The Low Flow Calculator
The Low-Flow Calculator Tool is a web application that tells you how river flow might be affected by future climate change in your area. The tool is based on a linear regression which predicts the low-flow statistic 
for a watershed from the physiographic and climatic characteristics of that watershed. The linear regression equation used by the tool is
Equation 1
with the physiographic and climate explanatory variables including the baseflow recession constant (Kb2); the drainage area (DA), the soil organic matter content (OMH),the 10th percentile of summer monthly precipitation (Pminsum), the average April precipitation (Pavgapr), and the 90th percentile of fall monthly temperature (Tmaxfal).
The linear regression in Equation 1 was developed from a stepwise procedure using the low-flow statistic and characteristics from160 watersheds in the Mid-Atlantic region of the United States. The linear regression has a high adjusted coefficient of determination of 0.935, a mean square error of 0.18, and all parameters significant at p <0.001.
The regression equation is used to predict future low flow in the Mid-Atlantic region based on the assumption that only climate characteristics will change in the future. With this assumption, and using the linear regression equation in Equation 1, we can estimate the fractional chane in low flow resulting from a change in climate using the relationship
Equation 2
where all superscripts of 2 indicate future characteristics and all subscripts of 1 indicate present day characteristics. When the user selects a river, general circulation model (GCM), and scenario (A2 or B2), the tool first identifies the changes in rainfall and temperature as predicted by the selected GCM and scenario. The changes in rainfall and temperature are entered into Equation 2 to estimate the fractional change in low flow. The tool then multiplies the estimated fractional change in low flow with the present day low flow to predict future low flow. Results are presented as both a bar graph showing the change in low flow over time as well as in a tabular form.
For more information contact Mary Schoen.