Dawson, D., VanLandeghem, M.M., Asquith, W.H., and Patiño, R. 2015. Long-term trends in reservoir water quality and quantity in two major river basins of the southern Great Plains, USA. Lake and Reservoir Management 31: 254-279.
Temporal and geographic patterns in water quality and storage were assessed for 11 major reservoirs from two river basins in Texas, Brazos River and Colorado River using a long-term dataset [maximum period of record (POR), 1965–2010]. Water quality data were simplified for analysis by spatially pooling within reservoirs while maintaining their depth structure and then estimating depth-corrected (1 meter) monthly values by LOWESS smoothing. Reservoir inflow and precipitation data were collected from available gages and weather stations. Trend analyses (seasonal or standard Kendall Tau as appropriate) were applied to each water-quality variable with sufficient data for analysis. Principal component analyses (PCA) were conducted to examine patterns in reservoir water quality data within and across basins. Lastly, quantile regression was used to assess temporal changes in reservoir inflow and watershed precipitation variability. Trend analyses indicated that storage decreased whereas total phosphorus increased in most reservoirs. Overall, warmest or coldest temperature increased in seven reservoirs, decreased in one reservoir, and did not change in three reservoirs. The most common salinity trend observed was one of no change and when change occurred, its direction was inconsistent among reservoirs. As represented by specific conductance, chloride and sulfate, salinity dominated the variance structure of water quality data in PCA and inspection of PC biplots indicated that this variance can be attributed to the presence of high- and low-salinity reservoirs in both basins. Median and extreme (90 and 10 % quantiles) values of inflow decreased for most reservoirs and were more pronounced in the Colorado than the Brazos basin, but no trends in precipitation were detected over the study POR. Overall, the present results describe a landscape in the Brazos and Colorado River basins where reservoir inflow and storage have generally decreased, eutrophication has generally increased and, in most reservoirs, temperature has increased within the last several decades. Other chemical traits of reservoir water, such as salinity, have remained relatively stable.