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Fluid Journal : Fluid Journal 1996-1998
Dr. T.K. Hartz Drip Irrigation Improves Drip Irrigation Improves Drip Irrigation Improves Drip Irrigation Improves Drip Irrigation Improves N Efficiency N Efficiency N Efficiency N Efficiency N Efficiency Trend in California is drip irrigation to improve water/N management and protect environment. Winter 1996 Summary: To realize benefits from drip irrigation systems requires careful manage- ment of both water and fertilizer inputs. Determination of both crop irrigation and nitrogen fertigation requirements is essential. Factors such as water loss rate, plant growth habits, system inefficiencies, nitrogen cycling, soil N availability, crop N needs, and irrigation water N content must be carefully monitored. The tendency to use excessive water and fertilizer in drip irrigation systems can be shortsighted and environmentally undesirable. The use of drip (trickle) irrigation for vegetable production is rapidly increasing. By 1995, nearly 100,000 acres of vegetables were drip-irrigated in California. There are a number of factors driving this conversion to drip irrigation, most important among them: • increased crop yield potential • water conservation • more efficient nutrient management. To realize these benefits requires careful management of both water and fertilizer inputs. We'll cover some of the basics of drip irrigation and fertigation management for vegetable production. Determining water requirement Evapotranspiration. Environmental variables such as solar radiation, air temperature, relative humidity, and wind speed interact to influence the rate of water loss from plants and soil. Historically, the most common technique for integrating these environmental variables and estimat- ing evapotranspiration (ETo, the combined water loss from plants and soil) has been the Class A pan evaporometer. This device is simply an open pan of water in which the depth of daily evaporation is measured. In California, however, we are fortunate to have the California Irrigation Management Information System (CIMIS), a network of computerized weather stations that measure the important environmental variables that govern water loss. From these measurement a daily ETo value is calculated, which has been very useful in estimating actual crop water needs. Crop growth stage. There are consider- able differences in growth habit, rooting depth, sensitivity to water stress, etc. among vegetable crops. However, their water use characteristics are more similar than one might expect. By far, the most significant factor in determining water needs is crop growth stage. A convenient way to account for crop growth stage is to estimate, by sighting down the row, the percentage of the field surface covered by foliage (canopy). Calculation for crop water need is: ETo (cumulative from last irrigation) x canopy spread (inches) divided by row width (inches). Until substantial foliage cover occurs, however, it is critical to back up this estimation procedure with a direct soil moisture measurement. One of the limitations of this way of calculating is it is difficult to get an estimate on very young plants. System inefficiency. No drip system delivers equal amounts of water to all portions of a field. There is a natural tendency to irrigate based on the drier areas. Generally, an additional 10 to 15 percent should be factored in to cover system inefficiency. A more difficult problem is the inefficiency of buried drip systems to supply shallow-rooted crops such as celery or lettuce that may not be able to reach all the applied water. The problem can be minimized by: 1) using relatively flat beds to minimize drip tube depth, 2) forming tightly pressed beds to improve capillary water movement, and 3) irrigating often, using
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