Spatially-distributed modeling of hydrology and nitrogen export from watersheds

Water and land management is a topic of great importance, and the impact of these management decisions play a direct role in the environmental and economic sustainability of the lands in which our lives and livelihoods depend. A comprehensive set of tools, used to accurately predict the impact of land use is needed in order to make well informed decisions, to plan our land use strategies.; DHSVM, the Distributed Hydrology Soil Vegetation Model, is used to accurately simulate the hydrologic process of watersheds. The DHSVM Solute Export Model (D-SEM), an adaptation of DHSVM created in support of this dissertation, integrates biogeochemical modeling research into DHSVM and leverages heterogeneous landscape data, such as topography, vegetation cover, and soil type, to predict hydrologic flow and nutrient export from a watershed level.; D-SEM provides the intelligence needed to perform landuse scenario analysis. The model's primary interest is in hydrologic modeling and dissolved nitrogen species, predicting how landuse changes may affect concentrations and loads of chemicals into streams and determine the relative nitrogen contributions from human, vegetation, and atmospheric sources.; In the first part of the dissertation, DHSVM is used to assess the impact of land use changes on the hydrologic regime of the Mae Chaem River in northwest Thailand. Three forest-to-crop expansion scenarios and one crop-to-forest reversal scenario were developed with emphasis on influences of elevation bands and irrigation diversion.; D-SEM is then applied as a test-of-concept to two dissimilar Hood Canal sub-basins, the Big Beef Creek basin which is high in anthropogenic activities and North Fork Skokomish River basin which is pristine. Hood Canal suffers from low dissolved oxygen levels caused by the fjord's characteristics and algal blooms fed by nitrogen rich waters. The application of D-SEM on these two basins will not only aid in the understanding of the Hood Canal dissolved oxygen problem, but will also help show that D-SEM is portable, and application of D-SEM will be possible to a variety of basins with no adjustments necessary. This is important because it will allow D-SEM to be applied to basins with low levels of field sampling, and also basins with hypothetical land-use changes, with a higher degree of confidence in the results generated.