Abstract
Measurements of evaporation from barley using a neutron moisture meter are compared with estimates from the Penman formula. Values of aerodynamic resistance ( r a) in 1971 are calculated from profile measurements of wind and temperature and empirical relationships are derived between r a, crop height and wind speed. The surface diffusive resistance ( r s) is calculated throughout the 1971 season (using the generalised Penman formula derived by Monteith, 1965) from measurements of net radiation, soil heat flux, temperature, vapour pressure ( r a, assumed to be equal to the resistance to transfer of water vapour) and evaporation either obtained by the Bowen ratio method or from a weighing lysimeter. A relationship is then derived between r s and leaf area index and a function of soil moisture deficit. The same formula is used in conjunction with the empirical values of r a and r s to derive estimates of evaporation in 1970. These show good agreement with the measured values. The aim is to obtain results of practical use and the empirical relationships are therefore kept as simple as possible, using only easily measured or estimated quantities.
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