Slide 6 of 7
Assuming a table of transform coefficients exists which describes the land-water interaction, their application in a new region requires spatial description of the region by means of paper maps or data layers or coverages in a GIS. All the available water quantity and quality data are also assembled. The results of applying the transform coefficients to the input variables are spatially added up or accumulated so that at each point on the water system, the water characteristics resulting from drainage from all upstream areas are known. Then the calculated values are compared to the observed data and adjustments made to the transform coefficients if necessary. For example, if the computed water yield at a given streamflow gaging station does not match the observed yield, an adjustment factor can be applied to the runoff coefficient in the upstream drainage area, so that the computed and observed values agree. Once the analysis is locally calibrated to observed data, the effects of land management changes can be evaluated, usually by looking at new land use maps, by placing structures in the landscape, or by applying regulatory or educational programs.