Use of the chloride ion in determining hydrologic-basin water budgets: a 3-yr case study in the San Juan Mountains, Colorado, U.S.A

Measurement of chloride concentration and water equivalent in precipitation and recharge at a site can be extrapolated to determine available moisture in a nearby basin. This method also may be extrapolated to a basin with similar climatic characteristics if precipitation, vegetation, and topographic data are available. The average accuracy of the total of evaporation, recharge, and runoff (assuming no storage) was similar to 10% of total precipitation. Soil moisture measurements indicate that the entire 10% error in moisture balance can be attributed to annual changes in storage. Data requirements for the method are considerably less than data requirements for energy budget methods to determine available moisture. Potential applications of the method to hydrologic problem solving are: estimating total available moisture from chloride concentrations in groundwater or surface water or both; modelling paleoclimate scenarios and evaluating their correctness by comparison with paleogroundwater chloride concentrations; and providing an independent comparison for water budgets obtained by energy budget methods. Obviously, the method cannot readily be applied to systems with a lithologic source of chloride. Most systems consisting primarily of tuff, intrusive volcanic rock, nonmarine sediments, quartzite, and other metamorphic rocks will be suitable for application of the model.