In the second chapter the Agricultural Drainage And Pesticide Transport (ADAPT) model was compared to the water management simulation models DRAINMOD, SWATREN, and PREFLO. SWATREN and PREFLO are one-dimensional finite-difference models that function on a daily time step. ADAPT and DRAINMOD are one-dimensional mass balance models with daily and hourly time steps, respectively. ADAPT, an extension of the computer model GLEAMS, also provides chemical transport information. All four models were tested against field data from Aurora, North Carolina. Observed water table depth data were collected during 1973 through 1977 from a water table management field experiment with three subsurface drain spacing treatments of 7.5 m, 15 m, and 30 m. Both the standard error of estimate and the average absolute deviation were computed between measured and predicted midpoint water table depths. For the five-year period ADAPT, DRAINMOD, SWATREN, and PREFLO had standard errors of estimated water table depth of 0.18, 0.19, 0.19, and 0.18 m and absolute deviations of 0.14, 0.14, 0.14, and 0.14 m, respectively. The results show good agreement between the models for this experimental site and encourage the further adoption of ADAPT to predict chemical transport. The Agricultural Drainage And Pesticide Transport (ADAPT) model was compared to the water management simulation models DRAINMOD, SWATREN, and PREFLO in chapter 3. SWATREN and PREFLO are one-dimensional finite-difference models that function on a daily time step. ADAPT and DRAINMOD are one-dimensional mass balance models with daily and hourly time steps, respectively. ADAPT, an extension of the computer model GLEAMS, also provides chemical transport information. All four models were tested against field data from Aurora, North Carolina. Observed water table depth data were collected during 1973 through 1977 from a water table management field experiment with three subsurface drain spacing treatments of 7.5 m, 15 m, and 30 m. Both the standard error of estimate and the average absolute deviation were computed between measured and predicted midpoint water table depths. For the five-year period ADAPT, DRAINMOD, SWATREN, and PREFLO had standard errors of estimated water table depth of 0.18, 0.19, 0.19, and 0.18 m and absolute deviations of 0.14, 0.14, 0.14, and 0.14 m, respectively. The results show good agreement between the models for this experimental site and encourage the further adoption of ADAPT to predict chemical transport. In the fourth chapter the Agricultural Drainage And Pesticide Transport (ADAPT) computer simulation model was modified to partition subsurface drainage flow into two partitions: a component of flow that passes strictly above the drain and a path that passes through the soil matrix below drain depth before rising back up to enter the drain. It has been shown in previous papers that this two layer model does a good job of predicting water table depth midway between the drains. Since this model has pesticide and nutrient transport components, it is important to make close approximations of water transport within the soil layers. The two layer model gave improved predictions of drainage nitrate mass prediction over the single layer model. A surface and subsurface drainage experiment on silty clay soil in northwest Ohio was used to gauge this effect. A sensitivity analysis was performed on ADAPT denitrification modeling. It was found that the two layer method required no adjustment factor and it out performed single layer versions of ADAPT in it's prediction of nitrogen effluent concentration. In 1990, the multi-agency Management Systems Evaluation Areas Program was established in the Midwest Region of the United States. As part of this study a systems approach was used to evaluate solute transport through the vadose zone into the Scioto River buried valley aquifer in southern Ohio. The Scioto River flows about 1.2 km from the plot site, which is located on a 260 ha farm in Pike County, south-central Ohio. Fluventic hapludoll and fluventic eutrochrept silt loams are the predominant soil series and overlie sands that grade into gravel at a depth of 3 to 5 m. The water table in this unconfined aquifer normally ranges from 4 to 6 m below the soil surface. Multiport wells were installed with sampling ports at average depths of 4.2, 5.7, 7.0, and 8.4 m below the ground surface. A centrally located well in each of three 10 ha plots had ports at 12.4 and 18.6 m. Three vacuum lysimeters were installed at depths of 1, 2 and 3 m in each of the plots to sample soil water in the vadose zone. Nitrate concentration measurements were collected from the wells and lysimeters approximately monthly. This chapter describes the multiport well and lysimeter monitoring systems and presents an analysis of well and lysimeter nitrate data that were obtained over a period of 6 years. Nitrate concentration data found in samples taken from the multiport wells were not found to be related to agricultural management system differences over the period of investigation. The 3 m gravel layer serves to inhibit unsaturated flow to lower layers and the aquifer.