AGRICULTURAL PRACTICES INFLUENCE FLOW REGIMES OF HEADWATER STREAMS IN WESTERN IOWA

Authors: Tomer, Mark; Meek, David; Kramer, Larry

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/31/2005
Publication Date: 9/27/2005
Citation: Tomer, M.D., Meek, D.W., Kramer, L.A. 2005. Agricultural practices influence flow regimes of headwater streams in western iowa. Journal of Environmental Quality. 34:1547-1558.

Interpretive Summary: Agricultural tillage influences runoff and infiltration, but consequent effects on watershed hydrology are poorly documented. This study evaluated 25-yr (1971-1995) hydrologic records from four first-order watersheds in Iowa's loess hills. Two watersheds were under conventional tillage, and two were under conservation (ridge) tillage, one of which was terraced. Statistical analyses were used to determine how conservation treatments influenced stream hydrology; and to evaluate variations in discharge, baseflow, and runoff at multi-year, annual, and shorter time scales. The ridge-tilled watershed (non-terraced) had 47% less runoff and 36% more baseflow than the conventional watershed of similar landform and slope. Recovery of baseflow after drought was quicker in the conservation watersheds, which had 67% greater baseflow during the driest two years. The steepest watershed had the greatest runoff and baseflow during short (less than one month), wet periods. Cyclic variations (seasonal and multi-year) occurred in all flow variables. Under ridge-till, baseflow had greater seasonal variablity, therefore conservation practices can increase the amount and the seasonality of agricultural-pollutants movement through the subsurface. However, conservation practices had benefits for increasing the stability of streamflow, so that floods and drought had less impact on streamflow under conservation practices. Large flows were decreased and small flows increased under conservation practices. Although total streamflow from the conservation watersheds was greater than from the conventionally tilled watersheds, the statistical variation in streamflow was smaller. Results are important to water resource planners and conservationists who need better information on how agricultural conservation practices influence hydrology at the watershed scale.

Technical Abstract: Agricultural tillage influences runoff and infiltration, but consequent effects on watershed hydrology are poorly documented. This study evaluated 25-yr (1971-1995) hydrologic records from four first-order watersheds in Iowa's loess hills. Two watersheds were under conventional tillage, and two were under conservation (ridge) tillage, one of which was terraced. All four watersheds grew corn every year. Flow-duration statistics and autoregressive modeling were used to determine how conservation treatments influenced stream hydrology. The autoregressive modeling characterized variations in discharge, baseflow, and runoff at multi-year, annual, and shorter time scales. The ridge-tilled watershed (non-terraced) had 47% less runoff and 36% more baseflow than the conventional watershed of similar landform and slope. Recovery of baseflow after drought was quicker in the conservation watersheds, as evidenced by 365-day moving average plots, and 67% greater baseflow during the driest two years. The two conventional watersheds were similar, except the steeper watershed discharged more runoff and baseflow during short (<30-d), wet periods. Significant multi-year and annual cycles occurred in all variables. Under ridge-till, seasonal (annual-cycle) variations in baseflow had greater amplitude, showing the seasonality of subsurface contaminant movement could increase under conservation practices. However, deviations from the modeled cycles of baseflow were also more persistent under conservation practices, indicating baseflow was more stable. Indeed, flow-duration curves showed wet-weather discharge decreased and dry-weather discharge increased under conservation practices. Although mean discharge increased in the conservation watersheds, variance and skewness of daily values were smaller. Ridge tillage with or without terraces increased stream discharge but reduced its variability.