Safe yield

The abstraction of water from underground aquifer at a greater rate than it is being recharged leads to the lowering of water table and upsets the equilibrium between discharge and recharge. If we compare groundwater with surface water, surface water is renewable in nature. Generally renew within few days to weeks. On the other hand, through groundwater is also renewable in nature, it takes longer time to renew, sometime up to decades or centuries. The concept of safe yield has been used to express the quantity of water that can be withdrawn from the ground without damaging an aquifer as a water source. The damage includes (i) reduction in groundwater withdrawn, (ii) reduction in ecological base flow, (iii) land subsidence, (iv) depletion of groundwater reservoir, etc . The definition of safe yield can also be written as “The maximum quantity of water that can be guaranteed during a critical dry period is known as the safe yield or firm yield.” The definition given by Todd (1959) is “Safe yield is the basin draft on a groundwater supply which can be continued indefinitely without harming the supply or basin landowners”.

Fig. 28.1 Effect of withdrawn on groundwater balance

Fig. 28.1 shows the effect of withdrawn of water on groundwater balance. The Fig. 28.1 (A) shows the natural condition of an aquifer. In this case, the amount of water recharge naturally is equal to the amount of water drain out from the aquifer. The aquifer is naturally at steady condition. This is also known as pristine groundwater system. The Fig. 28.1(B) shows the condition of the aquifer under stress condition. In this case, the pumping is less and thus there is balance among the amount of water recharge, the natural drainage and the amount of water withdrawn. The aquifer is in equilibrium condition and this system is known as developed groundwater system. The Fig. 28.1(C) shows the condition when water withdrawn from an aquifer is much higher than the natural recharge. In this case, the storage of the aquifer will be depleted with time and the aquifer may not be in a position to yield any water in future. In this case, the draft is more than the safe yield of the aquifer which is called as overdraft. This system is known as depleted groundwater system. This can give rise to pollution or cause serious problems due to severely increasing pumping lift. Indeed, this eventually may lead to the exhaustion of a well.

Estimation of safe yield is a complex problem and should consider the climatic, geological and hydrological conditions. Due to the variation of climatic and hydrologic conditions with time, the safe yield is likely to vary appreciably with time. The safe yield, G, often is expressed as follows:

(28.1)