Inherent advantages make controlled sub irrigation an attractive proposition to the irrigator if he can devise the means of execution. The advantages are the avoidance of evaporative losses of open water or wet soil surfaces and the elimination of the impedance caused to cultivation by pipes and ditches.

Natural sub-irrigation

Natural sub-irrigation is so called because the conditions, which make it possible, are geographical and topographical. These are near-level terrain and a deep topsoil of very high lateral permeability at 2 m to 7 m depth by an impermeable stratum.

Sub irrigation is limited usually to areas where the soils are relatively permeable for a considerable depth, where surface slopes are gentle and where natural sub drainage is restricted. It must be practical to hold the water table. Thus a first analysis is to determine the possible lateral flow from the area when the water table is at its desired elevation.

From the Darcy's equation of continuity

where

q = flow in m^{3} / s

k = hydraulic conductivity, m^{3} m^{2} = m / s

a = the cross sectional area in square meters through which flow occurs at right angles to it in the direction of flow, and

i = f/1

Example 1: It is desired to maintain the water table at some predetermined depth below ground surface in a highly permeable soil underlain by a restricting layer. The water table roughly parallels the ground surface, slopes about 1 in 176, and intersects the bottom of a nearby stream which is the only natural drainage way. The average depth to shale or tight clay was found to be about 15 m. Assume k for the soils is 0.3 x 10^{4} m/s. Calculate the outflow.

Solution:

Cross sectional area from 1 km long section paralleling the creek and through which natural drainage from the irrigated area would have to flow would be 1000 m x 15 in =15,000 m^{2}.

If this is not considered adequate water removal then some drainage system must be installed.

The total water requirement for the area is the drainage plus the consumptive use requirement of the crops grown.