A New Physically-Based Spatially-Distributed Groundwater Flow Module for SWAT+

Ryan Bailey,Katrin Bieger,David Bosch,Jeffrey Arnold

doi:10.3390/hydrology7040075

Abstract

Watershed models are used worldwide to assist with water and nutrient management under conditions of changing climate, land use, and population. Of these models, the Soil and Water Assessment Tool (SWAT) and SWAT+ are the most widely used, although their performance in groundwater-driven watersheds can sometimes be poor due to a simplistic representation of groundwater processes. The purpose of this paper is to introduce a new physically-based spatially-distributed groundwater flow module called gwflow for the SWAT+ watershed model. The module is embedded in the SWAT+ modeling code and is intended to replace the current SWAT+ aquifer module. The model accounts for recharge from SWAT+ Hydrologic Response Units (HRUs), lateral flow within the aquifer, Evapotranspiration (ET) from shallow groundwater, groundwater pumping, groundwater–surface water interactions through the streambed, and saturation excess flow. Groundwater head and groundwater storage are solved throughout the watershed domain using a water balance equation for each grid cell. The modified SWAT+ modeling code is applied to the Little River Experimental Watershed (LREW) (327 km2) in southern Georgia, USA for demonstration purposes. Using the gwflow module for the LREW increased run-time by 20% compared to the original SWAT+ modeling code. Results from an uncalibrated model are compared against streamflow discharge and groundwater head time series. Although further calibration is required if the LREW model is to be used for scenario analysis, results highlight the capabilities of the new SWAT+ code to simulate both land surface and subsurface hydrological processes and represent the watershed-wide water balance. Using the modified SWAT+ model can provide physically realistic groundwater flow gradients, fluxes, and interactions with streams for modeling studies that assess water supply and conservation practices. This paper also serves as a tutorial on modeling groundwater flow for general watershed modelers.

Highlights

The SWAT (Soil and Water Assessment Tool) watershed model [1] is used frequently worldwide to assess water supply, nutrient dynamics, and sediment dynamics under scenarios of climate change, water management practices, and conservation practices
Groundwater head changes only according to soil recharge and groundwater discharge to streams; in reality, there are many other sources and sinks of groundwater; a single groundwater head is computed for each Hydrologic Response Unit (HRU); groundwater flow to streams is based on the presence of a groundwater divide and an assumption of steady-state conditions; groundwater flow to streams is simulated only if groundwater storage exceeds a user-specified threshold, rather than due to hydraulic gradients; seepage from streams to the aquifer due to hydraulic gradients is not simulated; each aquifer is treated as a homogeneous system in which aquifer properties do not vary in space
Groundwater head fluctuations are shown at the eight monitoring well locations, in Figure comparison with the of observed groundwater head of values from the(site

Summary

Introduction

The SWAT (Soil and Water Assessment Tool) watershed model [1] is used frequently worldwide to assess water supply, nutrient dynamics, and sediment dynamics under scenarios of climate change, water management practices, and conservation practices. The SWAT+ model [2] has been presented as an alternative modeling tool with an emphasis on connectivity between spatial features (hydrologic response units, aquifers, channels, reservoirs, ponds, canals, pumps, etc.) within the watershed. Both models, often are limited in groundwater-driven watersheds due to the use of simplified, steady-state flow equations to represent water table fluctuation, groundwater. SWAT+ have been linked to physically-based, spatially-distributed groundwater flow models, most notably MODFLOW [6], which simulates flow in heterogeneous three-dimensional (3D) aquifer systems. To the authors’ knowledge, only one study [17] has linked

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