Abstract

Abstract. Interactions between shallow groundwater and land surface processes play an important role in the ecohydrology of riparian zones. Some recent land surface models (LSMs) incorporate groundwater-land surface interactions using parameterizations at varying levels of detail. In this paper, we examine the sensitivity of land surface evapotranspiration (ET) to water table depth, soil texture, and two commonly used soil hydraulic parameter datasets using four models with varying levels of complexity. The selected models are Hydrus-1D, which solves the pressure-based Richards equation, the Integrated Biosphere Simulator (IBIS), which simulates interactions among multiple soil layers using a (water-content) variant of the Richards equation, and two forms of a steady-state capillary flux model coupled with a single-bucket soil moisture model. These models are first evaluated using field observations of climate, soil moisture, and groundwater levels at a semi-arid site in south-central Nebraska, USA. All four models are found to compare reasonably well with observations, particularly when the effects of groundwater are included. We then examine the sensitivity of modelled ET to water table depth for various model formulations, node spacings, and soil textures (using soil hydraulic parameter values from two different sources, namely Rawls and Clapp-Hornberger). The results indicate a strong influence of soil texture and water table depth on groundwater contributions to ET. Furthermore, differences in texture-specific, class-averaged soil parameters obtained from the two literature sources lead to large differences in the simulated depth and thickness of the "critical zone" (i.e., the zone within which variations in water table depth strongly impact surface ET). Depending on the depth-to-groundwater, this can also lead to large discrepancies in simulated ET (in some cases by more than a factor of two). When the Clapp-Hornberger soil parameter dataset is used, the critical zone becomes significantly deeper, and surface ET rates become much higher, resulting in a stronger influence of deep groundwater on the land surface energy and water balance. In general, we find that the simulated sensitivity of ET to the choice of soil hydraulic parameter dataset is greater than the sensitivity to soil texture defined within each dataset, or even to the choice of model formulation. Thus, our findings underscore the need for future modelling and field-based studies to improve the predictability of groundwater-land surface interactions in numerical models, particularly as it relates to the parameterization of soil hydraulic properties.

Highlights

  • Shallow groundwater in river valleys, riparian zones, and wetlands interacts with soil, vegetation, and climate through capillary rise and direct root water uptake from the water table, influencing land surface processes

  • Model values for soil hydraulic parameters are obtained from two soil texture-based lookup tables that are commonly used by land surface models (LSMs) (Table 1), namely the parameter sets of Rawls et al (1982) and Clapp and Hornberger (1978)

  • In a numerical modelling study using a fully coupled groundwater/vadose zone/land surface model, Kollet and Maxwell (2008) described the “critical zone” as the region in which a strong correlation exists between ETa/ETp and water table depth, and they found this zone to occur at depths of 100–500 cm in their study area (Oklahoma, USA; generally loam and loamy sand soil textures)

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Summary

Introduction

Shallow groundwater in river valleys, riparian zones, and wetlands interacts with soil, vegetation, and climate through capillary rise and direct root water uptake from the water table, influencing land surface processes. Unlike deep water table conditions, a shallow groundwater table maintains elevated soil moisture in the root zone (Chen and Hu, 2004). Since land surface processes (e.g., evapotranspiration, runoff, and infiltration) are strongly dependent on soil moisture, incorporating groundwater in land surface models (LSMs) is crucial for realistic representations of hydrologic processes in watersheds (Niu et al, 2007; Yeh and Eltahir, 2005; York, 2002; Maxwell and Kollet, 2008). Soylu et al.: Quantifying the impact of groundwater depth on evapotranspiration absence of detailed field observations, numerical models are currently being used to explore the role of groundwater in simulated land surface fluxes (Fan et al, 2007; Liang et al, 2003; Maxwell et al, 2007)

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