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

Abstract

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 onedimensional 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 with 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.

Capturing soil-water and groundwater interactions with an iterative feedback coupling scheme: new HYDRUS package for MODFLOW

“…Note that the equation switching method takes full advantage of the θ -and h-form REs, which is different from the traditional primary variable switching schemes (Diersch and Perrochet, 1999;Forsyth et al, 1995;Zha et al, 2013a). In our work, the switching-RE approach is incorporated into a new HYDRUS package.…”

“…However, the θ -form RE is not applicable to saturated and heterogeneous soils (Crevoisier et al, 2009;Zha et al, 2013b). In this work, to take advantage of both forms of RE, the governing equations, rather than primary variables (Diersch and Perrochet, 1999;Forsyth et al, 1995;Zha et al, 2013a), are switched at each node according to its saturation degree.…”

Capturing soil-water and groundwater interactions with an iterative feedback coupling scheme: New HYDRYS package for MODFLOW

“…190 Assume that the large-scale phreatic aquifer is numerically represented by the activated top layer in a two-dimensional groundwater model, At small spatial and temporal scales, e.g., within a macro step time ∆T = T J+1 −T J and a local area of interest with thickness of 0 s M z z   , the specific storage term in Eqn. (1) is vertically integrated into a transient one-dimensional expression (Dettmann and Bechtold, 2016),…”

“…The second concern lies in the scale-mismatching problem. For groundwater models (Harbaugh et al, 2017;Langevin et al, 75 2017;Lin et al, 2010;McDonald and Harbaugh, 1988), the specific yield at the phreatic surface is usually represented by a simple large-scale parameter; while for soil-water models (Niswonger et al, 2006;Šimůnek et al, 2009;Thoms et al, 2006), the small-scale phreatic water release is influenced by the water table depth and the unsaturated soil moisture profile (Dettmann and Bechtold, 2016;Nachabe, 2002). Delivering small-scale solutions of the soil-water models onto the interfacial boundary Hydrol.…”

Deriving Effective Soil Water Retention Characteristics from Shallow Water Table Fluctuations in Peatlands

“…Water table dynamics at high water tables are strongly affected by a partially inundated, uneven soil surface that increases the specific yield of the system and permits the initiation of surface runoff before the whole soil surface is inundated. Also, at lower water tables in the absence of partial inundation, the specific yield is influenced by the microrelief through the non‐uniform vertical distribution of the soil volume and the resulting effect on soil water retention (Dettmann and Bechtold, 2016). Dettmann and Bechtold (2016) gave a one‐dimensional analytical expression combining the soil WRC and the microrelief effect on the spatially averaged specific yield ( S y ).…”