The Utah Division of Water Resources released draft appendices for its 2026 State Water Plan on July 20, providing a statistical analysis of water budget trends across the state. The report details decades of data regarding precipitation, evapotranspiration, and water depletions, specifically focusing on large basins including the Utah portions of the Great Salt Lake Basin.
Analyzing Natural System Evapotranspiration Trends
In the draft 2026 Appendices, the Division of Water Resources conducted a statistical trend analysis to evaluate the direction and strength of several hydrologic factors over the past 20 to 30 years. The division assessed evapotranspiration for natural systems throughout Utah using satellite data from the Moderate Resolution Imaging Spectroradiometer (MODIS) dataset. This assessment utilized the MODIS Terra Net Evapotranspiration Gap-Filled 8-Day 500-meter product, covering the entire state from 2001 to 2024.
To ensure accuracy in natural system estimates, the division applied a mask that excluded urban lands, built-up areas, water bodies, and permanent snow or ice. This process allowed for an estimate of evapotranspiration derived solely from natural system areas. While the analysis showed a Sen slope fit for evapotranspiration, the division noted that the trend was not statistically significant, reporting a p-value of 0.06.
Precipitation and Basin Data Metrics
The Division of Water Resources also evaluated annual precipitation using Daymet data for the years 1989 to 2023, supplemented by North American Land Data Assimilation System (NLDAS) data for 2024. These datasets served as primary input variables for the division's water budget model. Precipitation figures were reported by water year, which runs from October to September, and grouped into categories including the state boundaries, 11 hydrologic subbasins, and large basins such as the Colorado River, Sevier Lake, and the Utah portions of the Great Salt Lake.
According to the draft report, none of the precipitation trends analyzed across these various regions or basins reached statistical significance, with all reported p-values exceeding 0.05. The analysis utilized Kendall’s tau correlation coefficient to measure the strength of these relationships, classifying trends as strong, weak, or non-existent based on specific statistical thresholds.
Shifts in Water Reporting Methodology
The 2026 State Water Plan report highlighted critical changes in how municipal and industrial (M&I) water use is tracked. A significant shift occurred in 2015 when the Division of Water Rights enhanced M&I reporting and the Division of $&$ Water Resources updated its estimation methods for secondary water use. Because this change impacts long-term trend analysis, the division recommended against analyzing M&I diversions prior to 2015.
Instead, researchers focused on broad surface water and groundwater diversions, alongside M&I depletions, which are less susceptible to the 2015 reporting shifts. The division also noted a recent methodological update regarding outdoor M&I depletion rate assumptions, where modeled depletion was increased from 40% to 91%. The division clarified that this change does not indicate an actual increase in water use, but rather reflects a more accurate calculation method applied consistently across the historical record from 1989 to 2024.
Historical Variability of Great Salt Lake
The Great Salt Lake remains a central focus of Utah's hydrologic landscape, characterized by its extreme physical variability. According to the Utah Geological Survey, the lake is the largest in North America without an outlet to the ocean and is known for significant expansions and contractions. Historical records indicate that since lake elevations were first recorded, the lake has nearly doubled in depth and more than tripled in area at certain times.
This volatility extends to the lake's chemistry. The Utah Geological Survey reports that salinity levels change both seasonally and annually, with salt concentrations often reaching several times the level of the ocean. In certain areas, the lake can reach salt saturation. Human intervention, including the construction of dikes and causeways, has further fragmented the lake into a mosaic of different salinities, colors, and biological organisms.
Geological Legacy of Lake Bonneville
The geological history of the region is also tied to the much larger, ancient Lake Bonneville. The Utah Geological Survey provides information regarding the relationship between the modern Great Salt Lake and its predecessor, Lake Bonneville, noting that features such as mountain terraces serve as evidence of past lake elevations.
The survey's records indicate that the Great Salt Lake is a complex ecosystem involving various minerals, wildlife, and industrial uses. The presence of dikes and causeways has created distinct chemical fragments within the lake, influencing the types of organisms that can survive in different parts of the water body.
Statistical Foundations of Water Planning
As Utah continues to manage its water budget, the Division of Water Resources relies on robust statistical tools like Mann Kendall and Sen Slope tests to determine hydrologic trends. These tests, performed using R programming software, allow the division to assess whether changes in diversions and depletions are statistically significant, providing a foundation for the 2026 State Water Plan's approach to municipal and industrial supply modeling.
