QUANTIFYING GROUNDWATER–SURFACE WATER EXCHANGE FROM LOW-ALTITUDE REMOTE SENSING USING LARGE-SCALE PARTICLE IMAGE VELOCIMETRY

Groundwater and surface water are often treated as two separate resources, but these systems interact and exchange at the sediment/water interface of streams. This interaction has important implications concerning hydrogeologic problems, such as recharge rates of aquifers, contaminant transport, and riverine ecosystem health. The volume of this exchange can be measured using a simple mass balance as a positive or negative net differential discharge between two transects along a stream reach. The most common method to determine discharge involves multiple direct measurements of depth and average water velocity across a stream profile; however, this method is both time-consuming and costly. Additionally, applying this method for rapid-onset flood events can be impossible and dangerous. Large-scale particle velocimetry (LSPIV) is a relatively new method, that uses particle tracking software to quantify surface velocity over large flow areas from video footage. In low-altitude remote sensing studies, this method has been successfully used in determining stream discharge; however, it has yet to be evaluated as a method for determining groundwater exchange along a stream reach. The purpose of this research is to evaluate the accuracy of differential stream gauging using LSPIV as a potential time-saving, cost-effective approach to long-term monitoring of watersheds. Data for this research comes from two local third order streams in the greater Buffalo NY area where discharge data can be measured with both the LSPIV technique and direct measurement using the traditional method. Preliminary results from Elton Creek show that the accuracy varies depending on the conditions of video capture; in general by the angle of the camera, degree of illumination, and presence of traceable particles. These variables have been systematically tested using optical and thermal imagery in both controlled and dynamic environments. The degree of accuracy of LSPIV derived discharge measurements defines the minimum distance between profiles needed to quantify net groundwater flux: the error associated with the LSPIV discharge at each stream profile, cannot exceed the value of differential discharge. Even moderately accurate results can be useful for understanding the magnitude of groundwater exchange over a large area.