Evaluating soil water routing approaches in watershed-scale, ecohydrologic modelling

Authors: PIGNOTTI, GARETT - Purdue University; CHERKAUER, KEITH - Purdue University; CRAWFORD, MELBA - Purdue University; Williams, Mark; CHAUBEY, INDRAJEET - Purdue University

Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/20/2020
Publication Date: 1/6/2021
Citation: Pignotti, G., Cherkauer, K., Crawford, M., Williams, M.R., Chaubey, I. 2021. Evaluating soil water routing approaches in watershed-scale, ecohydrologic modelling. Hydrological Processes. 35(3). Article e14034. https://doi.org/10.1002/hyp.14034.
DOI: https://doi.org/10.1002/hyp.14034

Interpretive Summary: Water stored in the soil profile is important for controlling watershed hydrology and nutrient loss, but it is often difficult to model soil water processes given their complexity. In this study, we test several different approaches for simulating soil water within the Soil and Water Assessment Tool (SWAT) in watersheds in Indiana and Georgia to determine how the different approaches impacted model outputs and compare outputs to measured data. Different methods for simulating soil water processes resulted in improved model results within each watershed, but a lack of site-specific characterization of soil properties by the soils database at the site scale was found to severely limit the analysis. Changes to soil water processes within the model directly impacted the water balance; thus, predictions of nutrient fate and transport were also altered. Findings from this study highlight the need for continued efforts to enhance soil water model representation as an opportunity to improve model simulations for a range of predictions and applications.

Technical Abstract: Soil water dynamics are central in linking and regulating natural cycles in ecohydrology, however, mathematical representation of soil water processes in models is challenging given the complexity of these interactions. To assess the impacts of soil water simulation approaches on various model outputs, the Soil and Water Assessment Tool was modified to accommodate an alternative soil water percolation method and tested at two geographically and climatically distinct, instrumented watersheds in the United States. Soil water was evaluated at the site scale via measured observations, and hydrologic and biophysical outputs were analyzed at the watershed scale. Results demonstrated an improved Kling-Gupta Efficiency of up to 0.3 and a reduction in percent bias from 5 to 25% at the site scale, when soil water percolation was changed from a threshold, bucket-based approach to an alternative approach based on variable hydraulic conductivity. The primary difference between the approaches was attributed to the ability to simulate soil water content above field capacity for successive days; however, regardless of the approach, a lack of site-specific characterization of soil properties by the soils database at the site scale was found to severely limit the analysis. Differences in approach led to a regime shift in percolation from a few, high magnitude events to frequent, low magnitude events. At the watershed scale, the alternative approach reduced average annual percolation by 20 to 50 mm, directly impacting the water balance and subsequently biophysical predictions. For instance, annual denitrification increased 14 to 24 kg/ha for the new approach. Overall, the study demonstrates and highlights the need for continued efforts to enhance soil water model representation as an opportunity to improve model simulations for a range of predictions and applications.