Spring disharge monitoring at The Homeplace on Green River, Campbellsville
Accurate monitoring of the quantity of water discharging from any of Kentucky’s many karst springs often requires overcoming difficulties caused by the physical and hydrologic characteristics of each spring. Staff of the KGS Water Resources Section were confronted with this challenge during a project to investigate karst hydrogeology at The Homeplace on Green River farm, near Campbellsville. This research is being conducted in partnership with The Homeplace on Green River Inc., a 501(c)(3) organization promoting agricultural conservation education, and is funded by a Conservation Innovation Grant from the U.S. Department of Agriculture’s Natural Resources Conservation Service. It is the first phase of an effort to aid the development of new or improved approaches needed to adapt the NRCS’s “edge-of-field” monitoring techniques to assess water quality and soil health at crop fields and farms in karst areas dominated by sinkholes.
The spring at The Homeplace farm discharges from a small cave at the head of a steep ravine (See photo 1.) that drains to the Green River. Water from the spring flows first down a series of shallow pools and cascades formed on step-like layers of limestone and then in a narrow surface channel consisting of alternating small pools and riffles mostly filled with coarse, cherty gravel. The shallow depth of water, irregularities in channel shape, and constantly shifting gravel bedload make repeated accurate measurements of the spring’s discharge difficult. To overcome these obstacles and establish a continous discharge-monitoring station for the spring, KGS water researchers Chuck Taylor, Steve Webb, and Jason Backus installed an aluminum flume, a stilling well, and data logger and telemetry equipment at a site on the spring channel during the week of June 12 (Photos 2–4). The flume creates a stabilized, artificial section of channel that constricts the width of flow and increases the flow velocity and water level (depth) within it. This enables the discharge to be estimated to a high accuracy using a mathematical equation that relates the water level measured at a single point in the flume to the volume of water flowing through the flume per unit of time (in gallons per minute, for example).
Water levels are not measured directly within the flume, but are instead measured using a pressure transducer installed in the nearby stilling well, which is itself constructed of a PVC pipe capped and sealed to be water-proof but not air-tight, and partly buried upright in gravel near the channel (Photo 5). Water levels in the stilling well change directly in response to changes in water level in the flume as a result of hydraulic pressure transmitted through a connecting tube between the flume and the stilling well. To continously monitor the spring’s response to variability in weather conditions and precipitation, especially during storms when surface runoff drains from sinkholes into underground karst conduits, the pressure transducer’s data logger records changes in water levels every 15 minutes. Recorded water-level data are automatically transmitted once a day by cellular network telemetry to a web server that can be accessed at any time by computer at the KGS main office in Lexington.
The discharge-monitoring station survived a severe test during the recent passage of the remnants of Tropical Storm Cindy. Intense rainfall over a 24- to 36-hour period resulted in flash flooding throughout much of Kentucky, including the Campbellsville area. KGS Water Resources Section staff were able to successfully track the rapid, dramatic increase in water levels corresponding to the spring’s peak discharge and monitor the subsequent recession as the storm pulse passed and the spring began to return to base-flow conditions (See Fig. 1). As more storm-response data are collected at the discharge-monitoring station, it is anticipated that much will be learned about the spring’s dynamic hydrology and its sensitivity to potential contaminants transported through the subsurface karst drainage system.