Multivariate Analysis of the 2021 Boise Drought in the Context of Natural Human Systems
Authors: Martinez-Osario Jesus 
Published: December 2022

Droughts generally refer to lack of sufficient water to supply specific needs, and has several categories including meteorological, hydrologic, agricultural and socioeconomic droughts [22]. Drought is triggered by the lack of or reduced precipitation, but other factors including low soil moisture, groundwater depletion, insufficient snowpack, reduced surface storage, increased evaporation, and contaminated surface water also contribute to various drought categories [12, 27].

Droughts impact many functional aspects of a community including agricultural production, recreation, access to clean drinking water, and the health
of local ecosystems. Arid and semi-arid regions such as Idaho are specifically vulnerable to drought [12]. According to the Idaho State Department of
Agriculture, agriculture makes up approximately 18% of the state’s total economic output, and hence drought is a major concern in Idaho. As of October
12th, 2021, > 90% of Idaho was in a severe, extreme, or exceptional drought according to the U.S. Drought Monitor [32]. This drought was specifically
impactful for Idahoans, since a reasonable amount of snowpack and dam storage in the spring convinced local farmers to fully cultivate their farms and
they struggled to irrigate their crops later in the season.

I focused this research on a multivariate analysis of the 2021 Boise drought in the context of natural-built systems. I considered two natural storages: (1) snowpack and (2) atmospheric storage, i.e., spring precipitation, as well as the built storage facilities like reservoirs, specifically the three dams on tributaries flowing into the Boise River watershed. I obtained historical (1982-2021) data for the Boise River watershed in terms of snow water equivalent and total dam storage on April 1st for each of the years studied, as well as spring precipitation, which collectively supports irrigation of agriculture in the Treasure Valley [4, 6]. Both univariate and multivariate frequency analyses were conducted to obtain a nuanced understanding of the drivers of the 2021 Boise drought. This provides important insights for the future conditions of drought initiation and evolution in the region in a warming climate.

By utilizing univariate analysis, I noted that snow water equivalent, or SWE, stayed within the range of historical average at more than 30 percent occurrence probability. This gave way to a somewhat “normal” snowpack for the year of 2021. Similarly, cumulative dam storage for Arrow Rock Dam, Anderson Ranch Dam, and Lucky Peak Dam had a combined occurrence probability of 60 percent for that year, meaning it was more than 60% of years in the observation record. Spring precipitation, however, was strikingly low in the Boise River watershed. In fact, it was close to the record low (ranked second lowest in the period of observation), with less than a 10 percent occurrence probability for the area. Multivariate analysis revealed new information about combined effects of natural and built storages that collectively supply water to stakeholders in the Boise River basin. In terms of joint distribution of SWE-dam storage puts 2021 in the lower end of the bivariate distribution, but it was exceeded by 5 other years. In terms of natural storage (SWE-spring precipitation), 2021 claimed the worst rank on record. Combining precipitation and dam storage, 2021 was ranked second worst year (after 2002) in the bivariate distribution. In a trivariate analysis and in a strict AND scenario, 2021 was ranked the worst drought year on record when a combined effect of natural and built storage are considered. This means that there was no year on record with a worse condition in terms of ALL of the three water storages considered herein. My analysis revealed that while natural storages (snow and particularly spring precipitation) were low in 2021, built storage buffered some of the impacts of drought in the Treasure Valley, Idaho. Further analysis using climate projections showed that while spring precipitation in a moderate emission scenario may marginally increase, this is expected to reduce in a high emission scenario. This means that under a high emission scenario, droughts similar to that of 2021 may increase in Treasure Valley, combined with increasing evaporative demand as temperature increases, are expected to occur more frequently and can result in adverse societal impacts. Finding root causes of severe water shortages is crucial for the further understanding of droughts in the Treasure Valley area. Policymakers can use this information to implement detailed plans for the community and to avoid cases like the sudden, dangerously low water levels experienced in 2021 and plan more effectively for the future.