Evaporatron from soil can be a major component of crop water balance and land surface energy balance. A number of differerit appl~cations of the mlcrolysimeter method to rncaaure evaporation from soil have been used rn recent studies. Microlysimetcrs were used extcnsivelq In three sandy soils for this study. Measurement of evaporatlon from microlysimeters with difkrent dimensions and of different ages allows discussion of the sources of error inherent in the method. Thc evaporatlon recorded from microlysimetcrs of diarncters 214 rnm, 152 mm and 51 mm was not significantl) different. A comparison of 100 mm and 200 mni deep niicrolysime~ers showed that depth had no significant ~nfluence dur~ng the first ? day5 after cxtructlon from thc soil profile. For periods beginning 2 or more days after ram. significant drferences in evaporation owing t o depth may not occur for up to 7 days. Soil cores extracted at different times showed significant differences in evaporation immediately following a rain cvent, and no significant difftrenccs 2 or more days thereafter. This pcriod of significant difrerence was extended to about 4 days when the method was used w~rhin a crop (i.c. root extraction of water in the field sig~~rficant). A protocol for use of rnicrolysinieters is developed from these results.

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Evaporation from soil, Es, is important to land surface energy balance and has been estimated in many studies using a surface resistance approach. We investigate the accuracy of this approach using detailed measurement and simulation. Hourly evaporation rates were measured using microlysimeters and load cells at two semiarid sites with bare soil. A numerical model of water (liquid and vapor) and heat fluxes in a soil profile (the soil water, energy, and transpiration (SWEAT) model) provided an accurate simulation of measured evaporation rates. Using output from SWEAT, relationships between soil resistance rs and soil surface water content θs (0–20 and 0–50 mm) are determined and are then used to estimate Es. These rs‐based models performed well over a period of several days but provided poor estimates of Es on an hourly or even a daily basis. A characteristic divergence between measured Es rates and potential evaporation rates at a time during the early daylight hours was not well simulated by rs‐based models. Anrs(θs) function for a similar soil at a different location underestimated Es by about 60%, Our work suggests that rs calculated from both evaporative demand and near‐surface soil water content θs is likely to be more accurate.

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Abstract. A soil water energy and transpiration model (SWEAT) coupled with a microwave emission model (MICRO-SWEAT) was used to predict the microwave brightness temperature of both bare and corn plots during a drying cycle. The predicted microwave brightness temperatures compared favorably to measurements made with an L band (21 cm, 1.4 GHz) passive microwave radiometer. In addition, SWEAT successfully modeled time series of soil water content and soil temperature. The modeled brightness temperature for the bare soil was most sensitive to the parameters describing the soil water retention and conductivity characteristics. These were predicted by varying each parameter in turn until there was a minimum between the measured and modeled brightness temperature. The predicted parameters were in agreement with the measured values to within the experimental error. The microwave brightness temperatures estimated for the corn soil were sensitive to the vegetation parameters as well as to the soil hydraulic properties. This paper analyses microwave emission data collected during a 10-day drying period for a bare and a corn plot. MICRO-SWEAT is validated and used to estimate the soil hydraulic properties. 1689