The soil water balance is made for the complete effective rooting depth, including the evaporation layer:

0 0 ^ [ Pi - ( Qr)i ] + (In) i - (ETc)i - DPi f GWi 6 = 6i-1 +-1000®-' (5.86)

where 6i is the mean volumetric soil water content in the root zone (m3 m-3) on day i, 6i-1 is soil water content on the previous day, Pi is depth of precipitation on day i (mm), (Qr )i is the runoff from the soil surface on day i (mm), (In )i is net irrigation depth on day i (mm), (irrigation water infiltrating the soil), (ETc)i is the crop ET from Eq. (5.52) on day i (mm), DPj is any deep percolation on day i (mm), GWi is any upward contribution of water from a shallow water table on day i (mm), and (Zr)i is the rooting depth on day i (m). Estimation of GW, is well described by Martin and Gilley [74]. (Qr)i can be predicted using the SCS curve-number method [104].

Deep percolation is estimated as DPj = 0 when 6i < 6ul, and DPj = 1,000(0,- — 6ul)(Zr)i otherwise. In some applications, 6i may be allowed to exceed 6ul for one day before DPj > 0 to account for some ET from excess soil water before it drains from the root zone.

The depth of the effective root zone for any day i can be predicted as

Lrd for Ji < Jini + Lrd, where (Zr)i is the effective depth of the root zone on day i (m), (Zr)min is the initial effective depth of the root zone (generally at J = Jni), and (Zr)max is the maximum effective depth of the root zone (m). Ji is the day of the year [Eq. (5.30)] corresponding to day i and Jini is the day of the year corresponding to the date of planting or initiation of growth (or January 1 if a perennial is growing through all months of the year). Lrd is the length of the root development period (days). When Ji > Jini + length, (Zr)i = (Zr)max. Indicative values for (Zr)max are given in Table 5.1.

The latest date for scheduling irrigation to avoid water stress is when 6i equals 6t [Eqs. (5.83) and (5.86)]. However, irrigations often are scheduled when the MAD fraction of water is depleted, where MAD may be higher or lower than Fns [Eq. (5.83)]. In this case, irrigation is scheduled when

6i = 0MAD = (1 — MAD)(0ul — 0ll) + 0LL • (5.88)

The net irrigation depth to be applied then would be

The soil water balance currently is computed through crop-water simulation models that allow the selection of best irrigation scheduling alternatives [49-51]. Irrigation scheduling principles and applications are described in Section 5.3.

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