PREDICTING SHORT-TERM STORMFLOW RUNOFF BASED ON VADOSE ZONE ANTECEDENT SOIL MOISTURE USING GPR

Antecedent soil moisture - the soil water content (SWC) of the near surface vadose zone (d < 1 m) prior to a storm - is a first-order factor in the partitioning of precipitation between short term (hours to days) and longer term (days to weeks) groundwater discharge to rivers. While, in the past, a number of proxies have been used to estimate soil moisture, direct measurements of SWC have proven to be the best metric for runoff studies. This report describes the use of ground penetrating radar (GPR) to monitor antecedent SWC in order to predict storm-to-storm differences in event runoff from the Ten Mile River watershed, a typical USGS gaged, moderate-scale catchment of surface area 137.5 sq km (53.1 sq mi) in southern New England. Annual precipitation for the area is 117 +/- 20 cm/yr (46 +/- 8 in); mean stream discharge is 2.97 +/- 0.68 cubic m/s (105 +/- 24 cfs); resulting in a mean long term annual runoff coefficient of 58%. The questions we address here are two-fold: 1) How much does the relative event runoff fluctuate from storm-to-storm? 2) How much of this fluctuation is “driven” by antecedent soil moisture?

“Short term stormflow runoff efficiency” is the volume of the short term event flow determined from a standard streamflow separation, divided by the total input volume of precipitation for the storm over the watershed area. The runoff efficiency for individual storms is compared with the corresponding storm-by-storm antecedent SWC values from GPR data acquired from one to a few days preceding well-defined storm events. A simple linear least-squares regression implies that short term runoff efficiencies ranging from 3 to 30% are directly related to antecedent soil moisture at our reference site with a squared-correlation coefficient in excess 0.90, leading us to conclude that over 90% of the variance of the short term runoff efficiency about its mean value can be “explained” by variations of antecedent soil moisture.