DRAINMOD-P: A MODEL FOR SIMULATING PHOSPHORUS DYNAMICS AND TRANSPORT IN DRAINED AGRICULTURAL LANDS: II. MODEL TESTING

The recently developed phosphorus (P) model DRAINMOD-P was tested using a four-year dataset from a subsurface-drained field in northwest Ohio with significant potential for desiccation cracking or preferential flow. The model satisfactorily predicted subsurface drainage discharge, with a monthly Nash-Sutcliffe efficiency (NSE) of 0.59 and index of agreement (IOA) of 0.89. Lack of annual water budget closure was reported and was likely caused by uncertainty in measured surface runoff and/or modeling approaches representing macropore flow. More than 80% of predicted surface and subsurface P losses were in the particulate form. Surface runoff was the major pathway for P loss, contributing 78% of predicted total P (TP) load. On average, predicted macropore flow represented about 15% of drainage discharge and contributed 21% of DRP loss via subsurface drains. The performance of DRAINMOD-P in predicting monthly dissolved reactive P and TP losses through subsurface drains can be rated as poor (NSE = 0.33 and IOA = 0.60) and very good (NSE = 0.81 and IOA = 0.95), respectively. DRAINMOD-P demonstrated potential for simulating P fate and transport in drained cropland. More testing is needed to further examine newly incorporated hydrological and biogeochemical components of the model.