Quantifying Water Supply and Demand Across National Forest System Lands Within the Clark Fork River Watershed, Montana

Abstract

Water supply and demand across the western United States has been a source of contention since westward expansion. As climate change become more prevalent, the timing and quantity of water supply is changing and the demand for water is increasing. In the western US, water supply is divided among water users using the prior appropriation doctrine of “first in time, first in right” meaning those that used the water first historically have the priority to use the water when supply is low. Many headwater streams that supply water to downstream users originate on National Forest System lands. These small streams are critical habitat for many aquatic species and knowing how water in these headwater areas is allocated compared to water supply is important for land managers. Therefore, my study quantified water supply and demand on National Forest System lands using water rights spatial data and modeled historical streamflow rates. I developed a “Water Rights Selection Tool” to determine (1) points of diversion described on National Forest System lands within each state designated water Adjudication basin, (2) total allocated flow of those water rights based on watershed boundaries at the hydrologic unit code 10 level, and (3) total water supply on National Forest System lands using modeled flow metrics data. The output from the tool is a combined spatial data set that includes total allocated flow and total model estimated flow in each watershed. The percent allocated of each watershed was calculated from that data. The Clark Fork Study Area was chosen as the pilot location. It consisted of 30 watersheds within three Adjudication basins that make up the mainstem of the Clark Fork River in western Montana. I found that seven of 30 watersheds within the Clark Fork Study area were over-allocated on National Forest System lands meaning that more water was allocated to beneficial uses than estimated water supply available within that watershed. The Little Thompson River watershed had the highest level of over-allocation at 638%. Although the tool was successful at determining points of diversion, total allocated flow, and total water supply, several improvements need to be made before using it for wider replication across National Forest System lands.