One-dimensional expression to calculate specific yield for shallow groundwater systems with microrelief

Although the importance to account for microrelief in the calculation of specific yields for shallow groundwater systems is well recognized, the microrelief influence is often treated very simplified, which can cause considerable errors. We provide a general one-dimensional expression that correctly represents the effect of a microrelief on the total specific yield that is composed of the soil and surface specific yield. The one-dimensional expression can be applied for different soil hydraulic parameterizations and soil surface elevation frequency distributions. Applying different van Genuchten parameters and a simple linear microrelief model, we demonstrate that the specific yield is influenced by the microrelief not only when surface storage directly contributes to specific yield by (partial) inundation but also when water levels are lower than the minimum surface elevation. Compared to a simplified representation of the soil specific yield, in which a mean soil surface is assumed for the calculation of soil specific yield, the correct representation can lead to lower as well as higher soil specific yields depending on the specific interaction of the soil water retention characteristics and the microrelief. The new equation can be used to obtain more accurate evapotranspiration estimates from water level fluctuations and to account for the effect of microtopographic subgrid variability on simulated water levels of spatially-distributed hydrological models.