Evaluation of the WEPS hydrology sub-model for surface soil water content under different tillage practices in the inland Pacific Northwest, USA

Abstract

Surface soil water content (SSWC) is an important factor affecting wind erosion and crop management decisions, especially in semi-arid and arid regions that are often characterized by low rainfall and thus encounter severe wind erosion and insufficient seed-zone water for planting. Land management practices (e.g. tillage) play an important function in soil hydro-physical properties by modifying soil structure and soil surface characteristics. However, predicting SSWC spatio-temporal variability, and effects on wind erosion, in response to different land management practices is challenging. One tool available to soil conservationists is the Wind Erosion Prediction System (WEPS), which incorporates predictions of SSWC into a process-based model to simulate field-scale wind erosion. In this study, we test the WEPS hydrology sub-model, as typically applied by soil conservationists, to simulate the SSWC of agricultural land with disk, minimum tillage, and no-tillage treatments for winter wheat–summer fallow rotation at 12 sites from 2004 to 2019 in the inland Pacific Northwest (iPNW), USA. Despite widespread application of WEPS for conservation planning, little is known about the effectiveness of the WEPS hydrological sub-model when applied to contrasting tillage systems. The performance of the model differed among each tillage treatment. We found reasonable agreement between simulated SSWC and experimental measurements in the upper 10 cm across all periods as indicated by high value for the index of agreement (d = 0.7) for no-tillage. However, the model overestimated SSWC for disk and no-tillage across various layers in the upper 30 cm. The overestimation was due to the model improperly simulating some higher SSWC (>0.2 m³ m⁻³) values for both tillage treatments. The WEPS also overestimated SSWC for the minimum tillage treatment with d = 0.67. As SSWC suppresses wind erosion by increasing cohesion between soil grains, overestimating SSWC is likely to lead to underprediction of wind erosion risk by WEPS for the studied tillage systems. Care should, therefore, be taken by practitioners when using WEPS to assess potential wind erosion risks and benefits of tillage treatments in the iPNW. Our results contribute to the ongoing development of a serviceable tool for soil and water conservation planning in the iPNW.