USING GEOSTATISTICS TO SELECT GRID-CELL LAYOUTS FOR THE AGNPS MODEL

Currently, little guidance is available for the design of accurate gri d-cell layouts for spatially distributed models like the Agricultural Nonpoint Source Pollution (AGNPS) model. Inaccuracies in the grid-cell input could negate the advantages provided by the spatially distribut ed approach to watershed (field) representation. The objective of this study was to develop procedures for designing grid-cell layouts using geostatistical methods. Geostatistical spatial analysis tools were us ed to select a base cell size and locate cell sub-divisions for use in AGNPS. The overall spatial correlation structure of SCS Curve Number data was used to select the base cell size. The specific criterion use d to select the base cell size was the average squared residual of kri ged estimates for randomly selected locations within the study area. T he cell size with the minimum average squared residual was selected as the base cell size. Next, cells that wan-anted sub-divisions were ide ntified using the squared residual of kriged estimates for each cell i n the base cell layout. The grid-cell layout resulting from the applic ation of the geostatistical methods and the squared residual criteria had an average squared residual less than the grid-cell representation without sub-divisions. Furthermore, the cells that were sub-divided f ell within the most heterogeneous areas of the watershed. The geostati stical methods provided quantitative information about the error assoc iated with various grid-cell designs. This information was then used t o design accurate grid-cell layouts that preserved the spatial heterog eneity of the original data. Further development and application of th e geostatistical methods used to design grid-cell layouts will lead to greater improvements in the simulation accuracy of spatially distribu ted models including AGNPS.