Interaction of surface water and groundwater on agricultural plots: Insights from field measurements and numerical modelling

Understanding the interactions between ditch water levels, soil water and groundwater levels in drained lowlands is crucial for agricultural management, particularly under climate change with an increase both in flooding frequency and severity in winter and water scarcity for crop production in the vegetation season. The optimum quantity of water in the subsurface soil is the base for high yields, both on fields and on grassland. Water retention with weirs or sills in the ditches is a possible way to improve water availability in the adjacent areas. The aim of our study is to clarify the influence of ditch water management on groundwater dynamics and by this on soil water content. Observations were conducted over a transect near Werenzhain, a Lusation village (Germany, south of Brandenburg). Field measurements encompassed the collection of meteorological data essential for calculating potential evapotranspiration. Soil water content was meticulously monitored at various depths (10, 20, 30, 40, 60, and 100 cm), alongside soil tension measurements at depths of 30, 60, 90, and 120 cm. In the laboratory, soil hydraulic properties, texture, and bulk density were measured for these corresponding depths. Additionally, fluctuations in the groundwater level during the study period were observed down to 300 cm. First, measured groundwater levels were simulated using HYDRUS 2D to obtain the dynamics between ditch water and shallow groundwater. Thereafter, different sets of ditch water levels were used to predict the soil water and groundwater levels for vegetation season. Our approach successfully simulated data with observed soil water content, soil water tension, and groundwater level throughout the study period. Further, we could model scenarios for an optimized ditch water level management.