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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />, <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />1881 <br /> <br />some degree of control over transpiration losses through their <br />ability to close during climatic periods which favor high <br /> <br />evaporative demand. The ability to control excessive transpir- <br /> <br />ational losses through the stomata varies among plant species. The <br /> <br />amount of water required by a plant is therefore a function of crop <br /> <br />species, environmental and climatic conditions. <br /> <br />Consumptive use (evapotranspiration) is the sum of the water <br /> <br />evaporated from the soil and the water transpired by the plants <br /> <br />growing thereon. The consumptive use requirement may be derived <br />mdthematically using empirical equations or by measuring actual <br />water use with a lysimeter. Measurement of the evaporation from an <br />open pan filled with water may also be used as a basis to estimate the <br /> <br />consumptive use requirement for an area. The consumptive use <br /> <br />requirement is the amount of water required to meet plant needs when <br /> <br />growth is not limited by lack of water. Consumptive use <br />requirements reflect the plant growth environment and vary between <br />different geographic areas and climatic conditions. <br /> <br />Environmental growth conditions which act to reduce crop consumptive <br />use to levels below that needed to achieve full production <br />potential result in crop yield losses. The impact of reduced crop <br />consumptive use on yield is related to the type of crop and the timing <br />and amount of the water shortage. Generally, crops can be separated <br />into two broad groups: 1) those harvested for their vegetative <br />parts and 2) those harvested for their fruiting parts. General <br /> <br />3- 19 <br />