STATISTICAL LOW-FLOW ESTIMATION USING GIS ANALYSIS IN HUMID MONTANE REGIONS IN PUERTO-RICO

Statistical analysis of watershed parameters derived using a Geographi cal Information system (GIS) was done to develop equations for estimat ing the 7d-10yr, 30d-10yr, and 7d-2yr low flow for watersheds in humid montane regions of Puerto Rico. Digital elevation models and land use , geology soils, and stream network coverages were used to evaluate 21 geomorphic, 10 stream channel, 9 relief, 7 geology, 4 climate, and 2 soil parameters for each watershed. To assess which parameters should be used for further investigation, a correlation analysis was used to determine the independence and collinearity among these parameters and their relationship with low flows. Multiple regression analyses using the selected parameters were then performed to develop the statistica l models of low flows. The final models were selected in the basis of the Mallow Cp statistic, the adjusted R(2), the Press statistic, the d egree of collinearity, and an analysis of the residuals. In the final models, drainage density, the ratio of length of tributaries to the le ngth of the main channel, the percent of drainage area with northeast aspect, and the average weighted slope of the drainage were the most s ignificant parameters. The final models had adjusted standard errors o f 58.7 percent, 59.2 percent, and 48.6 percent for the 7d-10yr 30d-10y r, and 7d-2yr low flows respectively. For comparison, the best model b ased on watershed parameters that can be easily measured without a GIS had an adjusted standard error of 82.8 percent.