Soil Water Balance and Irrigation Water Requirements Soil Water Balance

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.

Was this article helpful?

0 0
Growing Soilless

Growing Soilless

This is an easy-to-follow, step-by-step guide to growing organic, healthy vegetable, herbs and house plants without soil. Clearly illustrated with black and white line drawings, the book covers every aspect of home hydroponic gardening.

Get My Free Ebook

Post a comment