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

^a USDA-ARS, Soil and Water Management Research Unit, St. Paul, MN 55108
^b Dep. of Agronomy, Iowa State Univ., Ames, IA 50011
^c Dep. of Agronomy, Kansas State Univ., Manhattan, KS 66506
^d State Key Lab. of Soil Erosion and Dryland Farming on the Loess Plateau, Institute of Soil and Water Conservation, Chinese Academy of Sciences, Yangling 712100, China, and Institute of Water Resources Research, Xian Univ. of Technology, Xian 710048, China

View larger version (49K): [in a new window]   Fig. 1. Samples of measured temperature increase differences (top panel) and temperature increase ratios (bottom panel) in sand, sandy loam, and silt loam soil for water fluxes from 0 to 20 cm h–1. The shaded regions in the bottom panel indicate the time window over which the temperature increase ratio was averaged.

View larger version (13K): [in a new window]   Fig. 2. Measured values of (a) maximum dimensionless temperature increase difference (MDTD) and (b) ln (Td/Tu) as functions of water flux through the soil columns.

View larger version (26K): [in a new window]   Fig. 3. Theoretical dimensionless temperature responses versus water flux for continuous heating (filled circles), for a 15-s heat pulse using the maximum dimensionless temperature increase difference (MDTD) method (open circles), and for a 15-s heat pulse using the ratio method (filled triangles). Thermal conductivity was 1.5 W m–1 K–1, volumetric heat capacity was 2.5 MJ m–3 K–1, needle spacing was 6 mm, and for the continuous heating scenario dimensionless temperature increase difference after 3600 s is shown.

View larger version (24K): [in a new window]   Fig. 4. Comparison of water flux estimated by the maximum dimensionless temperature increase difference (MDTD) method with water flux estimated by the ratio method. The inset expands the scale at the low end.

View larger version (20K): [in a new window]   Fig. 5. Water flux estimated by the ratio method versus water flux measured at the column outlet.

View larger version (38K): [in a new window]   Fig. 6. Measured temperature increases at the highest water flow rate for sand, sandy loam, and silt loam soil and modeled temperature increases using the original model (a = b = 1), an enhanced conduction model (a = 1/S), and a reduced convection model (b = S). Root mean square error (K) is shown in parentheses at the bottom of each pane.