Simulation of the soil water storage in risk situations. Management framework of the wetness conditions at a basin scale

Transcription

1 Simulation of the soil water storage in risk situations. Management framework of the wetness conditions at a basin scale U. Somorowska Faculty of Geography and Regional Studies, Warsaw University, Poland. Abstract Land surface hydrological conditions are considered to play an important role in the sustaining and restoring of natural vegetation. Prediction of the soil water availability under both average and extreme wetness conditions is essential especially for the water-related ecosystems. This research deals with an evaluation of the dynamic soil water resources in risk situations at a basin scale. Wetness conditions were evaluated for the Lasica basin, Poland, situated on the Mazovian Lowland, within the boundaries of the Kampinos National Park. The area has a status of the UNESCO MAB Biosphere Reserve. In this basin valuable marshland, swamp and woodland ecosystems are present but they were influenced by agricultural drainage in the past, Risk assessment of the occurrence of extreme wetness conditions focuses on the soil water resources stored in the wet zones of the basin. The extreme values of shallow groundwater levels and corresponding soil water resources are considered here as risk situations. Probability concept was applied to derive the Depth to the groundwater Duration Curves for wet and dry years as compared to the average year. Simulations of the maximum changes of the soil water storage were conducted for maximum stages of the groundwater levels. Estimates of the dynamic water storage have been linked with the depth to the groundwater table using the water storage decrease function. Maximum decrease of the soil water storage is presented in a spatially distributed way using raster-based GIS software, The results serve as a basis for restoration of wetness conditions and for mitigation of drought impact.

2 5 74 Risk Analysis III Introduction Soil water storage is often considered as a water resource available for natural vegetation or agricultural crops, The volume of soil water storage can be derived from soil moisture which varies in time and space, Predicting the spatial patterns of changes in soil water storage are critical for understanding and coping with the soil water deficits that can appear in dry conditions. Extreme dry wetness conditions are considered here as risk situations during which soil water deficits can develop, Soil moisture regimes influenced by shallow groundwater levels are investigated in this research. The volume of soil water deficits is related to the rate and duration of the groundwater level decrease, Long lasting risk situations have an influence on the fimctioning of ecosystems due to the limited availability of soil water. Soil moisture can be measured trough ground- or satellite-based systems or both. The potential exists nowadays to retrieve soil moisture estimates by remote sensing techniques [1]. However, there is still a challenge in developing this technology on an operational basis. Field studies are still of great importance as they provide reliable and direct estimates of this variable not only in the surface but also in the subsurface soil layers, Time Domain Reflectometry (TDR) technique applied nowadays in field equipment enables determining the volumetric soil moisture content directly in the field. Such conventional measurements provide information on soil moisture at a point from which spatial averages can be derived. It remains still a challenge to project the full range of natural soil moisture variability, In this research estimates of soil water storage were derived from representative point measurements. The soil water storage have been considered for two types of wetness conditions appearing in wet areas in the basin, Sites with very shallow groundwater levels have been considered separately from those with shallow groundwater levels. The effects of spatial and temporal variability of the soil water storage determined by field measurements and groundwater levels have been studied by means of simple water storage decrease function and the raster-based Geographical Information System. Study area The focus of this research is on the dynamic soil water resources of the lowland basin situated within a protected area of the Kampinos National Park (KNP) in Poland (fig. 1). In the area of the KNP substantial human interference in the natural environment had taken place in the past, before the area became protected. At the end of XIX century large areas of forests had been cut down, Drainage works started at the beginning of XX century, In the fifties of the XX century the embankments along the Vistula river were constructed. Last natural inundation of the KNP caused by high levels of the Vistula river took place in the thirties. Nowadays the main natural water income to the area is the precipitation that recharges the soil water resources and shallow groundwater levels. Land drainage, construction of the embankments, river regulations and expansion of agriculture have considerably modified the natural hydrological and

3 Risk Analysis III 575 ecological conditions. Large scale agricultural drainage and groundwave QP and Study srm ~ W.(,..s m Figure 1: Location of the study area in Poland. exploitation have led to the substantial changes in the groundwater regime and in th; soil moisture conditions. Declining qua;tity and ~uality of wat;r resources has resulted in the changes of the fimctioning or disappearing of ecosystems. In 1995 the Protection Plan was elaborated for the Kampinos National Park. Endeavors aim at slowing down runoff and raising groundwater levels, especially in wetland zones [2], Groundwater monitoring started in 1998 as a continuation of former investigations conducted in the region since 1956 [3]. The main aim is to monitor the current state of hydrologic conditions and, in perspective, to increase retention in the shallow soil layers. In 1995 monitoring of the soil water resources started in response to the strong requirement to control the dynamic soil water resources available for protected ecosystems [4]. The analysis of contemporary wetness conditions was based on measurements conducted in years [5]. Long-term variation of precipitation Since precipitation is considered the main factor influencing the recharge of shallow surface and subsurface soil layers, an assessment of long-term variation of precipitation was carried out. Climatically deviating periods were determined using the standardized cumulative annual deviation (SCAD) from mean Precipitation in the basin [6]. Two periods were considered, The period was investigated within which basin characteristics did not change significantly. Additionally the long-term precipitation series was reconstructed for the area of the Lasica basin with reference to the available instrumental period of for Warsaw (fig, 2). The course of SCAD is presented in fig, 3. The positive slopes indicate periods in which precipitation is above the mean and negative slopes below the mean. In the years the sequence of dry and wet years occurred,

4 5 7fj Risk Analysis III Eu 800 Figure 2: Variation of annual precipitation in the Lasica basin in the Kampinos National Park, Poland. Hydrological years Figure 3: Standardized cumulative annual deviation (SCAD) of precipitation in years in the Lasica basin in KNP, Poland. both for the period before substantial human interference took place (till the turn of XIX and XX centuries), as well as in the past fifty years, The mean annual precipitation in the basin calculated for the period was 563 mm and it does not deviate much from the mean calculated for the reconstructed series for years estimated as 562 mm, In years the reconstructed

5 Risk Analysis III 577 series expressed as a percentage of the long-term mean annual value was within therange ZO. Variation of precipitation in the long period provides an insight into the character of recharge of shallow soil layers. Natural recharge by precipitation incoming annually to the area has presewed its general character with a sequence of dry and wet years, The value of annual precipitation in a particular year can vary substantially from the mean but this is observed both in earlier periods as well as nowadays. Symptoms of drying such as lowering of groundwater levels, decreased retention of soil water storage and changes in the soil organic substrates should be attributed mainly to the drainage manipulations rather than to the variations of precipitation. These symptoms were directly caused by agricultural drainage schemes that lowered the drainage base in the basin. Since the fifties of the XX century an adjustment of groundwater levels to the new drainage scheme has taken place. Thus it can be assumed that contemporary variations of groundwater levels and retention of soil water storage are due to the variations of precipitation. An impact of wet and dry years on groundwater levels is investigated in the following section, Groundwater levels in risk situations Extreme groundwater levels characterized by the depth to the groundwater are considered in this research as risk situations, The risk of the occurrence of groundwater levels was evaluated as a less than or equal to probability, The analysis is restricted to the shallow and very shallow groundwater levels of wet zones of the basin, Depth to the groundwater Duration Curves The probability approach was applied using records of depth to the groundwater available for the period Empirical cumulative distributions were derived by means of plotting position method. Less than or equal to probability was calculated for wet years, dry years and average years , The derived distributions for wet and dry periods were then compared to the distribution derived for average years. They are assumed to represent average conditions. The relation established between depth to the groundwater and probability is called Depth to the groundwater Duration Curve (DDC), The basic shape of the DDC depends on aquifer characteristics as well as on the precipitation and land evaporation of the period for which DDC is determined. Hence DDC can be a tool to determine the changes of groundwater levels caused by dry and wet years. Synthetic DDCS with depth to the groundwater plotted against the less than or equal to probability are presented in fig, 4, separately for sites with shallow and very shallow groundwater levels, In the average year the depth to the shallow groundwater level equal to or less than mean annual value appears in 41% of time, For the dry year this probability reaches 57 Y., During approximately 20?.

6 S 7S Risk Analysis III (a) ~ r Ah J l?,, D yy H 956.OIM(M edn armud1value) I (b) -200 o -120 o , Less than or equal to probability (%) ---A verage > ear I w et year 4 b IIh v bm... T:;g;%... Tar$et...,..!,.,, 1J=..50,,!,,..!! ;m,.!!...,ṫ..,-: : -: t,=, R ----,.- / 1! ;0 10 r, 1 I I 1 i t -. I.,-.,. 1 ~ ~ ṃ 1,. / 4 // /, tlllllllll-ll~lmll Less than or equal to probability (%) Figure 4: Depth to the groundwater Duration Curves (DDCS): (a) for wet sites with shallow groundwater levels, and (b) for wet sites with very shallow groundwater levels. of time of the wet year the values equal to or less than mean annual value are expected to appear. For sites with very shallow groundwater levels the depth to the groundwater bigger than 50 cm lasts 45% of time in the wet year, 58 Y. of time in the average year and 73 % in the dry year. The depth to the groundwater not bigger than 50 cm is considered here as target value for re-establishment of wetland vegetation.

7 Risk Synthetic groundwater depletion curves Chosen groundwater depletion curves observed in the summer in years were analyzed for sites with shallow and very shallow groundwater levels. The variable rate of groundwater depletion depends on the aquifer characteristics as well as on evaporation and precipitation recharging shallow groundwater. Synthetic groundwater depletion curves were derived using a recession function in the following form Ht=Kt HO (1) in which Ht is groundwater depth at time t in cm below surface, K is a fitting coefficient characterizing the rate of groundwater depletion, t is time in days and HO is groundwater level in cm below surface at time t=o days. The value of K has been determined by the least squares fit for the summer depletion curves with the steepest slopes that appeared in the periods without precipitation. The average depletion coefficient K for the wet zone with very shallow water tables is and for the wet zone with shallow water tables it is Using these values the change in the groundwater level AH can be derived for a given duration At of the groundwater depletion from the eqn: AH= K At. (2) This relation was incorporated into the soil water storage decrease function described in the following section. Soil water storage in risk situations Maximum changes in the soil water storage The changes in the soil water storage can be estimated using the water storage decrease fimction: where AR is the change in water storage in mm, AH is the change in depth to the groundwater table in mm and p is the storage coefficient [5], The average value,of P estimated at sites with very shallow groundwater level was 0,0826 and at sites with shallow groundwater level. This stands for the water storage in the O-100 cm soil layer, Thus if AH is set to maximum differences in groundwater heads, then LR represents the maximum difference that can appear in the soil water storage by a given value of p, Having the spatial distribution of LH ~, and p, the spatial distribution of LRm,X can be derived. Spatial distribution of ~RmX for the western part of the basin s wet zones is shown in fig. 5.

8 58(I Risk Analysis III Figure 5: Spatial distribution of differences in soil water storage in the Lasica basin, Days Figure 6: Development of possible deficits of the soil water storage for shallow and very shallow groundwater levels. Soil water storage deficits Linking eqns (2) and (3) the decrease in the soil water storage can be estimated from the following equation: The value of p was set to at sites with very shallow groundwater levels and at sites with shallow groundwater levels, The value of K was set to at sites with very shallow groundwater levels and at sites with shallow groundwater levels. Using the eqn (4) synthetic curves representing development of soil water deficits were then derived for two types of wet zones (fig. 6).

9 Conclusions Risk Analysis III 581 The research provides an evaluation of the dynamic soil water resources in risk situations at a basin scale. Soil water resources were evaluated for the Lasica basin, Poland, located on the Mazovian Lowland, within boundaries of the Kampinos National Park. The analysis focuses on the soil water resources stored in wet zones that appear within low-lying flat areas, The east part of the basin which is affected by Warsaw agglomeration is not considered in this study. The research was based on the long-term series of precipitation ( ), groundwater levels ( ) and field measurements of soil moisture ( ). Precipitation is the main factor affecting the recharge of shallow surface and subsurface soil layers, Variations of precipitation characterized by the standardized cumulative annual deviation (SCAD) visualizes dry and wet years, both for the period before substantial human interference as well as in the past fifty years. The natural recharge by precipitation incoming annually to the area has preserved its general character with a sequence of dry and wet years. Contemporary variations of groundwater levels and retention of the soil water storage are affected by the variation of precipitation. An impact of wet and dry years on groundwater levels was investigated by means of Depth to the groundwater Duration Curves (DDCS), The probability of groundwater levels less than or equal to the target value at sites with very shallow groundwater level is 45 Yo, 58 %0 and 73?40 in the wet, average and dry years respectively. Thus especially in dry years the target value of the groundwater levels is not fidfilled for most of the year. The possible change of groundwater levels was characterized by synthetic groundwater depletion curves. On this basis development of soil water deficits have been simulated using average values of the storage coefficient p and groundwater depletion coefficient K. The 30 days period of groundwater depletion, if starting from the critical value of groundwater levels, may lead to the development of approximately 20 mm and 50 mm of soil water storage deficits in sites with very shallow and shallow groundwater levels respectively. These average deficits derived for two types of wet sites are results of groundwater level fluctuations and soil water retention capability, In longer periods of groundwater decrease the soil water storage deficits can increase. Simulated maximum changes in the soil water storage can reach the values over 150 mm as shown in a spatially distributed way in the basin. The simulated variability of the soil water storage reveals the range of wetness conditions to which contemporary vegetation is exposed. Redevelopment of old water-related ecosystems requires gradual restoration of wetness conditions throughout a reduction of soil water deficits.

10 582 Risk Analysis III Acknowledgement The author wishes to thank the Department of Enviromnental Sciences and Policy of the Central European University, Budapest, that has supported the soil moisture research by grant No References [1] Van Oevelen, P. J,, Soil moisture variability: a comparison between detailed field measurements and remote sensing measurement techniques. Hydrological Sciences Journal, 43(4), pp , [2] Kazirnierski, B., Pilichowska-Kazimierska, E. & Sikorska-Maykowska, M., Studium warunkow wodnych. Plan Ochrony Kampinoskiego Parku Narodowego (Study of water conditions. Protection Plan of the Kampinos National Park, in Polish), Narodowa Fundacja Ochrony Srodowiska (National Foundation of Environmental Protection): Warsaw, Poland, [3] Krogulec, E. & Sikorska-Maykowska, M., Optymalne warunki do projektowania monitoring lokalnego wod podziemnych i powierzchniowych na przykladzie Kampinoskiego Parku Narodowego (Optimal conditions of design of local surface and groundwater monitoring systew in Polish). Gospodarka Wodna (Water Management), 6, pp , [4] Somorowska, U., Extreme stages of the wetness conditions evaluated from direct soil moisture measurements in the Lasica catchment, Poland. Journal of the CEU Department of Environmental Sciences and Policy, 4, pp , [5] Somorowska, U., Extreme stages of the basin wetness conditions. Variability of the soil moisture in a lowland basin. Proc. of the Fourth International FRIEND Conference held at Cape Town, South Africa, March FRIEND 2002 Regional Hydrology: Bridging the Gap between Research and Practice. IAHS Publ. No. 274, [6] Van der Wateren - de Hoog, B., Quantification of catchment discharge sensitivity to climate variability. Nederlandse Geogray%che Studies, 233, Utrecht, 1997.