Understanding Tree Water Use Across the Snow-Rain Transition in Idaho's Mountain Watersheds: Feedbacks Between Stream Networks, Transpiration, and Basin Geomorphology

Warming trends are expected to reduce mountain snow pack, increase evapotranspiration, and thus diminish the sometimes limited water supplies of many intermountain streams and rivers. While it is believed that water that is transpired is no longer available for streamflow, it remains uncertain how the timing and quantity of transpiration differ between snow-dominated and rain-dominated elevations, and how alterations in transpiration in these regions affect surface water flow in mountain stream networks. To understand the spatiotemporal relationships of transpiration, we measured Douglas fir water use across the snow-rain transition line/elevation in the Pioneer Creek watershed of Idaho's Frank Church River of No Return Wilderness in 2014. We also recorded stream discharge and monitored surface flow areal extent in four subwatersheds with contrasting geomorphologic controls on the channel network, including moraine and fault controls. We sought to test the hypotheses that (1) Douglas fir trees at snow-dominated elevations would transpire less water each year, and do so later in the melt-season compared to Douglas fir trees at rain-dominated elevations, and (2), that patterns of stream network expansion and contraction will reflect patterns of timing in transpiration rates. Preliminary analyses suggest that transpiration timing is similar across all elevations, and that stream network extent varies minimally across a 20 to 60% variation in streamflow. Summer transpiration varied more strongly with tree size and age than with elevation. We present comparisons of drainage density across the sites at different flow rates, and relate them to geomorphic controls present within each basin. Understanding the present relationships of streamflow with transpiration across snowline contributes to more robust predictions of changes in water resources as a result of climate change.