Estimating minimum streamflow from measurements at ungauged sites in regions with streamflow‐gauging networks

Estimation of low flows in rivers continues to be a vexing problem despite advances
in statistical and process‐based hydrological models. We develop a method to
estimate minimum streamflow at seasonal to annual timescales from measured
streamflow based on regional similarity in the deviations of daily streamflow from
minimum streamflow for a period of interest. The method is applied to 1,019 gauged
sites in the Western United States for June to December 2015. The gauges were
clustered into six regions with distinct timing and magnitude of low flows. A gamma
distribution was fit each day to the deviations in specific discharge (daily streamflow
divided by drainage area) from minimum specific discharge for gauges in each region.
The Kolmogorov–Smirnov test identified days when the gamma distribution was
adequate to represent the distribution of deviations in a region. The performance
of the gamma distribution was evaluated at gauges by comparing daily estimates of
minimum streamflow with estimates from area‐based regression relations for minimum
streamflow. Each region had at least 8 days during the period when streamflow
measurements would provide better estimates than the regional regression equation,
but the number of such days varied by region depending on aridity and homogeneity
of streamflow within the region. Synoptic streamflow measurements at ungauged
sites have value for estimating minimum streamflow and improving the spatial
resolution of hydrological model in regions with streamflow‐gauging networks.