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Using a combination of a surface energy balance equation and an approximate expression of water vapor and sensible heat transfer, an equation is formulated relating potential evaporation for net radiation, ambient air properties, and surface roughness. As an improvement over the earlier Penman version, the proposed model contains no empirical constants or functions. Tests of the model in Phoenix, Arizona, using open water, wet bare soil, and well watered alfalfa, show excellent agreement of calculated and measured values on an hourly and daily basis under a variety of circumstances. Test conditions typically included advection of sensible heat to the evaporating surface in a large proportion to the latent heat flux, as well as high values of radiant flux, ambient temperatures, and vapor pressure deficits. The use of daily average values for the weather variables in the model, rather than hourly data, did not give appreciably different results or conclusions on a series of mostly clear days, suggesting that the combination model is not only accurate but also practical and generally applicable. (Key words: Evaporation; evapotranspiration; meteorology)

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Dryland crops grown in semiarid environments often do not completely cover the soil, leaving a portion of the soil surface exposed to a condition of rapid soil‐water evaporation. Quantitative separation of soil evaporation and crop transpiration is important if cultural practices or cultivars are to be evaluated. This study was designed to evaluate a combined energy and water balance model, ENWATBAL, to describe the concurrent heat and water fluxes in a row crop. Inputs to the model include soil and plant variables and daily weather data. Measurements were made for a period of 74 d over a cotton (Gossypium hirsutum L.) canopy during 1985 on an Olton soil (fine, mixed, thermic Aridic Paleustolls) at Lubbock, TX. Data collected included soil‐water content, soil temperature, root distribution, soil evaporation with microlysimeters, and leaf area index, for both an irrigated and a dryland plot. The values for daily evaporation and evapotranspiration calculated with the model were within 1 standard deviation of the measured values. Cumulative evaporation and evapotranspiration from the model agreed with measured values within 7% for the dryland and 8% for the irrigated plot. Estimated soil‐water and temperature profiles also agreed closely to measured values. Soil evaporation was found to be 30% of evapotranspiration, for both the irrigated and the dryland plot. The ENWATBAL model provides a reliable method of evaluating the effects of management practices and crop selection on the water‐use efficiency of crop production in a semiarid area.

Heat balanee tnethods of stetn flow measurement offer the opportunity to tneasure directly the mass flow rate of water in plants. We have tested one sueh approaeh; the constant power heat balance method of Sakuratani (1981). The results supported his statement of an approximate accuracy of 10% when measuring the transpiration rate of herbaceous plants. The response to sudden changes in stem flow rate is not instantaneous, but investigation of the time constant shows that it decreases as stem flow inereases, to the extent that, at flow rates typieal of daytime conditions the system is capable of accurately tracking changes in stem flow within 5 min or less.We describe a new gauge design that is relatively rugged, sitnple to use with an appropriate digital datalogger and suitable lor field use over prolonged periods of time. It does not injure or penetrate the stem, is amenable to eontinuous and direct reeor-ding of the mass flow rate of water in the stem and requires no calibration. A further refinement, which should itnprove both the aeeuracy and the dynamic response of the system, is pr-oposed.

The internal wiring of an existing stem or trunk flow gauge was redesigned to obtain greater accuracy of the gauge itself, eliminate errors due to signal loss in connecting cables, and reduce the number of channels and of the computing required of the datalogger. Tests of the gauge conducted on bald cypress (Taxodium distichum) and Ficus retusa (L.) Nitida trees, in a greenhouse and in an urban backyard, and under well‐watered and dry conditions gave daily sap mass flow rates that were within 5% of those obtained by direct weighing.

Experiments and calculations were performed to characterize the water relations of fritted clay, a material which has been found suitable for growing experimental plants. Its dry bulk density is 0.67 kg/liter, particle density 2.50 kg/liter, total porosity 0.73, and saturated hydraulic conductivity 9.5 × 10−4 m/sec. The desorption relation was measured and the unsaturated hydraulic conductivity as a function of water content was calculated. Much water drains from the saturated material by gravity. After drainage from commonly used containers, the material holds 0.31 by volume of plant‐available water, and has an air‐filled porosity of 0.28. Its water relations are excellent for plant growth purposes.