|Chung, S - IOWA STATE UNIVERSITY|
|Gassman, P - IOWA STATE UNIVERSITY|
|Williams, J - TEXAS A&M UNIVERSITY|
|Gu, R - IOWA STATE UNIVERSITY|
Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 20, 1998
Publication Date: N/A
Interpretive Summary: The Erosion Productivity Impact Calculator (EPIC) model was tested using long-term data from two southwest Iowa watersheds located near Treynor, Iowa. These watersheds represent the Deep Loess Soil Major Land Resource Area that covers western Iowa and northwestern Missouri. Water balance, sediment, and nutrient loss data were used from both watersheds that have been cropped with continuous corn and managed with contrasting tillage systems (conventional versus ridge tillage) for over two decades. EPIC was calibrated for 1988 to 1994 for both watersheds. Calibration revealed that standard parameters used in EPIC to divide rainfall between infiltration and surface runoff needed to be adjusted. This resulted from the fact that higher infiltration rates occurred under ridge till with greater residue. This was a confirmation of previous studies of conservation tillage. Validation of the model was performed for both watersheds over the period 1976 to 1987. The results showed that the model accurately replicated mos of the 12-year measured water balance, nutrient, and crop yield means. The results indicate that NRCS field personnel and other model users should be able to effectively apply EPIC to assess long-term average impacts of different tillage systems. The model should be used more cautiously for risk and other analyses that require reliance on simulated variability, especially on an event basis.
Technical Abstract: The Erosion Productivity Impact Calculator (EPIC) model was validated using long-term data collected for two southwest Iowa watersheds in the Deep Loess Soil Region, which have been cropped in continuous corn (Zea Mays L.) under two different tillage systems (conventional tillage versus ridge-till). The annual hydrologic balance was calibrated for both watersheds during 1988-94 by adjusting the runoff curve numbers and residu effects on soil evaporation. Model validation was performed for 1976-87, using both summary statistics (means or medians) and parametric and nonparametric statistical tests. The errors between the 12-year predicted and observed means or medians were less than 10% for nearly all of the hydrologic and environmental indicators, with the major exception of a nearly 44% overprediction of the N surface runoff loss for Watershed 2. The predicted N leaching rates, N losses in surface runoff, and sediment loss for the two watersheds clearly showed that EPIC was able to simulate the long-term impacts of tillage and residue cover on these processes. However, the results also revealed weaknesses in the model's ability to replicate year-to-year variability, with r**2 values generally below 50% and relatively weak goodness-of-fit statistics for some processes. This was due in part to simulating the watersheds in a homogeneous manner, which ignored complexities such as slope variation. Overall, the results show that EPIC was able to replicate the long-term relative differences between the two tillage systems and that the model is a useful tool for simulating different tillage systems in the region.