MEASURED AND SIMULATED SURFACE SOIL DRYING

The USDA initiated the Wind Erosion Prediction System (WEPS) to develo p improved technology for predicting wind erosion. A HYDROLOGY submode l has been developed for WEPS to simulate the soil energy and water ba lances. This study was conducted to evaluate the performance of the HY DROLOGY submodel in predicting surface soil drying. Water content was measured gravimetrically in a bare 5- by 30-m plot for 14 d after irri gation during July and August 1991. The plot was located 5 m directly north of a bare weighing lysimeter at the USDA-ARS Conservation and Pr oduction Research Laboratory at Bushland, TX. Hourly samples were take n from depth increments of 0 to 2, 2 to 6, 6 to 10, 10 to 34 and 30 to 50 mm. Furthermore, soil cores were taken to 900 mm at 6-h intervals. Water content was also measured daily at the lysimeter and between th e lysimeter and gravimetric sampling plot using a neutron probe to 2.1 m. The submodel accurately predicted that no deep percolation occurre d throughout the simulation period. Simulation results agreed well wit h the measured daily evaporation rates from the lysimeter (r(2) = 0.96 ). Furthermore, the submodel reasonably estimated the soil water conte nt profiles, particularly the status of soil water at the soil-atmosph ere interface. The mean absolute error, which describes the average ab solute deviation between measured and simulated soil water contents, w as 0.015 m(3) m(-3). The HYDROLOGY submodel of WEPS shows a potential to accurately simulate soil water dynamics, as needed for wind erosion modeling. The submodel successfully predicts the changes in water con tent at the soil surface, which relate to the susceptibility of the so il to wind erosion.