Agriculture Reference
In-Depth Information
Frequency of irrigation under this system depends on (
) the rate of water
depletion (
) the maximum allowable depletion (
) the volume of the active
rooting zone, (
) the soil water holding capacity and (
) the soil infiltration
rate.
Estimation of water depletion for scheduling purposes falls into three cate-
gories: (
) Calculation of ET
c
from climatic data which is usually provided
by local advisory services. (
) Measurement in changes in soil water content.
Gravimetric and matric potential (e.g. tensiometer) measurements have been
supplemented or replaced by, for example, neutron probe or electric capaci-
tance measurements often carried out by specialist contractors. With experi-
ence the moisture content of the soil can be judged by the feel of a sample. For
all soil measurements sampling representative of the rooting zone is essential.
(
) Pan evaporation based scheduling. Evaporation data is provided by local
advisory services. It is then adjusted by an empirically determined factor for
the area, crop and soil type, e.g.
.
for apples with full ground cover in Wash-
ington State (Evans,
), and by the percentage of the land actually covered
by the foliage of the trees being irrigated.
The maximum allowable depletion for apples has been assumed to be
-
).Thevolumeoftheeffectiverootingzonevariesenormously
with tree age, soil type and soil characteristics, and must be determined em-
pirically. Soil water holding capacity and infiltration rates are functions of soil
type and must be measured or estimated from the soil classification.
These concepts are also relevant to other forms of irrigation. Plant param-
eter based irrigation scheduling is currently being investigated.
It has not, so far, been possible to develop universally applicable systems to
determine irrigation requirement from the measurement of plant water status.
Osmotic adjustments by the leaves and fruits can influence the effects of any
givenleafwaterpotentialnotonlyonstomatalapertureandphotosynthesisbut
alsoonfruitgrowth.Afurthercomplicationisthatstomatalclosure,probablyin
response to root-sensed water stress, can actually increase leaf water potential
especially towards the end of the season ( Jones
et al.
,
%(Evans,
).
Infra-red thermometry can be used to determine when plants are show-
ing stomatal closure in response to water stress. Leaf temperature rises as
stomata close and evaporative cooling decreases. Crop water stress indices
(CWSIs) have therefore been calculated based on the difference between
measured leaf temperatures and the typical canopy temperature of a well-
watered crop. This technique is used as a basis for irrigation scheduling on
a number of crops. Jones
et al.
(
) have pointed out some problems in its
application in variable climates. The fact that several key aspects of apple tree
growth and fruiting are adversely affected by less severe water stress than that
which causes stomatal closure also suggest limits to the applicability of this
technique.
