A parsimonious soil-type based rainfall-runoff model simultaneously tested in four small agricultural catchments

Abstract

Discharge data from seven small agricultural catchments in the humid low-land region of Switzerland indicate that their hydrologic response is strongly influenced by the areal fractions of well and poorly drained soils. We report on a method to quantify the average flow contributions from well and poorly drained soil types during single runoff events. This study is based on data from four catchments with different areal fractions of well and poorly drained soils and with rainfall and discharge measured every hour. Each catchment was divided into four lumped hydrological response units (HRU) such as two soil types, one urban and one forested HRU. A slow and a fast flow component was assigned to each of the well and poorly drained HRUs. We calculate these flow components with a dynamic time-series model assuming that the areal fraction producing fast discharge depends on the soil water storage and the topographic index. The model is calibrated by simultaneously fitting the modeled catchment response in all four catchments with the restriction that equal HRU-types have equal parameter values in all catchments. We used a single 11-day observation period for calibration since it covers much of the discharge variability observed in the entire observation periods. The calibrated model was validated with two 10- and 11-day periods in four catchments and with four 6-months periods in one catchment only. Generally, the discharge dynamic in different catchments was reasonably well simulated for intermediate to high discharge peaks even for validation periods that are 15 times longer than the calibration record. The model showed particular limitations in periods with low antecedent soil moisture and for small peaks. In those cases, the model efficiency was low possibly indicating that the concept of the topographic index is less applicable under such conditions. However, to distinguish well and poorly drained pedological units as HRUs appears to be a sufficient parameterization that is necessary to reproduce the discharge dynamics in periods of intermediate and high discharge.