Streamflow responses to vegetation manipulations along a gradient of precipitation in the Colorado River Basin

The Colorado River Basin has been, and continues to be, the focus of a wide diversity of research efforts to learn more about the effects of natural and human-induced disturbances on the processes and functioning of the basin's upland watersheds. These watersheds are situated at the headwaters of streams and rivers that supply much of the water to downstream users in the western United States. Responses of streamflow to vegetation manipulations have been, and are, one of the research foci in this water-deficient part of the country. The watershed-scale research, led by the U.S. Forest Service and its cooperators, has spanned nearly a century and included an array of vegetation types along a wide range of precipitation gradients. Results from this research have shown that vegetation can be managed to enhance annual water yields while still providing the other natural resource benefits. Analyses of the research results suggest that the effect of vegetation manipulation on streamflow is associated with precipitation-elevational gradient and, therefore, vegetation type. An annual water yield increase between 25 and 100 mm could be achieved by implementing vegetation manipulations in the high elevation subalpine and mixed conifer forests, the ponderosa pine forests (in the Lower Basin), and portions of the low elevation chaparral shrublands. Negligible effects or small increases in water yield were observed for treating sagebrush, pinyon-juniper woodland and desert scrubs. Results from this research have improved our understanding of the basin's hydrology and provided much needed insights to manage forest to mitigate global climate change induced hydrologic impact and meet the increased needs of people living in the basin.