Back to the basics of understanding ET

Hydrocomplexity occurs when hydrologists realize that improved theoretical description of a hydrological process requires the representation of controlling features that hitherto had not been considered necessary. This paper makes a critical reappraisal of currently recommended methods for estimating the water requirements of irrigated crops, which reveals there is a fundamental theoretical inconsistency between present day understanding of the interaction between plant canopies and the atmosphere as represented by the Penman-Monteith (P-M) equation, and the procedures for estimating plant water requirements currently recommended by FAO. In the P-M equation, stomatal and aerodynamic controls on the transfer processes are expressed in terms of resistances which are embedded among the meteorological controls with crop-to-crop differences expressed in terms of different values for these resistances. However, the current procedure recommended by FAO for estimating crop water represents crop-to-crop differences as a simple multiplicative crop factor applied to an estimated evaporation rate calculated by the P-M equation for a single reference crop with fixed surface resistance and aerodynamic characteristics. Recent theoretical developments that allow adoption of the more robust P-M equation description of ET for all irrigated crops are reviewed along with an example application of this new approach to estimate the water requirements in the major irrigation districts of Australia. Broader adoption into irrigation practice of this method, which is known as the Matt Shuttleworth approach, is recommended on the grounds that it is consistent with presentday understanding of the evaporation process, is feasible and simple to apply, and will facilitate future adoption of realistic representations of the effect on evapotranspiration of plant stress and of crops with partial ground cover. However, when not all the weather variables needed to calculate crop evaporation rates are available, an estimate of reference crop evaporation may still have to be made by scaling down the measured evaporation loss from an evaporation pan by a "pan factor". In the past the value of this pan factor has been defined empirically but recent research into the physics which controls evaporation from the Class A evaporation pan has resulted in a physically-based equation that describes pan evaporation in terms of ambient climate variables. This equation, which has been verified experimentally, allows a formal definition of the pan factor that is used to investigate theoretically how ancillary measurements (or estimates) of temperature and wind speed at an evaporation pan site might be used to improve the accuracy of a pan-based estimate of reference crop evaporation.