Wxmax ze0UL 050llET0J5561

where Wx max is the maximum depth of water (mm) that can be evaporated from the soil when the upper 30 to 40 cm of the soil profile has been initially wetted completely, 6>ul is the soil water content at the drained upper limit of the soil (field capacity) (m3 m-3), 6>ll is the soil water content at the lower limit of root extraction (wilting point) (m3 m-3), and ze is the mean total depth of soil that is subject to drying by evaporation [100-200 mm]. If unknown, a value of ze = 150 mm is recommended. Typical values for 6>ul and 6>ll are given in Table 5.2. Soil water concepts are developed in Section 5.2.

The number of wetting (precipitation and irrigation) events Nw occurring during the initial period is determined by considering that two wetting events occurring on adjoining days should be counted as one event, and individual wetting events of less than 0.2 ET0 can be ignored.

The mean time interval between wetting events, tw (days), is determined by dividing the length of the initial growing stage tini (days) by the number of wetting events Nw, so that tw = tini/Nw. (5.62)

Pmean, the average depth (mm) of water added to the evaporating layer at each wetting event, is determined by dividing the sum of the precipitation and irrigation infiltration Pn + J2 In) occurring during all wetting events by the number of events Nw thus,

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