2016 Salt Lake County Watershed Symposium

Quantifying Interactions of Climate and Landscape on Water Resources
Growing populations and a changing climate are creating an uncertain future for water resources in the Western United States, including Salt Lake City and all of Utah. Planning for future population and climate conditions requires quantifying differential sensitivity of hydrologic partitioning to changes in climate. To address this challenge we ask: How do landscape characteristics influence the partitioning of precipitation into streamflow and plant available water along the Wasatch Front? And can historical observations of climate and streamflow provide inferences to the differential sensitivity of local watersheds to changes in climate?

The seven watersheds along the Wasatch Front (City Creek, Red Butte, Emigration, Parley’s, Millcreek, Big Cottonwood, and Little Cottonwood) provide an excellent research area to answer these questions. These watersheds are not only important to the Salt Lake area, providing over half of its water supply, but also have over 100 years of climate and streamflow response data to study the how the water balance is influenced by landscape. Additionally, there are many landscape differences between the watersheds: ranging in size from 19 km2 to 127 km2, in mean elevation from 1960 m to 2610 m, in mean slope steepness from 20° to 27°, in predominant slope aspect either north-facing or south-facing, in the type of soils present, and in the predominant bedrock geology.

Mean annual precipitation (790 mm to 1290 mm) and temperature (3.3°C to 6.9°C) vary primarily as a function of catchment elevation. Between 1900 and 2014 the average annual temperature across all watersheds has increased by 0.91°C, with most of the change occurring during the last fifty years. During the same time there has been no significant change in the amount of annual precipitation. Mean annual streamflow, normalized by catchment area, ranges from 150 mm to 820 mm with annual precipitation explaining between 43%-72% of the annual variability in streamflow. Surprisingly, the remaining variability is not correlated to annual or seasonal temperature even though the catchments have experienced notable warming. Instead, inter-annual variability in streamflow and water yield is significantly related to the rate of snow melt and the amount of subsurface storage in the watershed (derived from winter baseflow) in addition to the amount of annual precipitation. Specifically, higher antecedent baseflow and faster snowmelt both result in a preferential partitioning of precipitation to streamflow. This implies that the effects of a warming climate on the water resources of the seasonally snow dominated watersheds near Salt Lake City can be best understood through the context of the melting snowpack. Further, climate extremes, such as inter-annual drought, may leave a legacy effect on the subsurface storage in some watersheds, causing a delayed streamflow recovery in the years following the drought.