MS Thesis Defense
River Nitrogen Loads and Landscape Evapotranspiration as Influenced by Climate and Land Cover Changes in the Midwestern United States
The expansion of hypoxic zone in the Northern Gulf of Mexico has been attributed to the adoption of row crops and associated nitrogen management practices in the Mississippi River Basin. However, limited consideration has been given to the role of changing climate on increased N loads in various Midwestern rivers. This research quantified (1) climate and land cover change impacts on river flows and associated N loads in Midwestern United States, and (2) the extent of changes in landscape evapotranspiration (ET) from adoption of corn and soybeans over native prairies. Quantification of the first objective was done using the stepwise regression analysis on streamflow, baseflow, flow weighted N concentrations, and N loads with precipitation, area under soybean production and fertilizer rates as explanatory variables for seven Midwestern rivers. The annual analysis showed that higher N loads in recent years were due to increased precipitation. Furthermore, streamflows, baseflows and N loads were often controlled not only by the current year precipitation but also by the prior year precipitation. The prior year precipitation effects were in terms of increased or decreased soil wetness and thus higher or lower streamflows and associated N loads. Area under soybean production and fertilizer rates were generally not significant explanatory variables in the annual analysis. For the second objective, the ET from irrigated and non-irrigated corn, non-irrigated soybeans, and native prairies at various stage of management in Western Minnesota was evaluated using the satellite images in the METRIC model. The results showed that landscape ET from 8 June through 30 September 2015 followed the trend: wetland (671 mm)> non-irrigated corn (627 mm)>irrigated corn (601 mm)>non-irrigated soybeans (534 mm)>previously burned prairie (532 mm)>recently burned prairie (397 mm). Considering that soybeans also replaced low transpiring small grains such as oats and wheat suggests that large changeover of vegetative cover replacing native prairies and small grains with soybeans starting in the 1940s likely had minimal impact on landscape ET. In the presentation, I further discuss the implications of these results in finding alternatives to reduce N losses.