Agriculture Reference
In-Depth Information
Table 7.1
The relationship between soil and vine water status
Soil water tension (kPa) Soil status
Vine status
Vine condition
0
Saturation
Anaerobic root
environment
Unfavourable in pro-
longed situations
10
Field capacity
Transpiration is not
restricted by water
availability
Transpiration under
stomatal control
10-60
Readily available
water
Transpiration is not
restricted by water
availability
Transpiration under
stomatal control
60-200
Stress available water Transpiration is
restricted by water
availability
Transpiration under
stomatal and non
stomatal control
200-1500
Deficit available water Prolonged drought
conditions
Transpiration under
non stomatal control
when water is managed through a complex blend of water entitlement and comple-
mentary water purchase. As the cost of water increases as a result of scarcity, man-
agement of this resource further influence the net profit of this enterprise.
Irrigation has traditionally been managed using soil based methodologies, some-
times coupled with atmospheric demand and/or indicators of plant response to soil
water. In these methods, the soil is perceived as a water reservoir that is depleted
daily via soil evaporation and plant transpiration. Sandy soils hold less water than
clay soils and the ratio of sand silt and clay is often used as a predictor of the water
holding capacity of a soil.
A soil is called at Field Capacity (FC) or Drained Upper Limit (DUL) when
totally filled with water and subsequently allowed to drain under the influence of
gravity. By contrast, when water can no longer be extracted from the soil and plants
wilt, the soil is called at Permanent Wilting Point (PWP) or Lower Limit (LL).
The amount of water help by a soil between FC and PWP is directly related to its
texture and organic matter content and is called the Plant Available Water (PAW).
The fraction of PAW easily extracted by plants is called the Readily Available Wa-
ter (RAW) and will not lead to yield reduction. It is followed by the Stress Avail-
able Water (SAW), associated with the onset of yield loss and the Deficit Available
Water (DAW) associated with a significant loss of yield and plants. The tension
required to extract water from the soil is often used to assess vine water status, but
other instruments measure water fractions in volumetric terms (Table
7.1
).
Evaporative demand is evaluated using one of two common methods. The Pan
Evaporation method is a physical method where a change in water level is moni-
tored physically of electronically. The integrated impact of wind speed, ambient
temperature, relative humidity and solar radiation (collectively called the “evap-
orative demand”) is observed regularly. Alternatively, weather stations recording
the environmental components listed above can be used to compute the reference
evapotranspiration. Reference Evapotransiptation is the evapotranspiration of an
unstressed lucerne crop (alfalfa in the US) mowed and maintained at 10 cm. A range
of computational methods exist, mostly based on the work of Penman (Penman
