Water Resources Management - Practices of Irrigation and On-farm Water Management - page 146

irrigation efficiency up to 70% for country level demand assessment (Seckler, 1996).
Considering all these factors, the gross irrigation demand should be estimated.
5.2.1.3 Nonirrigated Evaporative Demand
Nonagricultural evaporative demand arises from areas under fisheries, forestry,
and environmental uses. Areas under fisheries include major and regional rivers,
perennial and seasonal standing water bodies (haor, baor, and beel), and fishponds.
Forest abstracts water from high and perched groundwater tables. The quantity
of water required to meet evapotranspiration demands of such trees is a net demand
and needs to be accounted for. Environmental demand arises from rivers as well as
urban, rural, and other areas (excluding areas under fisheries and forestry). Urban
areas under environmental use include parks, gardens, and playgrounds. Rural areas
include household trees, garden, graveyards, and playgrounds.
5.2.1.4 In-Stream Demand
Some flows are to be maintained in the major and regional rivers for navigation, fish-
eries, salinity control, chemical and biological dilution, and sustenance of aquatic
flora and fauna. These constitute in-stream demand, but they are not additive. The
in-stream flow is required to push the salinity front toward the salinity source (the
sea, which contains saline water) to arrest the environmental degradation. To salinity
control, a minimum flow should be maintained.
5.2.2 Estimation of Potential Supply of Water
At the same time that water needs are being assessed, water availability has to be
evaluated as to quantity, quality, and costs for various sources of water. While sur-
face water surveys are relatively easy and inexpensive, groundwater surveys are
more complex and costly as they involve test drilling, pumping, and geophysical
surveys, in addition to continuous data collection through yield and water level
measurements on wells.
5.2.2.1 Surface Water Resource
It is very important for surface water estimation that spatial as well as temporal
distribution of water is properly accounted for. It is not appropriate to calculate the
average for the purpose of planning.
In estimating effective volume of water in different available water bodies (such
as ponds, lakes, rivers), volume of water should be determined first during periods
(dry and wet) of the year. Then the assessment of environmental flow requirement
(such as fish culture, navigation, and other purposes) should be done. Effective stor-
age/volume of water for a particular season is the difference between actual volume
and environmental flow (EF) requirement, i.e.,