MEASURING WATER EXCHANGE BETWEEN SOIL AND ATMOSPHERE WITH TDR-MICROLYSIMETRY

Soil evaporation is an important hydrologic and energy transfer process, yet it has historically been difficult to measure. Weighing microlysimetry is a commonly employed and accepted technique that is accurate and inexpensive, but it is time consuming and labor intensive, with limited temporal resolution. We modified the method by fitting a time domain reflectometry (TDR) probe in each microlysimeter extending from the bottom to the top. When they are installed, the probes pierce a plastic film lining the bottom of the microlysimeter. The holes thus created allow free drainage when rainfall is sufficient to induce it, but they prevent unsaturated flow in either direction. The use of an automated TDR system allowed continuous, unattended measurement of both evaporative losses and rainfall additions in multiple lysimeters. The TDR data compared favorably with water content changes measured at discrete intervals by weighing, and also with independent measurements of daily evaporation losses and rainfall additions measured by the Bowen ratio method and a rain gauge. Short-term (e.g., hourly) rate estimation exhibited considerable scatter, but further improvement should be possible. The TDR-microlysimeters were replaced less frequently than is typically the case with standard microlysimeters, without serious consequences despite numerous rains. The method should be useful in many studies where information on soil/atmosphere water exchange is required.