Estimates of Evapotranspiration or Effective Moisture in Rocky Mountain Watersheds from Chloride Ion Concentrations in Stream Baseflow

The principle that atmospherically derived chloride is a conservative tracer in many watersheds can be used to calculate average annual evapotranspiration or effective moisture if estimates are available for (1) the average annual chloride input to the watershed, (2) the average annual precipitation, and (3) the baseflow chloride concentration are known. The method assumes that no long‐term storage of chloride occurs and there is no lithologic source of chloride, or that such source releases only insignificant amounts to groundwater compared to the atmospheric source. National Atmospheric Deposition Program estimates of chloride wet deposition, watershed precipitation records or hyetal map estimates of precipitation input to watersheds, and a single sample of chloride concentration in base flow were used to calculate evapotranspiration for diverse Rocky Mountain watersheds. This estimate was compared to evapotranspiration determined by subtracting mean discharge from precipitation. Of the 19 watersheds used to test the method, 13 agreed within 10%, 2 appear to have not met the lithology criterion, 1 appears to have not met the flow criterion, and 1 neither criterion. The method's greatest strength is the minimal data requirements and its greatest weakness is that for some watersheds it may be difficult to obtain reliable estimates of precipitation and chloride deposition. If reliable discharge data are available, the method may be used to estimate watershed‐average precipitation; this is especially useful in high‐altitude mountain watersheds where little or no precipitation data are available.