Specific Yield, Retention Define Total Porosity
Understanding the components of pore water storage in geologic formations is key.
- Specific Yield ($S_y$): This is the volume of water released from storage in an unconsolidated formation per unit surface area of the formation when the water table or piezometric surface declines by unit distance. It represents the drainable water.
- Specific Retention ($S_r$): This is the volume of water per unit surface area of the formation held within the formation, not released by gravity or capillary forces. It represents the retained water.
The total pore space in a formation, which can potentially hold water, is defined as its total porosity ($\phi_t$). For unconsolidated materials, the sum of the water that drains out under gravity (specific yield) and the water that remains held within the pores (specific retention) accounts for all the pore space.
The fundamental relationship is:
$ S_y + S_r = \phi_t $
Thus, the sum of specific yield and specific retention directly equals the formation's total porosity.
Important Questions from Groundwater Occurrence
An unconfined aquifer of areal extent $20 \text{ km} \times 20 \text{ km}$ has hydraulic conductivity of $4 \text{ m/day}$, porosity of $0.32$, and storage coefficient (specific yield) of $0.18$. If the initial saturated thickness of the aquifer is $30 \text{ m}$, and $4\times10^8 \text{ m}^3$ of water is extracted from the aquifer, then the decline in the saturated thickness is _____________ (rounded off to two decimal places) m.
An unconfined aquifer extends over an area of $1 \text{ km}^2$ and has hydraulic conductivity, total porosity and specific retention of $20 \text{ m per day}, 30\%$ and $10\%$, respectively. After pumping some groundwater from this aquifer, the water table dropped to a depth of $20 \text{ m}$ from the ground level. If the water table was initially at $14.5 \text{ m}$ below the ground level, the change in groundwater storage in million cubic meters would be_______
Match the following items between Column I and Column II with the most appropriate combinations:
Column I Column II P Properties of confined and semi- confined aquifers 1 Perched aquifer Q Property of an unconfined aquifer 2 Coefficient of storage R Property of semi-confined aquifer 3 Specific yield S A special case of an unconfined aquifer 4 Leakage factor A fully penetrating well is installed in a homogenous and isotropic confined aquifer. The aquifer has uniform thickness of 16 m and hydraulic conductivity of 25 m/d. Water is being pumped out from the well at a constant rate of $0.1 \text{ } m^3/s$ till steady state condition is reached. If a drawdown of 3.5 m is observed at a distance of 75 m from the well then the drawdown at a distance of 150 m from the well will be________ (in m, rounded off to two decimal places).
- One of the key assumptions made by Dupuit-Forchheimer about the flow behavior in the porous medium towards subsurface drains is
