GIS-BASED SOLUTE TRANSPORT MODELING APPLICATIONS - SCALE EFFECTS OF SOIL AND CLIMATE DATA INPUT

The Weather Generator (WGEN) and Chemical Movement through Layered Soi ls (CMLS) computer models were modified and combined with two sets of soil and climate inputs to evaluate the impact of input data map resol ution on model predictions, The basic soil and climate inputs required by WGEN and CMLS were acquired from either: (i) the USDA-NRCS State S oil Geographic Database (STATSGO) database; (ii) the USDA-NRCS (County ) Soil Survey Geographic (SSURGO) database; (iii) the Montana Agricult ural Potential System (MAPS) database (which divides Montana into appr oximately 18 000 twenty square kilometer cells and stores more than 20 0 different land and climate characteristics for each of these cells); and (iv) a series of fine-scale monthly climate surfaces developed by the authors (0.55 km(2) cell size) using thin-plate splines, publishe d climate station records, and USGS Digital Elevation Models (DEMs). F ifteen years of daily precipitation and evapotranspiration (ET) values were generated and combined with soil and pesticide inputs in CMLS to estimate the depth of picloram (4-amino-3,5,6-trichloro-2-pyridinecar boxylic acid) movement at the end of the growing season for every uniq ue combination (polygon) of soil and climate in a 320 km(2) area in Te ton County, Montana, Results indicate that: (i) the mean depths of pic loram movement predicted for the study area with the SSURGO (county) s oils and MAPS (coarse-scale) climate information and the two model run s using the fine-scale climate data were significantly different from the values predicted with the STATSGO (state) soils and MAPS climate d ata (based on a new variable containing the differences between the de pths of leaching predicted with the different input data by soil/clima te map unit and testing whether the mean difference was significantly different from zero at the 0.01 significance level); and (ii) CMLS ide ntified numerous (small) areas where the mean center of the picloram s olute front was likely to leach beyond the root zone when the county s oils information was used, This last measure may help to identify area s where potential chemical applications are likely to contaminate grou ndwaters.