It represents the volume of water that can be drained from a unit available volume of soil when the soil moisture pressure is decreased from atmospheric pressure to some specific negative pressure.

Q= Sh1 
         h2 f(h) dh
Relationship between drainable porosity and capillary pressure.

q= S h1 ah.dh = a/2 (h1square – h2 square) 

a = slope of the line
q= quality drained 
h1h2= hydraulic heads


It contains 35% moisture by dry weight when it is saturated and soil moisture pressure is equal to zero. When the soil moisture capillary pressure is increased to 100 cm of head the moisture content become 30% , bulk density of soil is 1.4, ( 1cc dry soil weights 1.4 gm). Calculate the drainable porosity of soil?

At saturation

1cc soil contains 1.4 X 0.35 = 0.49 of water per cc of soil.

Similarly at, 
100cm capillary pressure 
Soil moisture concentration = 1.4 X 0.30 = 0.42 cc of water cc of soil.  
:. Drainable porosity is 0.07 cc of water per cc of soil.



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