Submitted to: Soil and Water Conservation Society
Publication Type: Abstract Only
Publication Acceptance Date: May 1, 2012
Publication Date: July 25, 2012
Citation: Tomer, M.D., Moorman, T.B., Cole, K.J., Hatfield, J.L. 2012. Runoff and phosphorus loads from two Iowa fields with and without applied manure, 2000-2011 [abstract]. Soil and Water Conservation Society Conference Abstract Book. p. 22. Technical Abstract: Understanding the dynamics of field-edge runoff water quality and responses to changes in management practices and climate through monitoring will probably require decade-duration data sets. This study compared runoff volumes and phosphorus loads from two fields in central Iowa, where the glacial landscape is dominated by the Clarion-Nicollete-Webster soil association. Both fields were in corn-soybean rotation but only one received swine manure, which was applied following soybean harvest to meet the following corn crop's N requirement. Runoff flumes (H-type) were installed along grassed waterways prior to the 2000 growing season; runoff samples were collected on a flow-interval basis and analyzed for total P. Monitoring continued through 2010, providing 11 years of data. Results are only for rainfall-runoff events because sites were remote and consistent snowmelt monitoring was not feasible. Results provided data on 119 events from the manured site, and 94 events at the non-manured site, with 74 events common to both sites. Discharge through the monitoring flumes occurred on average less than 116 hours per year, but half the total discharge observed in the entire 11 years occurred during a total 48 hours of storm flow. This result emphasizes the challenge of monitoring ephemeral flows. Functions that described runoff fraction (the fraction of rainfall that was converted to runoff) as a function of storm size were virtually identical at the two sites. Average annual runoff was greater from the non-manured site (54 mm yr-1) than the manured site (37 mm yr-1), however, P loads were greater from the manured site 1.4 vs 1.1 kg P ha-1yr-1. The difference is small, especially given soils in the non-manured field approached P-deficiency by 2010 after years of maintenance rates of P-fertilizer. Interestingly, 40-56% of the runoff volume and 54-62% of the total P load during 2000-2010 resulted from runoff generated by 30-60 mm of precipitation. A total of 20-30% of the observed runoff and 12-16% of the total P load in both fields were generated from ten events that exceeded 60 mm rainfall. This result confirms that conservation systems designed to control runoff from 60 mm rainfall events on similar Midwest soils, which had about a one year return period during the study, could provide soil and water conservation to help mitigate a trend of increasing rainfall in the Midwest.