Water availability is the amount of water that can be appropriated from a given point on a given stream for new out-of-stream consumptive uses. It is obtained from the natural stream flow by subtracting existing in-stream flow requirements and out-of-stream consumptive uses. For a detailed description of the Water Availability Report program and the methodology used to develop it, you may review the report titled Determining Water Availability in Oregon.

Ideally a water availability calculation would be done for every watershed associated with a point of diversion or an in-stream water right. A watershed, in this case, includes all lands draining to the stream upstream of the point of diversion or the downstream end of an in-stream water right reach. Because there are so many water rights, the ideal approach is impractical.

The practical alternative is to limit the number of watersheds for which water availability is calculated. The delineation of these watersheds depends on the location of in-stream demands and on the physiography of affected streams. Generally watersheds are defined above the mouths of significant tributaries, on main channels above significant tributaries and for all in-stream demands.

These delineated watersheds are referred to as Water Availability Basins (WABs). Water availability is estimated at the downstream end, or pour point, of each WAB. Large drainage areas( e.g. the Rogue and Umpqua basins) are broken into a number of WABs. The WABs are nested, each upstream WAB being included in a WAB downstream. For water to be available in a given WAB, it must be available in all the other watersheds in which it is nested.