Actual crop water use (ET) can be measured (by means of water balance, sap flow or micro-meteorology) or estimated (by calculation) in a number of ways. The water balance approach involves measuring the change in water content (volumetric) of the soil profile (ΔW) over a period of time after allowing for rainfall (P), runoff (R)
and deep drainage (D), and finding ET by difference: ET ¼ P R D ΔW.
This can be done at different scales – from a whole catchment, when comparing changes in land use from, for example, rain forest to tea or oil palm, to an individual tree grown in a large container (known as a lysimeter). Changes in soil water content can be measured gravimetrically or with a neutron probe or a capacitance probe.
Evapotranspiration (ET) has two components: transpiration (T) and evaporation (E) from the soil (and crop) surface. Both processes occur simultaneously, and there is no easy way of distinguishing between the two.
When the crop is small, water is predominantly lost by evaporation from the soil surface (while it remains wet), but once the cropcanopy covers the ground T becomes the main process (Allen et al., 1998).The sap-flow method (of which there are several variations) involves measuring the rate of flow of water up the stem using a heat pulse (or by carrying out a heat balance for a section of tree trunk). It is well suited to tree crops and has been tried,for example, on citrus and olive.
Micro-meteorological methods, namely the Bowen-ratio and eddy-covariance methods, involve measuring the flux of water vapour above a crop using an array of sensors. These methods have been used with several of the fruit crops reviewed in this book, including citrus, date palm, passion fruit and mango.
In most practical situations, potential crop evapotranspiration (ETc) is estimated using a formula such as the Penman equation or the Penman–Monteith equation, both of which require standard weather data, or a well-sited evaporation pan such as the USWB Class ‘A’ pan (Epan). These give estimates of evaporation from a standard crop
surface, usually taken to be short grass or alfalfa, well supplied with water, now known as reference crop evapotranspiration (ETo) (Allen et al., 1998). To convert this to potential water use by a specific crop (ETc) a crop factor (Kc) is needed. This varies with the stage of development of the crop.
ET c ¼ Kc ET
Unfortunately, few researchers define precisely the methods they have used to calculate crop water use (there are several versions of the Penman equation). This can sometimes lead to confusion. The guidelines provided by Allen et al. (1998) are intended to help to standardise the approaches used internationally.
Similarly, in two hard-hitting but valuable papers, Allen et al. (2011a; 2011b) highlight the knowledge,
skill and awareness needed if large errors are to be avoided when attempting to
measure ET in the field. The advantages and disadvantages of each method are listed,
together with the basic quality standards to be met if the data are to be valid and
representative of the surroundings.
Allen et al. (2011b) also describe the ways in which data should be documented so that readers can judge their authenticity.These two papers should be essential reading for scientists working in this field
of study. Many papers would not get published if reviewers insisted on this level of quality control.