Evaluation of the heat pulse ratio method for measuring soil water flux

Soil water flux is an important hydrologic parameter, yet few techniques for measuring it in situ are available. Here we evaluate the heat pulse ratio method for measuring water flux. We conducted heat pulse measurements of flux in packed columns of sand, sandy loam, and silt loam soil. Water fluxes were calculated from the data following both a traditional temperature increase difference method and a new temperature increase ratio method. Both methods yielded similar estimates of flux, agreeing to within 0.84 cm h(-1) on average. The low flow detection limits for both methods were also similar and ranged from 0.1 to 0.4 cm h(-1). However, the ratio method was superior in that it permitted simpler calculations, reduced the number of required parameters by four, and exhibited two to three times greater precision. We 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 h(-1). However, the slopes of these relationships were less than one, ranging from 0.739 for the sand to 0.224 for the sandy loam. These slopes indicate that the sensitivity was less than predicted by the standard conduction-convection model. We have not discovered the cause of these errors, but we did find that the errors could not be explained by increasing the magnitude of the conduction term in the model as has been previously suggested. Instead, the errors could he explained by reducing the magnitude of the convection term. This finding can help direct future research efforts to improve the accuracy of the ratio method.