APPLICATION OF THE GEOGRAPHICAL INFORMATION-SYSTEMS APPROACH TO WATERSHED MASS-BALANCE STUDIES

This study was undertaken to test the utility of a geographical inform ation systems (GIS) approach to problems of watershed mass balance. Th is approach proved most useful in exploring the effects that watershed scale, lithology and land use have on chemical weathering rates, and in assessing whether mass balance calculations could be applied to lar ge multilithological watersheds. Water quality data from 52 stations w ere retrieved from STORET and a complete GIS database consisting of th e watershed divide, lithology and land use was compiled for each stati on. Water quality data were also obtained from 7 experimental watershe ds to develop a methodology to estimate annual fluxes from incomplete data sets. The methodology consists of preparing a composite of daily flux data, calculating a best fit sinusoid and integrating the equatio n to obtain an annual flux. Comparison with annual fluxes calculated f rom high resolution data sets suggests that this method predicts fluxe s within about 10% of the true annual flux. Annual magnesium fluxes (m oles km(-2) yr(-1)) were calculated for all stations and adjusted for fluxes from atmospheric deposition. Magnesium flux was found to be a s trong function of the amount of carbonate in the watershed, and silica fluxes were found to increase with the fraction of sandstone present in the watershed. All fluxes were strongly influenced by mining practi ces, with magnesium fluxes from affected watersheds being 6-10 times h igher than fluxes from comparable pristine watersheds. Mining practice s enhance chemical weathering by increasing the surface area of unweat hered rock to which water has access and by increasing acidity and rat e of mineral weathering. Fluxes were also found to increase with water shed size. This scale dependence is most likely caused by the sensitiv ity of weathering fluxes to even minor quantities of carbonates, which are likely to be found in all lithologies at larger scales. Mass bala nces were carried out in watersheds where gauged sub-watersheds made u p more than 95% of the area. The calculations show large magnesium flu x and water balance discrepancies. These errors may be a result of sig nificant groundwater inputs to streams between gauges. The results sug gest that improvements in how we measure discharge and estimate fluxes may be required before we can apply mass balance techniques to larger scales. (C) 1997 by John Wiley & Sons, Ltd.