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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.

SUMMARYA Linke-Feussner radiometer was used to measure the radiative temperature of natural surfaces, with small errors caused by the divergence of radiative flux and by its angular variation. O n typical cloudless summer days, the maximum temperature of tall crops and of open water was close to maximum air temperature; a bare soil surface exceeded air temperature by 20°C; short grass was the coldest surface at night. A linear variation of net (total) radiation with net short-wave radiation was established for several tall crops, and the increase in long-wave loss per unit increase of net radiation (heating coefficient) was 0.08. Daily totals of net radiation as a percentage of solar radiation income were : 37 per cent (bare soil), 11 per cent (short grass), 46 per cent (tall crop), and 53 per cent (water).When the aerodynamic character of a crop is known, the effective resistance of the stomata to water-vapour diffusion can be related theoretically to the difference between surface and air temperature. For rough vegetation, estimated stomata1 resistance is 0.4 to 0.8 sec cm-I and the estimated heating coefficient is almost independent of wind speed at about 0.1.

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SUMMARYIncoming short-wave radiation S, reflected short-wave radiation u S and net radiation R were measured over bare soil and crops from 1957 to 1959, and net long-wave radiation ( L ) was deduced from

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Measurements of Zo and other aerodynamic qualities of three crops in Germany (pine forest, lucerne, potatoes) are combined with British records and with those of Tanner and Pelton, permitting the following generalizations. For pliable agricultural crops the roughness length decreases with increasing wind speed (to % for I to 3 m sec-•), and in this range, Zo --crop height/10. Calculations of eddy conductivity (l/r,) give values 3 times that of open water for the farm crops, and 30 times that of open water for the trees. From measured components of the energy balance the surface resistances of the crops are calculated using Monteith's empirical relation. Values of r, for the trees-between 1.0 and 1.5 sec cm-•-are always 2 to 4 times as great as for the farm crops, indicating very effective stomatal control of transpiration by pine leaves. Including the two specific crop parameters in Penman's combination formula gives estimates of evaporation for open water, pine forest, potatoes, and lucerne that agree within 5-15% with measurements in southern England and in California. R, L, E, Symbols Net radiation, Soil-heat flux, Short-wave radiation from sun and sky, Net long-wave radiative loss of the surface, Evaporation, Latent heat of evaporation, Evaporation in energy units, cal cm -• sec -• Air temperature at height z, øC cal cm -• sec -• cal cm -• sec -• cal cm -• sec -• cal cm -• sec -• g cm -2 sec-1 585 cal g-• e, Vapor pressure of air at height z, mb e, (T), Saturation vapor pressure at temperature T, mb iX, Slope of the saturation vapor pressure curve, mb øC -• % .;• Psychrometric constant, 0.66 mb øC -• oc, Volume heat capacity of dry air, 2.9 10 -4 cal cm-• oC-•

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