Surface Resistance of Crop Canopies

Three analytic and two emperic methods are described to calculate the surface resistance rs of crops transpiring at a measured rate. The profile method is applicable when detailed temperature, humidity, and wind profiles are measured; for the residual method, measurements of surface temperature, wind, and humidity are enough; for the heat balance method, the calculation is based on the ratio of potential to actual evaporation. For rough estimates, an empiric equation of Monteith or a relation between leaf area and surface resistance can be used. In southern England and in California all three analytic methods agree closely. Hourly values of rs in California demonstrate the effect of water stress on an irrigated grass canopy by midday, whereas in England the rs of a barley crop is maintained constant for almost the whole day. From Aslyng's measurements of evaporation, the relation of rs to soil-water potential is calculated and used to show how relative rates of transpiration and photosynthesis may change in response to water stress. For an equatorial rain forest in Kenya, mean monthly rs varied systematically with soil moisture deficit, ranging between 0.3 and 1.1, sec cm−1.

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