2.6 Irrigation Scheduling and Management 2.6.1
Irrigation management
Irrigation management can be defined as the process of
implementation of suitable operation and maintenance in order to meet the
objectives of the concerned irrigation system and monitoring of the activities
to assure that the objectives are met.
Three implications can be drawn from the above definition.
First of all, that irrigation management is not a routine job. The management
decisions have to be made with great care, as they have to match with the
operation and maintenance objectives. Secondly, even though the overall goal
may be the same, objectives vary from system to system, hence management
decisions have to take account of these inter system differences. Thirdly, that
monitoring is an integral part of management thus management decisions have to
be continuously refined according to the feedback obtained from monitoring and
evaluation.
Irrigation Management is one of the major challenges for the
irrigation professionals. It is important as it decides the benefit derived
from the irrigation system.
2.6.2 Irrigation scheduling
Irrigation performance can be improved either by means of
developing new application systems (drip, sprinkler, etc.) or by a more
accurate irrigation scheduling. For any crop, schedule implies the
determination of time and volume of water application to meet a specified
management objective.
Jensen (1981) defines irrigation scheduling as a planning and
decision-making activity that the farm manager or operator of an irrigation
farm is involved in before and during most of the growing season for each crop
that is grown. It could also be defined as the use of water management
strategies to prevent over application of water while minimizing yield loss due
to water shortage or drought stress. He further indicated four types of data
needed for irrigation decision making:
1) Current level and expected change in available soil water for
each field over the next 5 to 10 days.
2) Current estimates of the probable latest date of the next
irrigation on each field to avoid adverse effects of plant water stress.
3) The amount of water that should be applied to each field,
which will achieve high irrigation efficiency.
4) Some indication of the adverse effects of irrigation a few
days early or late.
Irrigation scheduling requires a particular attention because
of its influence on irrigation efficiency and its consequences on the
environment. The water holding capacity of the soil and the suction that the
cultivated crop can develop on the soil water are good guides for irrigation
scheduling. The techniques used currently for irrigation scheduling are
diverse. Relative to the equipments that are used for these techniques, they
can be sophisticated or very simple. Methods based on direct measurements of
plant water status have always attracted the attention of irrigation research
as a tool for irrigation timing, but getting accurate and representative data
for these parameters has always been very difficult (Cremona et al.,
2000). Based on the soil-plant-water relations, Kramer (1983) suggested
that the determination of the time and quantity of water to supply by
irrigation could be obtained by either one of the three fundamental methods
namely:
· Determination of soil moisture
· Estimation of the water used by plants from climatic
data
· Measure of water stress that has affected the crop
Irrigation scheduling to satisfy the water requirement of
plants must conform to the hydrology of the milieu and to the objectives set by
the irrigation practice. Njila (1999), stated that most irrigation managers in
Cameroon prefer to irrigate their crops following a pre-established
calendar.
Whatever the context, Fonteh & Assoumou (1996), Tron et
al. (2000), present two fundamental questions that need to be answered in
any irrigation scheduling program:
- When to irrigate?
- How much water to apply?
Smith et al, (1996) classified scheduling options into
two different categories as follows:
a) Timing options - related to when irrigation is to be
applied:
1) Each irrigation defined by user; this type is used to
evaluate irrigation practices and to simulate any alternative irrigation
schedule.
2) Irrigation at critical depletion (100 % depletion of
readily available soil moisture). Resulting in minimum irrigations, but
irregular and therefore unpractical irrigation intervals.
3) Irrigation below or above critical depletion (% depletion
of readily available soil moisture). Useful to set a safety level above
critical soil moisture or allow a critical stress level.
4) Irrigation at fixed intervals per stage, suitable in
particular in a gravity system with rotational water distribution, may result
in some over-irrigation in the initial stages and under-irrigation in the peak
season.
5) Irrigation at given crop evapotranspiration reduction
(%).
6) Irrigation at given yield reduction (%).
7) No irrigation, only rainfall.
b) Application options - how much water is to be given
per irrigation turn:
1) Each irrigation depth is defined by the user, as determined
from field or simulated data.
2) Refill soil to field capacity, to bring soil moisture
content back to field capacity, thus equal to the depleted soil moisture in the
root zone, as the depletion in the root zone will normally vary over the
growing season with changing root depth and allowable depletion levels.
3) Refill below or above field capacity. Useful to allow for
leaching for salinity control
(above field capacity) or to accommodate possible rainfall (below
field capacity).
Irrigation scheduling schemes should take into account factors
such as the soil properties that affect soil moisture-holding capacity. James
et al. (1982) for example, reported that irrigation scheduling with a
soil of low water-holding capacity would have to be more frequent with smaller
amounts applied each time for best efficiency.
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