Estimating Hydrologic and Ecological Responses to Increased Climate Variability using Water Year Type Classification

Abstract:

Water management frameworks that were designed assuming stationary climate conditions will be increasingly difficult to implement in non-stationary climates, presenting a barrier to climate change adaptation and efficient water management for people and ecosystems. Hydrologic indices, or water year classification systems as they are also called, categorize streamflow by year type, such as wet, dry, or normal, compared to historical averages. Numerical thresholds separate water year types, often set by weighted winter and spring runoff volume for rivers. Year type classification is tied to water resources planning, helping to answer the question of whether there is ‘enough’ water, and water allocations to competing water uses are typically adjusted based on water year type. Hydrologic indices were developed for Utah’s Bear, Weber, and Jordan-Provo rivers using long-term reconstructed streamflow from tree-ring data (~1500 to present). Tree-ring data have considerably more variability than measured streamflow from the 20th Century, indicating droughts and wet periods could be longer, more intense, and of greater magnitude than more recent records suggest. Tree-ring reconstructed streamflow is also a promising surrogate to represent increasing future hydroclimatic variability. The Bear, Weber, and Jordan-Provo rivers make up approximately 95% of the streamflow to the Great Salt Lake and supply water to the Salt Lake City, Ogden, and Logan metropolitan areas. The frequency of water year types changes with increased hydroclimatic variability represented in tree-ring reconstructed streamflow when historical year type thresholds or historical year type distributions are maintained. In turn, this affects allocations to urban, agricultural, and environmental water uses, and highlights potential adaptation strategies for watershed-scale water resources management with increased climate variability from climate change.