Agriculture Reference
In-Depth Information
Box 6.10
Regulated Deficit Irrigation and Irrigation Scheduling
First, calculate the stored soil water S at FC . An example of how to do this is
given in appendix 9. Suppose that the soil is a uniform sandy loam for which S
300 mm/m depth of soil. From table 6.3, the RAW and DAW per m depth are 50
and 30 mm, respectively. Hence
Critical SWD to trigger irrigation
80 mm (B6.10.1)
Since the FC value is 300 mm, the critical SWD is reached when the stored soil
water is at the lower stress limit, S sl :
S sl
( RAW
DAW )
220 mm (B6.10.2)
The regulated deficit range for this soil is 30 mm/m depth, and the value of S ns at
the no-stress limit is
S ns
300
80
300
50
250 mm
(B6.10.3)
50 mm/m depth of soil.
When the soil has reached the lower stress limit, 30 mm of irrigation should be
applied to bring it back to the no-stress limit. Therefore, 30 mm is the depth of
water to be applied per ha and is equivalent to 30 L/m 2 of soil area. This
calculation assumes that the vine roots explore all the soil to a depth of 1 m.
Vine root distribution depends on a number of factors, the most important of
which follow:
1. Vine age (the older the vine, the deeper its roots), variety or clone, and
rootstock (but see factor 2)
2. Soil properties, whether there is a physical or chemical impediment
3. Method of irrigation (drip, minisprinklers, overhead sprinklers, furrow, or
flood), and its scheduling (frequent and shallow versus infrequent and deep)
Factors 1 and 2 are discussed in more detail in chapter 7. Factor 3 is
discussed in section 6.6. Appendix 10 gives examples of RDI volumes calculated
for different root distributions, according to soil type, number of vines per ha, and
method of irrigation.
Thus, the SWD should fluctuate between
80 and
used instead of E o , a different factor, called a crop coefficient C c , is used to calcu-
late E a as follows:
E a E p C c (6.13)
Because E p E o , crop coefficients are larger than crop factors for the same
conditions, as shown in table 6.4. The measurement of crop factors and coeffi-
cients is discussed in box 6.11.
The development of a SWD is calculated using equation 6.11 for a conve-
nient time period, usually 1 week. Assuming the soil is at FC at bud burst (max-
imum stored soil water S ), for week 1
S P E a SWD 1
(6.14)
and for week 2
SWD 2 SWD 1 ( P E a )
(6.15)
and so on.
 
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