By C. J. Phene
Many drip irrigation systems are permanently or semi-permanently installed for irrigation of trees and vine crops, but field and vegetable crops are normally irrigated with systems which are installed and retrieved annually. The storage of laterals poses extreme logistical problems. The annual handling is detrimental to the life expectancy of the system, even under the best handling conditions.
Methods for permanently installing subsurface drip irrigation systems will facilitate agricultural operations, improve conservation of natural resources, increase productivity, minimize labor requirements, and permit further expansion of drip irrigation.
Four subsurface drip irrigation systems have been designed, installed and successfully operated by the Water Management Research laboratory of the USDA–ARS, Fresno, with several crops. The oldest one, located at the California State University, Fresno (CSUF) Research farm was installed in 1981 and used for three years to grow tomatoes. We also successfully double–cropped broccoli with tomatoes during one of the three years. Cowpeas (two years) and cantaloupes (one year) have been grown with the same system.
Subsurface drip irrigation offers many advantages beyond those of surface drip irrigation:
1. Permanent installation below the plow depth provides considerable labor savings and irrigation can be applied while equipment is in the field.
2. The top 15–20 cm of soil remains dry; hence, evaporation of water from the soil surface will be limited to vapor diffusion because of the mulching effect of the dry soil and less salts will accumulate at the surface.
3. Any soil surface crusts which usually cause infiltration problems will be bypassed and infiltration will not be a problem. Nonuniformity of application usually associated with water running off the surface or ponding will be eliminated, hence water distribution throughout the field along the laterals should be improved.
4. The subsurface drip system is buried and not handled annually. The subsurface system is out of the sunlight and not subjected to constant wetting and drying and heating and cooling, therefore it is expected that the system will last longer than one which is on the surface and exposed to the changing environment.
5. Equipment traffic through the field will be easier and less cumbersome because all pipes and laterals are buried. In addition, the soil surface is kept dry after the initial irrigation for germination, thus, traction through the field should be improved and less soil compaction should result
6. Water and nutrients are applied directly to the root zone. In addition, the roots will take up and use nutrients more efficiently provided that the irrigation and fertilization schedules are adequate.
7. Application of fumigants and/or pesticides through the subsurface drip system will provide enhanced use of chemicals for weeds and pest control; particularly if the surface is irrigated simultaneously to seal the soil surface temporarily.
8. Since the topsoil is kept dry during most of the growing season, occurrence of fruit rot and soil borne diseases enhanced by wet soils should be minimized. Germination of shallow weed seeds will be decreased because of lack of necessary soil water.
9. Double cropping will be easier because laterals remain in place.