Specific Yield and Specific Retention

The porosity of a soil defines the maximum amount of water that a soil can contain when it is saturated. The specific yield is the portion of that water that n = (V - Vs)/V, = VJVt

Porosity

Porosity

1/16 1/8 1/4 1/2 1 2 4 8 16 32 64 128 256 Maximum 10% grain size, mm

FIGURE 2.3 Porosity, specific retention, and specific yield variations with grain size (in situ consolidated soils, coastal basin, California).

(ft

is

-a

-a

•a

c

c

c

<0

<0

<0

w

tfi

-a

<D

<D

<c

<8

C

C

O

CO

LL

LL

1/16 1/8 1/4 1/2 1 2 4 8 16 32 64 128 256 Maximum 10% grain size, mm

FIGURE 2.3 Porosity, specific retention, and specific yield variations with grain size (in situ consolidated soils, coastal basin, California).

will drain under the influence of gravity. The specific retention is the portion of the soil-water that will remain as a film and in very small voids. The porosity, therefore, is the sum of the specific yield and the specific retention. The relationship between the porosity, specific yield, and specific retention is illustrated for typical in situ, consolidated California soils in Figure 2.3. The specific yield is used when defining aquifer properties, particularly in calculating groundwater mounding beneath ponds and wastewater application sites. For relatively coarse-textured soils and deep water tables, it is acceptable to assume a constant value for the specific yield. Because the calculations are not especially sensitive to small changes in specific yield, it is usually satisfactory to estimate it from other properties, as shown in Figure 2.3 and Figure 2.4. Neither Figure 2.3 nor Figure 2.4 should be used to indicate the hydraulic properties of the medium in subsurface flow constructed wetlands. Groundwater mound analysis can be more complicated for finer-textured soils because of capillarity effects in the soil as the water table moves higher (Childs, 1969; Duke, 1972).

0 0

Post a comment