Water Resources Management - Practices of Irrigation and On-farm Water Management - page 148

Safe yield = maximum allowable fluctuation of water table (MAWT)
× specific yield of the aquifer

Here, MAWT refers the difference between maximum allowable depletion or low-
ering of water table (WT) during pumping, and the present depth to water table, i.e.,

MAWT = Maximum allowable lowering of WT − present depth to WT

Here, it is to be mentioned that the use of historically observed maximum fluctu-
ation in annual water table in safe-yield or recharge estimation may result in an
underestimated value of recharge. This is because the annual fluctuation can be
increased by withdrawal of groundwater for irrigation during dry season, thus creat-
ing further scope for increased recharge. That is, the potential recharge, which may
be defined as the mean annual/seasonal volume of surface water that could reach
the aquifer, is the sum of actual recharge to the aquifer and the rejected recharge.
Rejected recharge is that fraction of water available at the surface which can not
infiltrate because the water table is near/at the surface.
Specific yield should be determined by free drainage from the collected samples
of the aquifer (through bore logs). Specific yield is an important parameter in safe
yield or groundwater potential estimation using this approach. The value of spe-
cific yield varies widely within the physiographic zones and even in a zone. So,
the specific yield should be determined for each zone and the mean of representa-
tive number of samples should be used, that the data represent the actual aquifer
characteristics.
For each of the approaches mentioned above, the total area of a country or a
region should be divided into suitable physiographic units on the basis of some
criteria such as geological condition, topography, physical and hydraulic character-
istics of the topsoil, and/or depth of flooding. Available or potential recharge for
each unit should be calculated.

5.2.3 Issues of Groundwater Development in Saline/Coastal Areas

Freshwater in the saline and coastal area may occur in single or two zones (shal-
low and deep freshwater) (Saleh and Nishat, 1989 ; Rashid, 2008 ) . The development
of freshwater in such area requires careful planning. Localized pressure reduction
caused by tube-well abstraction may cause upconing of saline water interface. To
mitigate salinization of the aquifer, it is essential to control groundwater abstrac-
tion, adopt suitable well design procedures and have a thorough knowledge of the
hydrogeology of the aquifer.

5.2.4 Environmental Flow Assessment

5.2.4.1 Concept of Environmental Flow

Management of river flows should attempt to ensure minimum natural flows in order
to maintain the conditions that supply goods and services and ensure biodiversity.