Temporal dynamics of hydrological impacts of adjustable weirs in an open ditch: an experiment-modeling study on a shallow groundwater environment

Abstract. Adjustable weirs have been used to block ditch waters especially during wet seasons to retain more shallow groundwater in agricultural fields and forests. However, to fully verify the effectiveness of weirs, quantifying the weirs’ hydrological benefits to vegetation and nearby surface waters is important. Our study quantified the hydrological impacts of these weirs on the water balance across seasons under wet and dry years. We performed field experiments in a 2% sloped parcel with a single ditch and two weirs in Merelbeke-Melle, Belgium. Then, we used a 3D subsurface flow model (HYDRUS 5) to represent the phreatic and vadose zones underlying the regional hillslope. They were calibrated using water table levels monitored across the parcel in 2022–2026, covering both wet and dry years. Afterwards, we simulated separate scenarios for empty and full ditches under shallow and deep rooting depth parameterizations.

The observed water tables show little responses with opening and closing of weirs. Meanwhile, simulations involving weir management exhibited higher water tables especially closer to the ditch due to reduced field drainage that retain more water during wet periods and also infiltration of ditch waters during dry periods. Yet this boosted water availability was partitioned mainly to runoff, storage, and excess (added) field drainage downstream of the weirs. The stream downslope (Gondebeek stream) had only little additional baseflows. Moreover, the vegetation experienced both wet and drought stresses that inhibited uptake of water gained from water management. Thus, hydrological models on weir management should properly consider the presence and sources of infiltrating ditch waters, vegetation dynamics in the field, and the meteorological context (seasonality, interannual variability).