Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/5/2012
Publication Date: 1/3/2013
Citation: Hernandez-Santana, V., Zhou, X., Helmers, M.J., Asbjornsen, H., Kolka, R., Tomer, M.D. 2013. Native prairie filter strips reduce runoff from hillslopes under annual row-crop systems, Iowa USA. Journal of Hydrology. 477:94-103. Interpretive Summary: Practices that reduce runoff volumes from cropped hillslopes can help achieve flood mitigation, which is an important goal in flood-prone watersheds of the US Midwest. This study evaluated how prairie filter strips can reduce runoff from hillslopes being cropped in corn and soybean (row-crop) systems in Iowa, and how the area and position of the filter strips on the slope affect runoff reduction. Runoff data were collected from a replicated experiment including 12 small (ephemeral) watersheds with 6-10% slopes during 2008-2010. Results showed that runoff was reduced by 37% by the prairie strips compared to 100% row crop. Comparisons of 10% and 20% of the watershed area in filter strip showed no difference. In other words, a buffer at the base of the slope comprising 10% of the watershed area was as effective as increasing the proportion to 20% through either placing the buffer area at the base of the slope or distributing that area across the hillslope in multiple contour strips. In addition, the filter strips became more effective in attenuating runoff as the experiment proceeded through three years of observation. Therefore, results improved what we know about filter strip attenuation of runoff on two counts. First, buffer effectiveness increased during an establishment period of three years. Second, on runoff-prone Midwest soils during a period of ample rainfall and severe flooding, buffers did not have to exceed 10% of the watershed area to effectively reduce runoff volume from row-cropped hillslopes. These results are of interest to USDA action agencies, agricultural commodity and conservation groups, and the general public who are interested in discerning how conservation practices can contribute to mitigation of runoff and its potential to contribute to flooding.
Technical Abstract: Intensively managed annual cropping systems have produced high crop yields but have often produced significant ecosystem services alteration; in particular, hydrologic regulation loss. Reconversion of annual agricultural systems to perennial vegetation can lead to hydrologic function restoration, but its effect is still not well understood. Therefore, our objective was to assess the effects of strategic introduction of different amounts and location of native prairie vegetation (NPV) within agricultural landscapes on hydrological regulation. The study was conducted in Iowa (USA), and consisted of a fully balanced, replicated, and incomplete block design whereby 12 zero-order ephemeral flow watersheds received four treatments consisting of varying proportions (0%, 10%, and 20%) of prairie vegetation located in different watershed positions (footslope vs. contour strips). Runoff volume and rate were measured from 2008 to 2010 (April-October) with an H-Flume installed in each catchment and automated ISCO samplers. A total of 129 runoff events were studied and an average runoff volume reduction of 37%, based on the three treatments with NPV compared to watersheds with row crops, was found for the whole study period. We observed a progressively greater reduction across the three years of the study, as the perennial strips became established with the greatest differences among treatments in 2010. The differences among the watersheds were attributed mainly to NPV amount and position, with the 10% NPV at footslope treatment having the greatest runoff reduction, probably because the portion of NPV filter strip that actually contacts watershed runoff was greater with the 10% NPV at footslope. We observed greater reductions in runoff in spring and fall likely because while perennial prairie plants were active, crops were not. High antecedent soil moisture sometimes led to little benefit of the NPV treatments, but in general the NPV treatments were effective during both small and large events. We concluded that small amounts of NPV strategically incorporated into corn-soybean watersheds in the Midwest can be used to effectively reduce runoff.