GLEAMS-ASTERISK, OPUS, AND PRZM-2 MODEL PREDICTED VERSUS MEASURED RUNOFF FROM A COASTAL-PLAIN LOAMY SAND

Comprehensive models for agrichemical transport necessarily include ru noff predictions to partition rainfall between infiltration and runoff as this ability is fundamental to predictions of chemical runoff and leaching. We compared GLEAMS, Opus, and PRZM-2 model runoff prediction s with runoff measured in a precisely controlled field site used for c hemical runoff studies. In 1992 and 1993, two 14.5 m x 42.9 m corn (Ze a mays, L.) field plots with 3% slope on Tifton loamy sand (fine-loamy , siliceous, thermic Plinthic Kandiudult) received six severe, artific ial rainfall events over the growing season with each event consisting of a 25 mm h(-1) rainfall for 2 h. Runoff was monitored continuously using a collector and flume. Model performance criteria included sensi tivity analysis, graphical comparison and statistical analysis includi ng mean, ratio of means, root mean square error (RMSE), and a paired d ifference t-test. Observed runoff averaged 20% of added rainfall. Lowe st values occurred with freshly plowed soil or full canopy covet; whil e 24 to 34% runoff occurred when nearly bare soils had crusted over. U sing an initial moisture condition-II curve number (CN) of 85, GLEAMS and Opus predicted runoff within 10%, overall, and produced a pattern of high and low runoff that closely followed observed. PRZM-2 overpred icted runoff by 90%, overall, and predicted its highest runoff when ob served runoff was lowest. Paired difference t-tests indicated a signif icant difference between measured and predicted runoff for PRZM-2 (p<0 .001 at alpha = 0.05), but none for GLEAMS (p = 0.761) or Opus (p = 0. 194). Mean, ratio of means, and RMSE showed that GLEAMS and Opus perfo rmed better than PRZM-2. All three models were very sensitive to CN va lues which were empirical and subjective, but less sensitive to measur able soil physical properties. With careful parameterization, GLEAMS a nd Opus could be used to simulate runoff from similar row-crop and soi l conditions.