Role of climate variability on subsurface drainage and streamflow patterns in agricultural watersheds

Subsurface (tile) drainage is an important water management practice for agricultural watersheds in the Midwest because it lowers seasonally high water table levels and enables the land to be utilized for row crops. At the same time this practice poses problems for the environment because it increases the load of nitrates entering local bodies of water and alters streamflow patterns. Most existing simulation tools for subsurface drainage are field-scale tools used for investigating the effect of tile spacing and depth, crop type and management practices on drainage and water quality. Many hydrology models designed for use at watershed or river basin scales ignore the role of subsurface drainage in hydrology, meaning that the role of drainage and drainage management methods on river flow are less understood. We tested a new drainage algorithm within the Variable Infiltration Capacity model to determine its ability to adequately simulate subsurface tile drainage. Output from a single grid cell was compared to the record of observed tile drainage and water table data from the Southeast Purdue Agricultural Center (SEPAC). Subsequently, we applied the evaluated model code for watershed scale tests using the Upper White River Watershed at Nora, Indiana study area to evaluate changes in streamflow metrics incorporated through extensively tile drained areas. The VIC drainage model is more representative of agriculturally drained watersheds because it produces higher model performance metrics than the control simulations. The results indicated that tile drainage systems increase streamflow flashiness, increase peak flows, and decrease annual low flows where the average change between the metrics were 50 %, 95%, and 67% respectively. The model was also equipped to handle agricultural land where seasonal water table heights are controlled under Drainage Water Management (DWM). Future climate data from the Geophysical Fluid Dynamic Laboratory model was used to force the model simulations under high (A2), moderate (A1B), and mild emissions (B1) for two model simulations: 1) landscapes using conventional tile drainage practices and 2.) a setup for drainage systems using DWM. We found that DWM mitigates the effects of conventional drainage by increasing field-scale seasonal water conservation. Water conservation was calculated based on the streamflow differences between the conventional and DWM scenarios. Water conserved by DWM during the growing season will decrease in all emissions scenarios throughout the next century due to climate change effects. There was a noticeable decline in growing season water conservation from DWM which decreased from approximately 7.1 million cubic meters annually between the years of 2010 and 2039 to about 6.6 million cubic meters by the late 21st century (2070-2099)