Evaluating and Estimating DRAINMOD’s Effective Rooting Depth for Corn

Abstract

<abstract> <bold>Abstract.</bold> Evapotranspiration (ET) is the largest component of the water balance after precipitation; the magnitude of ET ranges from about 40% of annual precipitation in humid regions, up to 100% in arid regions. Estimation of ET has historically been a challenging endeavor, and numerous approaches exist. The estimation of ET in hydrologic models has seen various levels of rigor, from quite high (e.g., SWAT) to quite low (e.g., SWMM), and some models allow user-provided ET estimations (e.g., DRAINMOD). Accurate estimation of ET is a crucial component of water quality modeling, where good model performance begins with accurate hydrologic modeling. Within a model, ET estimation can be sensitive to many parameters, including weather, soil, and crop-related inputs. Some hydrologic models simulate the development of crop roots over the course of a season. The drainage model, DRAINMOD, provides a user input for root depth development, but it is unclear to what extent the maximum rooting depth impacts plant water uptake, and how this differs from the effective rooting depth required by DRAINMOD (the maximum depth below the soil surface from which water can be drawn to meet ET demand). Although some researchers have considered rooting depth to be one of the less sensitive parameters within DRAINMOD, we have found DRAINMOD estimates of ET to be sensitive to rooting depth. A literature review of the spatial and temporal development of corn roots suggests that root growth can be modeled using one or more various parameters such as the crop growth stage, water table depth, and growing degree days. A new method of generating effective rooting depths for use in DRAINMOD is proposed, with the overall goal of improving the ET estimates of the model. DRAINMOD was originally developed as a tool for selecting profitable agricultural drainage system spacing and depth. The role of effective rooting depth may be of particular importance in light of the increased use of DRAINMOD for hydrologic and water quality investigations.