Large Plot Tracing of Subsurface Flow in the Edwards Aquifer Epikarst

An experimental rainfall simulation plot (7 x 14 m) in the Edwards Aquifer region of Texas was established in dense juniper land cover to measure the effects of brush clearing on runoff. This project includes the monitoring of lateral subsurface flow using a trench (2.5 m deep x 8 m long) located at the downhill end of the plot. Two tracer tests with fluorescent dyes were conducted using simulated rainfall to assess the discrete flow paths discharging into the trench. Uranine was applied in a band at the uphill end of the plot, eosine was applied around the base of a tree in the center of the plot, and Phloxine B was applied in a narrow band at the downhill end of the plot. During the tracer tests, samples were collected at regular intervals from sixteen locations along the trench face where water flowed into the trench. A fluorescence analysis of water samples using a luminescence-spectrometer produced concentration values for each dye. An analysis of the tracer test results revealed: (1) not all areas in the plot contributed to the shallow subsurface flow into the trench, (2) subsurface flow paths displayed a high degree of interconnection, even over short distances, (3) conduits showed strong linkages to multiple plot regions, and (4) conduits showed strong connection with the area surrounding juniper vegetation, with rapid water flow from this area. These results demonstrated that the high variability in flow paths and interconnection encountered in the Edwards Aquifer exists even at the plot scale and highlights the need for greater research into vegetation-flow path interactions in shallow epikarst.