A Statistical Technique for Interpreting Streamflow Timing Using Streambed Sediment Thermographs

streamflow. They visually identified changes in streambed temperature wave amplitudes to determine the start and A moving standard deviation (MSD) technique is developed to end of streamflow events. Although this visual inspecinfer the onset and cessation of ephemeral streamflow using temperation technique has been successfully demonstrated in ture data from the upper 2.25 m of streambed sediments. During periods of streamflow, shifting of the predominant thermal-transport ephemeral streamflow settings for multiple day events, mechanism within the sediments from conduction to advection prothe technique is both subjective and time-consuming duced changes in the amplitude of the vertically propagating diurnal when analyzing large data sets. The technique can also temperature waves. Analytical expressions describing propagation of be ineffective for identifying stream flow events that conductive and advective diurnal temperature waves through streambed are 24 h in duration, which is typical of ephemeral sediments are presented for identifying depths with the largest changes streams in southern Arizona, for example. Finally, this in the diurnal temperature wave amplitude between periods of flow method of analysis requires relatively shallow measureand no flow. The MSD statistical technique was developed to identify ment depths to take advantage of reductions in the the thermal amplitude changes from bed sediment thermographs and diurnal temperature wave amplitude that occur as heat to infer streamflow timing. The accuracy of the MSD technique is is lost to the standing water above the sediments. Such quantified using direct streamflow and streambed water content measurements. Accuracy of the technique was most sensitive to the MSD shallow installed instruments are especially susceptible window length and the threshold parameter separating periods of to removal by scour of the channel sediments. conductive and advective heat transport. An alternative calibration The first objective of this investigation was to present procedure was developed using temperature measurements alone. a description of heat transport that considers the effects The average error for streamflow timing was approximately 400 min of both conductive and advective temperature transport. for each event. The results show that temperature sensors may be This description will expand the capability of this method deployed at a range of sediment depths depending on streamflow to a greater number of ephemeral streams by allowing stage and soil thermal and hydraulic properties, and that the MSD for monitoring of temperature at deeper depths where procedure can provide an objective and repeatable means to quantify shallow temperature measurements are impractical, such streamflow timing. as reaches that experience significant scour (Constantz et al., 2003). The second objective was to introduce an automated technique for identifying the onset and R the continual presence and absence of cessation of ephemeral streamflow based on a statistical streamflow within ephemeral channels in large analysis of streambed thermographs. semiarid and arid basins for use in water balance and The following section presents two analytical expreshydrologic models requires an inexpensive and reliable sions describing conductive and advective heat transport method for broad distribution. Difficulties arise when as well as the hydrological conditions necessary to use monitoring streamflow timing and extent within ephembed sediment thermographs for streamflow timing. We eral channels using traditional streamflow timing techcontinue by introducing the moving standard deviation niques because of the flashy nature of streamflow events technique for analyzing bed sediment thermographs. and shifting elevations of the channel surface (Constantz Finally, we evaluate the moving standard deviation techand Thomas, 1997; Blasch et al., 2002). Consequently, nique by using bed sediment thermographs collected measurement of streamflow presence using subsurface from an array of temperature sensors within an ephemmethods has been proposed (Constantz and Thomas, eral stream. 1997). Because streambed temperature measurements are inexpensive and easy to obtain relative to other subsurface measurements, streambed thermographs have OPTIMAL DEPTH SELECTION been introduced as a means to infer streamflow timing FOR PLACEMENT OF and extent in ephemeral streams (Constantz et al., 2001). TEMPERATURE SENSORS Constantz et al. (2001) placed thermal sensors at shalWhen streamflow is absent from a channel, radiant low depths (15 cm) to identify reductions in the ampliheating of the surface sediments is transported into the tude of the diurnal temperature wave in the presence of sediment profile primarily through conduction (Fig. 1). The diurnal temperature, often represented as a sinusoiK.W. Blasch and J.P. Hoffmann, USGS, Tucson, AZ 85719; K.W. dal function, propagates through the sediments with a Blasch and T.P.A. Ferré, Department of Hydrology and Water Redecreasing amplitude and increasing time lag with depth sources, University of Arizona, Tucson, AZ 85721. Received 16 Oct. (Van Wijk and De Vries, 1963). The magnitude of the 2003. Original Research Paper. *Corresponding author (kblasch@usgs. one-dimensional diurnal temperature wave as a function gov). Published in Vadose Zone Journal 3:936–946 (2004).  Soil Science Society of America Abbreviations: MSD, moving standard deviation; TDR, time domain reflectometry. 677 S. Segoe Rd., Madison, WI 53711 USA