Modeling water flow and mass transport in a loess catchment

For more than ten years, scientists of the Universities of Karlsruhe, Heide lberg and Bayreuth have investigated the water dynamics as well as transpor t of matter in the Weiherbach catchment, a loess area of about 6.3 km(2) lo cated in the "Kraichgau" of Southwest Germany. The investigations had the f ollowing scientific objectives: to identify the processes governing water a nd solute dynamics on different spatial and temporal scales in a small rura l loess catchment, to develop a physically based numerical model to cope wi th all hydrological situations, i.e simulating the extreme cases of very sm all runoff and flood runoff events as well as simulating soil water flow an d transport, based on the obtained understanding of processes as well as to provide a data set for hydrological research for the intensively used rura l Loess areas of Central Europe. Modeling approaches for the processes dominating water and mass balance on the plot, hillslope and small catchment scale were developed based on proce ss studies, tested against observations and implemented into the physically based, distributed model CATFLOW. During the observation period rainfall-r unoff activity was low. The distribution of soil types was found to be stro ngly influenced by erosion leading to a typical hillslope soil catena. Pref erential flow in earthworm burrows turned out to be crucial for solute tran sport in the soil on the plot and the hillslope scale, especially for leach ing of surface applied pesticides, but also For runoff generation on the ca tchment scale. However, pesticide as well as phosphate loads in the Weiherb ach creek during rainfall-runoff events were strongly determined by losses of sewer pipelines. which drain paved farm courtyards and pesticide loss du e to runoff from courtyards. Those entrance paths are difficult to model in physical terms. A simplified, effective model approach for preferential flow based on field and laboratory measurements yielded simulation results in good accordance with short term observations of tracer transport on the plot scale as well as long term observations of tracer transport on the hillslope scale. Assum ing that the soil catena of each hillslope may be substituted by the same t ypical soil catena a long term simulation of catchment scale water dynamics yielded results in acceptable agreement with the observed rainfall-runoff dynamics, soil water dynamics as well as evapotranspiration. However, a fal sification of the developed modeling approach was not possible, because of the uncertainty of the obtained parameter values due to measurement errors and due to the enormous variability of state variables and parameters in th e Weiherbach soils, (C) 2001 Elsevier Science Ltd. All rights reserved.