Modeling is a very important tool for developing nonpoint source (NPS) pollution control plans. Current NPS models were developed for temperate conditions and, thus, do not appropriately represent tropical conditions. The objective of this research was to develop or adapt a nonpoint source pollution model to simulate transformations and losses of nitrogen (N) and phosphorus (P) in leachate and runoff from crop fields under humid tropical conditions. An extensive literature synthesis identified appropriate relationships for representing hydrologic and NPS processes in the tropics, as well as soil and climate conditions that differ from temperate conditions and impact NPS pollution. The GLEAMS model was selected for adaptation. Changes to the model included calculation of potential evapotranspiration (ET); changes in initial and default values of N and P pools, C:N ratio of soil organic matter, and soil P sorption; changes in simulation of transformations between N and P pools, along with the effect of temperature; and inclusion of a nitrate retardation factor (Ncrit) and pH in the calculation of N transformation and movement. The adapted model, called TROPGLEAMS, was evaluated through model verification, application, and sensitivity analysis. Model verification comprised a mass balance of nutrients and analysis of the variation of variable values in time. Model validation included application of the GLEAMS and TROPGLEAMS to three sites in Brazil: a set of lysimeters planted with sugarcane in Piracicaba, SP; a set of plots planted with sugarcane in Piracicaba; and a set of plots planted in a wheat-soybean rotation in Lages, SC. Model sensitivity to temperature, Ncrit and pH were evaluated in the sensitivity analysis. Model evaluation indicated that TROPGLEAMS is more accurate than GLEAMS in simulating fate and transport of nutrients under tropical conditions. Prediction of actual ET, effect of tillage on losses of N and P in runoff, and N and P kinetics was improved with TROPGLEAMS compared to GLEAMS. However, based on data from the Lages study, TROPGLEAMS did not simulate losses of nutrients in runoff well. Improvements in the model, especially related to losses in runoff, and application of TROPGLEAMS to different areas of the humid tropics are recommended. / Ph. D.

Efforts to control surface runoff and erosion from forest roads at stream crossings are critical for the protection of aquatic ecosystems in forests. In this research, annual and event-based sediment delivery rates were estimated for reopened legacy roads at forest stream crossings in the Virginia Piedmont physiographic region, USA. Sediment delivery rates were compared among stream crossing approaches with diverse intensities of best management practice (BMP) implementations for surface cover and water control. Model predictions from the Water Erosion Prediction Project (WEPP) were compared to field observations of surface runoff and sediment delivery to evaluate model performance. Annual sediment delivery rates from reopened (bare) legacy road approaches to stream crossings were 7.5 times higher than those of completely graveled approaches. Sediment delivery rates ranged from 34 to 287 Mg ha⁻¹ year⁻¹ for the bare approaches and from 10 to 16 Mg ha⁻¹ year⁻¹ for the graveled approaches. Event-based surface runoff and associated total suspended solids (TSS) concentrations were compared among a succession of gravel surfacing treatments that represented increasing intensities of BMP implementations on reopened approaches. The three treatments were No Gravel (10-19% cover), Low Gravel (34-60% cover), and High Gravel (50-99% cover). Median TSS concentration of surface runoff for the No Gravel treatment (2.84 g L⁻¹) was greater than Low Gravel (1.10 g L⁻¹) and High Gravel (0.82 g L⁻¹) by factors of 2.6 and 3.5, respectively. WEPP predictions of event-based sediment yield show clear differences among the different road surface treatments, but prediction intervals were wide, reflecting substantial prediction uncertainty. These findings show that reopened legacy roads and associated stream crossing approaches can deliver significant quantities of sediment if roads are not adequately closed or maintained and that corrective best management practices (BMPs), such as gravel and appropriate spacing of water control structures, can reduce sediment delivery to streams. Watershed management decisions that hinge upon WEPP predictions of sediment yield from forest roads must necessarily take into account a wide range of potential erosion rates for specific management scenarios. / Ph. D.