John P. Hoffmann

New field method to determine streamflow timing using electrical resistance sensors

can become buried or damaged by moving sediment or debris. Consequently, streamflow timing sensors deElectrical resistance (ER) sensors were constructed to monitor ployed within the vadose zone have been shown to be streambed saturation to infer ephemeral streamflow timing. The senadvantageous under these circumstances (Constantz et sors were evaluated in an ephemeral stream through comparison with temperature-based methods, a stream gauge, and soil water content al., 2001). sensors. The ER sensors were more accurate at estimating streamflow Soil water content methods detect infiltration and timing and the resultant data required less interpretation than data percolation of water through the sediments, which may from temperature-based methods. Accuracy was equivalent to timing be used to infer timing of streamflow (Blasch et al., unmethods using stream gauge and soil water content measurements. published data). Placing sensors in the subsurface reThe ER sensors are advantageous for use in ephemeral stream chanduces the possibility that they will be damaged or lost nels because they are inexpensive, deployable above or below the during flow. Logging instrumentation, however, must sediment surface, insensitive to depth, and do not require connecting be placed on or near the bank with cables extending to wires to an external datalogger or power source. On the basis of these the buried sensors. results, we conclude that ER sensors may be used to monitor changes Temperature methods enable inference of streamflow in soil water content within the vadose zone. Additionally, the sensors can be used to infer the presence of surface water in diversion canals, timing on the basis of the combined transport of heat and storm-water sewers, and in the form of overland runoff. fluid within the bed sediments (Constantz and Thomas, 1996, 1997; Ronan et al., 1998; Constantz et al., 2001). Recent development of small ( 10 cm3), inexpensive, waterproof temperature sensors with integrated data G the erratic and variable nature of ephemeral and intermittent streamflow in arid and semiarid storage enable measurement and storage of temperature values without the need for external connecting basins, long-term collection of streamflow timing is necessary for obtaining information on extreme flow events wires. This advantage enables in situ temperature monitoring in ephemeral channels with unstable beds over and seasons. Streamflow timing in channels and arroyos is used to accurately model fluid transport through the large areas with high spatial resolution. While temperature methods have been used successunsaturated zone beneath ephemeral streams and to constrain channel recharge estimation, a primary compofully to monitor the timing of streamflow in ephemeral channels, the methods have limitations. Specifically, cernent of aquifer replenishment. Additionally, streamflow timing is a necessary component for designing stormtain conditions are required for streamflow to produce a readily identifiable thermal signal. For example, water water and flood-control networks in flood-prone environments. with the same temperature as the channel will not produce an identifiable signal. In ephemeral stream chanCurrent methods used to estimate streamflow timing include flow-rated stream gauges, velocity meters, soil nels subject to repeated scour and deposition, changes in sediment surface elevation complicate the application water content sensors, and temperature sensors (Latkovich and Leavesly, 1993; Constantz et al., 2001; Blasch of numerical methods used for interpretation of the temperature data. et al., unpublished data). These methods have met with varying success depending upon channel morphology, In this investigation, we converted commercially available temperature sensors into ER sensors to monitor bed sediment characteristics, frequency and duration of streamflow, and other requirements (e.g., magnitude of water content and tested their utility for streamflow detection. Advantages of the ER approach include functemperature signal). Stream gauges and velocity meters accurately detertionality above or below the channel surface, functionality in all streamflow temperatures, lack of connecting mine streamflow timing, but generally are not suitable for ephemeral channels that experience changes in chanwires, and minimal interpretation of data. These same attributes necessary for streamflow timing are also adnel morphology (Tadayon et al., 2000). Stream gauges and velocity meters installed at the bed sediment surface vantageous for monitoring sediment saturation in other similar vadose zone applications such as irrigated fields, fluctuating water tables, and postburn environments. K. Blasch, U.S. Geological Survey, 520 North Park Ave., Suite 221, Tucson, AZ 85719, and Dep. of Hydrology and Water Resources, BACKGROUND AND THEORY J.W. Harshbarger 122, 1133 East North Campus Drive, P.O. Box 210011, Univ. of Arizona, Tucson, AZ 85721; T.P.A. Ferré, Dep. of An electrical resistance measurement in a porous medium Hydrology and Water Resources, J.W. Harshbarger 122, 1133 East can be idealized as a measurement of three resistances in North Campus Drive, P.O. Box 210011, Univ. of Arizona, Tucson, series: the bulk electrical resistance of the medium, which AZ 85721; A.H. Christensen, U.S. Geological Survey, 5735 Kearny includes solid grains and pore water, (Rm), and a contact resisVilla Road, San Diego, CA 92123; J.P. Hoffmann, U.S. Geological Survey, 520 North Park Ave., Suite 221, Tucson, AZ 85719. Received 14 May 2002. *Corresponding author (kblasch@usgs.gov). Abbreviations: ER, electrical resistance [sensors]; PVC, polyvinyl chloride; TDR, time domain reflectometry. Published in Vadose Zone Journal 1:289–299 (2002).

A Statistical Technique for Interpreting Streamflow Timing Using Streambed Sediment Thermographs

A moving standard deviation (MSD) technique is developed to infer the onset and cessation of ephemeral streamflow using temperature data from the upper 2.25 m of streambed sediments. During periods of streamflow, shifting of the predominant thermal-transport mechanism within the sediments from conduction to advection produced changes in the amplitude of the vertically propagating diurnal temperature waves. Analytical expressions describing propagation of conductive and advective diurnal temperature waves through streambed sediments are presented for identifying depths with the largest changes in the diurnal temperature wave amplitude between periods of flow and no flow. The MSD statistical technique was developed to identify the thermal amplitude changes from bed sediment thermographs and to infer streamflow timing. The accuracy of the MSD technique is quantified using direct streamflow and streambed water content measurements. Accuracy of the technique was most sensitive to the MSD window length and the threshold parameter separating periods of conductive and advective heat transport. An alternative calibration procedure was developed using temperature measurements alone. The average error for streamflow timing was approximately 400 min for each event. The results show that temperature sensors may be deployed at a range of sediment depths depending on streamflow stage and soil thermal and hydraulic properties, and that the MSD procedure can provide an objective and repeatable means to quantify streamflow timing.

Quantifying Ephemeral Streambed Infiltration from Downhole Temperature Measurements Collected Before and After Streamflow

A constant flux infiltration experiment was conducted to determine the feasibility of using downhole temperature measurements to estimate infiltration flux. Temperatures measured using a downhole thermistor within a 15.4-m-deep borehole compare well with temperatures measured with buried thermocouples in an adjacent borehole to 5 m depth. Numerical forward model simulations were conducted using VS2DI. A numerical sensitivity analysis showed that the temperature profile was most sensitive to the average temperature of the infiltrating water, the infiltration flux, and the specific heat capacity of dry soil. The high sensitivity of these variables allows for a simple sequential optimization to be used to estimate the average temperature of the infiltrating water, the water flux, and the specific heat capacity of dry soil from numerical inversion of temperature measurements. Downhole temperature measurements could be a useful complement to shallow streambed temperature methods, allowing for better quantification of the contribution of streambed infiltration to basin-scale recharge.

Integrated geophysical surveys for mapping lati-andesite intrusive bodies, Chino Valley, Arizona

Three different geophysical methods (magnetic, transient electromagnetic (TEM) and gravity) were used near Chino Valley, Arizona, USA in order to map a suspected lati-andesite intrusive body (plug) previously located by interpretation of aeromagnetic data. The magnetic and TEM surveys provided the best indication of the location and depth of the plug. The north-south spatial extent of this plug was estimated to be approximately 600 meters. The depth to the top of the plug was found from the TEM survey to be approximately 350 meters near the center of the survey. The location of the plug defined by the ground magnetic data is consistent with that from the TEM data. Gravity data mostly image the basin-basement interface with a small contribution from the plug of about 0.5 mGal. Results from this investigation can be used to help define the irregular subsurface topography caused by several intrusive lati-andesite plugs that could influence groundwater flow in the area.