EVALUATION OF HYDROLOGICAL PROCESSES AND ENVIRONMENTAL IMPACTS OF FREE AND CONTROLLED SUBSURFACE DRAINAGE
Controlled drainage is a management strategy designed to mitigate water quality issues caused by subsurface drainage. To improve controlled drainage system management and better understand its hydrological and environmental effects, this study analyzed water table recession rate, as well as drain flow, nitrate and phosphorus loads of both free and controlled drainage systems, and simulated the hydrology of a free drainage system to evaluate surface runoff and ponding at the Davis Purdue Agricultural Center located in Eastern Indiana.
Statistical analyses, including paired watershed approach and paired t-test, indicated that controlled drainage had a statistically significant effect (p-value <0.01) on the rate of water table fall and reduced the water table recession rate by 29% to 62%. The slower recession rate caused by controlled drainage can have negative impacts on crop growth and trafficability by causing the water table to remain at a detrimental level for longer. This finding can be used by farmers and other decision-makers to improve the management of controlled drainage systems by actively managing the system during storm events.
A method was developed to estimate drain flow during missing periods using the Hooghoudt equation and continuous water table observations. Estimated drain flow was combined with nutrient concentrations to show that controlled drainage decreased annual nitrate loads significantly (p<0.05) by 25% and 39% in two paired plots, while annual soluble reactive phosphorus (SRP) and total phosphorus (TP) loads were not significantly different. These results underscore the potential of controlled drainage to reduce nitrate losses from drained landscapes with the higher level of outlet control during the non-growing season (winter) providing about 70% of annual water quality benefits and the lower level used during the growing season (summer) providing about 30%.
Three different methods including monitored water table depth, a digital photo time series and the DRAINMOD model simulations were used to determine the generation process of surface ponding and runoff and the frequency of incidence. The estimated annual water balance indicated that only 7% of annual precipitation contributed to surface runoff. Results from both simulations and observations indicated that all of the ponding events were generated as a result of saturation excess process rather than infiltration excess.
Overall, nitrate transport through controlled drainage was lower than free drainage, indicating the drainage water quality benefits of controlled drainage, but water table remained at a higher level for longer when drainage was controlled. This can have negative impacts on crop yields, when water table is above a detrimental level, and can also increase the potential of nutrient transport through surface runoff since the saturation excess was the main reason for generating runoff at this field.