Constraining evapotranspiration and understanding ET drivers and limitations in a mountain headwaters system

Abstract

Mountain regions provide much of the world’s water supply. Although these regions are essential for downstream water delivery, they are challenging to study due to numerous heterogeneities. These heterogeneities include steep topographic gradients, varying land cover, and spatially-diverse atmospheric drivers. For these reasons, high-elevation mountain watersheds are difficult to study using only point observations or remote sensing products. Estimating evapotranspiration (ET) is particularly challenging in these regions as commonly used measurement techniques are not designed for the diversity present in mountain regions, and as a result, ET is poorly understood in headwaters regions. However, ET constitutes a large portion of water leaving these systems and understanding this flux is vital to regulating downstream water availability, understanding of overall ecosystem function, and water and energy balance estimates. Using a combination of observation and modeling, this dissertation seeks to better quantify ET in complex, mountain regions as well as contribute toward better characterization of controls on ET at the hill slope scale. This work suggests that at the point-scale, in riparian areas of the watershed, ET is water-limited rather than energy limited and that groundwater supplies a large portion of water used for ET. At the hill slope scale, ET is largely dependent on convergence zone moisture and the presence of convergence zones acts as a control on ET magnitudes. This work also shows that on average across the hill slope, ET is driven mainly by rain or snow throughout the year with some fraction of older groundwater contributing to ET, but this contribution of older groundwater to ET increases with decreasing altitude and the bottom of the hill slope taps into older water than the top of the hill slope. This work aims to help bridge the gap between small-scale observations and coarse-resolution products to better constrain ET across similar headwaters regions leading to better estimates of downstream water availability.