EVAPORATION FROM BARE SOIL IN A TEMPERATE HUMID CLIMATE - MEASUREMENT USING MICRO-LYSIMETERS AND TIME-DOMAIN REFLECTOMETRY

In humid areas soil evaporation is an important factor in relation to soil tillage and timing of irrigation early in the growing season. Direct measurements of evaporation from a loamy sand were made using micro-lysimeters. The accuracy of daily measurement was about +/- 0.5 mm H2O day(-1). The micro-lysimeter method was not valid in periods with high precipitation. The use of time domain reflectometry (TDR) for measuring soil water content was investigated using a manual interpretation of the trace. The precision of changes in soil water content calculated from daily measurements with TDR was about 1.3 mm H2O, when using probes of 50 cm length. However, improved precision may be obtained by the use of an automatic interpretation of the trace. Estimates of daily evaporation from bare soil calculated from the water balance equation and measurements of soil water content with TDR were compared with measurements with micro-lysimeters. The TDR technique was suitable for estimating bare soil evaporation when the soil water content was integrated over a 0-50 cm soil profile and drainage had ceased at the lower depths of the profile. Evaporation during a 13 day drying period in spring, just after the soil had been fully rewetted, was about 26 mm. During a 23 day drying period later in the season the evaporation from the bare soil was about 30 mm. In both periods the accumulated evaporation was rather high and equivalent to about 65% and 50% of the accumulated potential evapotranspiration in the first and second drying period, respectively, even though the soil water content in the 0-50 cm profile was well below field capacity at the beginning of the second drying period.