Specific Yield and Specific Retention | Civil Engineering Optional Notes for UPSC PDF Download

Specific Yield and Specific Retention in Hydrogeology

• If the water in the connected pores of a material is allowed to drain due to gravity, not all of it is released because some water clings to the solids. This phenomenon is described by the term specific yield, S_y.
• Specific yield (S_y) is the ratio of the volume of water that drains by gravity (V_D) to the total volume of the sample (V_T).
• In simple terms, specific yield (S_y) = VD / VT.
• Water that remains in the material after gravity drainage, sticking to surfaces or hanging in the pore spaces, is called pendular water or residual water.
• The volume of retained water, or pendular water, depends on the number and size of the pore spaces and the surface area of the grains that can hold water.
• Specific retention (S_r) is the fraction of water retained in a porous material after gravity drainage, expressed as the ratio of the volume retained (V_R) to the total sample volume (V_T).
• Specific retention (S_r) = V_R / V_T = V_I - V_D, where VI is the volume of the effective pore space and VD is the volume drained. Sr can also be calculated as the effective porosity minus the specific yield (S_r = n_e - S_y).

Key Equations

Definitions

• S_y: Specific yield (dimensionless)
• V_D: Volume of water that drains by gravity (L³)
• V_T: Volume of sample (L³)

Illustrative Example

• Imagine a sponge soaked in water. When you squeeze it, not all the water drips out, some remains absorbed in the sponge. The water that drips out is like specific yield, while the water held within the sponge is like specific retention.

Hydrogeologic Properties of Earth Materials

• Represents the volume of water retained against gravity after drainage ceases.
• Measured in cubic units (L³).

Volume of Water Retained (V_R)

• Indicates the amount of water retained after drainage stops.
• Measured in cubic units (L³).

Volume of Sample (V_T)

• Refers to the total volume of the earth material sample.
• Measured in cubic units (L³).

Effective porosity, specific yield, and specific retention are interrelated. They each represent a ratio of water volume to the total volume of an earth material. When any two parameters are known, the third can be calculated.

Relationship Among Parameters

• The equation n_e = S_y + S_r illustrates their connection.

Understanding Specific Yield

• Specific yield is crucial for groundwater professionals as it signifies the volume of water entering or leaving a groundwater system during recharge or pumping.

For example, in a 1000m x 1000m area with sand having an effective porosity of 25% and a specific yield of 15%, if the water table drops by four meters, 600,000m³ of water would be drained from the sand.
After drainage, 400,000m³ of water remains on the grain surfaces against gravity due to capillary tension.

Specific Yield and Specific Retention

• Consider two rooms: one filled with soccer-ball-sized glass spheres and the other with 1 cm diameter marbles.
• Both rooms have equal porosity (48%) due to the same cubic arrangement of balls.
• The room with larger glass spheres yields more drained water during saturation and drainage.
• Even though pore volumes are equal, the marble room's smaller pores and larger surface area retain more water through capillary forces.

• Field capacity, denoted as S r, describes the water retained after drainage, available for plants.
• It signifies soil water holding capacity and water retention capacity.
• Plants can reduce porewater pressure near roots to extract water from soil.
• Over time, evaporation may reduce soil moisture content, leading to levels below field capacity.

Specific Yield Measurements

• Specific yield measurements from various laboratory and field techniques are consolidated in Table 3, offering insights into the specific yield of common earth materials. Specific yield can also be assessed by observing the water table's response to well pumping and analyzing water level variations using equations and models.
• Table 3 - Summary of specific yield values for common earth materials as compiled by Morris and Johnson (1967) along with additional data from Rivera (2014), Freeze and Cherry (1979), and Domenico and Schwartz (1998). "NA" indicates data not available.

Measurements of Specific Yield for Common Earth Materials

• Clay (27 samples) - 1% to 18%
• Silt (299 samples) - 1% to 40%
• Loess (5 samples) - 14% to 22%
• Eolian sand (14 samples) - 32% to 47%
• Sand (fine, 287 samples) - 1% to 46%
• Sand (medium, 297 samples) - 16% to 46%
• Sand (coarse, 143 samples) - 18% to 43%
• Gravel (fine, 33 samples) - 13% to 40%
• Gravel (medium, 13 samples) - 17% to 44%
• Gravel (coarse, 9 samples) - 13% to 25%
• Shale - 0.5% to 5%
• Siltstone (13 samples) - 1% to 33%
• Sandstone (fine-grained, 47 samples) - 2% to 40%
• Sandstone (medium-grained, 10 samples) - 12% to 41%
• Limestone and dolomite (32 samples) - 0% to 36%
• Karstic limestone - 2% to 15%
• Fresh granite and gneiss - Less than 0.1%
• Weathered granite/gneiss - 0.5% to 5%
• Fractured basalt - 2% to 10%
• Vesicular basalt - 5% to 15%
• Tuff (90 samples) - 2% to 47%