A FRAMEWORK FOR ENVIRONMENTAL WATER REQUIREMENTS FOR GROUNDWATER DEPENDENT ECOSYSTEMS

In many parts of the US, the Forest Service is facing increasing requests to access water off of National Forest System lands for human uses, such as irrigation and public water supply. Inevitably, these requests put the Forest Service in the position of having to evaluate whether water can be removed from a particular hydrologic system while maintaining the viability of dependent ecosystems, and how much can be removed over what periods of time. The “Framework for the Determination of Environmental Water Requirements for Ground Water Dependent Ecosystems” addresses water requirements for springs, groundwater discharge wetlands, and areas of terrestrial vegetation that are supported by a shallow water table (phreatophytes). Strongly groundwater dependent systems occupy only a small percentage (1% or 2%) of the land area. However, they are highly significant from a conservation perspective because in many landscapes biodiversity is focused on these areas of stable, generally high-quality water.

Environmental Water Requirements (EWRs) define the water regime necessary to protect biologically relevant goals, criteria, and indicators that prevent significant harm to groundwater-dependent aquatic and wetland habitats while at the same time considering human livelihoods and well-being. Much research has gone toward deriving EWRs for rivers. Few studies, however, have addressed water requirements for GDEs. The Framework is designed to answer the following key questions:

· Which populations or species of an ecosystem are groundwater-dependent?

· If some populations or species of an ecosystem are groundwater-dependent, what is the degree of dependency?

· What is the nature of the aquifer(s) supplying water to the system?

· What attributes of groundwater (level, flux, quality) are important to sustain the system?

· What are the safe limits to changes in the attributes of groundwater?

A step-wise approach is followed that includes 1) a threat analysis and definition of management objectives, 2) understanding the biophysical setting and the form and degree of dependence on groundwater, 3) developing an environmental response function, and 4) setting an EWR with monitoring and evaluation criteria.