Temperature Studies
In a connected system, the exchange of water between the stream and shallow aquifer plays a key role in influencing temperature not only in streams, but also in their underlying sediments. As a result, analysis of subsurface temperature patterns can provide information about seepage flux. Studies, notably in North America, have used temperature monitoring in the stream and underlying sediments as a screening tool for identifying gaining and losing reaches (Silliman and Booth 1993; Stonestrom and Constanz 2003). Recently, heat as a tracer has been demonstrated to be a robust method for quantifying surface water-groundwater exchanges in a range of environments, from perennial streams in humid regions (Lapham 1989; Silliman and Booth 1993) to ephemeral channels in arid locations (Stonestrom and Constanz 2003).
Figure 1: Common temperature sensors used to measure sediment and stream temperatures (Stonestrom & Constanz, 2003)
Logging devices that measure temperature at specific time intervals and store the information in memory can be installed both within the stream and at different depths in the sediments below the stream bed. The sensors most often employed are thermocouples, thermistors, resistance temperature devices and integrated circuit sensors (Figure 1). The characteristics of each type of temperature logging equipment along with the advantages and disadvantages and installation methods are presented in Stonestrom and Constantz (2003).
The hydraulic transport of heat enables its use as a tracer with temperature monitoring especially suited for delineating fine-scale flow paths. The heat tracer method has been used to estimate groundwater velocity and aquifer hydraulic properties, and to identify areas of recharge and discharge (eg. Bouyoucos 1915; Suzuki 1960; Lee 1985; Lapham 1989; Silliman and Booth 1993; Conant, 2004). One way of using heat tracing in stream-aquifer studies is to compare the temporal patterns evident in stream and shallow sediment temperature. Stream temperatures have a characteristic diurnal pattern overprinting seasonal trends, being influenced by changes in solar radiation, air and ground temperature, rainfall and stream inflows that include groundwater discharge (Sinokrat and Stefan, 1993). These diurnal variations in temperature in the near-stream environment are often large and rapid, providing a clear thermal signal that is easy to measure. In contrast, the temperature of regional groundwater tends to be relatively constant at the daily scale. The movement of heat between surface water and groundwater systems is both advective (associated with fluid movement) and conductive (through the static solid/liquid phase). Ignoring the effect of insitu sources of thermal energy (such as from biological activity), the temperature pattern in the shallow stream sediment profile can be used to evaluate seepage flux.
Temperature Signatures
The temperature signatures for three potential forms of stream-aquifer connectivity (gaining, losing and neutral) have been hypothesised (Silliman and Booth 1993). In gaining stream reaches, the hydraulic gradient is upward as indicated by a groundwater level in piezometers that is higher than the stream stage (Figure 2a). Although the stream has a large diurnal temperature variation, the shallow sediment has only slight or no diurnal variation. The downward propagation of any surface temperature effects is moderated by water that is flowing up from depths where temperatures are constant at the daily time scale. At any given depth beneath the streambed, higher flows of groundwater to the stream lead to smaller variations in sediment temperature while smaller flow leads to larger variations. Consequently, shallow installation of temperature equipment is necessary to characterise gaining stream reaches, in order to detect significant temperature variations.
In losing stream reaches, the stream stage is higher than the groundwater level so that the potential hydraulic gradient is downward. This downward flow of water transports heat by advection from the stream, resulting in deeper propagation of diurnal temperature fluctuations into the sediment profile (Figure 2b). As a consequence, deeper installation of temperature equipment (inside the piezometer or beneath the stream bed) is necessary for losing streams to be characterised. Losing streams also tend to have larger daily temperature fluctuations than gaining reaches, due to the absence of any moderating effect from groundwater inflow (Constanz 1998).
Figure 2: Temperature variation of groundwater and stream under gaining (a) and losing stream (b) conditions (adopted from Stonestrom & Constanz, 2003)
In neutral reaches of the stream, thermal conduction will control stream sediment temperatures. This means that sediment temperature can vary due to changes in surface water temperatures, and will have an average that is between that of the surface water and groundwater. The distinct temperature signal of episodic infiltration associated with ephemeral streams has also been characterised (Ronan et al, 1998; Constanz et al, 2002).
Temperature Modelling
Estimating the water exchange between the stream and shallow aquifer requires knowledge of the hydraulic and thermal conductivity of the material and the hydraulic gradient as defined by the stream stage and groundwater level. Numerical models of heat flow, such as VS2DH (Healy and Ronan 1996) and SUTRA (Voss 1984) can be used to quantify seepage fluxes. These can supplement and help calibrate more traditional groundwater flow models. In particular, temperature modelling can help constrain estimates of hydraulic conductivity, which can vary over several orders of magnitude, as the thermal conductivity of sediments has a much smaller range of potential values. Stream sediments composed of sand and gravel can have a hydraulic conductivity six orders of magnitude higher than clay. In contrast, the thermal conductivity of porous materials depends upon the composition and arrangement of the solid phase with the potential range in thermal conductivity between coarse grained sand (2.2 W/m oC) and clay (1.4 W/m oC) being much smaller than that for hydraulic conductivity (Stonestrom and Constantz 2003). The work of Bravo et al (2002) is a recent example of using temperature data to constrain estimates of boundary fluxes and hydraulic conductivity in a groundwater flow model for a wetland system.
Fluctuations in temperature can also directly influence seepage rates due to its influence on water density. The hydraulic conductivity of the stream bed is both a function of the porous medium and the water itself, the latter in terms of density and dynamic viscosity. Hence, transmission rates through the sediment bed can increase with increased water temperature. This process was used to explain diurnal variations in seepage flux in losing reaches of a small alpine stream (Constanz 1998).
Advantages
- Temperature logging devices are robust, simple, relatively inexpensive and available for various scales of measurement. Once installed, loggers can also provide useful time-series data that can provide information on seasonal changes in seepage flux.
- The temperature signal arrives naturally and the temperature data are immediately available for inspection and interpretation.
- Temperature monitoring can be used as a screening tool for identifying gaining and losing stream reaches. Such a screening method can be valuable both as a rapid investigative tool for small studies and as a precursor to more detailed studies such as the design/installation of a groundwater monitoring network.
- Temperature studies are particularly useful in defining small-scale flow paths, such as are associated with stream banks or sand bars (Stonestrom and Constanz 2003).
Disadvantages
- Interpretation of the temperature data can be ambiguous when viewed in isolation. It is recommended that temperature monitoring be used in conjunction with other methods such as minipiezometers, seepage meters or hydrographic analysis when interpreting stream-aquifer connectivity. It can be difficult to separate out localised effects (such as associated with weirs or shallow throughflow) from the broader seepage domain.
- Temperature measurement is at a point in space and many measurements may be required to obtain information on spatial variability.
It is suggested that temperature loggers can be readily and cheaply incorporated into existing hydrographic networks to provide a supplementary dataset for understanding stream-aquifer connectivity. This is because the water level data can indicate the potential seepage direction and the temperature data can help estimate the magnitude of the seepage. It is also recommended that the existing temperature logging used to calibrate pressure transducers for monitoring water levels be upgraded to sufficient accuracy for heat transfer studies. Temperature monitoring would be particularly useful in estimating infiltration rates in Australian ephemeral streams, where conventional water level recording and hydrographic analysis is problematic. Routine recording of temperature data also has relevance to the investigation and management of aquatic ecosystems, notably within the hyporheic zone.
Data Availability
Stream temperature data can be routinely collected as part of the stream gauging network, as pressure transducers that measure stream level require such data for calibration. Hydrographic databases are available across Australia, mostly maintained by State and Territory water management agencies, refer Hydrology Data.
However, this temperature data is not commonly made available and may not be of adequate resolution for heat transfer studies. Groundwater temperature can also be recorded in the same way, as pressure transducers are a common logging device for groundwater levels. It would be rare for both groundwater and surface water temperatures to be monitored in close enough vicinity at a site for application in heat transfer studies. For instance, temperature monitoring in the shallow stream bed sediments at stream gauging stations is not routinely done.
Case Studies
Border Rivers, Qld/NSW
Daly River, NT
Juday Creek, Indiana USA
Willamette Basin, Oregon USA
References
Bouyocous G, 1915. Effects of temperature on some of the most important physical processes in soils. Michigan College of Agriculture Technical Bulletin 24, 63p.
Bravo HR, Feng J, Hunt RJ, 2002. Using groundwater temperature data to constrain parameter estimation in a groundwater flow model of a wetland system. Water Resources Research 38(8): 1153.
Convant B, 2004. Delineating and quantifying ground water discharge zones using streambed temperatures. Ground Water 42(2), 243-257
Constanz J, 1998. Interaction between stream temperature, streamflow, groundwater exchanges in alpine streams. Water Resources Research. 34, 1609-1616.
Constanz J, Stewart AE, Niswonger R, Sarma L, 2002. Analysis of temperature profiles for investigating stream losses beneath ephemeral channels. Water Resources Research 38(12): 1316.
Healy RW, Ronan AD, 1996. Documentation of the computer program VS2DH for simulation of energy transport in variably saturated porous media - Modification of the US Geological Survey's computer program VS2DT. US Geological Survey Water Resources Investigation Report 96-4230, 36p.
Lapham WW, 1989. Use of temperature profiles beneath streams to determine rates of vertical ground-water flow and vertical hydraulic conductivity. US Geological Survey Water Supply Paper 2337
Lee DR, 1985. Method for locating sediment anomalies in lakebeds that can be caused by groundwater inflow. Journal of Hydrology 79:187-193.
Ronan AD, Prudic DE, Thodal CE, Constanz J, 1998. Field study and simulation of diurnal temperature effects on infiltration and variably saturated flow beneath an ephemeral stream. Water Resources Research 34:2137-2153.
Silliman SE, Booth DF, 1993. Analysis of time-series measurements of sediment temperature for identification of gaining vs. losing portions of Juday Creek, Indiana. Journal of Hydrology. 146: 131-148.
Sinokrat BA, Stefan HG, 1993. Stream temperature dynamics: Measuring and modelling. Water Resources Research 29(7): 2299-2312.
Stonestrom DA, Constanz J (eds), 2003. Heat as a tool for studying the movement of ground water near streams. US Geological Survey Circular 1260. http://pubs.water.usgs.gov/circ1260
Suzuki S, 1960. Percolation measurements based on heat flow through soil with special reference to paddy fields. Journal of Geophysical Research 65(9): 2883-2885
Voss CI, 1984. A finite-element simulation model for saturated-unsaturated, fluid density-dependent ground-water flow with energy transport or chemically-reactive single species solute transport. US Geological Survey Water Resource Investigations Report 84-4369, 409p.