Agricultural Water Management Proceedings of a Workshop in Tunisia (2007) / Chapter Skim
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Optimizing Irrigation for Agricultural Water Management: Scientific Principles
Optimizing Irrigation for Agricultural Water Management: Scientific Principles
Pages 51-65
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From page 51... ...
Plants transpire pure water causing the soil solution to become concentrated with salts as transpiration proceeds. Because of this effect, occasionally irrigation must not only recharge the storage capacity, but additional water may be necessary to leach excessive salts from the root zone.
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From page 52... ...
The infiltration rate decreases with time and approaches a constant steady state rate as depicted in Figure 1. The soil water content at the time of water application affects the initial infiltration rate.
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From page 53... ...
If irrigation is programmed to restore the storage capacity in the parts of the field that receive the most water, the other parts of the field will be under irrigated, causing yield reduction in those under-irrigated areas. Conversely, if irrigation is programmed to recharge the storage capacity zones with the lowest infiltration rate, the other parts of the field will be excessively irrigated, leading to unrecoverable water loss to deep percolation.
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From page 54... ...
The plant root system can accommodate uneven water distribution and can extract more water where the soil water content is high. A tree with a large root system can accommodate considerable nonuniformity of the water application under the canopy.
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From page 55... ...
The uniformity of irrigation significantly affects the crop-water production function. The relationship between cotton lint yield and IW for various irrigation uniformities for climatic conditions of the San Joaquin Valley of California is presented in Figure 2.
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From page 56... ...
For a nonuniform irrigation there is a trade-off between irrigating for high crop yield and low deep water percolation. The salinity of irrigation water is another factor that affects the crop water production function.
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From page 57... ...
for waters of different salinities. The number on each curve represents the irrigation water salinity in dS/m.
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From page 58... ...
Irrigation water salinity may reach a level where maximum yield cannot be achieved regardless of the amount of water applied. Larger differences in yield for a given amount of AW or larger differences in AW for a given yield occur for the salt sensitive corn than the salt tolerant cotton.
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From page 59... ...
However, if the water is polluted it can impose a cost to society and create a negative externality. The crop-water production functions as depicted in Figures 2, 3 and 4 can be converted to benefit curves by multiplying the yield by the market price for the crop.
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From page 60... ...
Irrigation Scheduling Irrigation scheduling refers to the time, duration, and quantity of an irrigation. Although crop water production functions as depicted in the figures provide the scientific and economic basis for optimizing irrigation, farmers do not have such complete detailed information available to guide their irrigation management.
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From page 61... ...
The effect of cotton and water prices on the optimal AW and the gross returns net of water cost to the farmer were calculated for a measured crop-water production function for cotton grown in the San Joaquin Valley of California. In this case it was assumed that all of the AW infiltrated and there was zero runoff.
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From page 62... ...
Water quality must be considered as a significant factor but may have relatively little effect on total societal water availability. Water resource managers who are considering increasing irrigation efficiency as a major policy approach to increasing water supplies to meet future demands may be over estimating the potential increase in water supply.
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From page 63... ...
Maximum yields of salt tolerant cotton and wheat can be achieved with an irrigation water of 4 dS/m by applying very little more water than used for nonsaline water. In this case the leaching requirement concept to achieve maximum yield is appropriate.
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From page 64... ...
The foods that feed the masses of population are the lower valued crops. If the trend for agricultural production throughout the world was towards the higher valued crops, this could have potentially serious consequences on the ability to feed the human population.
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From page 65... ...
1985. Crop-water production function models for saline irrigation waters.
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