Weighing Lysimeter Data Confirm the Accuracy and Precision of the Heat-Pulse Technique for Measuring Daily Soil Evaporation

The heat-pulse technique, which is based on the sensible heat balance, has been developed for measuring soil-water evaporation (E). The accuracy and precision of the technique has been previously tested with the Bowen ratio method and micro-lysimeters, which are themselves subject to considerable uncertainties. This study evaluated the accuracy and precision of the heat-pulse technique using direct, continuous, and nondestructive E measurements from a weighing lysimeter. Multi-needle heat-pulse probes were installed in a silt loam soil at four locations in a 22.5-m(3) (3-m x 3-m x 2.5-m) weighing lysimeter to measure soil thermal conductivity, heat capacity, and temperature gradients in the 0- to 5-cm layer. Daily E rates were estimated with sensible heat balance theory. Independent daily E rates obtained directly from a weighing lysimeter were used to evaluate the accuracy of heat-pulse E estimates during a 10-d drying period. Potential daily evaporation (E-0) was also computed using the Penman-Monteith equation. The E/E-0 ratio was >0.9 for the first 4 d but fell sharply thereafter and approached a stable value of 0.4. There was a linear relationship between heat-pulse values and weighing lysimeter values, with a slope of 0.95 and an offset of 0.09 mm d(-1). The absolute error of the heat-pulse E ranged from -0.02 to 0.29 mm d(-1) (or -1 to 8% of the true value), and the RMSE was 0.15 mm d(-1). The SD of the heat-pulse E data ranged from 0.07 to 0.51 mm d(-1), and the coefficients of variation ranged from 3 to 14%. We concluded that the heat-pulse technique can provide daily E measurements with good accuracy and precision.