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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I' <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />Je~9~ar. <br /> <br />High evaporation negates the effecti veness of light <br /> <br />rains. <br /> <br />Transpiration in plants is the loss of water through the plant <br />stomata. Water vapor pressure inside the stomata is quite high. <br /> <br />When relative humidity is low, a large vapor pressure gradient is <br /> <br />produced. This vapor pressure gradient is often referred to as the <br />vapor pressure defici t (VPD). Since water vapor moveS from areas of <br /> <br />high to low vapor pressure, lower relative humidity will increase the <br /> <br />VPD and the transpiration rate. Humidity generally impacts crop <br />growth as a result of its effect on the transpiration rate which then <br /> <br />influences related plant growth processes. <br /> <br />Site specific relative humidity data are lacking for reservation <br />lands. However, data are available for sites at Cortez, Colorado <br /> <br /> <br />and Farmington, New Mexico. Since summertime humidity at both <br /> <br /> <br />Farmington and Cortez is very similar, it is assumed that conditions <br /> <br /> <br />at these sites will approximate those of the reservations areas. <br /> <br />Relative humidity normally ranges from approximately 60 percent <br />during cooler morning hours to less than 30 percent during late <br />afternoon. Summer thunder showers may temporarily increase <br />relative humidity, but such effects are generally localized and <br />short lived. <br /> <br />3- 30 <br />