Determining Ecological Baseflow Needs for Streams and Springs In Arid and Semi-Arid Regions

International efforts have been conducted to describe the ecological flow needs for regulated rivers. Water needs for ecosystems are greater than just consumptive use by riparian and aquatic vegetation, including the magnitude, frequency, duration and timing of flows and the depth and annual fluctuations of groundwater levels of baseflow supported streams. This approach was adapted and applied to an unregulated, baseflow dependent river in the arid to semi-arid Southwestern U.S. and was adapted to determine groundwater sources potentially at risk from climate, land management, or groundwater use changes. In June 2007 and April 2008, workshops with ecological, cultural, and physical experts from agencies, universities, tribes, and other organizations were convened. Flow-ecology response functions were developed with either conceptual or actual information for a baseflow dependent river, and scoring systems were developed to assign values to categories of risks to water sources. A reduction of baseflow to the river was predicted to lead to a decline in cottonwood and will tree abundance, decreases in riparian forest diversity, and increases in non-native tree species, such as tamarisk. These types of forest vegetation changes would likely cause reductions or loss of some bird species. Loss of riffle habitat through declines in groundwater discharge and the associated river levels would likely lead to declines in native fish and amphibian species. A research agenda was developed to develop techniques to monitor, assess and hopefully better manage the aquifers supporting the baseflow dependent river to prevent potential threshold responses of the ecosystems. The scoring system for categories of risk will allow for water managers to assess and prioritize potential impacts to the biological, historical, or cultural aspects of springs from groundwater abstraction. These approaches can be adapted to other baseflow dependent rivers or to assess risks to natural features associated with water sources in other regions.