WATER QUALITY IMPLICATIONS OF CROP WATER DYNAMICS

Authors: Hatfield, Jerry; Prueger, John

Submitted to: American Water Resources Association Conference Proceedings
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/14/2003
Publication Date: 5/14/2003
Citation: HATFIELD, J.L., PRUEGER, J.H. WATER QUALITY IMPLICATIONS OF CROP WATER DYNAMICS. AMERICAN WATER RESOURCES ASSOCIATION CONFERENCE PROCEEDINGS. 2003. CD-ROM. ALEXANDRIA, VA.

Interpretive Summary: Water quality impacts from agricultural practices are assumed to be functions of the changes in the soil cover or amount of soil disturbance. In cropping systems there is a seasonal change in the amount of crop cover in either permanent or annual vegetation and it has been assumed the effectiveness of these practices are directly related to the amount of vegetation present. These vegetative-based practices have water use patterns that change throughout the year and these patterns affect the amount of water storage capacity in the soil profile. We have conducted studies within an agricultural watershed near Ames since 1990 to quantify the daily and seasonal water balance of different tillage and cropping systems. Water use patterns vary among the years due to precipitation patterns and crop growth patterns. Across a landscape the soils that are lighter in color have less organic matter and a lower water holding capacity and in the spring begin to drain first, but also are the soils most susceptible to erosion because they are quickly filled to capacity during heavy rainfall events. To protect these soils requires a vegetative pattern that provides cover in the early spring before the soil profile is full. As we learn how to manage the soil water component and crop water use patterns, then the design of landscape practices that reduce the water quality impacts of agriculture is possible. This information can assist natural resource planners and land managers with information on management systems that have a positive water quality benefit.

Technical Abstract: Water quality impacts are a result of the soil water balance for any given soil and position within a watershed. These effects have been recognized, but there is little direct assessment of the impacts of crop water use patterns on water quality. Studies have been conducted within the Walnut Creek watershed since 1990 to quantify the daily and seasonal water balance of different tillage and cropping systems. Water use patterns vary among years due to differences in energy input and precipitation amounts and within a year due to tillage practice, crop residue cover, crop, and soil water holding capacity. Soils with lower organic matter content have a reduced soil water holding capacity and begin to drain earlier in the spring compared to higher organic matter content in soils. These lower organic matter soils are more subject to surface runoff conditions because they refill faster during intense precipitation events and cropping practices that can continually remove soil water will have a positive impact on environmental quality. Understanding the soil water balance at the watershed scale and the interactions with crop management will enhance water quality when best management practices are coupled with the understanding of the seasonal water balance.