The advantages of subsurface drip irrigation (SDI) with respect to less water evaporation, less water runoff, less weeds, less herbicides, less fertilizers, less humidity, less labor, less harvesting damage, less animal damage or human vandalism etc., are now well accepted in the agricultural industry.

In addition there have been many reports both by researchers and in the popular press of substantial increases in yields and/or quality with SDI. These improvements are not achieved by many growers who are still pleased with their use of SDI for the long list of advantages mentioned above. The purpose of this paper is to advance one theory as to the cause of this improvement and to encourage practices which may help other growers achieve the same dramatic results. The theory I advance is supported by research and observation that most of the growers who are achieving substantial yield benefits do have this practice in common.

Ideal growing conditions require a combination of soil, water, fertilizer and air. If the growing conditions can be managed so that the near optimal ratio of these components are maintained continuously throughout the root zone without either times of water saturation or deprivation, then increased yields can be anticipated. Of course if stress is required as part of the growing cycle that needs to be equally precisely managed.

High frequency SDI can do exactly as required.
What is high frequency SDI?
High frequency SDI irrigation is the frequent application of water (several times per day) through the SDI system, to replace water lost by soil and plants by evapotranspiration.

How does SDI work?
Forces controlling the movement of water are mostly due to the capillary forces which are equal in all directions, and gravity which is constant and downwards. The capillary force decreases as soil wets. Hence in dry soil the capillary force is much greater than the gravitational force and tends to move water equally in all directions. As the soil becomes wetter, the soil pores become saturated and the capillary force is weakened allowing gravitational forces to dominate and the water moves mainly downwards. This rather simple and basic concept implies that irrigation water should be applied in short pulses so that the movement of the water in the soil is controlled mainly by the capillary action.

How frequent is high frequency?
The coarser the soil texture the more frequent and shorter the irrigation cycles must be. For example, in sandy loam soil with a bulk density of .054 lb./cu. in. and the SDI system buried at 18" depth irrigation should not be more than 50 minutes in order to prevent surfacing of the water. In clay loam soil this could be increased to 90 minutes, but rarely more than 120 minutes. The system should then be left off for at least the equal time as it was on.

The photographs on page 3 demonstrates the comparative performance of SDI vs. surface drip. This photograph and the following computer simulation are taken from research done by Phene and Ben-Asher while at the USDA / ARS in Fresno.

In the photographs and computer simulation the ratios of wetted areas are as follows:

SDI Surface Drip Difference

0.36 m
0.40 m –10%
1.629 m
1.005 m2 +62%
0.195 m3
0.134 m3 +46%

The same amount of water has been applied to the 46% larger wetted volume with SDI as with surface drip. Clearly there will be more air in the larger wetted volume with SDI. It is as simple as that!

Of course, good management of the amount of water and fertilizers applied are still essential for a successful SDI system. For example the concentration of phosphorous is generally low at greater depths so it is quite likely that it will be necessary to apply more phosphorous with SDI. Salt management may be important under some conditions.

While SDI will help all growers experience the list of advantages of less water evaporation, less water runoff, less weeds, less herbicides, less fertilizers, less humidity, less labor, less harvesting damage, less animal damage or human vandalism etc.; high frequency SDI is the tool that the skilled grower can use which combined with his good management will result in substantial increases in yield and/or quality. This tool will help a competent grower be even more successful.

Acknowledgment: This paper has borrowed freely from the works of Dr. Claude Phene, SDI+, Fresno, California, who is a consultant who specializes in the management of SDI systems.