Evaluation of the Heat Pulse Ratio Method for Measuring Soil Water Flux

ods for measuring water flux (Hopmans et al., 2002; Mori et al., 2003; Ren et al., 2000; Wang et al., 2002). Soil water flux is an important hydrologic parameter, yet few techHeat pulse methods for measuring water flux are niques for measuring it in situ are available. Here we evaluate the based on measuring the convective transport of a heat heat pulse ratio method for measuring water flux. We conducted heat pulse measurements of flux in packed columns of sand, sandy loam, pulse introduced by a small heater. If the relationship and silt loam soil. Water fluxes were calculated from the data following between the water flux and the convective transport of both a traditional temperature increase difference method and a new heat is known, then flux can be determined. Traditiontemperature increase ratio method. Both methods yielded similar ally, heat pulse methods have relied on the difference beestimates of flux, agreeing to within 0.84 cm h 1 on average. The low tween temperature increases at points downstream and flow detection limits for both methods were also similar and ranged upstream of the heat source as the indicator of water flux from 0.1 to 0.4 cm h 1. However, the ratio method was superior in (Byrne et al., 1967, 1968; Kawanishi, 1983; Melville et al., that it permitted simpler calculations, reduced the number of required 1985; Ren et al., 2000). However, the mathematical form parameters by four, and exhibited two to three times greater precision. of the relationship between flux and the temperature inWe found strong linear relationships (r 2 0.98, standard error 0.4 cm h 1) between estimated and imposed water fluxes up to 40 cm crease difference is complicated (Kluitenberg and Warh 1. However, the slopes of these relationships were less than one, rick, 2001; Ren et al., 2000). This complexity is an obstaranging from 0.739 for the sand to 0.224 for the sandy loam. These cle to the implementation of heat pulse methods. Wang slopes indicate that the sensitivity was less than predicted by the et al. (2002) proposed that the ratio of temperature instandard conduction–convection model. We have not discovered the creases at points downstream and upstream of the heat cause of these errors, but we did find that the errors could not be source would serve as a better indicator of water flux. explained by increasing the magnitude of the conduction term in the Wang et al. (2002) showed theoretically that using the model as has been previously suggested. Instead, the errors could be temperature increase ratio would result in greatly simexplained by reducing the magnitude of the convection term. This plified data analysis if the temperature sensors were equifinding can help direct future research efforts to improve the accuracy of the ratio method. distant from the heat source. However, few data with which to evaluate the Wang et al. (2002) theoretical finding have been reported (Mori et al., 2003). If the heat pulse ratio method suggested by Wang et al. (2002) S water flux can vary in time and space by many can in fact enable precise and accurate in situ monitoring orders of magnitude and is a principal variable in subof water flux, then this method will be useful in a wide surface chemical transport, ground water hydrology, and range of hydrologic research endeavors. the soil water balance. Measurements of water flux in The main objective of this paper is to provide an emsoil and other porous geologic materials are difficult to pirical evaluation of the ratio method. Specifically, soil obtain, and the lack of suitable measurement techniques water flux estimates from the ratio method will be comcomplicates the study of many important research probpared with those from the traditional temperature inlems. For these reasons, scientists continue to search for more effective methods to measure water flux in situ. crease difference method, and the relative merits of each Recent encouraging developments include the automethod will be considered. Also, flux estimates from the mated equilibrium tension lysimeter (Brye et al., 1999; ratio method will be compared with independent flux Masarik et al., 2004), the controlled-suction period lymeasurements, and the differences between the two will simeter (Kosugi and Katsuyama, 2004), the vadose zone be examined. fluxmeter (Gee et al., 2003, 2002), and heat pulse meth-