Evaluation of the root zone water quality model for conditions in Central Nebraska

The Root Zone Water Quality Model (RZWQM) was evaluated for a region that h as a long history of irrigated corn (Zea mays L.). Imprecise irrigation and plentiful fertilizer applications have contributed to the buildup of NO3-N in ground water, where concentrations often exceed 30 mg L-1. Experiments were conducted to evaluate the effects of management practices on corn yiel d, N uptake, plant biomass, leaf area development, soil water content, and soil N. Field-measured results from 1992 were used to calibrate RZWQM. The model was evaluated by simulating three irrigation levels and five fertiliz er rates for 1993 and 1994. The model simulated the soil water pattern sati sfactorily during most of the growing season; however, the simulated soil p rofile water content was lower than field conditions in the spring and fall . Transpiration appears to be excessive when the leaf area index (LAI) is s mall. Simulated yields exceeded measured values, with the largest errors fo r small fertilizer applications in 1993 and for large fertilizer applicatio ns in 1994 The mean error between measured and modeled yields was 2.9 Mg ha (-1), with a root mean square error (RMSE) equal to 46% of the mean yield f or all treatments over the 3 years. Estimates of LAI and aboveground biomas s were closer to measured values, with mean errors of 5% for leaf area and 1% for biomass. The relative RMSE for LAI and biomass was 26 and 17%, respe ctively. The model underestimated aboveground N uptake, with a mean error o f 25 kg N ha(-1) and a relative RMSE of 41%. While development of the compr ehensive RZWQM has been constructive, improvements are still needed. Excess ive depletions in the spring and fall overstate storage of off-season preci pitation, and this leads to underestimating annual leaching losses. Combini ng excessive yield predictions with low N uptake estimates exaggerates the N use efficiency. These discrepancies will have a profound effect on the si mulated impact of management practices. Leaching losses and yield effects w ill be understated, which may lead to development of policies that affect p roducers more severely than the model implies.