Agriculture Reference
In-Depth Information
As an approximation the water retained against a pressure of
bar usually
corresponds to the permanent wilting point.
Available water capacity is the difference between field capacity and the
permanent wilting point. The soil moisture deficit is the difference between
the amount of water held at field capacity and the amount held at the time
considered.
The above concepts are simplest in their application to uniform soils in
which the roots are evenly distributed to a standard depth and may give
misleading results if this is assumed to be the case when it is not. It is essential
to characterize both the soil and the root distribution.
Cover crops and weeds can use a large part of total water resources. In the
early orchard years, when the soil area around the base of the trees receives
very high levels of solar irradiation, grass or weeds growing there can compete
very effectively for water. Holloway and White (
) found that even a sparse
cover of clipped annual weeds allowed to extend up to the trunk of newly
planted apple trees reduced their shoot extension growth to only a third of
that of clean-cultivated controls, irrespective of fertilizer treatment. Atkinson
and Thomas (
) showed that for
-year-old trees the soil water deficit in the
surface
cm,
cm from the trunk, was about
cm under overall herbicide
management but almost
cm under grass by mid-September in England.
Evaporation and evapotranspiration
Evaporation (
E
)
Evaporation is the conversion of water to water vapour. This requires energy,
known as the latent heat of vaporization. The rate of evaporation varies with
incoming energy and some other factors, so a key step in determining potential
crop water needs is to characterize evaporative water consumption at the
relevant sites.
Direct observations of evaporation can be made using evaporation pans.
The small capacities and shallow depths of these allow proportionately more
advected heat to be absorbed than in the natural situation, so pan evaporation
gives an overestimate, and a pan coefficient specific to the pan design has to
be applied. Generally the standard British pan has a coefficient of
.
and
that of the US Weather Bureau Class A pan is about
.
, but there can be
wide variation.
Alternatively potential evaporation from an open water surface (
E
) can be
calculated from widely available meteorological data (Penman,
). The key
factors determining evaporation rate are as follows. Evaporation increases as
solar radiation increases. As water vaporizes, the boundary layer between the
evaporating surface and the air becomes saturated and must be replaced by
