Application Of Distributed Fiber-Optic Temperature Sensing To Study Surface/Groundwater Interactions In A Mountain Catchment
Cowie, Rory 1 ; Williams, Mark 2 ; Anderson, Suzanne 3
1 University of Colorado, Dept. of Geography and INSTAAR
2 University of Colorado, Dept. of Geography and INSTAAR
3 University of Colorado, Dept. of Geography and INSTAAR
Future changes in climate will generally limit water availability in mountain environments because of decreased snowpack, earlier spring snowmelt, and increased evaporation. Knowledge of current surface/groundwater interactions in mountain catchments must be improved before we can forecast how future changes in climate will affect streamflow. Watersheds within the crystalline rocks of the Rocky Mountains are characterized by steep slopes, thin soils, and highly fractured bedrock. Groundwater flow occurs primarily through fractures, reducing the effectiveness of the classic porous medium approach for understanding surface/groundwater interactions.
I intend to use distributed fiber-optic temperature sensing (DTS) to locate and quantify groundwater inputs to a headwater stream in Gordon Gulch, a montane catchment in the Boulder Creek watershed, Colorado. The DTS system continuously records stream temperature every meter along a 1-km fiber-optic cable with a precision of ±0.01°C (Selker et al., 2006a). Groundwater inputs are quantified from downstream changes in temperature with a two-component mixing model. Concurrent sampling of the stream and known springs for isotopic and geochemical composition, analyzed with end-member mixing analysis (eg Liu et al., 2004), will complement the temperature data.
Groundwater temperatures measured in springs in Gordon Gulch since May 2008 have remained relatively constant (~ 6°C), while main channel stream surface water temperature has ranged from 0 to 14°C due to both shallow flowpath inputs and energy exchange with the atmosphere and radiation. Constant temperature groundwater flows can be distinguished from the diurnally fluctuating surface water temperatures in low flow headwater streams (figure 1). I intend to run a fiber optic cable along the streambed, and employ Ramen-backscatter DTS along multimode fiber optic cable to acquire high-resolution temperature data.
Quantification of localized groundwater inputs to the stream, in combination with isotopic and geochemical tracers, will increase our knowledge concerning outstanding questions in the hydrology of fracture-flow systems in the Rocky Mountains, (a) including the size of the groundwater reservoir, (b) residence time of groundwater, (c) fracture-flow system, and (d) surface/groundwater interactions. This research will be beneficial to water resource managers in understanding how varying climactic conditions will impact water availability, especially in watersheds where groundwater significantly contributes to discharge.
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Selker, J. S., L. Thevenaz, H. Huwald, A Mallet, W. Luxemburg, N. van de Giesen, M. Stejskal, J. Zeman, M. Westhoff, and M. B. Parlange, 2006a, Distributed fiber optic temperature sensing for hydrologic systems: Water Resources Research, v. 42, doi: W12202 10.1029/2006wr005326.
Selker, J. S., Van de Giesen, N., Westhoff, M. C., Luxemburg, W., and M. B. Parlange, 2006b, Fiber optics opens window on stream dynamics: Geophys. Res. Lett., V. 33, L24401, doi: 10.1029/2006GL027979.