Specific Yield (Unconfined Aquifer)

Measures the fraction of water that will drain from a saturated unconfined aquifer under gravity, key to estimating usable groundwater storage.

Written by the Vorzep Team

GeologyHydrogeologyGroundwater

⟹ Solve⟷S_y, V_w, V_t

Water Drained (V_w)

m³

Total Volume (V_t)

m³

Specific Yield (S_y)

dimensionless

Please fix the errors above.

Solve for: Specific Yield (S_y) Water Drained (V_w) Total Volume (V_t)

Presets:

Specific Yield—

—

V_w: —V_t: —S_y: —

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Specific Yield Gauge—

Low (< 0.10) Moderate (0.10–0.30) High (> 0.30)

S_y = V_w / V_t · Specific yield is the ratio of water that drains from a saturated aquifer under gravity to the total volume

Variables

Symbol	Quantity	Unit
Sy	Specific yield
Volume of Water Drained	Water released by gravity drainage	m3
Total Volume of Aquifer	Total aquifer volume dewatered	m3

What it means

Specific yield (Sy) is the ratio of the volume of water that will drain from an unconfined aquifer under gravity to the total volume of the aquifer. It represents the fraction of pore space that can contribute to groundwater flow. Sy is typically less than porosity. It is used in groundwater balance calculations, for estimating water table fluctuations, and for managing groundwater resources. Understanding Sy is important for hydrogeologists to evaluate aquifer productivity and to assess sustainable yields.

Worked example

Specific Yield – Two Detailed Examples

Real‑World

Scenario: A hydrogeologist conducts a pumping test in an unconfined aquifer. They measure the volume of water drained from the aquifer as 15,000 m³ over a total aquifer volume of 100,000 m³. The specific yield Sy = Volume Drained / Aquifer Volume = 15,000 / 100,000 = 0.15 (15%). This parameter indicates the amount of water that can be extracted from storage by gravity drainage. It is essential for estimating sustainable groundwater yields.

Parameter	Value
Volume Drained (m³)	15000
Aquifer Volume (m³)	100000

1Sy = 15000 / 100000 = 0.15

Result 0.15 (15%) ✓ Specific yield

Scenario: A farmer evaluates a shallow aquifer for irrigation. A pumping test shows 8,000 m³ drained from an aquifer volume of 50,000 m³. The specific yield is Sy = 8000 / 50000 = 0.16 (16%). This moderate specific yield suggests the aquifer can provide sufficient water for agriculture during the dry season, but careful management is required to avoid over‑extraction and water table decline.

Parameter	Value
Drained	8000
Volume	50000

1Sy = 8000 / 50000 = 0.16

Result 0.16 (16%) ✓ Agricultural yield

Insight: Specific yield is the volume of water that drains from an unconfined aquifer per unit volume of aquifer due to gravity. It is typically between 0.05 and 0.30, depending on the soil and rock type.

Common mistakes

Specific yield Sy: The volume of water that drains from an unconfined aquifer per unit area per unit decline in water table.
Volume drained: The water released by gravity – not all water is drainable.
Total volume: The volume of aquifer considered – both saturated and unsaturated.
Units: Sy is dimensionless (fraction).
Depends on: Grain size, sorting, and degree of saturation – not a constant.

Applications

Specific yield (Sy) is the volume of water drained from an unconfined aquifer per unit decline in water table, expressed as a fraction of the total aquifer volume. This parameter is essential for estimating groundwater recharge and for modelling water‑table fluctuations. Hydrogeologists use Sy to determine the amount of water available from an unconfined aquifer for pumping, to design water‑supply wells, and to assess the impact of droughts. By measuring specific yield, professionals can better manage groundwater resources, especially in semi‑arid regions. Understanding Sy is also critical for contaminant transport and for evaluating the vulnerability of unconfined aquifers.

Estimation of usable groundwater storage in unconfined aquifers
Water‑table fluctuation and recharge analysis
Design of wells and pumping schemes
Assessment of aquifer vulnerability to contamination
Groundwater resource management in arid and semi‑arid regions