Impact of drainage water management on watershed nitrate load in west central Indiana

Srinivasulu Ale, Purdue University

Abstract

Subsurface drainage, popularly known as tile drainage, is an essential water management practice in the Midwestern region of the United States to convert poorly drained areas into highly productive crop land. Despite many agronomic and environmental benefits, subsurface drainage also increases nitrate losses to the receiving streams. Development of hypoxia in the Gulf of Mexico has been attributed to nitrate loading from the Midwest. The concern about negative environmental effects of subsurface drainage has led to the development of practices such as drainage water management (also called controlled drainage), which has a potential to reduce nitrate loads generated by the existing drainage systems by limiting drain flow during some periods, while maintaining drainage intensity during critical periods of the crop growth cycle. Although drainage water management practices have been adopted at several locations in the Midwest, strategies for operation of drainage water management systems are not well-defined; watershed-level controls on the movement of water and nitrogen in the subsurface drained areas are not well-studied; and excepting a few studies, detailed watershed-scale impacts of drainage water management on nitrate load reduction have not been evaluated. This study addresses these research gaps. The overarching goal of this study is to assess the potential impacts of drainage water management on nitrate load reduction at field and watershed scales. A combination of field observations of watershed processes, Geographic Information System (GIS) analysis and hydrologic modeling were used to achieve the research goal. The deterministic field-scale model, DRAINMOD was used in the study. The DRAINMOD model was calibrated using the long-term drain flow and nitrate concentration data from the continuous corn (CC) and corn-soybean (CS) rotation drainage lysimeter plots at the Purdue University's Water Quality Field Station (WQFS). A strategy for operation of a hypothetical drainage water management system at WQFS was identified and used for drainage water management simulations. For the soil and drainage conditions similar to the WQFS, during the crop season, raising the outlet 0 to 20 days after planting to a height of 50 cm above the drain (40 cm from the surface) and lowering the outlet 4 to 6 weeks before the harvest were identified as preferable strategies for drainage water management. During the non-growing season, completely closing the outlet from the first week of November to the last week of March was identified as the preferable strategy. This operational strategy reduced simulated annual drain flows at the WQFS by 60% while maintaining crop yield. The simulated effects of fertilizer application rate, crop rotation and drainage water management on nitrate load from the subsurface drains at the WQFS were evaluated. Decreasing the fertilizer application from high (224 and 202 kg N/ha) to low (179 and 157 kg N/ha) rates reduced nitrate losses by 29 and 14% in the CC and CS rotations, respectively. Predicted annual nitrate losses in the CS treatments were 14 to 40% higher than the CC treatments. Higher mineralization, lower immobilization and more drain flow in soybean years than corn years could have resulted in higher nitrate losses from CS treatments as compared to CC treatments. Drainage water management simulations at the WQFS predicted 46 and 58% reductions in nitrate load from the CC and CS treatments, respectively. The potential impacts of drainage water management on reduction of nitrate load from the Hoagland watershed, a low-gradient agricultural watershed in west central Indiana, were assessed. The estimated annual edge-of-field and watershed outlet nitrate loads from the Hoagland watershed were 285 and 231 metric tons, respectively, indicating a 19% reduction in nitrate load due to in-stream attenuation. The estimated watershed nitrate load with drainage water management was 113 metric tons, a 51% reduction when compared to conventional drainage.

Degree

Ph.D.

Advisors

Bowling, Purdue University.

Subject Area

Soil sciences|Agricultural engineering|Water Resource Management|Environmental engineering

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