The physical bases of irrigation control

Irrigation designed to replace transpiration losses can be controlled if transpiration rates can be adequately estimated. As a particular form of natural evaporation, transpiration is dominantly a weather-controlled phenomenon in which plant character plays only a minor part, and rates can be calculated from weather data. The physical principles, involving energy supply and turbulent transport of vapour are outlined for open water, first because they are most clearly revealed for open water, and second because for S.E. England it has been possible to convert estimated open water evaporation into estimated transpiration by using an empirical conversion factor. By an extension of the principles and the introduction of stomatal and daylength factors, it has proved possible to eliminate local factors and to estimate transpiration rate directly from weather data without first calculating the rate for a hypothetical open water surface. The special case of orchard crops is separately treated. Field checks, chiefly in the more extreme climate of southern Australia, have been satisfactory, but only by accepting somewhat arbitrary values of stomatal conductance for diffusive flow of water vapour. The checks are equally successful for short crops and for orchard crops. [Author's summary.].