How Do Crops Manipulate Water Supply and Demand?

The supply of water provided by the root system of a crop stand is defined in terms of the rate at which water is extracted by a root front moving downwards with a constant velocity, the available water per unit soil volume, and a time constant that is inversely proportional to root density. The demand for water, often identified with a potential transpiration rate, is defined in terms of a maximum crop growth rate multiplied by the conservative ratio of transpiration to dry-matter production. From experimental evidence, supported by theory, this ratio is proportional to the mean saturation vapour-pressure deficit. As hypothesized, the root front accelerates during seedling establishment to keep demand and supply in balance. Once a maximum root velocity is reached (ca. 2-4 cm d^-1) transpiration is limited by water supply, except when the soil behind the root front is wetted by rain or irrigation, when it is limited by demand. Irrigation amounts and timing can both be estimated from this scheme.