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Short-term changes in the magnitude, frequency and temporal distribution of floods in the Eastern Mediterranean region during the last 45 years — Nahal Oren, Mt. Carmel, Israel
Geomorphology 84 (2007) 181 – 191 www.elsevier.com/locate/geomorph
Short-term changes in the magnitude, frequency and temporal distribution of floods in the Eastern Mediterranean region during the last 45 years — Nahal Oren, Mt. Carmel, Israel Lea Wittenberg a,⁎, Haim Kutiel a , Noam Greenbaum a,b , Moshe Inbar a b
a Department of Geography and Environmental Studies, University of Haifa, Haifa 31905, Israel Department of Natural Resources and Environmental Management, University of Haifa, Haifa 31905, Israel
Received 15 April 2005; received in revised form 2 January 2006; accepted 2 January 2006 Available online 28 August 2006
Abstract Short-term changes in Eastern Mediterranean precipitation affecting flow regime were documented in Nahal Oren, a 35 km2 ephemeral stream in Mt. Carmel, a 500 m high mountain ridge located at the NW coast of Israel. The rainy winter of the Mediterranean type climate (Csa) in Mt. Carmel is characterized by average annual rainfall of 550 mm at the coastal plain to 750 mm at the highest elevation while the summer is hot and dry. Stream flow generates after accumulated rainfall of 120–150 mm while “large floods”, defined as flows with peak discharge of N 5 m3 s− 1 and/or N 150,000 m3 in volume, are generated in response to rainfall of over 100 mm. Hence, large floods in Nahal Oren stream occur not earlier than December. Precipitation and flow data were divided into two sub-periods: 1957–1969 and 1991–2003 and compared to each other. The results indicate a clear increase in the frequency of large floods, their magnitudes and volumes during the second period with no parallel change in the annual precipitation. Similarly, an increase in storm rainfall–runoff ratio from b 5% to N 15% and a decrease in the threshold rainfall for channel flow by 16–25% were documented. These short-term variations in flooding behavior are explained by the clear decrease in the length of the rainy season and by the resulting significant shortening in the duration of the dry-spells. The increase in the number of large rainfall events and the large floods in each hydrological year together with the increasing number of years with no floods indicate strengthening of their uncertainty of behavior. © 2006 Elsevier B.V. All rights reserved. Keywords: Mediterranean streams; Flow regime; Rainy season length; Rainfall–runoff ratio; Floods; Wet-spells; Dry-spells; Mt. Carmel
1. Introduction In the Mediterranean region flow regime is characterized by strong temporal variability of the water flow. The effect of centennial to decadal-scale fluctuations in climate and riverine processes is well
⁎ Corresponding author. E-mail address: [email protected] (L. Wittenberg). 0169-555X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2006.01.046
established in the Mediterranean basin (Vita-Finzi, 1969; Macklin et al., 1995; Maas and Macklin, 2002; Macklin et al., 2002). Various research methods including geomorphological, sedimentological (e.g. Maas and Macklin, 2002) and a range of geochronological methods (e.g. Fuller et al., 1996; Gob et al., 2003) demonstrate significant linkage between longterm climate fluctuations and patterns of river development, especially in the European part of the Mediterranean basin (Maas et al., 1998). However, the
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influence of the annual and decadal-scale climate changes on flood frequency and magnitude in small Mediterranean streams is relatively poorly documented. Evidence for intensified flooding in Crete during the last two decades is associated with short-term climate fluctuations related to the North Atlantic Oscillation (NAO, Maas and Macklin, 2002) or in southern France to a significant increase in temperature and humidity (Ludwig et al., 2004). Palaeoflood and historic-floods records in dry parts of Europe (e.g. Tagus River — Spain) also indicate periods with increased flood magnitude and or frequencies associated with increased winter precipitation in the Iberian Peninsula (Benito et al., 2003). In many Mediterranean streams most of the water discharge occurs during very brief flash floods (MartinVide et al., 1999; Wittenberg et al., 2004). The intensity and frequency of the floods vary greatly from year-toyear depending on the frequency and intensity of the rainfall (Sabater et al., 1995); the magnitude of peak discharge is frequently about one order of magnitude greater than for rivers in non-Mediterranean areas (Ludwig et al., 2004). Under the current climatological conditions, runoff coefficients within the Mediterranean region are negligible (e.g. Lopez-Bermudez et al., 1998). In the western drainage basins of Israel, annual runoff coefficients are less than 3%. However, changes in rainfall amounts, intensities, annual distribution and number of rain days, could considerably affect flow regimes (Arnell and Reynard, 1996). Hydrological models for upland catchments suggest that of all environmental and climatological factors, increases in storm magnitude, rather than storm frequency may have a great impact on the extent of floods (Coulthard et al., 2000). In Mt. Carmel, a typical Mediterranean region, an event-based rainfall–runoff model (Garti et al., 1999) indicates that rain spells of 120 mm are essential for stream flow initiation with a recurrence interval of 5% (1:20), while large floods will commence following rain spells of 160 mm with a recurrence interval of 2% (1:50). The model, however, is based on rainfall–runoff relationships developed from post-flood peak flow estimations and frequency analyses of rainfall rather than on continuous field monitoring due to the lack of measured flow data. Field-based assessments of flow stage are available mainly for exceptional floods, for the purpose of flooddamage estimations (TAHAL, 1971; Garti et al., 1992, 1995, 1999). Based on spatially disrupted yet prolonged rainfall records such as that of the Technion (1921–2003) in the Mt. Carmel area it is suggested
that during the last century, 6–8 large floods occurred following extreme rainstorms (Garti et al., 1999). However, during the last two decades a considerable increase in flooding has been documented (Wittenberg et al., 2004). This study aimed at analyzing short-term variations of the rainfall regime and their impact on the consequent flow regime. The present study compares and analyzes the occurrence of large flow events and the possible climatic explanation for the timing and magnitude of extreme floods in Nahal Oren, the largest ephemeral stream system in the Mt. Carmel area. 2. Study area Mt. Carmel is a distinctive triangular mountain ridge in NW Israel. It covers approximately 250 km2 with the highest peak at 546 m; characterized by its proximity to the sea, and its distinctive borders with the adjacent lowland (Fig. 1). The permeable lithology is composed of upper Cretaceous carbonate rocks, mainly limestone, dolomite, chalk, marl and local exposures of volcanic tuff. The shallow stoney soils are determined mainly by the lithology: Terra-Rosa soil covers the limestone and dolomite while Rendzina soil characterizes chalk and marl. Slopes are steep and exceed, in places, gradients of 50%. The vegetation of Mt. Carmel is a typical Mediterranean forest and matoral composed mainly of a complex of pine (Pinus halepensis), oak (Quercus calliprinos), Pistacia lentiscus and associations. The Mediterranean type climate at Mt. Carmel is characterized by dry and hot summers and rainy winters. In Northern Israel precipitation begins during October and ends in May; most rainstorms occur between November and March. Autumn precipitation is often convective in nature with relatively high rainfall intensities while winter rainfalls (December–February) are mainly a result of frontal activities related to wide synoptic systems (Sharon and Kutiel, 1986). The average annual rainfall in Mt. Carmel ranges from 550 mm near the coastal plain to 750 mm at the highest elevations (Fig. 1). The ephemeral stream systems of Mount Carmel experience sporadic flow events which occur usually in early December following high intensity rainstorms or later in the winter once the soil is well saturated. Flow generation depends largely on the cumulative rainfall since the beginning of the rain-season. Flows in the ephemeral channels of Mt. Carmel are commonly characterized by a rapid rise in water level, a peak discharge within a few hours, and a swift recession followed by a relatively long “tail”. The typical lag time is generally a few hours, depending strongly on the extent of the drainage basin, antecedent
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soil moisture and rainstorm characteristics. Flow event duration ranges between several hours to several days, where upon the channel returns to its previous dry condition. Only in some channels spring discharge sustains base flow in limited segments which may last for a few days and up to a couple of weeks.
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2.1. Nahal Oren catchment The Oren catchment − 35 km2 is a typical mountainous, ephemeral stream system of Mt. Carmel draining its western flanks into the Mediterranean Sea with a general channel gradient of 3% (Fig. 1b). The
Fig. 1. Location map of Mt. Carmel area. Rainfall data and location of Nahal Oren catchment (adapted after Soffer and Kipnis, 1980) (a), Nahal Oren Basin, location of the monitoring equipment (b).
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karstic nature of Nahal Oren catchment is well presented by the relatively low drainage density of 3.2 km/ km2. Its major tributary, Nahal Bustan (9 km2) joins the main channel near its outlet from the Carmel mountains into the coastal plain. Continuous rainfall data for Nahal Oren catchment are derived from two gauging stations at Bet Oren (1940–2003) and at the University of Haifa (1976– 2003). Three hydrometric stations were installed in 1957 by the Israel Hydrological Service (Ministry of Agriculture). Two stations along the main channel: the lower station, Oren Bridge (25 km2 ), is located upstream of the confluence with Nahal Bustan and the upper station, The Pond (18 km2), is located further 3.5 km upstream. The third gauging station is located at Nahal Bustan (Fig. 1b). These stations operated during the period 1957–1964. Indirect assessment of large flow events continued until 1969; their peak discharges were determined using slope–area method and high water marks. The Oren Bridge hydrometric station was reoperated in 1991. All three stations were reinstalled by the University of Haifa in 2001, and continuous measured hydrological data is available since then. 3. Methodology Two 12-year periods of measured flow data in Nahal Oren: 1957–1969 and 1991–2003 were analyzed and compared. Results of flood frequency analysis (based on the available data) using Log Pearson type III distribution indicate that the 5 year flood in Nahal Oren has a peak discharge of 5 m3 s− 1 . The volume of such a flood was estimated at 150 * 103 m 3 , using discharge–volume regression (Fig. 2). Accordingly, flows with higher peak discharges and/or volumes were defined here as “large” floods.
3.1. Rainfall data Annual rainfall data were analyzed for the Bet Oren and the University of Haifa rainfall gauging stations. The record of the Haifa University was also analyzed for the annual number of rainy days and the length of the rainy season. 3.2. Dry-spells Dry-spells are defined as a series of consecutive days without any measurable rain amount. Likewise, a wet-spell or a rain-spell is defined as a series of consecutive days with a measurable rain amount. The Mediterranean climate is characterized by a long summer dry-spell. However, even during the rainy season most of the days are dry. For example, in northern Israel, where the rainy season usually spreads over a period of 7 months from October to April, there are about 70 rainy days/year. A rain day is defined as a day in which at least 0.1 mm was measured. This means that two thirds of the days during the rainy season are dry. Therefore, the distribution of the dry days or dry-spells is crucial to evaluate the rainfall regime in a Mediterranean climate. Kutiel (1985) proposed a multimodal rainfall distribution in Israel based on the temporal distribution of dry-spells. In that study, each day was attributed a number that represented the number of days since the last rain day. A rainy day was designated as zero, the first dry day after a rain-spell was attributed with 1, the next day with 2 and so on, until the next rainy day which got the value of 0. As there is a long summer dry-spell the hydrological calendar was used, i.e., from September 1st to August 31st of the next year. This methodology was applied in the present study on daily rainfall data from the meteorological station at the University of Haifa (Fig. 1a). The available data were for 28 rainy seasons during the period 1976/ 1977–2003/2004. Thus, for each day, 28 values were attributed and sorted in an ascending order. This enabled to calculate for each day the median (and other percentiles) dry-spell length. 4. Results
Fig. 2. Regression between peak discharge and corresponding flow volume — Nahal Oren, the Bridge station.
The hydrological record from Nahal Oren shows striking differences between the two periods (Table 1) although the number of flows in both periods was alike: each period included 20–22 floods. On average two flows per year, 3 years with no floods and three large floods were documented during
L. Wittenberg et al. / Geomorphology 84 (2007) 181–191 Table 1 Comparison of various hydrological parameters of the periods 1957– 1969 and 1991–2003 in Nahal Oren (Oren Bridge station) 1957–1969 1991–2003 Number of annual flow events Number of dry years Range of peak discharge (m3 s− 1) Maximal peak discharge (m3 s− 1) Maximal flood volume, 103 (m3 year− 1) Maximal annual flow volume, 103 (m3 year− 1) (excluding 1969 flood) Annual runoff coefficient (%) Maximal runoff coefficient (%) Number of measured large floods Average annual volume, 103 (m3 year− 1) (excluding 1969 flood) a b
0–2 2 0.7–4.3 4.3 (90 a) 2500 a
0–9 5 0.6–20 20 580
218 b
840
0.3 b 4.2 b 2 58 b
3.7 24.6 7 610
1969 flood (estimated). For the period 1957–1964.
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estimated recurrence interval of 100 years was 4.5 times larger than the second largest flood on record, but it was reconstructed downstream of the Oren Bridge station where the drainage area is 35 km2 — about 40% larger than the drainage area at the upper Bridge station. Maximum annual flow volume for the first period was approximately 218 * 103 m3 year− 1 compared to more than 840 * 103 m3 year− 1 in the second research period (except for the estimated volume of the 1969 flood). In the second period 1991–2003, 5 years without floods alternates with 7 years with flows, 3 of which had 5–6 flow events. This period included seven large floods, six of which had peak discharges between 5 and 20 m3 s− 1. Annual and maximal runoff coefficients were in one order of magnitude higher during the second period. 4.1. Large floods
the first period (1957–1969) in comparison to average of 3.8 flows/year, seven dry years and seven large floods during the second period (1991–2003). However, we assume on the basis of the rainfall data of storms N 150 mm, that during the first period another four undocumented flow events probably occurred between 1965 and 1969. Measured peak discharges of all floods during the first period were b5 m3 s− 1 (excluding the 1969 flood), all of which had recurrence intervals of less than 5 years. The 1969 flood − 90 m3 s− 1 with an
Nine large events were documented in Nahal Oren since the onset of the hydrological measurements (Fig. 3). Table 2 presents the hydrological characteristics and average storm precipitation for these floods at the Bridge station (25 km2). The measured rainfall threshold for the initiation of flow event ranges between 45 and 60 mm/rain-spell. These amounts generate low and moderate flows which usually decrease downstream and may even disappear. However, large flows are usually initiated following
Fig. 3. Hydrological characteristics of large floods in Nahal Oren, 1957–2003. A large event is defined by peak discharge N5 m3 s− 1 and/or volume N150 106 m3 (the volume of the flood of 22–24.1.69 is estimated using discharge–volume regression).
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Table 2 Hydrological characteristics of the “large” floods and storm precipitation in Nahal Oren Date of rainstorm
Peak discharge (m3 s− 1)
Specific peak discharge (m3 s− 1 km− 2)
Volume (103 m3)
Storm rainfall amount (mm)
Rainfall–runoff ratio (%)
22–25.12.1961 22–24.1.1969 7–9.12.1991 31.1–7.2.1992 4–6.12.2001 7–10.1.2002 17–20.12.2002 21–22.2.2003 24–27.3.2003
4.3 90.0 a 20.0 6.8 17.0 8.0 5.5 5.0 3.3
0.17 2.57 0.80 0.27 0.68 0.32 0.22 0.20 0.13
159 2500 b 560 b 204 575 c 360 c 180 169 265
153.1 149.0 144.0 162.7 155.7 111.5 140.5 70.9 177.2
4.1 47.5 15.5 5.0 14.8 12.9 5.1 9.5 6.0
a b c
Reconstructed at Road 4 (35 km2). Estimated using discharge–volume regression. Calculated.
rain-spells of N 100 mm (and up to 177 mm; Table 1) but may also occur in response to rain-spells of N 70 mm during the end of winter (February) and the beginning of spring (March) due to high moisture content in the soil. In contrast, not every large rainstorm produces flow event of any extent. Magnitude and volume of floods are strongly related to rainfall intensities and seasonality, i.e. the duration of high-intensity rainfall exceeds tens of minutes to generate channel flow.
Storm rainfall–runoff ratio ranged between 4.1% and 15.5% (excluding the unknown 1969 flood) and specific peak discharges ranged between 0.13 and 2.57 m3 s− 1 km− 2 . During the period 1957–1969, except for two dry years, Nahal Oren was flowing, in average, twice a year, but only two “large” floods which exceeded the threshold volume were recorded, one of them, the 1969 flood with an estimated volume of 2.5 * 106 m3 . All peak flows were b 5 m3 s − 1 , except
Fig. 4. Annual course of dry spells at the University of Haifa. The vertical axis represents the number of days since the last rainy day (Q1 = first quartile, Q2 = median, Q3 = third quartile).
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Fig. 5. The course of the median rainy season at the University of Haifa (adapted after Paz and Kutiel, 2003).
for the 1969 flood (90 m3 s− 1 ), the largest peak discharge on record (TAHAL, 1971). The rainfall threshold for the initiation of channel flows during this period was 50–60 mm. The period of 1991–2003 which include five dry winters with no flows on the one hand, included three winters with 5 to 6 floods, on the other hand: 1991/1992 with five flows, two of which were “large” flows; the winter of 2001/2002 – six flows, two of which were “large” flows, and the winter of 2002/2003 – five flows, with three large events. These winters were characterized by very high annual rainfall amounts — 165%, 126% and 135% of the average, respectively. Peak discharges ranged between 3.3 and 20 m3 s − 1 and the volumes ranged between 169 and 575 * 103 m3 . The annual rainfall in the dry years varied from below average to somewhat above average. Storm rainfall–runoff ratio increased from less than 5% during 1957–1969 (excluding the 1969 flood with an unknown volume) to 5–15.5% during 1991–2003. The rainfall threshold for the initiation of channel flows during this period was 45–50 mm. The lack of flow events, despite these amounts of precipitation, stems largely from the spatial distribution of rainfall within the basin. 4.2. Rainfall analyses and distribution of dry spells Fig. 4 presents the annual course of the median and the two quartiles of dry-spell length. This figure reflects well the characteristics of the Mediterranean climate with a very long summer dry-spell and relatively short dry-spells during the cold season. The rainy season length is defined as the time elapsed since the date when 10% of the annual total was accumulated until the date when 90% of the annual total was
accumulated (Paz and Kutiel, 2003). This definition is due to the fact that the first and the last 10% of the annual total spread over a long period. The median date of the first rain day is October 4 and the median date of accumulated 10% is November 12. The median date of the last rain day is May 12 and the median date of accumulated 90% is March 11 (Fig. 5). This means that if the rainy season length is defined as the period elapsed from the first to the last rain day, this period will be longer than 7 months. However, if the rainy season length is defined as the time elapsed since the date when 10% of the annual total accumulated until the date when 90% were accumulated, this period will be about 4 months. The last 10% of the annual total accumulated in a period of 62 days (March 11–May 12). This period is longer than the period needed for accumulation of half of the annual total (from 30% to 80%), 61 days (December 4–February 1). The beginning and the end of the rainy season in a Mediterranean climate are very sporadic in time. Therefore, we defined the rainy season length (RSL) as the period that comprises the core of the rainy season, 80% of the annual total (10%–90%). Analyses of time series of annual totals of the University of Haifa station (Fig. 6a) reveal no temporal trend as can be observed from the trend lines. Similarly, no trend of change is true also for time series of the number of rain days at the University of Haifa meteorological station (Fig. 6b). This means that there was no change also in the mean daily totals over the analyzed period. However, a clear and significant reduction of the RSL can be observed in Fig. 6c. This shortening of the rainy season at an average rate of more than 1 day/ year, is primarily due to a later beginning of the rainy season rather than an earlier ending.
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Fig. 6. Time series of total rainfall (a), number of rainy days (b), and rainy season length (c), at the University of Haifa.
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5. Discussion The measured hydrological data from the Nahal Oren catchment reflect clear hydrological differences between the two 12-year periods, 1957–1969 and 1991–2003. Flood magnitudes, i.e., peak discharges and flow volumes (excluding the 1969 flood) are significantly higher during the second period. On the other hand the number of dry years (years with no flows) are only two during the first period and five during the second period. This means that the same number of floods would have occurred during a smaller number of years. Only two large floods occurred during the first period versus seven during the second period. Peak discharges of all floods during the first period were relatively low (b 5 m3 s− 1 ) and their recurrence intervals were less than 5 years. The 1969 flood was exceptionally larger than all the other floods on record (estimated as the 100-year flood). All flow volumes during the first period were b 158 * 103 m3 (except for the estimated volume of the 1969 flood). In contrast to the first period the second period 1991–2003, included 5 years without floods alternating with 7 years with flows, 3 of which had 5–6 floods. This period included seven large floods with peak discharges between 5 and 20 m3 s− 1 . All these floods were “large” also in terms of their volumes and ranged between 0.169 and 0.575 * 106 m3 . The annual volumes of the flows during the second period are much larger accordingly. The threshold rainfall amount for channel flow generation decreased by 16–25% during the second period from about 60 mm during the first period to 45–50 mm. These clear, short-term changes in the hydrological regime and flow characteristics occurred in spite of the fact that no significant change in annual rainfall amount was observed in any of the rain-gauging stations in the vicinity of Nahal Oren catchment nor in other gauging stations in Mt. Carmel during the last 50 years (Halfon, 2004). Similarly, no significant change was documented in rain-spell length and daily average rainfall per each storm. However, the significant shortening of the rainy season (Fig. 6c) without a parallel change in annual total is well represented by the shortening of the dry spells within the rainy season (Kutiel, 1985). This means an increase in the frequency of rain-spells which may generate floods. Such shortening of the rainy season which means an increase in the frequency of rain-spells, while all other rainfall parameters remain constant, should be represented by increased frequency and magnitude of flood episodes. In addition, each rain-spell during the latter period falls over relatively wetter slopes because of the shortened dryspells, which causes a decrease of both evapotranspiration
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and soil drainage between successive rain-spells. This leads to higher antecedent moisture conditions for each rain-spell which, in turn, enhance runoff generation as indicated by the increased storm rainfall–runoff ratio from b 5% to over 15% and by the 16–25% decrease in the threshold rainfall amount for generation of channel flow. This was also confirmed by Maas and Macklin (2002) who reported on a reduction of floods frequencies in Crete due to shorter rainstorm durations. Generation of relatively more large floods may also be attributed to a possible increase in the distribution of the high intensity rain-spells. The increase in the number of dry years on the one hand and the number of years with 5–6 floods on the other hand, may represent hydrological instability and increased uncertainty of the larger rain-spells and floods in Nahal Oren catchment. This trend parallels an increase in rainfall regime uncertainty in northern Israel documented by Paz and Kutiel (2003). These changes in the rainfall regime and their hydrological consequences are mainly due to changes in atmospheric circulation, air masses stability and moisture content and not to human activity or changes in vegetation cover or type. An increase in monthly rainfall concentration was also noted in the Mediterranean parts of Spain (De Luis et al., 2001). Their results agree with GCM predictions of a decrease in rainfall volume, changes in seasonal distribution and an increase in rainfall intensity in some areas across the Mediterranean basin (Palutikof, 1996). The agreement between model prediction and observations suggests that this trend could be associated with global warming (Palutikof, 1996). Similarly, Maas and Macklin (2002) demonstrated a strong linkage between increased flooding periods in the Aragena Gorge, Crete, and the North Atlantic Oscillation index. Our results conform to those of Maas and Macklin (2002) suggesting that river catchments in the Mediterranean are sensitive to short-term climate variability. 6. Conclusions The results of this study provide several conclusions related to short-term changes in the frequency and magnitude of the flows in Nahal Oren catchment and the possible reasons for these changes. These shortterm changes indicate instability of the hydrological regime. Large floods are generated in response to rain-spells with over 100 mm. When rain-spells are scattered, irrespective of the total annual amount, no flow will develop in the streams. Flow generates after accumulated rainfall of 120–150 mm. Hence, large floods in Nahal Oren catchment rarely occur prior to December.
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Comparing two periods of continuous hydrological data: 1957–1969 and 1991–2003 shows the following changes in flow regime and flood characteristics: 1. Mean annual number of floods has increased from about 1.8/year to 3.8 2. The annual number of floods increased up to 5–6 during the wetter years. 3. Increase in the frequency of large floods (magnitude and volume) from two floods to seven floods. 4. Increase in peak discharges and flow volumes of the floods by up to 4.5 and 3.5 times, respectively. 5. Increase in the number of years with no floods from 2 years to 5 years. 6. Increase in storm rainfall–runoff ratio from b 5% to 5–15.5%. 7. Decrease in the threshold rainfall amount for channel flow generation by 16–25%, from 60 mm to 45– 50 mm. Although annual rainfall totals, rain-spell length and daily average rainfall per storm have not changed during the study period these trends in the hydrological regime and characteristics of the floods may be explained by the following trends of the rain-spells and rainfall regime: 1. The length of the rainy season became significantly shorter. 2. The length of the dry spells is significantly reduced. 3. The number of large rain-spell in each hydrological year increased, parallel with an increase in the uncertainty of behavior. References Arnell, N.W., Reynard, N.S., 1996. The effect of climate change due to global warming on river flows in Great Britain. Journal of Hydrology 183, 397–424. Benito, G., Sopen, A., Sanchez-Moya, Y., Machado, M.J., PerezGonzalez, A., 2003. Palaeofood record of the Tagus River (Central Spain) during the late Pleistocene and Holocene. Quaternary Science Reviews 22, 1737–1756. Coulthard, T.J., Kirkby, M.J., Macklin, M.G., 2000. Modeling geomorphic response to environmental change in an upland catchment. Hydrological Processes 14, 2031–2045. De Luis, M., Garcia-Cano, M.F., Cortna, J., Raventos, J., GonzalezHidalgo, J.C., Sanchez, J.R., 2001. Climatic trends, disturbances and short-term vegetation dynamics in a Mediterranean shrubland. Forest Ecology and Management 147, 25–37. Fuller, I.C., Macklin, M.G., Passmore, D.G., Brewer, P.A., Lewin, J., Wintle, A.G., 1996. Geochronologies and environmental records of Quaternary fluvial sequences in the Guadalope basin, northeast Spain, based on luminescence dating. In: Branson, J., Brown, A.G., Gregory, K.J. (Eds.), Global Continental Changes: the Context of
Palaeohydrology. Geological Society, London, Special Publication, vol. 115, pp. 99–120. Garti, R., Arbel, S., Getker, M., Ganani, A., 1992. Peak Discharges and Flood Volumes in Secondary Catchments, Winter 1991/92. Special Report M-41. Soil Conservation Station, Ministry of Agriculture. (in Hebrew). Garti, R., Getker, M., Arbel, S., Lazanov, M., Gotesman, M., Fisik, E., 1995. Exceptional Hydrological Events in Northern Israel, 1994– 1995. Special Report M-51. Soil Conservation Station, Ministry of Agriculture. (in Hebrew). Garti, R., Morin, E., Getker, M., Arbel, S., Gefen, O., Tamir, M., Gotesman, M., Farhi, R., Fisik, E., 1999. The Rare Rainstorm of the 31 December 1998 and the Associated Floods in the Mt. Carmel Catchments. Special Report M-65. Soil Conservation Station, Ministry of Agriculture. (in Hebrew). Gob, F., Petit, F., Bravard, J.P., Ozer, A., Gob, A., 2003. Lichenometric application to historical and subrecent dynamics and sediment transport of a Corsican stream (Figarella River-France). Quaternary Science Reviews 22, 2111–2124. Halfon, N., 2004. Translocation of the orographic rainfall enhancement from the ridge towards the lee slopes in northern Israel (Hebrew with an English abstract). Unpublished M.A. Thesis, Department of Geography and Environmental Studies, University of Haifa. 145 pp. Kutiel, H., 1985. The multimodality of the rainfall course in Israel, a reflect by the distribution of dry spells. Archives for Meteorology, Geophysics, and Bioclimatology. Series B, Climatology, Environmental Meteorology, Radiation Research 36, 15–27. Lopez-Bermudez, F., Romero-Diaz, A., Martinez-Fernandez, J., Martinez-Fernandez, J., 1998. Vegetation and soil erosion under a semi-arid Mediterranean climate: a case study from Murcia (Spain). Geomorphology 24, 51–58. Ludwig, W., Serrat, P., Cesmat, L., Garcia-Esteves, J., 2004. Evaluating the impact of the recent temperature increase on the hydrology of the Têt River (Southern France). Journal of Hydrology 289, 204–221. Maas, G.S., Macklin, M.G., 2002. The impact of recent climate change on flooding and sediment supply within a Mediterranean mountain catchment, southwestern Crete, Greece. Earth Surface Processes and Landforms 27, 1087–1105. Maas, G.S., Macklin, M.G., Kirkby, M.J., 1998. Late Pleistocene and Holocene river development in Mediterranean steepland environments, southwest Crete. In: Benito, G., Baker, V.R., Gregory, K.J. (Eds.), Palaeohydrology and Environmental Change. Wiley, Chichester, pp. 153–166. Macklin, M.G., Lewin, J., Woodward, J.C., 1995. Quaternary fluvial systems in the Mediterranean basin. In: Lewin, J., Macklin, M.G., Woodward, J.C. (Eds.), Mediterranean Quaternary River Environments. Balkema, Rotterdam, pp. 1–25. Macklin, M.G., Fuller, I.C., Lewin, J., Maas, G.S., Passmore, D.G., Rose, J., Woodward, J.C., Black, S., Hamlin, R.H.B., Rowan, J.S., 2002. Correlation of fluvial sequences in the Mediterranean basin over the last 200 ka and their relationship to climate change. Quaternary Science Reviews 21, 1633–1641. Martin-Vide, J.P., Ninerola, D., Bateman, A., Navarro, A., Velasco, E., 1999. Runoff and sediment transport in torrential ephemeral stream of the Mediterranean Coast. Journal of Hydrology 225, 118–129. Palutikof, J.P., 1996. Scenarios of future rainfall over the Mediterranean: is the region drying? Book of abstracts. Proceedings of the International Conference on Mediterranean Desertification, CreteHellas, 29 October–1 November. Paz, S., Kutiel, H., 2003. Rainfall Regime Uncertainty (RRU) in a Mediterranean Region. The Israel Journal of Earth-Sciences 52 (2), 47–63.
L. Wittenberg et al. / Geomorphology 84 (2007) 181–191 Sabater, S., Guasch, H., Marti, E., Armengo, J., Sabater, S., 1995. The river Ter: a Mediterranean river case study in Spain. In: Cushing, C.E., Cummins, K.W., Minshall, G.W. (Eds.), River and Stream Ecosystem. Elsevier, Amsterdam, pp. 419–438. Sharon, D., Kutiel, H., 1986. The distribution of rainfall intensity in Israel, its regional and seasonal variations and its climatological evaluation. Journal of Climatology 6, 277–291. Soffer, A., Kipnis, B., 1980. Atlas of Haifa and Mount Carmel. University of Haifa, Israel. 122 pp. TAHAL, 1971. The Floods of Winter 1968/69. Department of Drainage, TAHAL, Haifa. 21 pp., (in Hebrew).
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Vita-Finzi, C., 1969. The Mediterranean Valleys. Geological Changes in Historical Times. Cambridge University Press, Cambridge. 140 pp. Wittenberg, l., Greenbaum, N., Paz, S., Kutiel, H., 2004. Frequency of large flood in the Eastern Mediterranean Region: a comparison between two 12-year periods — Nahal Oren, Mt. Carmel, Israel. In: Maos, J.O., Inbar, M., Shmueli, D.F. (Eds.), Contemporary Israeli Geography, Department of Geography and Environmental Studies. University of Haifa, Israel, pp. 129–138.
Short-term changes in the magnitude, frequency and temporal distribution of floods in the Eastern Mediterranean region during the last 45 years — Nahal Oren, Mt. Carmel, Israel Lea Wittenberg a,⁎, Haim Kutiel a , Noam Greenbaum a,b , Moshe Inbar a b
a Department of Geography and Environmental Studies, University of Haifa, Haifa 31905, Israel Department of Natural Resources and Environmental Management, University of Haifa, Haifa 31905, Israel
Received 15 April 2005; received in revised form 2 January 2006; accepted 2 January 2006 Available online 28 August 2006
Abstract Short-term changes in Eastern Mediterranean precipitation affecting flow regime were documented in Nahal Oren, a 35 km2 ephemeral stream in Mt. Carmel, a 500 m high mountain ridge located at the NW coast of Israel. The rainy winter of the Mediterranean type climate (Csa) in Mt. Carmel is characterized by average annual rainfall of 550 mm at the coastal plain to 750 mm at the highest elevation while the summer is hot and dry. Stream flow generates after accumulated rainfall of 120–150 mm while “large floods”, defined as flows with peak discharge of N 5 m3 s− 1 and/or N 150,000 m3 in volume, are generated in response to rainfall of over 100 mm. Hence, large floods in Nahal Oren stream occur not earlier than December. Precipitation and flow data were divided into two sub-periods: 1957–1969 and 1991–2003 and compared to each other. The results indicate a clear increase in the frequency of large floods, their magnitudes and volumes during the second period with no parallel change in the annual precipitation. Similarly, an increase in storm rainfall–runoff ratio from b 5% to N 15% and a decrease in the threshold rainfall for channel flow by 16–25% were documented. These short-term variations in flooding behavior are explained by the clear decrease in the length of the rainy season and by the resulting significant shortening in the duration of the dry-spells. The increase in the number of large rainfall events and the large floods in each hydrological year together with the increasing number of years with no floods indicate strengthening of their uncertainty of behavior. © 2006 Elsevier B.V. All rights reserved. Keywords: Mediterranean streams; Flow regime; Rainy season length; Rainfall–runoff ratio; Floods; Wet-spells; Dry-spells; Mt. Carmel
1. Introduction In the Mediterranean region flow regime is characterized by strong temporal variability of the water flow. The effect of centennial to decadal-scale fluctuations in climate and riverine processes is well
⁎ Corresponding author. E-mail address: [email protected] (L. Wittenberg). 0169-555X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2006.01.046
established in the Mediterranean basin (Vita-Finzi, 1969; Macklin et al., 1995; Maas and Macklin, 2002; Macklin et al., 2002). Various research methods including geomorphological, sedimentological (e.g. Maas and Macklin, 2002) and a range of geochronological methods (e.g. Fuller et al., 1996; Gob et al., 2003) demonstrate significant linkage between longterm climate fluctuations and patterns of river development, especially in the European part of the Mediterranean basin (Maas et al., 1998). However, the
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influence of the annual and decadal-scale climate changes on flood frequency and magnitude in small Mediterranean streams is relatively poorly documented. Evidence for intensified flooding in Crete during the last two decades is associated with short-term climate fluctuations related to the North Atlantic Oscillation (NAO, Maas and Macklin, 2002) or in southern France to a significant increase in temperature and humidity (Ludwig et al., 2004). Palaeoflood and historic-floods records in dry parts of Europe (e.g. Tagus River — Spain) also indicate periods with increased flood magnitude and or frequencies associated with increased winter precipitation in the Iberian Peninsula (Benito et al., 2003). In many Mediterranean streams most of the water discharge occurs during very brief flash floods (MartinVide et al., 1999; Wittenberg et al., 2004). The intensity and frequency of the floods vary greatly from year-toyear depending on the frequency and intensity of the rainfall (Sabater et al., 1995); the magnitude of peak discharge is frequently about one order of magnitude greater than for rivers in non-Mediterranean areas (Ludwig et al., 2004). Under the current climatological conditions, runoff coefficients within the Mediterranean region are negligible (e.g. Lopez-Bermudez et al., 1998). In the western drainage basins of Israel, annual runoff coefficients are less than 3%. However, changes in rainfall amounts, intensities, annual distribution and number of rain days, could considerably affect flow regimes (Arnell and Reynard, 1996). Hydrological models for upland catchments suggest that of all environmental and climatological factors, increases in storm magnitude, rather than storm frequency may have a great impact on the extent of floods (Coulthard et al., 2000). In Mt. Carmel, a typical Mediterranean region, an event-based rainfall–runoff model (Garti et al., 1999) indicates that rain spells of 120 mm are essential for stream flow initiation with a recurrence interval of 5% (1:20), while large floods will commence following rain spells of 160 mm with a recurrence interval of 2% (1:50). The model, however, is based on rainfall–runoff relationships developed from post-flood peak flow estimations and frequency analyses of rainfall rather than on continuous field monitoring due to the lack of measured flow data. Field-based assessments of flow stage are available mainly for exceptional floods, for the purpose of flooddamage estimations (TAHAL, 1971; Garti et al., 1992, 1995, 1999). Based on spatially disrupted yet prolonged rainfall records such as that of the Technion (1921–2003) in the Mt. Carmel area it is suggested
that during the last century, 6–8 large floods occurred following extreme rainstorms (Garti et al., 1999). However, during the last two decades a considerable increase in flooding has been documented (Wittenberg et al., 2004). This study aimed at analyzing short-term variations of the rainfall regime and their impact on the consequent flow regime. The present study compares and analyzes the occurrence of large flow events and the possible climatic explanation for the timing and magnitude of extreme floods in Nahal Oren, the largest ephemeral stream system in the Mt. Carmel area. 2. Study area Mt. Carmel is a distinctive triangular mountain ridge in NW Israel. It covers approximately 250 km2 with the highest peak at 546 m; characterized by its proximity to the sea, and its distinctive borders with the adjacent lowland (Fig. 1). The permeable lithology is composed of upper Cretaceous carbonate rocks, mainly limestone, dolomite, chalk, marl and local exposures of volcanic tuff. The shallow stoney soils are determined mainly by the lithology: Terra-Rosa soil covers the limestone and dolomite while Rendzina soil characterizes chalk and marl. Slopes are steep and exceed, in places, gradients of 50%. The vegetation of Mt. Carmel is a typical Mediterranean forest and matoral composed mainly of a complex of pine (Pinus halepensis), oak (Quercus calliprinos), Pistacia lentiscus and associations. The Mediterranean type climate at Mt. Carmel is characterized by dry and hot summers and rainy winters. In Northern Israel precipitation begins during October and ends in May; most rainstorms occur between November and March. Autumn precipitation is often convective in nature with relatively high rainfall intensities while winter rainfalls (December–February) are mainly a result of frontal activities related to wide synoptic systems (Sharon and Kutiel, 1986). The average annual rainfall in Mt. Carmel ranges from 550 mm near the coastal plain to 750 mm at the highest elevations (Fig. 1). The ephemeral stream systems of Mount Carmel experience sporadic flow events which occur usually in early December following high intensity rainstorms or later in the winter once the soil is well saturated. Flow generation depends largely on the cumulative rainfall since the beginning of the rain-season. Flows in the ephemeral channels of Mt. Carmel are commonly characterized by a rapid rise in water level, a peak discharge within a few hours, and a swift recession followed by a relatively long “tail”. The typical lag time is generally a few hours, depending strongly on the extent of the drainage basin, antecedent
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soil moisture and rainstorm characteristics. Flow event duration ranges between several hours to several days, where upon the channel returns to its previous dry condition. Only in some channels spring discharge sustains base flow in limited segments which may last for a few days and up to a couple of weeks.
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2.1. Nahal Oren catchment The Oren catchment − 35 km2 is a typical mountainous, ephemeral stream system of Mt. Carmel draining its western flanks into the Mediterranean Sea with a general channel gradient of 3% (Fig. 1b). The
Fig. 1. Location map of Mt. Carmel area. Rainfall data and location of Nahal Oren catchment (adapted after Soffer and Kipnis, 1980) (a), Nahal Oren Basin, location of the monitoring equipment (b).
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karstic nature of Nahal Oren catchment is well presented by the relatively low drainage density of 3.2 km/ km2. Its major tributary, Nahal Bustan (9 km2) joins the main channel near its outlet from the Carmel mountains into the coastal plain. Continuous rainfall data for Nahal Oren catchment are derived from two gauging stations at Bet Oren (1940–2003) and at the University of Haifa (1976– 2003). Three hydrometric stations were installed in 1957 by the Israel Hydrological Service (Ministry of Agriculture). Two stations along the main channel: the lower station, Oren Bridge (25 km2 ), is located upstream of the confluence with Nahal Bustan and the upper station, The Pond (18 km2), is located further 3.5 km upstream. The third gauging station is located at Nahal Bustan (Fig. 1b). These stations operated during the period 1957–1964. Indirect assessment of large flow events continued until 1969; their peak discharges were determined using slope–area method and high water marks. The Oren Bridge hydrometric station was reoperated in 1991. All three stations were reinstalled by the University of Haifa in 2001, and continuous measured hydrological data is available since then. 3. Methodology Two 12-year periods of measured flow data in Nahal Oren: 1957–1969 and 1991–2003 were analyzed and compared. Results of flood frequency analysis (based on the available data) using Log Pearson type III distribution indicate that the 5 year flood in Nahal Oren has a peak discharge of 5 m3 s− 1 . The volume of such a flood was estimated at 150 * 103 m 3 , using discharge–volume regression (Fig. 2). Accordingly, flows with higher peak discharges and/or volumes were defined here as “large” floods.
3.1. Rainfall data Annual rainfall data were analyzed for the Bet Oren and the University of Haifa rainfall gauging stations. The record of the Haifa University was also analyzed for the annual number of rainy days and the length of the rainy season. 3.2. Dry-spells Dry-spells are defined as a series of consecutive days without any measurable rain amount. Likewise, a wet-spell or a rain-spell is defined as a series of consecutive days with a measurable rain amount. The Mediterranean climate is characterized by a long summer dry-spell. However, even during the rainy season most of the days are dry. For example, in northern Israel, where the rainy season usually spreads over a period of 7 months from October to April, there are about 70 rainy days/year. A rain day is defined as a day in which at least 0.1 mm was measured. This means that two thirds of the days during the rainy season are dry. Therefore, the distribution of the dry days or dry-spells is crucial to evaluate the rainfall regime in a Mediterranean climate. Kutiel (1985) proposed a multimodal rainfall distribution in Israel based on the temporal distribution of dry-spells. In that study, each day was attributed a number that represented the number of days since the last rain day. A rainy day was designated as zero, the first dry day after a rain-spell was attributed with 1, the next day with 2 and so on, until the next rainy day which got the value of 0. As there is a long summer dry-spell the hydrological calendar was used, i.e., from September 1st to August 31st of the next year. This methodology was applied in the present study on daily rainfall data from the meteorological station at the University of Haifa (Fig. 1a). The available data were for 28 rainy seasons during the period 1976/ 1977–2003/2004. Thus, for each day, 28 values were attributed and sorted in an ascending order. This enabled to calculate for each day the median (and other percentiles) dry-spell length. 4. Results
Fig. 2. Regression between peak discharge and corresponding flow volume — Nahal Oren, the Bridge station.
The hydrological record from Nahal Oren shows striking differences between the two periods (Table 1) although the number of flows in both periods was alike: each period included 20–22 floods. On average two flows per year, 3 years with no floods and three large floods were documented during
L. Wittenberg et al. / Geomorphology 84 (2007) 181–191 Table 1 Comparison of various hydrological parameters of the periods 1957– 1969 and 1991–2003 in Nahal Oren (Oren Bridge station) 1957–1969 1991–2003 Number of annual flow events Number of dry years Range of peak discharge (m3 s− 1) Maximal peak discharge (m3 s− 1) Maximal flood volume, 103 (m3 year− 1) Maximal annual flow volume, 103 (m3 year− 1) (excluding 1969 flood) Annual runoff coefficient (%) Maximal runoff coefficient (%) Number of measured large floods Average annual volume, 103 (m3 year− 1) (excluding 1969 flood) a b
0–2 2 0.7–4.3 4.3 (90 a) 2500 a
0–9 5 0.6–20 20 580
218 b
840
0.3 b 4.2 b 2 58 b
3.7 24.6 7 610
1969 flood (estimated). For the period 1957–1964.
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estimated recurrence interval of 100 years was 4.5 times larger than the second largest flood on record, but it was reconstructed downstream of the Oren Bridge station where the drainage area is 35 km2 — about 40% larger than the drainage area at the upper Bridge station. Maximum annual flow volume for the first period was approximately 218 * 103 m3 year− 1 compared to more than 840 * 103 m3 year− 1 in the second research period (except for the estimated volume of the 1969 flood). In the second period 1991–2003, 5 years without floods alternates with 7 years with flows, 3 of which had 5–6 flow events. This period included seven large floods, six of which had peak discharges between 5 and 20 m3 s− 1. Annual and maximal runoff coefficients were in one order of magnitude higher during the second period. 4.1. Large floods
the first period (1957–1969) in comparison to average of 3.8 flows/year, seven dry years and seven large floods during the second period (1991–2003). However, we assume on the basis of the rainfall data of storms N 150 mm, that during the first period another four undocumented flow events probably occurred between 1965 and 1969. Measured peak discharges of all floods during the first period were b5 m3 s− 1 (excluding the 1969 flood), all of which had recurrence intervals of less than 5 years. The 1969 flood − 90 m3 s− 1 with an
Nine large events were documented in Nahal Oren since the onset of the hydrological measurements (Fig. 3). Table 2 presents the hydrological characteristics and average storm precipitation for these floods at the Bridge station (25 km2). The measured rainfall threshold for the initiation of flow event ranges between 45 and 60 mm/rain-spell. These amounts generate low and moderate flows which usually decrease downstream and may even disappear. However, large flows are usually initiated following
Fig. 3. Hydrological characteristics of large floods in Nahal Oren, 1957–2003. A large event is defined by peak discharge N5 m3 s− 1 and/or volume N150 106 m3 (the volume of the flood of 22–24.1.69 is estimated using discharge–volume regression).
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Table 2 Hydrological characteristics of the “large” floods and storm precipitation in Nahal Oren Date of rainstorm
Peak discharge (m3 s− 1)
Specific peak discharge (m3 s− 1 km− 2)
Volume (103 m3)
Storm rainfall amount (mm)
Rainfall–runoff ratio (%)
22–25.12.1961 22–24.1.1969 7–9.12.1991 31.1–7.2.1992 4–6.12.2001 7–10.1.2002 17–20.12.2002 21–22.2.2003 24–27.3.2003
4.3 90.0 a 20.0 6.8 17.0 8.0 5.5 5.0 3.3
0.17 2.57 0.80 0.27 0.68 0.32 0.22 0.20 0.13
159 2500 b 560 b 204 575 c 360 c 180 169 265
153.1 149.0 144.0 162.7 155.7 111.5 140.5 70.9 177.2
4.1 47.5 15.5 5.0 14.8 12.9 5.1 9.5 6.0
a b c
Reconstructed at Road 4 (35 km2). Estimated using discharge–volume regression. Calculated.
rain-spells of N 100 mm (and up to 177 mm; Table 1) but may also occur in response to rain-spells of N 70 mm during the end of winter (February) and the beginning of spring (March) due to high moisture content in the soil. In contrast, not every large rainstorm produces flow event of any extent. Magnitude and volume of floods are strongly related to rainfall intensities and seasonality, i.e. the duration of high-intensity rainfall exceeds tens of minutes to generate channel flow.
Storm rainfall–runoff ratio ranged between 4.1% and 15.5% (excluding the unknown 1969 flood) and specific peak discharges ranged between 0.13 and 2.57 m3 s− 1 km− 2 . During the period 1957–1969, except for two dry years, Nahal Oren was flowing, in average, twice a year, but only two “large” floods which exceeded the threshold volume were recorded, one of them, the 1969 flood with an estimated volume of 2.5 * 106 m3 . All peak flows were b 5 m3 s − 1 , except
Fig. 4. Annual course of dry spells at the University of Haifa. The vertical axis represents the number of days since the last rainy day (Q1 = first quartile, Q2 = median, Q3 = third quartile).
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Fig. 5. The course of the median rainy season at the University of Haifa (adapted after Paz and Kutiel, 2003).
for the 1969 flood (90 m3 s− 1 ), the largest peak discharge on record (TAHAL, 1971). The rainfall threshold for the initiation of channel flows during this period was 50–60 mm. The period of 1991–2003 which include five dry winters with no flows on the one hand, included three winters with 5 to 6 floods, on the other hand: 1991/1992 with five flows, two of which were “large” flows; the winter of 2001/2002 – six flows, two of which were “large” flows, and the winter of 2002/2003 – five flows, with three large events. These winters were characterized by very high annual rainfall amounts — 165%, 126% and 135% of the average, respectively. Peak discharges ranged between 3.3 and 20 m3 s − 1 and the volumes ranged between 169 and 575 * 103 m3 . The annual rainfall in the dry years varied from below average to somewhat above average. Storm rainfall–runoff ratio increased from less than 5% during 1957–1969 (excluding the 1969 flood with an unknown volume) to 5–15.5% during 1991–2003. The rainfall threshold for the initiation of channel flows during this period was 45–50 mm. The lack of flow events, despite these amounts of precipitation, stems largely from the spatial distribution of rainfall within the basin. 4.2. Rainfall analyses and distribution of dry spells Fig. 4 presents the annual course of the median and the two quartiles of dry-spell length. This figure reflects well the characteristics of the Mediterranean climate with a very long summer dry-spell and relatively short dry-spells during the cold season. The rainy season length is defined as the time elapsed since the date when 10% of the annual total was accumulated until the date when 90% of the annual total was
accumulated (Paz and Kutiel, 2003). This definition is due to the fact that the first and the last 10% of the annual total spread over a long period. The median date of the first rain day is October 4 and the median date of accumulated 10% is November 12. The median date of the last rain day is May 12 and the median date of accumulated 90% is March 11 (Fig. 5). This means that if the rainy season length is defined as the period elapsed from the first to the last rain day, this period will be longer than 7 months. However, if the rainy season length is defined as the time elapsed since the date when 10% of the annual total accumulated until the date when 90% were accumulated, this period will be about 4 months. The last 10% of the annual total accumulated in a period of 62 days (March 11–May 12). This period is longer than the period needed for accumulation of half of the annual total (from 30% to 80%), 61 days (December 4–February 1). The beginning and the end of the rainy season in a Mediterranean climate are very sporadic in time. Therefore, we defined the rainy season length (RSL) as the period that comprises the core of the rainy season, 80% of the annual total (10%–90%). Analyses of time series of annual totals of the University of Haifa station (Fig. 6a) reveal no temporal trend as can be observed from the trend lines. Similarly, no trend of change is true also for time series of the number of rain days at the University of Haifa meteorological station (Fig. 6b). This means that there was no change also in the mean daily totals over the analyzed period. However, a clear and significant reduction of the RSL can be observed in Fig. 6c. This shortening of the rainy season at an average rate of more than 1 day/ year, is primarily due to a later beginning of the rainy season rather than an earlier ending.
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Fig. 6. Time series of total rainfall (a), number of rainy days (b), and rainy season length (c), at the University of Haifa.
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5. Discussion The measured hydrological data from the Nahal Oren catchment reflect clear hydrological differences between the two 12-year periods, 1957–1969 and 1991–2003. Flood magnitudes, i.e., peak discharges and flow volumes (excluding the 1969 flood) are significantly higher during the second period. On the other hand the number of dry years (years with no flows) are only two during the first period and five during the second period. This means that the same number of floods would have occurred during a smaller number of years. Only two large floods occurred during the first period versus seven during the second period. Peak discharges of all floods during the first period were relatively low (b 5 m3 s− 1 ) and their recurrence intervals were less than 5 years. The 1969 flood was exceptionally larger than all the other floods on record (estimated as the 100-year flood). All flow volumes during the first period were b 158 * 103 m3 (except for the estimated volume of the 1969 flood). In contrast to the first period the second period 1991–2003, included 5 years without floods alternating with 7 years with flows, 3 of which had 5–6 floods. This period included seven large floods with peak discharges between 5 and 20 m3 s− 1 . All these floods were “large” also in terms of their volumes and ranged between 0.169 and 0.575 * 106 m3 . The annual volumes of the flows during the second period are much larger accordingly. The threshold rainfall amount for channel flow generation decreased by 16–25% during the second period from about 60 mm during the first period to 45–50 mm. These clear, short-term changes in the hydrological regime and flow characteristics occurred in spite of the fact that no significant change in annual rainfall amount was observed in any of the rain-gauging stations in the vicinity of Nahal Oren catchment nor in other gauging stations in Mt. Carmel during the last 50 years (Halfon, 2004). Similarly, no significant change was documented in rain-spell length and daily average rainfall per each storm. However, the significant shortening of the rainy season (Fig. 6c) without a parallel change in annual total is well represented by the shortening of the dry spells within the rainy season (Kutiel, 1985). This means an increase in the frequency of rain-spells which may generate floods. Such shortening of the rainy season which means an increase in the frequency of rain-spells, while all other rainfall parameters remain constant, should be represented by increased frequency and magnitude of flood episodes. In addition, each rain-spell during the latter period falls over relatively wetter slopes because of the shortened dryspells, which causes a decrease of both evapotranspiration
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and soil drainage between successive rain-spells. This leads to higher antecedent moisture conditions for each rain-spell which, in turn, enhance runoff generation as indicated by the increased storm rainfall–runoff ratio from b 5% to over 15% and by the 16–25% decrease in the threshold rainfall amount for generation of channel flow. This was also confirmed by Maas and Macklin (2002) who reported on a reduction of floods frequencies in Crete due to shorter rainstorm durations. Generation of relatively more large floods may also be attributed to a possible increase in the distribution of the high intensity rain-spells. The increase in the number of dry years on the one hand and the number of years with 5–6 floods on the other hand, may represent hydrological instability and increased uncertainty of the larger rain-spells and floods in Nahal Oren catchment. This trend parallels an increase in rainfall regime uncertainty in northern Israel documented by Paz and Kutiel (2003). These changes in the rainfall regime and their hydrological consequences are mainly due to changes in atmospheric circulation, air masses stability and moisture content and not to human activity or changes in vegetation cover or type. An increase in monthly rainfall concentration was also noted in the Mediterranean parts of Spain (De Luis et al., 2001). Their results agree with GCM predictions of a decrease in rainfall volume, changes in seasonal distribution and an increase in rainfall intensity in some areas across the Mediterranean basin (Palutikof, 1996). The agreement between model prediction and observations suggests that this trend could be associated with global warming (Palutikof, 1996). Similarly, Maas and Macklin (2002) demonstrated a strong linkage between increased flooding periods in the Aragena Gorge, Crete, and the North Atlantic Oscillation index. Our results conform to those of Maas and Macklin (2002) suggesting that river catchments in the Mediterranean are sensitive to short-term climate variability. 6. Conclusions The results of this study provide several conclusions related to short-term changes in the frequency and magnitude of the flows in Nahal Oren catchment and the possible reasons for these changes. These shortterm changes indicate instability of the hydrological regime. Large floods are generated in response to rain-spells with over 100 mm. When rain-spells are scattered, irrespective of the total annual amount, no flow will develop in the streams. Flow generates after accumulated rainfall of 120–150 mm. Hence, large floods in Nahal Oren catchment rarely occur prior to December.
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Comparing two periods of continuous hydrological data: 1957–1969 and 1991–2003 shows the following changes in flow regime and flood characteristics: 1. Mean annual number of floods has increased from about 1.8/year to 3.8 2. The annual number of floods increased up to 5–6 during the wetter years. 3. Increase in the frequency of large floods (magnitude and volume) from two floods to seven floods. 4. Increase in peak discharges and flow volumes of the floods by up to 4.5 and 3.5 times, respectively. 5. Increase in the number of years with no floods from 2 years to 5 years. 6. Increase in storm rainfall–runoff ratio from b 5% to 5–15.5%. 7. Decrease in the threshold rainfall amount for channel flow generation by 16–25%, from 60 mm to 45– 50 mm. Although annual rainfall totals, rain-spell length and daily average rainfall per storm have not changed during the study period these trends in the hydrological regime and characteristics of the floods may be explained by the following trends of the rain-spells and rainfall regime: 1. The length of the rainy season became significantly shorter. 2. The length of the dry spells is significantly reduced. 3. The number of large rain-spell in each hydrological year increased, parallel with an increase in the uncertainty of behavior. References Arnell, N.W., Reynard, N.S., 1996. The effect of climate change due to global warming on river flows in Great Britain. Journal of Hydrology 183, 397–424. Benito, G., Sopen, A., Sanchez-Moya, Y., Machado, M.J., PerezGonzalez, A., 2003. Palaeofood record of the Tagus River (Central Spain) during the late Pleistocene and Holocene. Quaternary Science Reviews 22, 1737–1756. Coulthard, T.J., Kirkby, M.J., Macklin, M.G., 2000. Modeling geomorphic response to environmental change in an upland catchment. Hydrological Processes 14, 2031–2045. De Luis, M., Garcia-Cano, M.F., Cortna, J., Raventos, J., GonzalezHidalgo, J.C., Sanchez, J.R., 2001. Climatic trends, disturbances and short-term vegetation dynamics in a Mediterranean shrubland. Forest Ecology and Management 147, 25–37. Fuller, I.C., Macklin, M.G., Passmore, D.G., Brewer, P.A., Lewin, J., Wintle, A.G., 1996. Geochronologies and environmental records of Quaternary fluvial sequences in the Guadalope basin, northeast Spain, based on luminescence dating. In: Branson, J., Brown, A.G., Gregory, K.J. (Eds.), Global Continental Changes: the Context of
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