Water Resources Management - Practices of Irrigation and On-farm Water Management - page 145

An evaluation of future demand should be based on the results of a recent com-
prehensive survey of water users. Forecasts of future uses of water resources on the
basis of various “scenarios” for exploitation should also be undertaken. These can
be achieved by means of mathematical models.
Demands for water in a region can be classified into the following sectors:
(i) demand for domestic, industrial, and commercial uses
(ii) irrigation demand
(iii) evaporative demand for fisheries, forestry, and environment, and
(iv) in-stream demand
5.2.1.1 Demand for Domestic, Industrial, and Commercial Uses
Domestic water per capita varies substantially between urban and rural areas, and
depends on income, standard of living, mode of water supply, availability and qual-
ity of water, etc. An approach of demand estimation may be based on standard
of living. Average per capita consumption for metropolitan, town, and rural areas
should be estimated separately. In urban area, typically the per capita consump-
tion varies between 100 and 150 l/day; and in rural areas it varies between 50 and
100 l/day. In long-term planning purposes, change in income and a cultural shift
in water uses of the rural population should be considered. A system loss of about
10-15% should also be taken into account.
Commercial and industrial demands are typically 8-11% and 10-15%, respec-
tively, of the total water supply for the piped distribution system (World Bank,
1997). A return of 30-50% may be considered, but this is not useful in most cases.
5.2.1.2 Irrigation Demand
The usefulness of an estimate of future irrigation demand largely depends on how
closely prediction can be made of areas under different crops, cropping sequences
and intensity, and crop calendar (the time and length of growing period). The prob-
able climatic scenario is also required to compute crop evapo-transpirative demand.
Agricultural crops also require water for seedbed preparation, land preparation, salt
leaching etc. These water requirements are fulfilled either from rainfall or from irri-
gation supply. Not all the rains received over the year or growing period become
useful for these purposes. Effective rainfall should be calculated or estimated using
appropriate techniques. In meeting evapo-transpirative demand, some seepage and
percolation loss will occur from the field, specially in case of rice crop. The mea-
surement and/or estimation techniques of these components have been described in
detail in an earlier chapter (in Volume 1). There is a difference between the quantity
of water diverted from a source for irrigation and that reached at the field plot or
used in actual evapotranspiration. This difference is a loss from the viewpoint of
irrigation water supplier. The irrigation efficiency, which is a ratio of usage to sup-
ply, varied depending on soil, climate, crop and method of application, and usually
between 60 and 70%. The International Water Management Institute (IWMI) used