Modification of the Field Moisture Balance Water Quality model (FMBWQ) to simulate subsurface drainage and preferential flow
The general objective of this study was to add the capability of simulating subsurface drainage and preferential flow to FMBWQ. The modified FMBWQ was calibrated and validated for an agricultural plot in the Iowa State University North East Research Center near Nashua, Iowa (hereafter referred to as the Nashua site). The study has been undertaken as part of the Iowa Management System Evaluation Area Project (MSEA). FMBM, the predecessor of FMBWQ, was originally developed for deep loess soils in Western Iowa (Anderson et al., 1978) which weren't artificially drained. However, the Nashua site has tile drainage to minimize variability in soil drainage. Although FMBWQ had an existing tile flow subroutine,it had not been fully developed and tested. Initial runs using the original tile flow subroutine gave unsatisfactory predictions of tile flow. Therefore, there was a need to improve FMBWQ's tile flow subroutine in order to adequately simulate hydrologic processes at the Nashua site. Kanwar et al. (1993) noted that there was rapid increase in tile flows at the Nashua site after heavy rains which shows the preferential movement of water through macropores. Considering this, it was also deemed necessary to add a preferential flow subroutine to FMBWQ. Under these modeling conditions, the following specific objectives were laid out: 1) to improve or replace the existing tile flow subroutine of FMBWQ; 2) to add a simplified preferential flow subroutine to FMBWQ; 3) to calibrate the modified FMBWQ for tile flow predictions using Nashua data for the 1990 season; and 4) to validate the modified FMBWQ for tile flow predictions using Nashua data for the 1991 and 1992 seasons. For this study, no attempts were made to simulate pesticide transport since it was more important to ensure adequate hydrologic predictions beforehand.
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