Quantifying the contribution of tile drainage to basin-scale water yield using analytical methods and a numerical model

Authors: SCHILLING, KEITH - University Of Iowa; GASSMAN, PHILIP - University Of Iowa; ARENAS-AMADO, ANTONIO - University Of Iowa; JONES, CHRISTOPHER - University Of Iowa; Arnold, Jeffrey

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/23/2018
Publication Date: 12/3/2018
Citation: Schilling, K.E., Gassman, P.W., Arenas-Amado, A., Jones, C.S., Arnold, J.G. 2018. Quantifying the contribution of tile drainage to basin-scale water yield using analytical methods and a numerical model. Water Resources Research. 657:297-309. https://doi.org/10.1016/j.scitotenv.2018.11.340.
DOI: https://doi.org/10.1016/j.scitotenv.2018.11.340

Interpretive Summary: Much of the U.S. Midwest corn belt was previously swamp and wetland until it was drained with subsurface tiles and drainage ditches. It is now some of the most productive agricultural land in the world. However, the subsurface drainage provides a direct conduit for the transport of nutrients to water bodies. A better understanding of water sources contributing to river discharge is needed if mitigation and control strategies are going to be successfully targeted to reduce downstream nutrient export. In this study, three different techniques were used to quantify the contribution of tile flow to total river flow that include: 1) hydrograph separation, 2) chemical and isotopic tracers, and 3) the watershed scale model SWAT (Soil and Water Assessment Tool). All three were applied to the Boone River watershed (2,370 square kilometers) in north central Iowa. A synthesis of the techniques suggest that tile drainage contributed 46-54% of the annual river flow. Tile drainage contributes to river flows from watersheds ranging in size from 20 to 16,000 square kilometers, with the contribution decreasing as watershed area increases. The study demonstrated that tile drainage is a significant component of river flows at a variety of scales and is higher that previously reported. With this information, more efficient and effective management strategies to reduce nutrient discharge into water bodies can be developed.

Technical Abstract: The Des Moines Lobe (DML) of north-central Iowa has been artificially drained by subsurface drains and surface ditches to provide some of the most productive agricultural land in the world. Herein we report on the use of hydrograph separation, end-member mixing analysis (EMMA) models and the numerical model Soil and Water Assessment Tool (SWAT) to quantify the contribution of tile drainage to basin-scale water yields at various scales with the 2370 km2 Boone River watershed. Discharge from three drainage district tiles contributes primarily baseflow but surface runoff also comprises a substantial portion of the water yield at monthly and annual scales. Baseflow from drainage district tiles was highly synchronized with baseflow in the Boone River. EMMA and SWAT methods suggested that tile drainage provided approximately 46 to 54% of annual discharge in the Boone River and during the March to June period, accounted for a majority of flow in the river. In the BRW subbasin of Lyons Creek, approximately 66% of the annual flow was sourced from tile drainage. Within the DML region, tile drainage contributes to basin-scale water yields at scales ranging from 40 to 16,000 km2, with downstream effects diminishing with increasing watershed size. The contribution and scaling of tile drainage flows to basin-scale water yield reported herein greatly expands the downstream effects of drainage tile influence beyond what has been previously reported. By combining multiple approaches, we demonstrate that tile drainage is a significant component of the annual and seasonal water yield in intensively-drained basins at a variety of spatial scales.