2016 Salt Lake County Watershed Symposium

Water Losses in SLC During Warm, Dry Years!
Water is a critical resource for human development, economic well-being, and sustainability. Growing population and expanding agricultural activities have made water extraction for anthropogenic use a major flux in the hydrological cycle. In order to successfully meet the rising demands, water managers have resorted to over-exploitation of regional surface water resources, large scale inter-basin transfer, and extraction from subsurface aquifers making sustainable water management practices a major challenge, thus endangering water availability for our future generations.The severity of these effects, and the need to better understand connections between climate, water extraction, water use, and water use impacts, is strongest in areas of climatic aridity and substantial land-use change, such as the rapidly urbanizing areas of Utah. To understand these connections, we collected and analyzed stable isotopic ratios of more than 800 urban tap water samples in a series of semiannual water surveys (spring and fall, 2013 to 2015) across the Salt Lake Valley (SLV) of northern Utah. We observed strong and structured spatiotemporal variation in tap water isotopic compositions across the region which we attribute to complex distribution systems, varying water management practices and multiple sources used across the valley.

Isotopic mass balance indicated significant inter- and intra-annual variability in water losses within the distribution network due to evaporation from surface water resources supplying the SLV. Our calculation suggests that evaporative losses from the SLV water supply increased from 1 - 1.5% of the total water flux in 2013 to 4 - 6% in 2015. Using yearly water consumption data for the SLV obtained from the Utah Division of Water Rights, these values translate into > 4 million gallons of evaporative loss per day in 2015. This enhanced loss to the atmosphere would equate to $2.25 million of revenue loss in 2015 (calculated at current rates of $1.16 per unit within Salt Lake City; 1 unit = 748 gallons) if translated into reduced extractions, or significant ecological impacts if extraction remained unchanged.

Our isotopic assessment highlights aspects of the municipal water systems supplying the SLV that are germane to planning for and understanding these future water resource challenges. Our calculations show significant increase in evaporative losses within the system over the three-year sampling period, likely attributable to the atypically warm and dry weather that persisted throughout the study. Given that majority of municipal water used within the SLV is currently sourced from surface water and the proposed development of water resources to satisfy future demands associated with population growth focuses primarily on surface water systems of the Central Utah Project and Bear River, SLV communities are and will continue to have strong exposure to changes in evaporative losses from these systems.