Monitoring and modeling of fluxes on Kalahari - setup and strategy of the Kalahari Monitoring project. Serowe study case, Botswana

摘要

The ongoing discussion about the presence and the rates of recharge in Botswana, which constrains groundwater sustainability in the country scale, has led to the initiation of a new recharge project in Botswana called Kalahari Monitoring Project. In contrast to previous attempts this project focuses on temporal flux monitoring by using automated data acquisition systems (ADAS). The framework of that project is discussed on the base of the example of the Serowe study area, located at the eastern fringe of Kalahari, where an extensive monitoring network was installed to provide data for spatio-temporal flux assessment. This network allows measurements of saturated, unsaturated and surface zone fluxes. It consists of groundwater table fluctuation monitoring in 21 piezometers, soil moisture and soil suction pressure monitoring in 7 identical profiles comprising measurements at 0.5, 2, 4, 6, 8 m b.g.s., one deep suction pressure profile down to 76 m (sensors at 15 different depth levels), transpiration monitoring using 51 sap velocity thermal dissipation probes installed at 9 ADAS locations and monitoring of climatic variables for potential and actual evapotranspiration in 10 towers scattered over the study area. This data is either interpreted directly (rainfall, transpiration) or used in 1 -D models to calibrate surface and subsurface fluxes such as evapotranspiration, groundwater evapotranspiration and recharge. For spatial data assessment the remote sensing (RS) method is proposed. The evapotranspiration is obtained with RS solution of energy balance, transpiration by RS upscaling of the sap flow measurements and recharge by RS and GIS modeling. The final integration of spatial and temporal data for spatio-temporal flux assessment is carried out by transient groundwater model calibration with spatio-temporally variable recharge and groundwater evapotranspiration. The aspect of partitioning of tree transpiration fluxes into saturated and unsaturated zone is tackled by isotopic depth dependent tracing of groundwater and tree response analysis. The preliminary results of this study indicate already that the net recharge in incidental hydrological seasons can be substantially higher than the average recharge defined by isotopic and chemical methods. In other years however, the net recharge is usually negative due to the typical excess of groundwater evapotranspiration over the recharge, which is mainly due to the substantial role of transpiration in the overall groundwater balance.