Watershed area ratio accurately predicts daily streamflow in nested catchments in the Catskills, New York

•The Catskills region of New York is the principle water supply for New York City.•Daily flow in nested catchments scales according to watershed area.•Area ratio alone explains 93% of the variance in daily flow in nested catchments.•Flows at ungaged sites can be estimated accurately using area ratio and downstream flow.

Study regionThe Catskills region of New York State is largely forested and dominated hydrologically by stream watersheds with few natural lakes. The area experiences intensive water resources management and ecosystem monitoring due to its strategic role as the principal water supply for New York City.Study focusWe analyzed average daily flows in nested and non-nested pairs of gaged watersheds in the Catskills to assess whether daily flow in ungaged watersheds can be calculated based on watershed area ratios.New hydrological insights for the regionWatershed area ratio was the most important basin parameter for estimating flow at upstream sites based on downstream flow. The area ratio alone explained 93% of the variance in the slopes of relationships between upstream and downstream flows. Regression analysis indicated that flow at any upstream point can be estimated by multiplying the flow at a downstream reference gage by the watershed area ratio. This method accurately predicted upstream flows at area ratios as low as 0.005. We also observed a very strong relationship (R2 = 0.79) between area ratio and flow–flow slopes in non-nested catchments. Our results indicate that a simple flow estimation method based on watershed area ratios is justifiable, and indeed preferred, for the estimation of daily streamflow in ungaged watersheds in the Catskills region.