Snowmelt supports more of transpiration at the bottom than top of an alpine hillslope in the East River Watershed, Colorado

Abstract:

In the snow-dominated catchments of the Rocky Mountains, trees rely heavily on winter precipitation through the summer months and into the fall. However, over the summer as the snowmelt pulse ages and moves downslope, it presumably does not support transpiration demands equally on hillslopes. Here, we present data from a network of sapflux sensors and stable isotope measurements along a ~400 m hillslope transect in the East River Watershed of Colorado. The network crosses through the transition from aspen to conifer (fir and spruce) and spans sites that vary by a factor of 2 in terms of April snow depth. We combine the sap flux data with LiDAR-derived tree crown measurements, to produce estimates of plot-level transpiration. This data was then integrated with results from a stable isotope mixing model applied to xylem water measurements to estimate the rate of snowmelt-derived transpiration. The results show that during the early portion of the growing season, the contribution of snow to transpiration is proportionate to the size of the site’s snowpack, which leads to higher and lower snow use at the top and bottom of the hillslope, respectively. This pattern reverses later in the growing season, such that there is higher use of snowmelt at the bottom of the hillslope whereas the upper part of the hillslope relies more on summer rains. By the end of the growing season there is ubiquitous reliance on snowmelt, showing that summer rain contributions had largely been evaporated or transpired away but a small pool of residual snowmelt from the current or prior year remains present to support the low level of fall transpiration. The observations frame a simple conceptual model of snowmelt moving downslope and continually subsidizing transpiration in the lower portion of the hillslope months after the snow had melted out. This conceptual model is tested with particle tracking simulations that separates the snow and precipitation water sources for the hillslope using the EcoSLIM model.