Agriculture Reference
In-Depth Information
Table 4.4
The Range of
RAW, DAW
, and
PAW
for Soils of Different Texture (measured in
millimeters per centimeter depth of soil)
Soiltexture
RAW
DAW
RAW
+
DAW
PAW
Loamysand
0.55
0.15
0.70
0.86
Sandyloam
0.64
0.28
0.92
1.15
Sandyclay
loam
0.71
0.41
1.12
1.43
Clay loam
0.65
0.51
1.16
1.48
Heavy clay
0.41
0.18
0.59
1.20
Note. RAW
= readily available water;
DAW
= deficit available water;
PAW
= plant available water.
CompiledfromNicholas(2004).
Managing Soil Water with Irrigation
HowVineGrowth,WaterStress,andGrapeFlavorAreRelated
Water stress is a vine's physiological response to a limitation in water supply.
AstandardrubricofOldWorldviticultureisthatvinesmustexperiencesome
water stress to enhance the sensory sensations of the wines produced. Better wine
qualityhasbeenlinkedwithloweryields,sothatAppellationd'OrigineContrôllée
wines in France, for example, normally cannot be made from irrigated vines. This
concepthassomefollowinginothercountriessuchasChileandSouthAfrica,
whereviticulturehasbeenestablishedforalongtime.Nevertheless,irrigation
is now used to supplement the water supply to vines in many vineyards around
the world where summers are hot and dry, because commercial wine production
wouldnotbepossiblewithoutit.ExamplesarefoundintheLaMancharegion,
Spain,theCentralValleyofCalifornia,theMurray-Darlingregion,Australia,and
thefast-expandingvineyardareasintheNingxiaregionofChina.
hereareseveralwaysofassessingvinewaterstress(describedlater).Inthis
sectionitisdiscussedintermsofavailablesoilwater.Whensoilwateriswithinthe
RAW
range(seeigure4.13),vinesarewellwateredandexhibitactivelygrowing
shoot tips and normal stem internode expansion. However, if water is too readily
available, especially during the period of rapid shoot growth from flowering to
veraison, excess vigor may result, as evidenced by long shoots and a large leaf area
andcanopydensitythatcauseexcessivefruitshading(igure4.14).Vinesdisplay-
ing too much canopy relative to the amount of fruit carried are said to be “out of
balance.” For example, a balanced vine should have a leaf area to fruit weight ratio
of 1 to 1.5 m
2
per kg, which provides sufficient leaf area for satisfactory photosyn-
thesis and sugar accumulation but not so much that ripening is retarded.
Whenmoderatewaterstressoccurs(soilwatercontentatthebottomofthe
DAW
rangeshowninigure4.13),shootgrowthslows,internodesshorten,andshoottips
maybecomeadullgray-green.Atthehottestpartoftheday,leavesbecomelaccid(a
signoftemporarywilting)andwarmtothetouch.Withmoreseverestress(soilwater
