Date of Award
Doctor of Philosophy
Civil, Construction, and Environmental Engineering
Roy R. Gu
Excessive nutrient and sediment export from agricultural basins with intensive row crop cultivation have been identified as persistent problems leading to higher levels of nitrate nitrogen and reduced levels of dissolved oxygen in the water bodies. Water quality thus gets degraded and becomes less suitable for human use and potential threat to the aquatic life and environment. Development of bio-fuel technology further increases the demand of grain that will result in more land under row crop cultivation, which supposedly would worsen the situation regarding water quality. In this study, application of contour and riparian buffer strips and strategic conversion of row crop to biomass yielding switchgrass to conserve the basin scale water quality is evaluated using the Soil and Water Assessment Tool (SWAT) model. Finally SWAT hydrological parameters have been developed for a small portion of Iowa that would be instrumental in development of TMDL's.
The use of contour and riparian buffer strips planted with perennial vegetation has been found to improve surface water quality by reducing NO3-N and sediment outflow from cropland to a river. Modeling such a system to compare alternative layout and different strip sizes often faces challenges in flow routing scheme. The hillslope scheme in Soil and Water Assessment Tool (SWAT) offers the flexibility of allowing the flow from a crop area to be routed through a buffer and/or contour strip, in which a thin sheet flow represents more closely the natural condition of a watershed. SWAT was applied to the Walnut Creek Watershed and the hillslope option was used to examine the effectiveness of contour and riparian buffer strips in reducing NO3-N outflows from crop fields to the river. Numerical experiments were conducted to identify potential subbasins in the watershed that have high water quality impact and to examine the effects of strip size and location on NO3-N reduction in the subbasins under various meteorological conditions (dry, average and wet). Variable sizes of contour and riparian buffer strips (10%, 20%, 30% and 50%, respectively, of a subbasin area) planted with perennial switchgrass were used to simulate the effects of strip size on stream water quality. Simulation results showed that a filter strip having 10%-50% of the subbasin area could lead to 55%-90% NO3-N reduction in the subbasin during an average rainfall year. Strips occupying 10-20% of the subbasin area were found to be more efficient in reducing NO3-N when placed along the contour than that when placed along the river. Varying the area and location of the contour and buffer strip affects NO3-N outflow and crop yields as well since it takes the land out of production. The size of the filter strip has economic implications in deciding how much land area to dedicate to prevent NO3-N loss to a desired limit or vice versa. The results of this study can assist in cost-benefit analysis and decision-making in best management practices for environmental protection.
SWAT was then applied to the Upper Mississippi River (UMRB) to study the perpetuation of the current trend of growing corn to meet the increasing corn demand for ethanol industry. A hypothetical case of converting the entire UMRB agricultural land into corn production was simulated by SWAT. Though very unlikely, this study provided a guideline to identify the highest nitrate contributing subbasins that could be used for switchgrass production instead of corn. Such conversion would yield economic value from cellulosic ethanol from swtichgrass and at the same time there would be an improvement in water quality. High impact subbasins were identified based on the total nitrate output of each subbasin. Converting them to switchgrass production was found to reduce nitrate nitrogen yield of up to 14 kg/ha and sediment reduction of up to 5 tons/ha. In many cases, switchgrass reduced up to 71% of total nitrate nitrogen yield and almost 99% of sediment. The Production-Economy-Environment matrix analysis of growing switchgrass for various rates of fertilizer application and its consequences on the yield of biomass and environment was performed. It demonstrated that the efficacy of rate of fertilizer application and its relationship to economy and environment was not proportionate. It underscores the importance of such analysis to design an optimum amount of fertilizer to be used. Conversely, it can be used to determine the rate of fertilizer application for a desired gain or desired target in environmental quality. A simple economic analysis found out that there was a significant economic gain from the cellulosic ethanol compared to corn ethanol. It was concluded that even though the economic benefits of bio-energy crops were marginal, the bio-energy crops are yet a potentially viable solution for the degrading water environment in the waterways of Upper Mississippi River Basin and the Gulf of Mexico.
Finally, Soil and Water Assessment Tool (SWAT) was set up, calibrated and validated for the Maquoketa (4867 km2) and Beaver Creek (905 km2) watersheds to develop SWAT hydrologic parameters specific to one of the six principal Iowa landform regions. These landforms (eco-regions) cover the majority of the intensively cropped regions in the state and are based on similar bio-physical characteristics that are assumed to have a corresponding specific range of SWAT input parameters unique to each one of them. Having a readily usable set of SWAT hydrological parameters would make the modeling part of TMDL development easier. Using the observed data of 1995-2008, calibration of SWAT for Maquoketa gave the annual and monthly flow Nash-Sutcliffe's efficiency (E) of 0.89 and 0.83 and coefficient of determination (R2) value of 0.94 and 0.86. Without making any further changes to the model parameters, model validation on Beaver Creek gave the monthly flow E of 0.73 and 0.82 and R2 value of 0.96 and 0.87 that was well over acceptable limit. A sensitivity analysis on Beaver Creek was performed by modifying the land use distribution similar to Maquoketa and the results showed that SWAT model was performing coherently in both the watersheds. Thus a SWAT hydrological parameter set was recommended for the Iowan Surface landform region.
Mahesh Kumar Sahu
Sahu, Mahesh Kumar, "Basin Scale Water Quality Conservation: Impacts of filter strips, bio-fuel development and hydrological parameters" (2010). Graduate Theses and Dissertations. 11768.