mean temperature trends over the study area for the 1976–2010 warm season (March–October). Significant trends are indicated by 10% p-value contours. Warm-season streamflow trends at each gauge (indicated by upward- or downward-pointing triangles) are presented for comparison.
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Fig. 6. Trend slopes (mm/yr) indicating total precipitation trends over the study area for the 1976–2010 warm season (March–October). Significant trends are indicated by 10% p-value contours. Warm-season streamflow trends (indicated by upward- or downward-pointing triangles) are presented for comparison.
trends did not appear to be stronger or more significant in the ARB versus the surrounding watersheds, as was found with streamflow. Cold-season temperatures in the ARB exhibited no significant trend overall, but a non-significant increasing tendency was noted over much of the basin. Fig. 6 reveals that the majority of the study region experienced decreasing trends and tendencies in warm-season precipitation between 1976 and 2010. Similar observations were noted for the cold months of November to February. The upper and mid-ARB showed the strongest decreasing precipitation trends, significant at the 10% level. Decreasing warm-season streamflow patterns clearly reflected decreases in precipitation, as the majority of significant decreasing trends for both variables were concentrated in the region between the mountainous headwaters and the mid-ARB. In addition, increasing streamflow tendencies in the lower ARB corresponded to non-significant increasing tendencies in precipitation detected in this area. Trend slopes tended to be stronger for streamflow than for precipitation, as shown in Fig. 7. Unlike temperature trend results, precipitation trends did appear to be stronger and more negative in the ARB compared to the surrounding watersheds, in agreement with streamflow observations. Figs. 5 and 6 suggest that, for the 1976–2010 warm season, precipitation was a much stronger driver of streamflow in the ARB and surrounding regions than was temperature. Further analysis of the relationship between climate and streamflow was accomplished through trend analysis of temperature and precipitation data spatially averaged over each WSC gauged watershed. The localized effects of changing temperature and precipitation on runoff trends in this region are illustrated in Fig. 7, which shows that warm season precipitation trends accounted for more than twice the amount of variability in warm season runoff than did warm-season temperature trends (R2 of 62% versus 30%, respectively). Following the approach of McCabe and Wolock (2011), a multiple linear regression (MLR) model was developed to demonstrate the sensitivity or ‘‘elasticity’’ of streamflow in the ARB and surrounding regions to temperature and precipitation over the 1976–2010 warm season. The model expressed trend in runoff (mm/yr) as a function of trend in temperature (°C/yr) and trend in precipitation (mm/yr). The result of MLR over the entire study region defined runoff trend as:
Y runoff ¼ 0:46 13:38xtemp þ 5:39xprecip
Fig. 7. Scatterplots of trend slopes for temperature (°C/yr) and precipitation (mm/ yr) against runoff (mm/yr) for each WSC gauged watershed for the 1976–2010 warm season (March–October). Percent variance of runoff explained by each climate variable is given by the R2 value.
The R2 value of the MLR was 0.62. The MLR model temperature trend parameter was negative, indicating an overall negative relationship between temperature and runoff, as would be expected. As eluded to by the results of Figs. 5 and 7, however, this parameter was not significant at the 10% significance level, thus warm season runoff was not significantly affected by changes in temperature in this region for this period. The precipitation trend parameter, on the other hand, was positively and significantly (10% significance level) related to runoff over the entire study region, indicating a strong connection between trend in precipitation and trend in runoff. Analogous models with 10% significant precipitation trend parameters and non-significant temperature trend parameters were established for both the ARB (R2 = 0.69) and surrounding watersheds (R2 = 0.48). The higher R2 value for the ARB MLR model reflects the tendency of precipitation and streamflow trends to be more similar in the ARB than in the surrounding watersheds.
5. Discussion of results The overall decreasing trends observed for mean annual and warm season flow, flow during the summer months and maximum flow for both the ARB and surrounding watersheds may have even more critical implications than would at first appear. Schindler and
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Donahue (2006) indicated that while streamflow in major rivers feeding Canada’s Western Prairie Provinces (WPP) has significantly decreased over the past century, this period also appears to have been one of the wettest centuries of the past two millennia. Despite an average air temperature warming of 1–4 °C in the WPP over the past 80–118 years, 18th and 19th century droughts are reported to have been more frequent and more severe than any dry spell experienced during the 20th century. Several additional studies (e.g., Johnston et al., 2009; Wolfe et al., 2011) have confirmed that climate during the 20th century was unusually stable and moist. Wolfe et al. (2008) observed that freshwater in western Canada was only in a similar plentiful supply during one other multi-centennial interval (2000–1500 B.P.) of the past 5200 years, and that the transition from water abundance to scarcity can occur as fast as within one human lifespan. The Athabasca River has seen summer flows in its lower reaches decline by 30% since 1970 (Schindler and Donahue, 2006). Most large glaciers feeding the ARB headwaters have experienced shrinkage by approximately 25% during the last century and, if climate change continues as per projections, could lose an additional 80–90% of their volume by 2100 (Marshall et al., 2011); glacial sources to the Athabasca River may therefore eventually cease to exist. Numerous reports have indicated that glacial retreat in southwestern Canada has already advanced to the point where we may start to see significant decreases in runoff from the headwaters (Demuth et al., 2008; Moore et al., 2009; Peters et al., 2013). The ARB may already be showing signs of this; the three headwater gauges examined in this study (WSC stations 07AA001, 07AA002 and 07AD002) displayed decreasing tendencies annually and during the warm season for all periods with available data (see Fig. 2 for warm season variable trend results), resulting in significant decreases in mid-ARB runoff. Although the purpose of trend analysis is not to predict future occurrences, the direction and behavior of streamflow in the ARB and surrounding regions since at least 1976 suggest that declining water availability could very well continue into the future. Kerkhoven and Gan (2011) indicated that 21st century warming will result in further reduced mean annual flows due to declines in the spring snowpack, and that the hydrologic response of the ARB is more sensitive to projected temperature than precipitation changes. Here, however, we have shown that historical changes in Athabasca River region streamflow and runoff depend much more strongly on precipitation than on temperature. Unfortunately, the quality and certainty of climate-model projections of precipitation are less reliable than those of air temperature, thereby restricting the ability to project future streamflow. The impacts of these projections are critical when considering the growth rate of communities and industry in northeastern Alberta. The petroleum sector is the largest water user in the ARB, using primarily surface water for bitumen extraction via mining processes and in situ steam assisted gravity drainage (SAGD) systems. Current operations typically require two to four barrels of freshwater per barrel of oil produced through surface mining, and approximately 0.5 barrels of freshwater per barrel of oil produced by SAGD operations, although a great deal more water is often used during the earlier stages of these developments (Alberta Environment, 2013b). The petroleum industry accounts for 92% of licensed surface water use in the ARB, approximately 35.5% of which was actually used in 2005. The sector’s use of surface water has, however, been forecasted to increase by between 120% and 165% of its 2005 usage by 2025 (Mannix et al., 2010). Increases in surface water use could be of particular importance during the winter low flow months given the small capacity for storage in the ARB upstream of Lake Athabasca. As the oil industry continues to expand, the number of workers needed to run these operations also continues to grow, resulting in
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further water demands in the ARB for municipal needs. According to the provincial agency responsible for its management, water allocations in the ARB are growing at a rate nine times faster than the provincial average, having increased by 88% since 2000 (Mannix et al., 2010). These findings have important implications for water resources managers in western Canada and particularly in Alberta, as historical hydrologic and climatic data indicate that we may be heading into a period unprecedented over the past millennium. Recent tree-ring reconstructions of drought history in the eastern Rockies and Canadian Prairies have indicated that while the 20th century was representative of drought frequency over the long term, there are droughts of greater severity and especially duration in the proxy record (Axelson et al., 2009; Sauchyn et al., 2011). If similar droughts were to occur in the future, it is not clear what impacts they would have on activities that rely heavily on inputs from summer moisture (i.e., agriculture, hydropower, consumptive supplies) (St. George et al., 2009), especially given that at least some of the pre-instrumental periods of stable water deficits sustained droughts that current water use and management could not endure (Fleming and Sauchyn, 2013). Moreover, the hydrograph of the 21st century may become more extreme than that of the ‘‘Medieval Megadrought’’ (900–1300 A.D.) – a period described by flashy spring freshet and extremely low summer flows – when reduced glacial meltwater contributions were partly compensated by abundant snowmelt runoff (Wolfe et al., 2008). Wolfe et al. (2011) suggest that water allocations not be based solely on the short instrumental record (see also Sauchyn et al., 2011; Fleming and Sauchyn, 2013) as this will likely lead to an over-estimation of water availability in the Athabasca River region, particularly as the effects of climate warming are felt. 6. Conclusions An examination of trends in hydrologic variables for gauging stations in and around the ARB revealed generally decreasing trends over three time periods that were more prominent in the ARB than in the surrounding watersheds. Results indicated that at least some of the observed reductions in streamflow were likely a result of non-climatic factors, based on a comparison of results for RHBN and non-RHBN stations, while an analysis of trends in meteorological data over the study region revealed a strong relationship between precipitation and streamflow. A review of some of the relevant literature suggests that the recent hydroclimatic regime in the ARB has been unusually wet in comparison to conditions that have occurred in the past, and could occur again in the future. It is therefore reasonable for water managers to plan based on the possibility of substantially less runoff in the years to come. Moreover, climate-model projections for precipitation still require a great deal of improvement, thus future runoff in this region remains difficult to predict. Acknowledgements The authors gratefully acknowledge the reviewers for their helpful comments and feedback on this work. This work was supported by funds from Environment Canada and the Natural Sciences and Engineering Research Council of Canada (NSERC). References Abdul Aziz, O.I., Burn, D.H., 2006. Trends and variability in the hydrological regime of the Mackenzie River Basin. J. Hydrol. 319, 282–294. Alberta Energy, 2013. Facts and Statistics.
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