Water is a vital and scarce resource in the southwestern U.S. The sustainability of water resources depends on the health of high-elevation forests that are the source of most water in southwestern streams, rivers, and aquifers (Barr et al. 1956; Flerchinger and Cooley 2000; Scanlon et al. 2006). Forests support water supplies for cities and towns, irrigated agriculture, aquatic and riparian ecosystems, recreational opportunities, and sites of historical and cultural significance.
Fire suppression, logging, and grazing beginning in the late 19th century have led to a decline in the health of southwestern forests (Covington et al. 1997). Before Euro-American settlement, frequent, low-intensity fires maintained a sparse distribution of trees in ponderosa pine and mixed-conifer forests. The exclusion of fire led to substantial increases in tree density and basal area and an increased risk of high- intensity, stand replacing wildfires (Fulé et al. 1997; Fulé et al. 2003). Forest restoration seeks to return forests to a more natural condition by thinning trees to reduce density and conducting prescribed burns in the understory or managing natural or human-caused fires to restore a low-intensity fire regime (Covington et al. 1997). The past and present structure and restoration of southwestern forests is described in detail by Friederici (2003) and in Ecological Restoration Institute working papers no. 22 for ponderosa pine and no. 28 for mixed conifer.
The immediate goal of forest restoration is to reduce wildfire risk, but improved watershed health and function is often a secondary goal. The pace and extent of forest restoration has increased since Congress established the Collaborative Forest Landscape Restoration Program in 2009, which provides funding to science-based ecosystem restoration projects. A number of forest restoration projects to protect municipal water supplies are planned or underway in fire-adapted forests in the Southwest, including the Flagstaff Watershed Protection Project, the Upper South Platte Watershed Protection and Restoration Project near Denver, and the Santa Fe Municipal Watershed Project. Forest restoration in the Southwest will expand to the landscape scale in the coming decades. The Four Forest Restoration Initiative (4FRI) plans to conduct thinning and prescribed burning on 2.4 million acres of forest in Arizona (USDA 2013). Large-scale restoration is also being planned in New Mexico through the Rio Grande Water Fund (RGWF 2014). With the increase in scale of forest restoration, it is possible that restoration treatments will affect major river basins and regional aquifers.
This working paper summarizes research relevant to understanding the effect of restoration treatments on the hydrologic cycle of southwestern forests. An overview of forest hydrology in the Southwest is presented, followed by discussions of forest restoration and wildfire effects on water quantity, water quality, and hydrologic function.