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

Abstract

Core Ideas Multi‐needle heat‐pulse probes are applied for measuring daily evaporation rate. Measurement accuracy and precision are confirmed with weighing lysimeter data. The probes provide a new way to monitor daily evaporation rate continuously. 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‐m3 (3‐m × 3‐m × 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 (E0) was also computed using the Penman–Monteith equation. The E/E0 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.