Analytic Probability Distributions for Snow Dominated Low Flow

The hydrological regime of high mountainous catchments is characterized by a pronounced winter base flow resulting from the interplay of i) high elevation areas that experience permanent freezing conditions during the winter and of ii) lower elevation areas that show a slow release of water during intermittent melt conditions. Knowledge about this winter low flow is crucial for water resources management in mountain environments and the related socio-economic, environmental and ecological services. Generalizing the stochastic framework employed by Botter et al. (2007), we propose a novel analytical description of the probability distribution function (pdfs) of winter low flow for Alpine catchments. The proposed framework links the stochastic properties of precipitation (rain and snow) and air temperature to the streamflow dynamics through a small number of parameters that can be directly estimated from observed discharge, precipitation and air temperatures. The effect of snow dynamics on the flow regime is specifically included by incorporating the temporary disconnection of high-elevation areas that experience freezing conditions over the entire winter season, and the delay produced on streamflow formation by the temporary accumulation (and later melting) of snow at lower elevations. In a first step, the size of this temporarily nonresponsive area is calibrated on observed streamflow. Based on case studies from the Swiss and the Italian Alps, the regional pattern of the extension of the nonresponsive area is analyzed. Overall, the proposed analytic model reproduces the observed streamflow pdfs remarkably well. This approach marks a progress toward the characterization of base flow resulting from snow accumulation and melting processes and, more generally, the statistical characterization of catchment streamflow variability. Reference: Botter, G., A. Porporato, I. Rodriguez-Iturbe, and A. Rinaldo (2007), Basin-scale soil moisture dynamics and the probabilistic characterization of carrier hydrologic flows: Slow, leaching-prone components of the hydrologic response, Water Resour. Res.,43, W02417, doi:10.1029/2006wr005043.