Water Resources Research Act Program

Principal Investigators: Anna Crockett

Abstract: Water is a fundamental necessity for sustained life and is therefore a critical component of both social andecological systems. The uncertainty of climate change, through changes in water availability, compounds thestress on these systems, particularly in the arid Western United States. In the West, climate change and watersupply are inextricably linked through snowpack. Snowpack serves as a reservoir, storing water and providing itto downstream users, particularly in mountainous regions such as Western Montana. A warming climate,particularly in the winter and spring has been linked to declines in spring snowpack in much of the mountain West(Mote et al., 2005). Timing of snow-derived runoff is predicted to change significantly due to reduced springsnowpack and early onset melt, resulting in a decrease in runoff, less late season flows, and less water availablefor the dry season (Li et al., 2017). It is hypothesized that longer growing seasons and increased potentialevapotranspiration due to higher atmospheric water demands will also contribute to streamflow declines(Abatzoglou et al., 2014).Although climate change on its own is one driver of changing patterns of water availability, governanceof water use is another major factor that will determine the fate of future water supplies. Climate change impactsare already detectable in the West (Barnett et al., 2008), however, linking climate change with the legal system forallocating water in western states has been technically difficult, creating challenges for future water allocationneeds and current planning (Tarlock, 2018). Administrative uncertainty adds another layer of complexity. As anexample, in Montana the negotiation and settlement of tribal water compacts such as the Confederated Salish andKootenai Tribes (CSKT) water compact can result in large tribal water rights that are senior to previously senior,non-tribal irrigation rights, changing the priority of water users and potentially altering local hydrology givenchanges in priority use.Contests between competing water uses for agricultural, urban, and ecological outcomes will likelyincrease as water sources become less dependable and Western populations grow. In particular, climate changewill undoubtedly add to the competition between consumptive and non-consumptive uses. Agriculture accountsfor the overwhelming majority of surface water rights in Montana at 96% of total withdrawals (USGS, 2015). Achanging climate may render this current balance of water use untenable. Given a shrinking water supply, thislevel of agricultural water use will disadvantage ecological water needs, including water available for fish andriparian habitat. Water rights, no matter where they are geographically, are inherently uncertain as climate changewill have impacts on the availability and timing of surface water flows and groundwater recharge. Priorappropriation law, the idea that water rights are ‘first in time, first in right,’ has not explicitly faced theseuncertainties in its century and a half history in the American West; however, climate change and a growingWestern population has and will continue to expand demand on changing distribution and timing of watersupplies. It is imperative that the administration of prior appropriation water rights be rigorously assessed inconjunction with potential climate change impacts to water resources to increase our understanding of how watermay need to be reallocated, and to plan for how best to administer water law in the future.My thesis research seeks to address the uncertainty of climate change’s impacts on water supply as itrelates to the administration of prior appropriation water rights. Specifically, I propose to conduct a case study ofinstream flow policies in Montana. I will use this case study to determine the spatial and temporal strain onCrockett Page 2instream flows given current conditions and future climate predictions. I hope to explore whether instream flowpolicies, either in their current or modified form, could serve to balance water use between agricultural demandfor water and aquatic species/habitat water needs given uncertainty of future availability and timing of waterresources in Montana. I will approach this problem through an analysis of irrigation and instream flow waterrights using a spatially explicit dataset of Montana water rights integrated into a hydro-economic model.