Estimation of natural dissolved-solids discharge in the Upper Colorado River basin, Western United States

A statistical method was developed to estimate monthly natural dissolved-solids discharge at selected sites in the Upper Colorado River Basin. Natural dissolved-solids discharge was defined as the rate of inorganic-solute flow past a specific site that would have occurred if there had been no water-resources development in the basin upstream from the site. The method used weighted least-squares regression to fit a model of dissolved-solids discharge as a function of streamflow and several variables representing development. After the model had been calibrated for an individual site, the development variables were assigned a value of zero to yield the relation between dissolved-solids discharge and streamflow for conditions of no upstream development. Natural dissolved-solids discharge was calculated using this relation and estimates of natural streamflow provided by the U.S. Bureau of Reclamation. Limitations of the method included lack of data to verify estimates of the natural dissolved-solids discharge and to represent all the effects of development adequately. However, model statistics indicated a good fit to historical data. Also, the estimates of natural dissolved-solids-discharge had the expected monthly distribution and were consistent between upstream and downstream sites. (USGS)