[1] Raman spectra distributed temperature sensing (DTS) by fiber-optic cables has recently shown considerable promise for the measuring and monitoring of surface and near-surface hydrologic processes such as groundwater-surface water interaction, borehole circulation, snow hydrology, soil moisture studies, and land surface energy exchanges. DTS systems uniquely provide the opportunity to monitor water, air, and media temperatures in a variety of systems at much higher spatial and temporal frequencies than any previous measurement method. As these instruments were originally designed for fire and pipeline monitoring, their extension to the typical conditions encountered by hydrologists requires a working knowledge of the theory of operation, limitations, and system accuracies, as well as the practical aspects of designing either short-or long-term experiments in remote or challenging terrain. This work focuses on providing the hydrologic user with sufficient knowledge and specifications to allow sound decisions on the application and deployment of DTS systems.
Abstract. Two experiments were performed to investigate flow beneath an ephemeral stream and to estimate streambed infiltration rates. Discharge and stream-area measurements were used to determine infiltration rates. Stream and subsurface temperatures were used to interpret subsurface flow through variably saturated sediments beneath the stream. Spatial variations in subsurface temperatures suggest that flow beneath the streambed is dependent on the orientation of the stream in the canyon and the layering of the sediments. Streamflow and infiltration rates vary diurnally: Streamflow is lowest in late afternoon when stream temperature is greatest and highest in early morning when stream temperature is least. The lower afternoon streamflow is attributed to increased infiltration rates; evapotranspiration is insufficient to account for the decreased streamflow. The increased infiltration rates are attributed to viscosity effects on hydraulic conductivity from increased stream temperatures. The first set of field data was used to calibrate a two-dimensional variably saturated flow model that includes heat transport. The model was calibrated to (1) temperature fluctuations in the subsurface and (2) infiltration rates determined from measured streamflow losses. The second set of field data was to evaluate the ability to predict infiltration rates on the basis of temperature measurements alone. Results indicate that the variably saturated subsurface flow depends on downcanyon layering of the sediments. They also support the field observations in indicating that diurnal changes in infiltration can be explained by temperature dependence of hydraulic conductivity. Over the range of temperatures and flows monitored, diurnal stream temperature changes can be used to estimate streambed infiltration rates. It is often impractical to maintain equipment for determining infiltration rates by traditional means; however, once a model is calibrated using both infiltration and temperature data, only relatively inexpensive temperature monitoring can later yield infiltration rates that are within the correct order of magnitude.
This report presents data collected as part of an investigation to delineate groundwater systems and to characterize the groundwater quality in the southeast part of the Lake Tahoe basin in Nevada. The data include: (1) Bouguer-gravity values from 31 gravity stations; (2) 59 well-drillers' reports, including water-level data; (3) information regarding U.S. Geological Survey hydrologic-data sites and 16 wells having historical water-quality analyses, including water levels measured in wells and discharge measurements of streams and springs; (4) particle-size distributions for 30 augered samples of unconsolidated sediment from 20 well bores; (5) 22 results of previous analyses of water from 16 wells; and (6) results of additional analyses to determine the physical properties, major dissolved chemical constituents, dissolved iron, total and dissolved species of nitrogen and phosphorus, total or dissolved organic carbon, and selected naturally occurring radionuclides for water samples collected during this study from 30 wells, 9 streams, and 3 springs. Maximum and median dissolved-nutrient concentrations in the 48 water samples collected from 42 sites during 1986-87 are: nitrate-nitrogen, 8.2 and 0.
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