A possibility for drought mitigation in wetlands
Introduction: Most of the wetlands in the North-East German Lowland are fens. After the last ice age they developed in spite of the climatic conditions with a mean annual precipitation of approximately 500 mm per year, because they got sufficient recharge from their basin and the discharge was blocked by natural barriers. In the last two centuries most of the fens were drained for agricultural land use. However, because of the low precipitation, the drainage systems were completed with a number of weirs as a prerequisite for intensive agricultural production in the 1970s and 1980s. Therefore, these regions have complex water resources management systems today, which are often integrated in the water resources management system of the whole river basin.
The last decade, with dry summers and hot temperatures, shows that there is an increasing risk of droughts in these wetlands. Climate models forecast an additional threat, with increasing temperatures and decreasing precipitation in the summer months for the next few decades in north-east Germany. On the other hand, there are a lot of ways of enabling these areas to be used once more with groundwater levels more typical for wetlands.
Study region: The Spreewald wetland is situated about 70 km south-east of Berlin, locatedwithin the Spree River basin. It is one of the most significant wetlands inGermany. The lowland region has an area of 320 km². It is characterized by alow mean annual precipitation of about 530 mm for the period 1961-1990(HAD) and rather high potential evapotranspiration (FAO grass referenceevapotranspiration) of about 610 mm for the same period.
The wetland soils are dominated by groundwater-influenced sands (49%), fens (33%) and loamy soils (18%). The land use of the wetland area is characterized by extensive grassland (44%), fields (23%) and forests (20%). The wetland has a very dense stream and ditch system of about 1,600 km in length with more than 600 weirs to regulate ditch water and groundwater levels. Therefore, the water balance of the Spreewald region is strongly influenced by the water management system within the wetland.
Method: The models used, WBalMo Spreewald and WBalMo Spree/Schwarze Elster,consider aspects of the water resources management in the wetland and in thebasin. WBalMo Spreewald is a combination of a water management model(WBalMo®, WASY) and a water budget model for wetlands with drainage /sub-irrigation systems (WABI, Dietrich et al). This combination is a solutionto fulfil the complex requirements of the wetland region. WBalMoSpree/Schwarze Elster is a complex water balance model with a great number ofdifferent water users in the basin.
Scenarios: The WBalMo models were used to determine the impacts of different watermanagement options on the water balance of the Spreewald wetland underchanging global conditions. Scenarios were defined as combinations of boundaryconditions and water resources management options in the wetland. In this paperthe boundary conditions for the wetland water balance are climate change(Wechsung et al) and two water resources management options in the SpreeRiver basin.
Results and discussion: The statistical evaluation of the model results was made for each 5-year period.In the following we compare and discuss the results of the period 2003-2007(P1) with the last period 2048-2052 (P10). In the figures the first period of thebasis scenario is also the reference status for the other scenarios. The bars in thefigures represent the 50th percentile of 500 values per month. The caps show therange between the 20th and 80th percentiles.
Boundary condition – climate change: The climatic boundary conditions are the same in all scenarios. For a betterinterpretation of the model results the impact of climate change is represented bythe climatic water balance. Already, the balance of P1 shows a deficit forthe months from April to September. But from June to August, especially, thedeficit will clearly increase up to the last period (P10). This increasing deficitinfluences the water demand in the wetland, but also the inflow from the basin.
Recharge from basin into the wetland: The inflow from the basin will decrease in the summer months up to 2050. The reasons are the changed climatic conditions as well as the planneddevelopment of the mining activities in the basin. The inflow from the MalxeRiver, especially, will decrease in the future, because today there are two largeopencast mines in this sub-basin, which are going to close by 2030. Then thepumping of mine water will stop and additional water is needed to refill theresidual mining pits.
Water demand of the wetland: The development of the wetland water demand is shown using the example ofJuly for all ten 5-year periods. The values are influenced by the water balanceparameters of precipitation, actual evapotranspiration and the water storagedeficits of the previous month.