Where the Vine Roots Live - Understanding Vineyard Soils - page 139

In the subsoil at 40 to 60 cm, RAW and DAW were1.3%and1.6%( θ = 0.013
and 0.016 m 3 /m 3 ),respectively,correspondingto2.6and3.2mm,respectively,in
this depth interval. Thus we see there is generally more RAW plus DAW in the
topsoil than in the subsoil, where the pores are narrower and water is held more
tightly.
Values of RAW and DAW should be calculated for the depth of interest in the
soil profile, which may be the depth for irrigation management, or the maximum
rooting depth in dry-grown vineyards. Table 4.3 shows results for the clay loam of
igure4.13,calculatedusingtheequationfor d given above, assuming that the 0 to
10-cmdataapplytothewholeAhorizon(40cmdeep)andthe40to60-cmdata
applytothewholeBhorizon(40to120-cmdeep).hemoreeasilyavailablewater
( RAW plus DAW )for0to40-cm(Ahorizon),40to120-cm(Bhorizon),andthe
wholeproile(0to120-cm)areshown.Alsoshownisthetotalplantavailable
water( PAW ),whichincludesallthewaterbetween FC and PWP for the relevant
soil depths.
Although the relationship between the amount of soil water and its matric
suction is mainly influenced by soil structure at small suctions, the effect of texture
is more important at large suctions. Because texture is a less changeable property
than structure, representative values of RAW, DAW, and PAW have been devel-
oped for soils of different texture. This information, shown in table 4.4, is used for
managing the water supply to grapevines under irrigation and also for “Managing
NaturalSoilVariabilityinaVineyard”(seechapter6).

Table 4.3 Calculated RAW, DAW , and PAW at Various Depths in the Clay Loam Soil of
figure 4.13

( RAW + DAW ) a
(mmperdepth
interval)

Soildepthinterval
(cm)andtexture

RAW a (mmper
depthinterval)

DAW a (mmper
depthinterval)

PAW a (mmper
depthinterval)

A horizon
0-10(loam) 9 9 18 20

10-40(clayloam) 27 27 54 60

Total 0-40 36 36 72 80

B horizon
40-120(clay)

10 12 23 48

Whole profile
0-120 46 49 95 128

Note. RAW = readily available water; DAW = deficit available water; PAW = plant available water.
a Calculatedusingtheequation d (mm)=10 × θ × z , where z is measured in centimeters. All numbers
rounded to no more than two significant figures.