DIURNAL VARIATION IN DISCHARGE FROM THREE SHALLOW KARST SPRINGS IN THE SAVOY EXPERIMENTAL WATERSHED, ARKANSAS OZARKS

This study describes the results of a series of spring-discharge measurements that were conducted at the Savoy Experimental Watershed (SEW). The study was conducted as part of the Research Experience for Undergraduates (REU) program and was funding by the National Science Foundation (NSF) is gratefully acknowledged. Geologically, SEW is underlain by a mid-Paleozoic, nearly flat-lying regolith-mantled karst, and includes an infrastructure of 10 weirs, 3 flumes, more than 100 wells, 30 springs and seeps, and a complete weather station.

Discharge measurements for Tree, Woodpecker, and Red Dawg Springs were conducted twice a day, during the early morning (7:00 am during July and August; 8:00 am during December because of late sunrise), and the early afternoon, at about 2:00 pm. Two data collection periods were chosen, reflecting the growing season (21 days in July and August) and the dormant season (5 days in December). Measurements were made with a 2000 mL graduated cylinder and stopwatch, with 10 repetitions per spring per twice daily measurement. Additional data collected were stage, measured from the notch of a 22.5-degree v-notch weir.

Results indicated that each of the three springs had maximum flow during the morning measurement during the growing season, and decreased in flow from 4.9 to 10.7% during the afternoon. All springs showed this diurnal reduction in flow during the period of maximum solar radiation, which also was reflected in the transducer measurements at each weir. This loss of water from the shallow ground-water system is interpreted to be evapotranspiration from the shallow ground-water system, and ranges from 5 to 25 mL/s over the course of a diurnal cycle for each spring. Specifically, Tree Spring decreased 5.5% between morning and afternoon during the growing season, Woodpecker decreased 10.7% between morning and afternoon for the growing season, and Red Dawg decreased 4.9% during the morning and afternoon during the growing season. Diurnal variations showed significantly reduced differences between morning and afternoon during the dormant season, 1.0%, 0.2%, and 0.9% respectively. These changes are remarkably consistent with precipitation data from tipping-bucket rain gages, provide valuable insight in the hydraulic functioning and water budgets of these shallow aquifers.