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Irrigation aims to enhance the soil productivity through
artificialmethods of diverting, drawing and conveying water to
supplement soil moistureswhen rainfalls are insufficient to meet
the crop growth needs. Irrigationwater distribution is a
practice to distribute water equitably and reasonablyat
appropriate time with adequate rate and amount, taking into
account theprinciple of balanced water supply-demand for the
cropped farms. This study attempts to develop a more
flexible water distribution modelwhich differentiates water
distribution operations in water deficiency andabundance
periods, under the situation of changing flows of water sources.
During the dry period, the irrigated water will be merely to
supply an amountof water just for sustaining of crop production;
while during the waterabundance period, the irrigation plans or
schemes will arrange to draw theavailable water within the limit
of canal capacity to suppl crop needs, livinguses and ecological
uses. These show the characteristics of the mobility ofthe
water distributions model presented in the study. In the
early days which the irrigation water distribution was
practicedby the irrigation associations in Taiwan, the field
irrigation waterrequirements where estimated in terms of
irrigation rates, which calculationswere originated from the
rough estimate of the area irrigable by an unit ofwater, based
on the experience of farmers and irrigation association staff.
The method of estimate of irrigation requirement has been used,
however forthe convenience of irrigation water management and
irrigation systemoperations by the irrigation associations.
Thus early days irrigationdistribution plans in a district were
drawn up based upon one criteria, i.e.irrigation rates,
regardless of the seasons and the growth stages of the crops.
This seems to be rather inappropriate from the viewpoint of crop
physiology.Thus, in recent years, Taiwan''s irrigation
associations have taken intoconsideration the meteorological and
soil parameters. That is cropevapotranspirations, rainfalls, and
soil percolations at diffent crop ageswhen estimating the
irrigation rates. Therefore it is no longer the singlefactor,
but rather the multiple factors varying with seasons and
locations ofirrigation, to better meet actual situations in the
farm lands. Although the current irrigation distribution
method adopted by theirrigation associations is indeed
convenient practical, it does have itsapplicability restrictions
facing the complicated field situations and supplyproblems of
water sources. Its advantage lies in the systemization of
waterrequirement criteria, which irrigation rates have
integrated factorsinfluencing irrigation. However, the method
is merely good for the waterdistribution under normal
conditions, but not drought period when emergencymeasures to
counter the abnormal situations are required. In Taiwan,
twotypical supply systems of water sources: reservoir supply
systems andrun-of-the-river systems. The discharges released
from the reservoir supplysystem can be more easily controlled,
and therefore more steady and stable.Diversions of water from
the natural river systems are less stable due tochanging river
flows. Since the existing irrigation water distribution followsa
mode of constant discharge within a fixed time period, it can
opportunityrespond to the flow changes at water sources. As a
result, the fairness andrationale of the practice is in doubt,
because the efficient use of waterresources is not attainable,
and disputes and troubles caused duringimplementation.
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