A computer model using the alternating direction implicit (ADI) finite difference method to study two‐dimensional coupled soil heat and water flow with a partial surface mulch cover is developed. A new, simplified computational procedure, which has only tridiagonal matrix problems, for the ADI method is introduced. The model uses a soil surface energy balance equation to determine soil surface boundary conditions for both heat and water flow. The inputs required for the computer simulations are weather data, soil thermal and hydraulic properties, and mulch data. Numerical experiments are performed to examine the effects of soil type, mulch width, and weather conditions on soil heat and water movement. For continuous evaporation and drainage, 10‐day simulations were performed for each combination of clay, loam, and sand soil and fractions of mulch cover of 0, 0.5, 0.8, and 1.0 of the row interval width. For repetitive evaporation and infiltration, 15‐day simulations were performed. The mulch cover greatly reduces evaporation loss and the amplitude of daily soil temperature, water content, and pressure head variations. Large spatial variations in temperature and soil water content are predicted near the interface of mulch and bare soil surface. The soil hydraulic properties have important roles in controlling soil surface water content. The present model reasonably describes the soil thermal and hydrologic environments and thus can be applied successfully in soil science and groundwater hydrology and can be extended to related disciplines.

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BSTRACT: A~unsteady~wo-dimens~onal (20) reservoir hydrodynamics and transport model is employed to slm~late contammate~denSity cu~ents m the Shasta Reservoir after a chemical spill into the Sacramento River, Cahf. Three flow regimes (plungmg flow, underflow, and interflow) and their occurrence are captured by the laterally~verag~d model. Transp~~and mixing processes in the temperature-stratified reservoir are analyzed through simulatIOns of flow velOCities, water temperature, and contaminant concentration. Flow behavior of the contaminan.t p~ume is desc?bed b~plun~e distance, separation depth, intruding thickness, and the spatial and tempor~l dtlutlon of chemicals. Simulation results are compared with field data for water temperature and contammant concentration collected in the reservoir during the emergency response to the spill. Relatively good ag~eement between fiel~measurements and predicted reservoir stratification and chemical dilution is obtained. It IS show~that the aeration system installed in the reservoir contributed to the downstream reduction of chemical conc~ntration to a. n~ndetectable level shortly after the spill. The 2D simulations and analyses improve understandmg and predictions of the movement of a conservative contaminant plume in a stratified reservoir. The res~lts can a~sist in cont~mi~ation c~ntrol and remediation after a toxic chemical spill, guide field sampling dunng the spill, and provide mformation useful for water quality management.

The Erosion Productivity Impact Calculator (EPIC) model is validated using long-term data collected for two southwest Iowa watersheds that have been cropped in continuous corn under two different tillage systems. The annual hydrologic balance was calibrated during 1988-94 by adjusting the runoff curve numbers and residue effects on soil evaporation. Model validation was performed for 1976-87 using both summary statistics and parametric and nonparametric statistical tests. Overall, results show that EPIC was able to replicate the long-term relative differences between the two tillage systems. ABSTRACT

The Erosion Productivity Impact Calculator (EPIC) model was validated using long-

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