PREDICTING WATER, SEDIMENT AND NO3-N LOADS UNDER SCENARIOS OF LAND-USE AND MANAGEMENT PRACTICES IN A FLAT WATERSHED

Authors: CHAPLOT, V - L'INST DE RECHERCHE (IRD); SALEH, A - TARLENTON STATE UNIVERSIT; Jaynes, Dan; Arnold, Jeffrey

Submitted to: Journal of Water Air and Soil Pollution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/3/2003
Publication Date: 5/1/2004
Citation: Chaplot, V., Saleh, A., Jaynes, D.B., Arnold, J.G. 2004. Predicting water, sediment and NO3-N loads under scenarios of land-use and management practices in a flat watershed. Journal of Water Air and Soil Pollution. 154(1-4):271-293.

Interpretive Summary: Changes in land use or farm management practices can affect water outflow, sediment, nutrient, and pesticide losses from agricultural watersheds. There is an increasing demand by decision makers and planners for information regarding the magnitude of these changes due to adoption of various agricultural practices. Because of the complexity of watershed systems and the large number of possible management practices to be considered, the development and use of computer models is required. We verified that a modified version of the ARS Soil and Water Assessment Tool (SWAT) model could successfully simulate the water flow and nitrate concentrations within the Walnut Creek watershed of central Iowa – a watershed that has been extensively tile drained and is representative of much of the U.S. cornbelt. We then used the validated model to evaluate the water quality impact of nine management scenarios over a 30-year period. We found that a percentage change in the nitrogen fertilizer rates used reduced nitrate losses in the stream by roughly an equal percentage. We also found that replacing corn with a winter wheat increased fall nitrate losses from the watershed, but replacing row crops with pastures greatly reduced nitrate losses from the watershed. These findings and the improved watershed model will be of interest and use to a wide range of scientists, regulators, and decision makers.

Technical Abstract: Changes in land-use or management practices may affect water outflow, sediment, nutrients and pesticides loads. Thus, there is an increasing demand for quantitative information at the watershed scale that would help decision makers or planners to make appropriate decisions. This paper evaluates by a modeling approach the impact of farming practices and land-use changes on water discharge, sediment and nitrate-nitrogen (NO3-N) loads at the outlet of a 51.29 km**2 watershed of central Iowa (Walnut Creek watershed). This intensively farmed (corn-soybean rotation) watershed is characterized by a flat topography with tiles and potholes. Nine scenarios of management practices (nitrogen application rates: increase of current rate by 20, 40%, decrease of current rate by 20, 40 and 60%; no tillage) and land-use changes (from corn-soybean rotation to winter wheat and pasture) were tested over a 30-year simulated period. The selected model (Soil and Water Assessment Tool, SWAT) was first validated using observed flow, sediment and nutrient loads from 1991 to 1998. Scenarios of nitrogen (N) application rates did not affect water and sediment annual budgets but did so for NO3-N loads. Lessening the N rate by 20, 40 and 60% in corn-soybean fields decreased mean NO3-N annual loads by 22, 50 and 95%, respectively, with greatest differences during late spring. On the other hand, increasing input N by 20 and 40% enhanced NO3-N loads by 25 and 49%, respectively. When replacing corn-soybean rotation with winter wheat, NO3-N loads increased in early fall, immediately after harvest. Pasture installation with or without fertilization lessened flow discharge, NO3-N and sediment delivery by 58, 97 and 50%, respectively. No-tillage practices did not significantly affect the water resource and sediment loads. Finally, such realistic predictions of the impact of farming systems scenarios over a long period are discussed regarding environmental processes involved.