The importance of seepage zones in predicting soil moisture content and surface runoff using GLEAMS and RZWQM

Seepage zones have been shown to be of critical importance in controlling contaminant export from agricultural catchments. To date, no multi-purpose agricultural water quality model has incorporated seepage zones into its process-level representations. We chose to test two commonly used models of agricultural water quality, Groundwater Loading Effects of Agricultural Management Systems (GLEAMS) and the Root Zone Water Quality Model (RZWQM), by seeing how well each predicted surface runoff and soil moisture content in two agricultural fields: one with and one without seepage zones. Daily simulated surface runoff and soil moisture content from both calibrated and default (or non-calibrated) GLEAMS and RZWQM were compared with three years of measured surface runoff and soil moisture content in the two fields. The results of the study show that GLEAMS and RZWQM, using default model parameters, were not capable of predicting surface runoff and soil moisture content in either field. Site-calibrated GLEAMS and RZWQM performed well in simulating surface runoff trends from the field with and without seepage zones, but they predicted soil moisture content poorly. Several statistical tests were used that showed that although both site-calibrated GLEAMS and RZWQM performed well, RZWQM performed better than GLEAMS and is better suited in assessing the effects of seepage zones on soil moisture content and surface runoff from agricultural fields.