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Ratio between throughfall and open-field bulk precipitation used for quality control in deposition monitoring
ARTICLE IN PRESS
Atmospheric Environment 38 (2004) 4869–4878
Ratio between throughfall and open-field bulk precipitation used for quality control in deposition monitoring Johan C. Knulst* IVL Swedish Environmental Research Institute, Aneboda, SE-360 30 Lammhult, Sweden Received 13 November 2003; received in revised form 30 April 2004; accepted 26 May 2004
Abstract In the Swedish regional throughfall monitoring network, set up to assess the atmospheric input of acidifying compounds to forests, throughfall collections are combined with open-field bulk precipitation collections throughout the nation. Bulk precipitation chemistry is routinely compared with throughfall chemistry to estimate deposition to the forest floor. The amount ratio between throughfall and bulk precipitation can give valuable information about the quality of the collected samples. In this study, the information provided by the ratios between throughfall and open field and amounts, supported by ion concentrations of chloride and sulphur is discussed for deciduous and coniferous forests while using summer or winter collectors. Distribution of common vs. uncommon ratios for amounts shows regional patterns on a temporal scale. On a local scale, ratios are primarily dependent on tree species and shape, forest stand structure, and meteorological events, in that order. Amount ratios in Sweden are strongly influenced by the precipitation form; either rain or snow. Occasions with less than 40% normal ratio amount occurred in the northern region, and mostly during winter months with snowfall dominating. Ion concentration ratio of chloride can show the influences of strong meteorological events, as well as marine influences on a regional scale. Sulphur ion concentration ratios reveal a south to north diminishing gradient, as well as a seasonal cycle. Ion concentration ratios are not stable on a local scale and cannot be used effectively as site specific quality indicators. They can, however, be used to check the reliability of calculated deposition. Recommendations on quality control handling are given. r 2004 Elsevier Ltd. All rights reserved. Keywords: Acidification; Regional monitoring; Pine; Spruce; Deciduous forest; Methodology
1. Introduction Environmental monitoring data should be collected with methods that allow for comparison and quality assurance over long term periods to enable trend studies (Wells and Cofino, 1997; UN-ECE and EU, 1998). To achieve reasonable results at lowest possible costs, the collection, sample handling and analytical processing *Tel.: +46-472-267780; fax: +46-472-267790. E-mail address: [email protected] (J.C. Knulst).
ought to be simple, cheap and reliable. The Swedish regional throughfall monitoring network (RTM) has used similar collection and analytical methods since investigations started in 1985 at over 100 rural permanent observation plots nationwide. Throughfall is defined as the water dripping from the forest canopies or falling to the forest floor in between the trees (Parker, 1983). In accordance with the ICPforest manual on deposition measurements (UN-ECE and EU, 1998) RTM throughfall measurements are combined with precipitation sampling at an open field
1352-2310/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2004.05.015
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location. Bulk precipitation is defined as the total (wet and dry) deposition collected in the open, generally a field or a clear cut forest area. Since the deposition of acidifying compounds is calculated from the measured amount of precipitation and measured concentrations of chemical substances in the sample, the accuracy of both measurements is crucial. Analytical accuracy of the concentrations measured in solutions is established in inter-calibration efforts (Mosello et al., 2002). Inter-comparison of the accuracy of bulk precipitation (Erisman et al., 2003) and throughfall collectors (Bleeker et al., 2003) used in the European ICP-forest network were made. Spatial variation in throughfall can be due to site conditions (Parker, 1983), tree species, stand structure (Aboal et al., 2000), and several other factors (Thimonier, 1998; Crockford and Richardson, 2000). Likewise, open field collectors reveal spatial variation due to collector size, shape, placement and local meteorological conditions (Erisman and Draaijers, 2003). Throughfall amounts are especially affected by crown interception due to forest top roughness (Erisman and Draaijers, 2003), crown wetability and water that runs along the branches and tree trunk, called stemflow (Parker, 1983), which generally entails a few percent of the total precipitation deposited in the forest stand, unless the forest stand contains large-crowned emergent trees, such as beech (Fagus sp.) (Thimonier, 1998) or micrometeorological conditions are such that crown interception is stimulated (Xiao et al., 2000b). The ratio between amounts of throughfall and incident bulk precipitation (TFBU ratio) from the same location is routinely used as one of several quality control indicators, which has been problematic at times. Sudden aberrations, with occasional higher throughfall than bulk precipitation amounts do occur, especially during winter months. In an attempt to find patterns in the deviations from expected ratio, a detailed analysis of the ratios is made for amounts. Patterns in concentrations of chloride and sulphur in bulk and throughfall collectors are studied to evaluate the possible deviations for amount TFBU ratios that may lead to faulty deposition calculations. Concentrations are affected by evaporation, tree interception (losses) and selective ion adsorption to different surfaces (gains), where variation
always is influenced by meteorology. The major findings are presented in this paper.
2. Methods 2.1. Field methodology 2.1.1. Throughfall collection RTM throughfall is collected in Norway spruce (Picea abies) stands, Scots pine (Pinus sylvestris) or deciduous stands of oak (Quercus sp.), birch (Betula sp.) or beech (Fagus sylvatica). Winter equipment is used during months with likely snowfall. Summertime collectors are polyethene (PE) funnels attached to aluminium foil covered carboys with PE liners, that are placed on wooden poles in an L-form pattern along two sides of a 30 30 m forest plot. See Table 1 for more details on used collectors. The contents of the 10 throughfall collectors are pooled into a 20-l PE plastic bucket of which the amount is known for each distance d between the water surface and the rim. d is measured with a clean measuring tape at opposite sides in the bucket, and then averaged. Amounts are read from a calibrated chart. Summertime throughfall collectors were discussed in (Bleeker et al., 2003). Wintertime collectors are buckets with PE liners, placed on poles at the same spots where summertime collectors sit during summer. In case the sample consists of liquid, the contents of the 10 collectors are pooled and treated as mentioned for summer equipment. Otherwise, the snow/ice filled liners are taken indoors and melted at room temperature, after which the meltwater is treated as during summertime collection. 2.1.2. Open field incident precipitation bulk collection At each location, one or two bulk precipitation collectors are placed in an open field near the permanent forest observation plot. The bulk collector used during summertime has been described in detail in (Erisman and Draaijers, 2003). The collector consists of a plastic PE funnel with 101.5 mm radius placed on an aluminium foil covered carboy with PE liner, which in turn is placed in a holder on top of a pole.
Table 1 Details of the summer and winter collection equipment used in the Swedish RTMN Collector type
Opening radius (mm)
Reservoir volume (L)
Surface area of materials (cm2)
Sampling height (m)
Summer throughfall Winter throughfall Summer bulk Winter bulk
77.5 107.0 101.5 97.5
2 5 5 5a
5370 6550 863 7350
0.8 0.8 1.8 1.8
a
Additional space for 36 l snow.
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Wintertime collector consists of a 1.2 m tall PE bag fitted in a ring-shaped holder (97.5 mm radius) at the top, and a summertime funnel and carboy at the bottom. 2.1.3. General Collections are made approximate the monthly intervals (mean7stdev, 3077 days). Amounts are either measured by transferring the solution to a clean graduated cylinder or by weighing the vessel with contents in the laboratory. Aliquots of the collected solution for analytical processing are transferred to small PE bottles prior to amount measurements. Aliquot amounts are weighed in at the laboratory and added to the field-measured amounts.
3. Site description Sweden (see Fig. 1) covers latitudes between 55 and 69 N. A vast regime of climatic conditions and vegetation types are present. Sweden’s topography consists of mountains in the northwest, bounded on the east by a plateau that slopes down to lowlands and
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plains in the east and south. The principal natural resources are forests, which cover about 23 million ha (SCB, 2000). The majority of Swedish forests are secondary cultivated growth, intensively managed for production purposes with even age distributions (Linder et al., 1997). The climate of northern Sweden is considerably more severe than that of the south primarily because it has higher altitudes and because the mountains cut off the moderating marine influence. The average temperature in February, the coldest month, is below freezing throughout Sweden, with an average temperature range in Stockholm of 5–1 C, in Gothenburg of 4–1 C, and in Pitea of 14–6 C. Precipitation generally falls as snow or hail between October and March. In July, the warmest month, the average temperature range is 14–22 C in Stockholm, 14–21 C in Gothenburg, and 12–21 C in Pitea. Salinity of the western seas (Kattegat and Skagerak) is considerably higher than that in the Baltic Sea in the east and Bay of Bothnia in the northeast, averaging 25–30, 8–12 and 5–10 PSU, respectively (SCB, 2000). The RTM sites are shown in Fig. 1. In 1996 several of the initial sites were abandoned. The 1996 changes in the network did not have noticeable effects on the measured parameters (Hallgren Larsson, 1999). Missing amount values are replaced by estimates for 3.3% of the winter and 2.8% of the summer collections made, using data from surrounding locations and trend studies for the disturbed collector.
4. Quality control procedure At the data analysis stage of the RTM scheme, collected data are quality checked by comparison of monthly results between stations from the same region. Calculated ionic balances, relative amounts in throughfall and bulk precipitation, sodium to chloride ratio in each sample, and some other quality assurance indicators (not described here) are used.
5. Data analysis methodology
Fig. 1. Map of Sweden with plot locations (dots) major regions (grey scales), county borders (black lines) and codes used in this study.
TFBU ratios are calculated for amounts (Eq. (1)), and between concentrations of chloride and sulphate–sulphur ions for monthly collections at each location where both open field incident bulk precipitation and throughfall were collected. For ratio comparison a best estimate value is established for each location by calculating averages for each type of equipment used, after removing outliers for concentrations or amounts outside the 2-sigma range (above twice the standard deviation from the mean value for the location) from the database. 3.4% of the TFBU ratios during winter collections, and 2.8% of summer
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equipment collections are omitted outliers. Patterns are identified by averaging TFBU ratios for type of equipment used, for regions, for type of primary forest vegetation, and proximity to nearest coastline. To identify temporal and spatial patterns, amount TFBU ratios were divided into 5 classes and plotted as percentages of each class for each month during 1997–2000. For large aberrations from the best estimate values at each location weather observations by the Swedish meteorological and hydrological institute (SMHI) and reports from field personnel were studied to identify the possible causes.
6. Results 6.1. General ratios The national range of monthly incident precipitation is 0–320 mm between 1990 and 2000, while regional maximum monthly amounts of 270, 320 and 229 mm are measured for southern, central and northern Sweden. National annual precipitation has increased by 16% between 1990 (mean 599 mm) and 2000 (mean 695 mm), based on RTM bulk measurements. Throughfall generally contains lower amounts than corresponding bulk precipitation collectors for the same exposure period, but in 5% of the winter and 3% of summer collections throughfall amounts exceeded bulk amounts. Average TFBU ratio for the different types of vegetation (Table 2) show that differences exist while using winter vs. summer equipment, and between the major regions of Sweden. The frequency of high or low TFBU ratio values has not changed between 1990–1995 and 1996–2000. TFBU ratios in the north are generally higher than those in the south, both for spruce and pine forest types either during winter or summer equipment use. The relationship between ratio and sum of throughfall and open field precipitation (Fig. 2) shows that all
acceptable instances when the amount TFBU ratios were higher than 2 occurred while the sum of open field precipitation and throughfall was below 100 mm month1. High sum amounts (>300 mm month1) do not affect the ratios negatively in the few cases that they occurred. The seasonal differences found for the TFBU ratios more often appear as lower values during winter for a wide range of total amounts, and as high ratios during the summer, especially during drier episodes with sum amounts of less than approx. 60 mm (Fig. 2). 6.2. Regional ratios 6.2.1. Amounts On a regional scale, the TFBU ratios vary between all spruce sites (Table 2). The seasonal differences exceed the regional variations. The occurrence of aberrant TFBU ratios (values either lower or higher than the best estimate value) is plotted by dividing the ratios into 5 classes with their 500 winter summer
400 300 Sum
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200 100 0 0
1
2
3
4
5
Ratio
Fig. 2. The ratios of throughfall to bulk incident precipitation amounts and the sum of both amounts collected on a monthly basis at southern Swedish monitoring sites with spruce forest plots (n=153) during 1990–2000. Data is presented separately for collections with winter and summer equipment.
Table 2 Average ratio 7 standard deviation between total amount of throughfall and bulk open field precipitation in three forest types of the Swedish RTMN in 1990–2000 Tree species and equipment
Number of locations
Southern (S) (n values)
Central (C) (n values)
Northern (N) (n values)
Spruce–winter Pine–winter Deciduous–winter Spruce–summer Pine–summer Deciduous–summer
S=81, S=21, S=10 S=84, S=26, S=10
0.4770.22 0.5570.15 0.5970.31 0.6270.16 0.6770.17 0.7470.15
0.4670.24 0.4870.19 N/A 0.6270.16 0.6870.17 N/A
0.5970.25 0.6270.24 N/A 0.6970.18 0.7470.17 N/A
C=57, N=22 C=19, N=4 C=57, N=22 C=20, N=4
(1994) (509) (283) (3328) (1118) (576)
(1865) (441) (1999) (503)
(931) (154) (675) (117)
N/A indicates that tree species plot was not available in the region. The number of values is given between parentheses. Outliers (see text for explanation) were excluded.
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frequency distribution for each month during 1997–2000 (Fig. 3). It becomes obvious that relatively low (ratio o=0.2) and rather high values (ratio >1.0) most often occur during winter months, which is also seen in Fig. 2. The monthly average amounts of precipitation shown in Fig. 3 reveal that 40–95% of the values in the normal TFBU ratio range (between 0.2 and 0.7) appear during months with 30–75 mm incident precipitation. Above 75 mm monthly precipitation, the normal TFBU ratio range occurs between 36% and 100%, while all normal range portions of less than 40% occur in the northern region, during winter (October through March), with the single exception of June 1997 (26.7% in normal ratio range). 100% of all TFBU ratios in the northern region were in the normal range in April 1998 and April 2000, with 79 and 76 mm month1 precipitation falling respectively. 6.2.2. Chloride ions Concentrations of chloride ions revealing extreme events appeared for example during the month of June 1993, August 1997 and April 2000 in several counties. The chloride ion concentrations in bulk collectors increase directly during the event, while they lag in throughfall. The ratios drop during the collection interval of the event, and are higher in the following collection. The effects of these extreme events are most clearly seen in the data collected in southwestern Sweden, the region receiving most precipitation, but also at sites in Blekinge (K county) and in the northern coastal region. No clear relationship between the amount and chloride concentration TFBU ratio is found. A relationship between chloride concentration TFBU ratio and closeness to coastline is apparent in southern Sweden (Table 3), less so in the north. Local variations in chloride ratios were much greater near the coast than at inland locations (Table 3). 6.2.3. Sulphur ions Sulphur ion concentrations and TFBU ratios for sulphur concentrations between throughfall and corresponding open field precipitation reveal no specific covariation with the amount of TFBU ratios. Sulphur TFBU ratios vary spatially, and have a temporal pattern. A gradient of sulphur TFBU ratios is found from south to north, with the highest values in the southern end. Sulphur ion concentrations in both throughfall and bulk precipitation throughout the nation have diminished during the whole investigation period, as well as in seasonal fluctuation amplitudes. Ratios between the two concentrations are generally the highest from March–May, while winter equipment is still in use, and lowest during August. Wintertime TFBU ratios are thus higher than summer ratios (Table 3).
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6.3. Local ratios At the highest resolution (location level) the variations in amount TFBU ratios reveal roughly a seasonal cycle (Fig. 4). December–March are commonly drier than June–September, and the highest ratios are generally found during periods with little precipitation (Figs. 2 and 4). Looking at the amount TFBU ratios at tree species level for the deciduous plots shows that differences clearly exist between summer and winter periods (Table 4). Slight variations occur between sites of the same dominant tree species (Table 5). For spruce a weak increase in site-specific average best estimate TFBU ratios is seen with increasing plot altitude. This is not observed for pine. The standard deviation between TFBU ratios for the same location and same deciduous tree species at different locations is greater during winter than during summertime, which is also the case for spruce stands but not for pine stands in southern and central regions (Table 2). Throughfall amounts vary least for the deciduous tree stands, more for pine stands and most for spruce stands, throughout the year. TFBU amount ratios in beech stands are lowest during the late winter–early spring (o0.4) and show high peaks (>0.8) during the late fall.
7. Discussion 7.1. General ratios The observed increase in precipitation during the study period did not alter the frequency of amount TFBU ratios that are higher or lower than the normal TFBU ratio range. Higher than normal monthly precipitation did not affect the amount TFBU ratio negatively, indicating that the total amount of incident precipitation is not primarily responsible for extraordinary TFBU ratios. If collectors frequently over-fill, the TFBU ratio is not reliable. The collectors in this study did not over-fill summertime, but some throughfall winter collections may have been over-filled with snow in the northern region. Volume adjustments were made to a few collections where this was noted by field personnel combined with suspicious looking TFBU amount ratios. Differences in best estimate TFBU ratios between sites are affected by the site specific factors, such as plot placement, stand characteristics, and general meteorological settings. In this study, the tree species is identified as the most important factor of the best estimate TFBU ratio. Response of different tree species to rain events varies extensively (Xiao et al., 2000b) in its turn influencing throughfall (Robertson et al., 2000).
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Fig. 3. Percentage of the 5 general classes of the ratios between amount of throughfall collected in spruce forest and amount of incident precipitation in bulk open field collector at 160 locations. All valid collections on a monthly basis are included for three major regions in Sweden. The all black diagrams give monthly mean incident precipitation amounts for the respective region.
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Table 3 Throughfall (TF) collected in Norway spruce stands and open field precipitation (BU) in southern Sweden between 1 January, 1990 and 31 December, 2000, as county mean values for months with summer (S) or winter equipment (W) and the mean TFBU ratio between throughfall and bulk precipitation (amount), chloride (Cl) and sulphur (SO4–S) ions Equipment
County
n=
TF Cl
BU Cl
TFBU Cl
TF SO4–S
BU SO4–S
TFBU SO4–S
TF Amount
BU Amount
TFBU Amount
S S S S S S W W W W W W
F G K LM N O F G K LM N O
331 382 337 441 367 1151 250 280 236 238 181 805
6.034 4.637 4.664 8.810 10.136 7.503 12.672 9.352 10.433 16.139 28.350 15.172
0.847 0.839 0.804 1.516 2.138 1.260 2.822 2.378 2.169 3.515 4.951 3.577
8.57 6.99 7.03 7.50 5.54 7.65 5.50 4.88 5.69 5.38 6.04 5.13
2.32 2.18 2.70 3.72 2.50 2.07 4.70 4.86 6.51 8.32 5.79 3.70
0.78 0.78 0.93 1.06 1.07 0.85 1.03 0.90 1.27 1.28 1.24 0.97
3.00 2.82 3.06 3.57 2.54 2.62 4.97 5.26 5.46 7.00 4.60 4.03
42 42 41 40 56 48 30 36 25 32 41 45
66 69 65 66 85 73 79 82 54 76 79 85
0.61 0.60 0.60 0.58 0.65 0.65 0.37 0.45 0.45 0.45 0.57 0.52
Fig. 4. Monthly throughfall and open field incident precipitation amounts collected at a location in the county of Varmland (S), central Sweden, during 1997–2000, and the ratios between these amounts. Amount of throughfall collected in forest plot ðmm month1 Þ : Equation 1: Amount of incident bulk precipitation collected at open field ðmm month1 Þ
The amount TFBU ratios during winter collections are quite different from the summer values. The frequent occurrence of low ratios during winter seems often due to higher than expected amounts collected in the winter bulk collection equipment. Low ratios are more frequent in summer, either due to higher tree interception and crown dripping than direct precipitation into the open
field collector, or small total amounts of precipitation during the collection period, resulting in nearly dry throughfall collectors. The retention time for winter precipitation in forest vegetation can easily exceed 30 days below freezing ambient temperatures. When snow and ice are collected in bulk open field collectors, the amount is directly
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Table 4 Average ratio 7 standard deviation between total amount of throughfall (TF) and OF collected in and near deciduous forest types of the Swedish RTMN in 1990–2000 Tree species
Locations (n=)
Winter equipment
Summer equipment
Birch Beech Oak
1 6 2
0.5970.24 (53) 0.5570.24 (146) 0.5170.19 (44)
0.7870.09 (75) 0.7470.13 (260) 0.7970.11 (70)
The number of values is given between parentheses. Outliers (see text for explanation) are excluded.
Table 5 Average ratio 7 standard deviation between total amount of throughfall (TF) and OF collected in and near the beech (Fagus sylvatica) forest plots of the Swedish RTMN in 1990–2000 Plot code and name
Tree age (2000) (year)
Winter equipment
Summer equipment
K 11, Komperskulla N 14, Djupeasen M 04, Ska¨r L 11, Akeboda L 12, Kampholma K 07, Ryssberget
75 80 85 100 102 120–125
0.5070.20 (26) 0.5970.26a(26) 0.8170.23 (26) 0.5370.20 (19) 0.5270.19 (17) 0.4970.21 (53)
0.8470.09 0.7070.19 0.7570.19 0.7770.15 0.7770.09 0.6970.09
(34) (36) (43) (38) (38) (75)
The number of values is given between parentheses. Outliers (see text for explanation) are excluded. a Open field and throughfall not always collected on the same day.
affected, while it may take an extended period of time before they melt or fall off the tree branches into the throughfall collectors at the forest plot. Thus, a precipitation detection gap can frequently exist at boreal forest locations. The trees in areas that annually receive a lot of snow have adapted their shape to lose the snow off their branches as quickly as possible. Where snowfall is uncommon, tree adaptation is less common, leading to a higher degree of snow retention in the tree crown, compared with adapted trees. This physical difference can also off-set snow collection duration. Another possible cause of concern is the condensation of air humidity onto cold surfaces of the collectors or the capture of fog droplets. Fog in forests affects throughfall measurements, especially at higher elevations (Zimmermann et al., 1999; Kowalski and Vong, 1999). Since the exposed collector surfaces are larger during winter than during the summer operations, especially on open fields (Table 1), and temperatures are generally lower than the ambient dew point temperature, this can cause higher collection values in our winter bulk collectors than in corresponding throughfall collectors. The unequal wind exposure of throughfall and bulk collectors may off-set the ratio differences for collected amounts even more. Dew or fog collection occurs most likely during the wet months of October–December, less so during the drier period of January through March. Especially RTM bulk amounts may be affected by this in wintertime, which was seen in the data. The RTM winter bulk collectors are found to collect slightly more than amounts collected
at nearby stations by SMHI (Uggla et al., 2003). Amounts collected with summer equipment correspond better with nearby collected SMHI precipitation (Uggla et al., 2003). Another possible cause of low winter TFBU ratios can be the enhanced tree interception of snow and rimfrost, instead deposited to the forest floor as stemflow during melting periods. On a regional scale, the amount TFBU ratios vary between spruce sites. Seasonal differences exceed the regional variations, pointing at meteorological causes being superior over the other influences, in support of Crockford and Richardson (2000) who reviewed partitioning of rainfall. The technical functionality of the equipment used can influence regional scale differences as well, as it is influenced by meteorology. The form, intensity and frequency of precipitation events are strongly responsible for the amount of throughfall that falls to the forest floor. When intensities are low, and events occur sparsely, it has been found that nearly all of the incident precipitation as rain falls unhindered to the forest floor. Initially coniferous tree surfaces are able to shed most water droplets until the dry particles are washed off. But when intensities increase or events occur closely after one another, the tree crown is wetted to a greater extent, which causes a greater loss of throughfall toward stemflow or tree crown water retention (Xiao et al., 2000a), also found in a southern Swedish 25-year old spruce stand (Alavi et al., 2001). In deciduous forests it is more common that low-volume rainfall leads to almost complete interception. When rain events
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continue over extended periods of time, the tree crown eventually becomes saturated and a balance is established between stemflow and throughfall (Draaijers and Erisman, 1995). Another physical factor of importance is the placement of the collectors in the forest plot. Placement of the collectors determines their collection ability of throughfall to a greater extent than the tree crown formation under which they are placed (Hansen, 1995). The closer the collector is placed to the tree trunk, lesser amount of throughfall will be collected (Thimonier, 1998). Since placement is the same throughout the whole investigation period, this component is a constant variable for each plot. Careful consideration has to be given to instances when collectors are moved around, which has not happened in this study. The regional differences in forest site physical appearance have a visible effect on the site specific TFBU ratios, even though generalizations are hampered by natural variability of the forest types, and the importance of meteorological conditions (Crockford and Richardson, 2000). Spruce trees in the northern region are narrower in crown diameter, with branches bend sharply down in a conical or spire-shape (Linder et al., 1997). At southern locations the spruce trees have wider crowns, with a greater needle biomass and more horizontal branches (Nilsson et al., 2002) than those in the northern region. Pine trees commonly have wider crowns in the northern region than in southern regions, leading to a slightly higher normal interception. In our study, the slightly higher amount TFBU ratios established for pine in the north than for the rest of the nation supports that. The amount TFBU ratios of deciduous plots are prone to be influenced by defoliation during fall, which does not occur simultaneously with the changing from summer to winter collectors. The exact dates of defoliation and leaf sprouting are not available. Thus, defoliation influences cannot be revealed in this study. The amount of chloride ions in incident open field precipitation is a good indicator for influences from the sea. Even though site specific collection of chloride exists in throughfall, the general amount of chloride ions in throughfall can tell about the coarse meteorological setting during exposure time. In addition to sample quality control by the ratio between sodium and chloride, the chloride concentration of the sample and deposited amount is used to indicate sea-salt input to the site. The variations in chloride concentrations on a spatial and temporal scale are indicative of extreme wind direction and speed, although the temporal variation usually is obscured by the monthly collection period. Deposition is therefore more appropriately reported as cumulative deposition on annual basis between 1 October and 30th September. Sometimes the extreme wind condition leads to an event that covers over the sum of minor events during the same exposure interval. These extreme events usually also have a strong effect on
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the total deposition during that interval. It is therefore useful to be able to detect such events. For the more detailed study of process importance on TFBU ratios, micro-meteorological data needs to be collected at or very near the investigation site. This, however, greatly increases the costs of the monitoring endeavour and cannot be applied on a widespread basis.
8. Conclusions The throughfall to bulk incident open field amount ratio is a valuable indicator of data quality. The ratio indicates site specific collector efficiency on a local scale and how it is affected by major meteorological events. Comparison between sites with the same forest type can indicate regional patterns, and help identify possible field operational disturbances that may cause faulty deposition calculations. In the evaluation process of our deposition data, automatic calculations of the described ratios, combined with ionic balances and manual intersites comparisons has proven a good way to detect ‘‘problem’’ samples. In case the computer program warns that a ratio is uncommon, possible reasons are sought manually. If the reason is of the kind discussed in this paper, and the aberration is not detected in samples from surrounding sites, an appropriate manual adjustment is made to the volume, marked in the database as ‘‘adjusted value’’ that is traceable during future data treatment.
Acknowledgements This study was performed as part of the Swedish EPA Throughfall Monitoring Program, with data from the Regional Throughfall Monitoring Program financed by Swedish air quality authorities. The valuable discussions with E. Hallgren Larsson and O. Westling, and the map of Sweden provided by E. Uggla are gratefully acknowledged, as were the reviewers comments on the manuscript.
References Aboal, J.R., Jimenez, M.S., Morales, D., Gil, P., 2000. Effects of thinning on throughfall in Canary islands pine forest— the role of fog. Journal of Hydrology 238 (3–4), 218–230. Alavi, G., Jansson, P.E., Ha¨llgren, J.E., Bergholm, J., 2001. Interception of a dense spruce forest, performance of a simplified canopy water balance model. Nordic Hydrology 32 (4–5), 265–284. Bleeker, A., Draaijers, G., Van Der Veen, D., Erisman, J.W., Mols, H., Fonteijn, P., Geusebroek, M., 2003. Field intercomparison of throughfall measurements performed within the framework of the Pan European intensive
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monitoring program of EU/ICP forest. Environmental Pollution 125 (2), 123–138. Crockford, R.H., Richardson, D.P., 2000. Partitioning of rainfall into throughfall, stemflow and interception: effect of forest type, ground cover and climate. Hydrological Processes 14 (16–17), 2903–2920. Draaijers, G.P.J., Erisman, J.W., 1995. A canopy budget model to assess atmospheric deposition from throughfall measurements. Water Air and Soil Pollution 85, 2253–2258. Erisman, J.W., Draaijers, G., 2003. Deposition to forests in Europe: most important factors influencing dry deposition and models used for generalisation. Environmental Pollution 124 (3), 379–388. Erisman, J.W., Mols, H., Fonteijn, P., Geusebroek, M., Draaijers, G., Bleeker, A., Van Der Veen, D., 2003. Field intercomparison of precipitation measurements performed within the framework of the Pan European intensive monitoring program of EU/ICP forest. Environmental Pollution 125 (2), 139–155. Hallgren Larsson, E., 1999. Forest observation plots in ICPforest level II network. IVL Swedish Environmental Research Institute Ltd., Aneboda. Hansen, K., 1995. In-canopy throughfall measurements in Norway spruce: water flow and consequences for ion fluxes. Water Air and Soil Pollution 85, 2259–2264. Kowalski, A.S., Vong, R.J., 1999. Near-surface fluxes of cloud water evolve vertically. Quarterly Journal of the Meteorological Society 125, 2663–2684. Linder, P., Elfving, B., Zackrisson, O., 1997. Stand structure and successional trends in virgin boreal forest reserves in Sweden. Forest Ecology and Management 98 (1), 17–33. Mosello, R., Derome, J., Derome, K., Ulrich, E., Dahlin, T., Marchetto, A., Tartari, G., 2002. Atmospheric deposition and soil solution working ring test 2002—Laboratory ring test for deposition and soil solution sample analyses between countries participating in the ICP forest level II monitoring programme. European Union/UN Economic commission for Europe, Fontainebleau, France. Nilsson, S.G., Niklasson, M., Hedin, J., Aronsson, G., Gutowski, J.M., Linder, P., Ljungberg, H., Mikusinski, G., Ranius, T., 2002. Densities of large living and dead trees in old-growth temperate and boreal forests. Forest Ecology and Management 161, 189–204.
Parker, G.G., 1983. Throughfall and stemflow in the forest nutrient cycle. Advances in Ecological Research 13, 58–135. Robertson, S.M.C., Hornung, M., Kennedy, V.H., 2000. Water chemistry of throughfall and soil water under four tree species at Gisburn, Northwest England, before and after Felling. Forest Ecology and Management 129 (1–3), 101–117. SCB, 2000. Environmental Statistical Yearbook 2000. Statistics Sweden, Stockholm. Thimonier, A., 1998. Measurement of atmospheric deposition under forest canopies: some recommendations for equipment and sampling design. Environmental Monitoring and Assessment 52 (3), 353–387. Uggla, E., Hallgren Larsson, E., Knulst, J., Westling, O., 2003. Comparison between measured and modeled deposition of sulphur and nitrogen in Sweden. (Ja¨mfo¨relse mellan uppma¨tt och modellbera¨knad deposition av svavel och kva¨ve i Sverige.) In Swedish, IVL Svenska Miljo¨institutet AB, Aneboda/Gothenburg. UN-ECE, EU, 1998. Manual on methods and criteria for harmonized sampling, assessment, monitoring and analysis of the effects of air pollution on forests. Federal Research Centre for Forestry and Forest Products, Hamburg, Germany. Wells, D.E., Cofino, W.P., 1997. Developments in quality assurance in marine environmental monitoring. The QUASIMEME II laboratory performance studies and EU quality assurance of sample handling (QUASH) project. Marine Pollution Bulletin 35, 146–155. Xiao, Q.F., Mcpherson, E.G., Ustin, S.L., Grismer, M.E., 2000a. A new approach to modeling tree rainfall interception. Journal of Geophysical Research: Atmosphere 105, 29173–29188. Xiao, Q.F., Mcpherson, E.G., Ustin, S.L., Grismer, M.E., Simpson, J.R., 2000b. Winter rainfall interception by two mature open-grown trees in Davis, California. Hydrological Processes 14 (4), 763–784. Zimmermann, L., Fruhauf, C., Bernhofer, C., 1999. The role of interception in the water budget of spruce stands in the Eastern Ore Mountains, Germany. Physics and Chemistry of the Earth, part B: Hydrology of Oceans and Atmosphere 24, 809–812.
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Ratio between throughfall and open-field bulk precipitation used for quality control in deposition monitoring Johan C. Knulst* IVL Swedish Environmental Research Institute, Aneboda, SE-360 30 Lammhult, Sweden Received 13 November 2003; received in revised form 30 April 2004; accepted 26 May 2004
Abstract In the Swedish regional throughfall monitoring network, set up to assess the atmospheric input of acidifying compounds to forests, throughfall collections are combined with open-field bulk precipitation collections throughout the nation. Bulk precipitation chemistry is routinely compared with throughfall chemistry to estimate deposition to the forest floor. The amount ratio between throughfall and bulk precipitation can give valuable information about the quality of the collected samples. In this study, the information provided by the ratios between throughfall and open field and amounts, supported by ion concentrations of chloride and sulphur is discussed for deciduous and coniferous forests while using summer or winter collectors. Distribution of common vs. uncommon ratios for amounts shows regional patterns on a temporal scale. On a local scale, ratios are primarily dependent on tree species and shape, forest stand structure, and meteorological events, in that order. Amount ratios in Sweden are strongly influenced by the precipitation form; either rain or snow. Occasions with less than 40% normal ratio amount occurred in the northern region, and mostly during winter months with snowfall dominating. Ion concentration ratio of chloride can show the influences of strong meteorological events, as well as marine influences on a regional scale. Sulphur ion concentration ratios reveal a south to north diminishing gradient, as well as a seasonal cycle. Ion concentration ratios are not stable on a local scale and cannot be used effectively as site specific quality indicators. They can, however, be used to check the reliability of calculated deposition. Recommendations on quality control handling are given. r 2004 Elsevier Ltd. All rights reserved. Keywords: Acidification; Regional monitoring; Pine; Spruce; Deciduous forest; Methodology
1. Introduction Environmental monitoring data should be collected with methods that allow for comparison and quality assurance over long term periods to enable trend studies (Wells and Cofino, 1997; UN-ECE and EU, 1998). To achieve reasonable results at lowest possible costs, the collection, sample handling and analytical processing *Tel.: +46-472-267780; fax: +46-472-267790. E-mail address: [email protected] (J.C. Knulst).
ought to be simple, cheap and reliable. The Swedish regional throughfall monitoring network (RTM) has used similar collection and analytical methods since investigations started in 1985 at over 100 rural permanent observation plots nationwide. Throughfall is defined as the water dripping from the forest canopies or falling to the forest floor in between the trees (Parker, 1983). In accordance with the ICPforest manual on deposition measurements (UN-ECE and EU, 1998) RTM throughfall measurements are combined with precipitation sampling at an open field
1352-2310/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2004.05.015
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location. Bulk precipitation is defined as the total (wet and dry) deposition collected in the open, generally a field or a clear cut forest area. Since the deposition of acidifying compounds is calculated from the measured amount of precipitation and measured concentrations of chemical substances in the sample, the accuracy of both measurements is crucial. Analytical accuracy of the concentrations measured in solutions is established in inter-calibration efforts (Mosello et al., 2002). Inter-comparison of the accuracy of bulk precipitation (Erisman et al., 2003) and throughfall collectors (Bleeker et al., 2003) used in the European ICP-forest network were made. Spatial variation in throughfall can be due to site conditions (Parker, 1983), tree species, stand structure (Aboal et al., 2000), and several other factors (Thimonier, 1998; Crockford and Richardson, 2000). Likewise, open field collectors reveal spatial variation due to collector size, shape, placement and local meteorological conditions (Erisman and Draaijers, 2003). Throughfall amounts are especially affected by crown interception due to forest top roughness (Erisman and Draaijers, 2003), crown wetability and water that runs along the branches and tree trunk, called stemflow (Parker, 1983), which generally entails a few percent of the total precipitation deposited in the forest stand, unless the forest stand contains large-crowned emergent trees, such as beech (Fagus sp.) (Thimonier, 1998) or micrometeorological conditions are such that crown interception is stimulated (Xiao et al., 2000b). The ratio between amounts of throughfall and incident bulk precipitation (TFBU ratio) from the same location is routinely used as one of several quality control indicators, which has been problematic at times. Sudden aberrations, with occasional higher throughfall than bulk precipitation amounts do occur, especially during winter months. In an attempt to find patterns in the deviations from expected ratio, a detailed analysis of the ratios is made for amounts. Patterns in concentrations of chloride and sulphur in bulk and throughfall collectors are studied to evaluate the possible deviations for amount TFBU ratios that may lead to faulty deposition calculations. Concentrations are affected by evaporation, tree interception (losses) and selective ion adsorption to different surfaces (gains), where variation
always is influenced by meteorology. The major findings are presented in this paper.
2. Methods 2.1. Field methodology 2.1.1. Throughfall collection RTM throughfall is collected in Norway spruce (Picea abies) stands, Scots pine (Pinus sylvestris) or deciduous stands of oak (Quercus sp.), birch (Betula sp.) or beech (Fagus sylvatica). Winter equipment is used during months with likely snowfall. Summertime collectors are polyethene (PE) funnels attached to aluminium foil covered carboys with PE liners, that are placed on wooden poles in an L-form pattern along two sides of a 30 30 m forest plot. See Table 1 for more details on used collectors. The contents of the 10 throughfall collectors are pooled into a 20-l PE plastic bucket of which the amount is known for each distance d between the water surface and the rim. d is measured with a clean measuring tape at opposite sides in the bucket, and then averaged. Amounts are read from a calibrated chart. Summertime throughfall collectors were discussed in (Bleeker et al., 2003). Wintertime collectors are buckets with PE liners, placed on poles at the same spots where summertime collectors sit during summer. In case the sample consists of liquid, the contents of the 10 collectors are pooled and treated as mentioned for summer equipment. Otherwise, the snow/ice filled liners are taken indoors and melted at room temperature, after which the meltwater is treated as during summertime collection. 2.1.2. Open field incident precipitation bulk collection At each location, one or two bulk precipitation collectors are placed in an open field near the permanent forest observation plot. The bulk collector used during summertime has been described in detail in (Erisman and Draaijers, 2003). The collector consists of a plastic PE funnel with 101.5 mm radius placed on an aluminium foil covered carboy with PE liner, which in turn is placed in a holder on top of a pole.
Table 1 Details of the summer and winter collection equipment used in the Swedish RTMN Collector type
Opening radius (mm)
Reservoir volume (L)
Surface area of materials (cm2)
Sampling height (m)
Summer throughfall Winter throughfall Summer bulk Winter bulk
77.5 107.0 101.5 97.5
2 5 5 5a
5370 6550 863 7350
0.8 0.8 1.8 1.8
a
Additional space for 36 l snow.
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Wintertime collector consists of a 1.2 m tall PE bag fitted in a ring-shaped holder (97.5 mm radius) at the top, and a summertime funnel and carboy at the bottom. 2.1.3. General Collections are made approximate the monthly intervals (mean7stdev, 3077 days). Amounts are either measured by transferring the solution to a clean graduated cylinder or by weighing the vessel with contents in the laboratory. Aliquots of the collected solution for analytical processing are transferred to small PE bottles prior to amount measurements. Aliquot amounts are weighed in at the laboratory and added to the field-measured amounts.
3. Site description Sweden (see Fig. 1) covers latitudes between 55 and 69 N. A vast regime of climatic conditions and vegetation types are present. Sweden’s topography consists of mountains in the northwest, bounded on the east by a plateau that slopes down to lowlands and
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plains in the east and south. The principal natural resources are forests, which cover about 23 million ha (SCB, 2000). The majority of Swedish forests are secondary cultivated growth, intensively managed for production purposes with even age distributions (Linder et al., 1997). The climate of northern Sweden is considerably more severe than that of the south primarily because it has higher altitudes and because the mountains cut off the moderating marine influence. The average temperature in February, the coldest month, is below freezing throughout Sweden, with an average temperature range in Stockholm of 5–1 C, in Gothenburg of 4–1 C, and in Pitea of 14–6 C. Precipitation generally falls as snow or hail between October and March. In July, the warmest month, the average temperature range is 14–22 C in Stockholm, 14–21 C in Gothenburg, and 12–21 C in Pitea. Salinity of the western seas (Kattegat and Skagerak) is considerably higher than that in the Baltic Sea in the east and Bay of Bothnia in the northeast, averaging 25–30, 8–12 and 5–10 PSU, respectively (SCB, 2000). The RTM sites are shown in Fig. 1. In 1996 several of the initial sites were abandoned. The 1996 changes in the network did not have noticeable effects on the measured parameters (Hallgren Larsson, 1999). Missing amount values are replaced by estimates for 3.3% of the winter and 2.8% of the summer collections made, using data from surrounding locations and trend studies for the disturbed collector.
4. Quality control procedure At the data analysis stage of the RTM scheme, collected data are quality checked by comparison of monthly results between stations from the same region. Calculated ionic balances, relative amounts in throughfall and bulk precipitation, sodium to chloride ratio in each sample, and some other quality assurance indicators (not described here) are used.
5. Data analysis methodology
Fig. 1. Map of Sweden with plot locations (dots) major regions (grey scales), county borders (black lines) and codes used in this study.
TFBU ratios are calculated for amounts (Eq. (1)), and between concentrations of chloride and sulphate–sulphur ions for monthly collections at each location where both open field incident bulk precipitation and throughfall were collected. For ratio comparison a best estimate value is established for each location by calculating averages for each type of equipment used, after removing outliers for concentrations or amounts outside the 2-sigma range (above twice the standard deviation from the mean value for the location) from the database. 3.4% of the TFBU ratios during winter collections, and 2.8% of summer
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equipment collections are omitted outliers. Patterns are identified by averaging TFBU ratios for type of equipment used, for regions, for type of primary forest vegetation, and proximity to nearest coastline. To identify temporal and spatial patterns, amount TFBU ratios were divided into 5 classes and plotted as percentages of each class for each month during 1997–2000. For large aberrations from the best estimate values at each location weather observations by the Swedish meteorological and hydrological institute (SMHI) and reports from field personnel were studied to identify the possible causes.
6. Results 6.1. General ratios The national range of monthly incident precipitation is 0–320 mm between 1990 and 2000, while regional maximum monthly amounts of 270, 320 and 229 mm are measured for southern, central and northern Sweden. National annual precipitation has increased by 16% between 1990 (mean 599 mm) and 2000 (mean 695 mm), based on RTM bulk measurements. Throughfall generally contains lower amounts than corresponding bulk precipitation collectors for the same exposure period, but in 5% of the winter and 3% of summer collections throughfall amounts exceeded bulk amounts. Average TFBU ratio for the different types of vegetation (Table 2) show that differences exist while using winter vs. summer equipment, and between the major regions of Sweden. The frequency of high or low TFBU ratio values has not changed between 1990–1995 and 1996–2000. TFBU ratios in the north are generally higher than those in the south, both for spruce and pine forest types either during winter or summer equipment use. The relationship between ratio and sum of throughfall and open field precipitation (Fig. 2) shows that all
acceptable instances when the amount TFBU ratios were higher than 2 occurred while the sum of open field precipitation and throughfall was below 100 mm month1. High sum amounts (>300 mm month1) do not affect the ratios negatively in the few cases that they occurred. The seasonal differences found for the TFBU ratios more often appear as lower values during winter for a wide range of total amounts, and as high ratios during the summer, especially during drier episodes with sum amounts of less than approx. 60 mm (Fig. 2). 6.2. Regional ratios 6.2.1. Amounts On a regional scale, the TFBU ratios vary between all spruce sites (Table 2). The seasonal differences exceed the regional variations. The occurrence of aberrant TFBU ratios (values either lower or higher than the best estimate value) is plotted by dividing the ratios into 5 classes with their 500 winter summer
400 300 Sum
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200 100 0 0
1
2
3
4
5
Ratio
Fig. 2. The ratios of throughfall to bulk incident precipitation amounts and the sum of both amounts collected on a monthly basis at southern Swedish monitoring sites with spruce forest plots (n=153) during 1990–2000. Data is presented separately for collections with winter and summer equipment.
Table 2 Average ratio 7 standard deviation between total amount of throughfall and bulk open field precipitation in three forest types of the Swedish RTMN in 1990–2000 Tree species and equipment
Number of locations
Southern (S) (n values)
Central (C) (n values)
Northern (N) (n values)
Spruce–winter Pine–winter Deciduous–winter Spruce–summer Pine–summer Deciduous–summer
S=81, S=21, S=10 S=84, S=26, S=10
0.4770.22 0.5570.15 0.5970.31 0.6270.16 0.6770.17 0.7470.15
0.4670.24 0.4870.19 N/A 0.6270.16 0.6870.17 N/A
0.5970.25 0.6270.24 N/A 0.6970.18 0.7470.17 N/A
C=57, N=22 C=19, N=4 C=57, N=22 C=20, N=4
(1994) (509) (283) (3328) (1118) (576)
(1865) (441) (1999) (503)
(931) (154) (675) (117)
N/A indicates that tree species plot was not available in the region. The number of values is given between parentheses. Outliers (see text for explanation) were excluded.
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frequency distribution for each month during 1997–2000 (Fig. 3). It becomes obvious that relatively low (ratio o=0.2) and rather high values (ratio >1.0) most often occur during winter months, which is also seen in Fig. 2. The monthly average amounts of precipitation shown in Fig. 3 reveal that 40–95% of the values in the normal TFBU ratio range (between 0.2 and 0.7) appear during months with 30–75 mm incident precipitation. Above 75 mm monthly precipitation, the normal TFBU ratio range occurs between 36% and 100%, while all normal range portions of less than 40% occur in the northern region, during winter (October through March), with the single exception of June 1997 (26.7% in normal ratio range). 100% of all TFBU ratios in the northern region were in the normal range in April 1998 and April 2000, with 79 and 76 mm month1 precipitation falling respectively. 6.2.2. Chloride ions Concentrations of chloride ions revealing extreme events appeared for example during the month of June 1993, August 1997 and April 2000 in several counties. The chloride ion concentrations in bulk collectors increase directly during the event, while they lag in throughfall. The ratios drop during the collection interval of the event, and are higher in the following collection. The effects of these extreme events are most clearly seen in the data collected in southwestern Sweden, the region receiving most precipitation, but also at sites in Blekinge (K county) and in the northern coastal region. No clear relationship between the amount and chloride concentration TFBU ratio is found. A relationship between chloride concentration TFBU ratio and closeness to coastline is apparent in southern Sweden (Table 3), less so in the north. Local variations in chloride ratios were much greater near the coast than at inland locations (Table 3). 6.2.3. Sulphur ions Sulphur ion concentrations and TFBU ratios for sulphur concentrations between throughfall and corresponding open field precipitation reveal no specific covariation with the amount of TFBU ratios. Sulphur TFBU ratios vary spatially, and have a temporal pattern. A gradient of sulphur TFBU ratios is found from south to north, with the highest values in the southern end. Sulphur ion concentrations in both throughfall and bulk precipitation throughout the nation have diminished during the whole investigation period, as well as in seasonal fluctuation amplitudes. Ratios between the two concentrations are generally the highest from March–May, while winter equipment is still in use, and lowest during August. Wintertime TFBU ratios are thus higher than summer ratios (Table 3).
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6.3. Local ratios At the highest resolution (location level) the variations in amount TFBU ratios reveal roughly a seasonal cycle (Fig. 4). December–March are commonly drier than June–September, and the highest ratios are generally found during periods with little precipitation (Figs. 2 and 4). Looking at the amount TFBU ratios at tree species level for the deciduous plots shows that differences clearly exist between summer and winter periods (Table 4). Slight variations occur between sites of the same dominant tree species (Table 5). For spruce a weak increase in site-specific average best estimate TFBU ratios is seen with increasing plot altitude. This is not observed for pine. The standard deviation between TFBU ratios for the same location and same deciduous tree species at different locations is greater during winter than during summertime, which is also the case for spruce stands but not for pine stands in southern and central regions (Table 2). Throughfall amounts vary least for the deciduous tree stands, more for pine stands and most for spruce stands, throughout the year. TFBU amount ratios in beech stands are lowest during the late winter–early spring (o0.4) and show high peaks (>0.8) during the late fall.
7. Discussion 7.1. General ratios The observed increase in precipitation during the study period did not alter the frequency of amount TFBU ratios that are higher or lower than the normal TFBU ratio range. Higher than normal monthly precipitation did not affect the amount TFBU ratio negatively, indicating that the total amount of incident precipitation is not primarily responsible for extraordinary TFBU ratios. If collectors frequently over-fill, the TFBU ratio is not reliable. The collectors in this study did not over-fill summertime, but some throughfall winter collections may have been over-filled with snow in the northern region. Volume adjustments were made to a few collections where this was noted by field personnel combined with suspicious looking TFBU amount ratios. Differences in best estimate TFBU ratios between sites are affected by the site specific factors, such as plot placement, stand characteristics, and general meteorological settings. In this study, the tree species is identified as the most important factor of the best estimate TFBU ratio. Response of different tree species to rain events varies extensively (Xiao et al., 2000b) in its turn influencing throughfall (Robertson et al., 2000).
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Fig. 3. Percentage of the 5 general classes of the ratios between amount of throughfall collected in spruce forest and amount of incident precipitation in bulk open field collector at 160 locations. All valid collections on a monthly basis are included for three major regions in Sweden. The all black diagrams give monthly mean incident precipitation amounts for the respective region.
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Table 3 Throughfall (TF) collected in Norway spruce stands and open field precipitation (BU) in southern Sweden between 1 January, 1990 and 31 December, 2000, as county mean values for months with summer (S) or winter equipment (W) and the mean TFBU ratio between throughfall and bulk precipitation (amount), chloride (Cl) and sulphur (SO4–S) ions Equipment
County
n=
TF Cl
BU Cl
TFBU Cl
TF SO4–S
BU SO4–S
TFBU SO4–S
TF Amount
BU Amount
TFBU Amount
S S S S S S W W W W W W
F G K LM N O F G K LM N O
331 382 337 441 367 1151 250 280 236 238 181 805
6.034 4.637 4.664 8.810 10.136 7.503 12.672 9.352 10.433 16.139 28.350 15.172
0.847 0.839 0.804 1.516 2.138 1.260 2.822 2.378 2.169 3.515 4.951 3.577
8.57 6.99 7.03 7.50 5.54 7.65 5.50 4.88 5.69 5.38 6.04 5.13
2.32 2.18 2.70 3.72 2.50 2.07 4.70 4.86 6.51 8.32 5.79 3.70
0.78 0.78 0.93 1.06 1.07 0.85 1.03 0.90 1.27 1.28 1.24 0.97
3.00 2.82 3.06 3.57 2.54 2.62 4.97 5.26 5.46 7.00 4.60 4.03
42 42 41 40 56 48 30 36 25 32 41 45
66 69 65 66 85 73 79 82 54 76 79 85
0.61 0.60 0.60 0.58 0.65 0.65 0.37 0.45 0.45 0.45 0.57 0.52
Fig. 4. Monthly throughfall and open field incident precipitation amounts collected at a location in the county of Varmland (S), central Sweden, during 1997–2000, and the ratios between these amounts. Amount of throughfall collected in forest plot ðmm month1 Þ : Equation 1: Amount of incident bulk precipitation collected at open field ðmm month1 Þ
The amount TFBU ratios during winter collections are quite different from the summer values. The frequent occurrence of low ratios during winter seems often due to higher than expected amounts collected in the winter bulk collection equipment. Low ratios are more frequent in summer, either due to higher tree interception and crown dripping than direct precipitation into the open
field collector, or small total amounts of precipitation during the collection period, resulting in nearly dry throughfall collectors. The retention time for winter precipitation in forest vegetation can easily exceed 30 days below freezing ambient temperatures. When snow and ice are collected in bulk open field collectors, the amount is directly
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Table 4 Average ratio 7 standard deviation between total amount of throughfall (TF) and OF collected in and near deciduous forest types of the Swedish RTMN in 1990–2000 Tree species
Locations (n=)
Winter equipment
Summer equipment
Birch Beech Oak
1 6 2
0.5970.24 (53) 0.5570.24 (146) 0.5170.19 (44)
0.7870.09 (75) 0.7470.13 (260) 0.7970.11 (70)
The number of values is given between parentheses. Outliers (see text for explanation) are excluded.
Table 5 Average ratio 7 standard deviation between total amount of throughfall (TF) and OF collected in and near the beech (Fagus sylvatica) forest plots of the Swedish RTMN in 1990–2000 Plot code and name
Tree age (2000) (year)
Winter equipment
Summer equipment
K 11, Komperskulla N 14, Djupeasen M 04, Ska¨r L 11, Akeboda L 12, Kampholma K 07, Ryssberget
75 80 85 100 102 120–125
0.5070.20 (26) 0.5970.26a(26) 0.8170.23 (26) 0.5370.20 (19) 0.5270.19 (17) 0.4970.21 (53)
0.8470.09 0.7070.19 0.7570.19 0.7770.15 0.7770.09 0.6970.09
(34) (36) (43) (38) (38) (75)
The number of values is given between parentheses. Outliers (see text for explanation) are excluded. a Open field and throughfall not always collected on the same day.
affected, while it may take an extended period of time before they melt or fall off the tree branches into the throughfall collectors at the forest plot. Thus, a precipitation detection gap can frequently exist at boreal forest locations. The trees in areas that annually receive a lot of snow have adapted their shape to lose the snow off their branches as quickly as possible. Where snowfall is uncommon, tree adaptation is less common, leading to a higher degree of snow retention in the tree crown, compared with adapted trees. This physical difference can also off-set snow collection duration. Another possible cause of concern is the condensation of air humidity onto cold surfaces of the collectors or the capture of fog droplets. Fog in forests affects throughfall measurements, especially at higher elevations (Zimmermann et al., 1999; Kowalski and Vong, 1999). Since the exposed collector surfaces are larger during winter than during the summer operations, especially on open fields (Table 1), and temperatures are generally lower than the ambient dew point temperature, this can cause higher collection values in our winter bulk collectors than in corresponding throughfall collectors. The unequal wind exposure of throughfall and bulk collectors may off-set the ratio differences for collected amounts even more. Dew or fog collection occurs most likely during the wet months of October–December, less so during the drier period of January through March. Especially RTM bulk amounts may be affected by this in wintertime, which was seen in the data. The RTM winter bulk collectors are found to collect slightly more than amounts collected
at nearby stations by SMHI (Uggla et al., 2003). Amounts collected with summer equipment correspond better with nearby collected SMHI precipitation (Uggla et al., 2003). Another possible cause of low winter TFBU ratios can be the enhanced tree interception of snow and rimfrost, instead deposited to the forest floor as stemflow during melting periods. On a regional scale, the amount TFBU ratios vary between spruce sites. Seasonal differences exceed the regional variations, pointing at meteorological causes being superior over the other influences, in support of Crockford and Richardson (2000) who reviewed partitioning of rainfall. The technical functionality of the equipment used can influence regional scale differences as well, as it is influenced by meteorology. The form, intensity and frequency of precipitation events are strongly responsible for the amount of throughfall that falls to the forest floor. When intensities are low, and events occur sparsely, it has been found that nearly all of the incident precipitation as rain falls unhindered to the forest floor. Initially coniferous tree surfaces are able to shed most water droplets until the dry particles are washed off. But when intensities increase or events occur closely after one another, the tree crown is wetted to a greater extent, which causes a greater loss of throughfall toward stemflow or tree crown water retention (Xiao et al., 2000a), also found in a southern Swedish 25-year old spruce stand (Alavi et al., 2001). In deciduous forests it is more common that low-volume rainfall leads to almost complete interception. When rain events
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continue over extended periods of time, the tree crown eventually becomes saturated and a balance is established between stemflow and throughfall (Draaijers and Erisman, 1995). Another physical factor of importance is the placement of the collectors in the forest plot. Placement of the collectors determines their collection ability of throughfall to a greater extent than the tree crown formation under which they are placed (Hansen, 1995). The closer the collector is placed to the tree trunk, lesser amount of throughfall will be collected (Thimonier, 1998). Since placement is the same throughout the whole investigation period, this component is a constant variable for each plot. Careful consideration has to be given to instances when collectors are moved around, which has not happened in this study. The regional differences in forest site physical appearance have a visible effect on the site specific TFBU ratios, even though generalizations are hampered by natural variability of the forest types, and the importance of meteorological conditions (Crockford and Richardson, 2000). Spruce trees in the northern region are narrower in crown diameter, with branches bend sharply down in a conical or spire-shape (Linder et al., 1997). At southern locations the spruce trees have wider crowns, with a greater needle biomass and more horizontal branches (Nilsson et al., 2002) than those in the northern region. Pine trees commonly have wider crowns in the northern region than in southern regions, leading to a slightly higher normal interception. In our study, the slightly higher amount TFBU ratios established for pine in the north than for the rest of the nation supports that. The amount TFBU ratios of deciduous plots are prone to be influenced by defoliation during fall, which does not occur simultaneously with the changing from summer to winter collectors. The exact dates of defoliation and leaf sprouting are not available. Thus, defoliation influences cannot be revealed in this study. The amount of chloride ions in incident open field precipitation is a good indicator for influences from the sea. Even though site specific collection of chloride exists in throughfall, the general amount of chloride ions in throughfall can tell about the coarse meteorological setting during exposure time. In addition to sample quality control by the ratio between sodium and chloride, the chloride concentration of the sample and deposited amount is used to indicate sea-salt input to the site. The variations in chloride concentrations on a spatial and temporal scale are indicative of extreme wind direction and speed, although the temporal variation usually is obscured by the monthly collection period. Deposition is therefore more appropriately reported as cumulative deposition on annual basis between 1 October and 30th September. Sometimes the extreme wind condition leads to an event that covers over the sum of minor events during the same exposure interval. These extreme events usually also have a strong effect on
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the total deposition during that interval. It is therefore useful to be able to detect such events. For the more detailed study of process importance on TFBU ratios, micro-meteorological data needs to be collected at or very near the investigation site. This, however, greatly increases the costs of the monitoring endeavour and cannot be applied on a widespread basis.
8. Conclusions The throughfall to bulk incident open field amount ratio is a valuable indicator of data quality. The ratio indicates site specific collector efficiency on a local scale and how it is affected by major meteorological events. Comparison between sites with the same forest type can indicate regional patterns, and help identify possible field operational disturbances that may cause faulty deposition calculations. In the evaluation process of our deposition data, automatic calculations of the described ratios, combined with ionic balances and manual intersites comparisons has proven a good way to detect ‘‘problem’’ samples. In case the computer program warns that a ratio is uncommon, possible reasons are sought manually. If the reason is of the kind discussed in this paper, and the aberration is not detected in samples from surrounding sites, an appropriate manual adjustment is made to the volume, marked in the database as ‘‘adjusted value’’ that is traceable during future data treatment.
Acknowledgements This study was performed as part of the Swedish EPA Throughfall Monitoring Program, with data from the Regional Throughfall Monitoring Program financed by Swedish air quality authorities. The valuable discussions with E. Hallgren Larsson and O. Westling, and the map of Sweden provided by E. Uggla are gratefully acknowledged, as were the reviewers comments on the manuscript.
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