State Water Resources Research Institute Program


Project ID: 2012SD212B
Title: Subsurface Drainage Impacts on Evapotranspiration and Water
Project Type: Research
Start Date: 3/01/2012
End Date: 2/28/2013
Congressional District: First
Focus Categories: Hydrology, Agriculture, Water Quantity
Keywords: Evapotranspiration, Subsurface Drainage, Hydrology, DRAINMOD
Principal Investigators: Hay, Christopher; Kjaersgaard, Jeppe H; Trooien, Todd P.
Federal Funds: $ 19,643
Non-Federal Matching Funds: $ 39,297
Abstract: Subsurface drainage has increased dramatically in eastern South Dakota with increases in precipitation, commodity prices, and land prices. Subsurface drainage improves agricultural production by increasing yields and reducing risk, but there are concerns about its environmental impacts. A key concern is to what extent does subsurface drainage contribute to downstream !ow alterations and flooding through changes in the amount and timing of water leaving the field. Changes in evapotranspiration (ET), as a result of drainage, are a primary determinant of the hydrologic alterations from subsurface drainage. However, the impacts of drainage on ET are not yet well understood. Lack of such knowledge is an important problem, because without it, we are limited in our ability to accurately quantify the impacts of subsurface drainage on watershed hydrology and !ooding.

DRAINMOD is a widely used computer simulation model designed to model the hydrology of high water table soils using water balance techniques. However, while DRAINMOD accounts for excess water stress limitations on crop yield from poor drainage, it doesn't account for reduced transpiration as a result of those yield reductions. The FAO 56 dual-crop coeffcient method for calculating evapotranspiration, which includes a soil-water balance, will be used for comparison with the DRAINMOD ET estimates. The FAO 56 dual-crop coeffcient method also allows for a crop stress coe$cient to adjust actual ET for crop stressors that can reduce ET, including excess water. ET calculated using the FAO 56 dual-crop coe$cient method, along with yield reductions estimated using DRAINMOD, will be used to develop plant stress coe$cients to account for excess water stress reductions on ET. The crop stress coeffcients will be integrated into FAO 56 dual-crop coe$cient method ET estimates. These new ET estimates will then be used along with DRAINMOD estimates of the water balance components, including ET, to estimate the impact that reduced transpiration as a result of crop yield reductions due to excess water stress has on water yield.

The overall goal of this proposal is to develop a method to account for the impact of yield reductions from poor drainage on evapotranspiration in drainage model simulations. Our central hypothesis, based on water productivity functions that relate crop yield and ET, is that current drainage model simulations overestimate ET under undrained or poorly drained conditions. The rationale for the proposed research is that once we are able to accurately simulate ET under undrained and poorly drained conditions, we can then better estimate the impacts that subsurface drainage development will have on hydrology. Our contribution here is expected to be an improved understanding of the impacts of subsurface drainage on ET. Once such knowledge is available, we can better evaluate the hydrologic impacts of increased subsurface drainage in eastern South Dakota.

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