photosynthesis, stomata on the surface of leaves are open to permit absorption of CO2 and release of O2. As long as the stomata are open, there will be a continuous loss of water vapor to the air, the quantity depending on the temperature and relative humidity of the air and the number of open stomata. If the soil water supply declines and insufficient water is available to satisfy the demand, the stomata close and photosynthesis ceases. At this point the temperature of the leaf may rise, since heat is no longer being lost by evaporation. If the heat being absorbed from the environment cannot be rejected as IR radiation and/or as sensible heat conducted to the air, the leaves may gradually wilt and die. In extremely hot dry climates where the heat load is too intense part of the time due to lack of sufficient ground water, the growth of many plants may be limited to periods of rainfall, so that annual droughts are survived in the seed stage. Other plants are adapted to storing water for long periods of time, and still others can survive surprisingly long periods of drying out. The effect of the receiving antenna's heat loss can be partly evaluated by considering the additional pressure on the water balance which would result if the energy absorbed by the plants were rejected by them as latent heat, that is, in terms of the extra water which would be needed for this purpose. About one half the total energy lost by the receiver would be absorbed by the plants on the ground below the receiving array. This would represent a heat rejection problem only during the daytime. The result, therefore, would be equivalent to increasing the potential evapo-transpiration by an amount equivalent to 25% of the heat rejected. The corresponding values are 10 to 65 mm per year for the range of mean energy loss values and about 170 mm for the assumed maximum of 50 w/m2 in the center of the beam. The effect on typical desert communities of this increase in potential evapo-transpiration may be judged by comparing it with the factors determining the water balance in these communities. The factors of interest are: (a) the potential evapo-transpiration, as calculated from the rainfall (humidity) and temperature; (b) the actual evapo-transpiration, which depends on the potential evapo-transpiration and the availability of soil water (and which can be roughly calculated knowing the rainfall and soil type); (c) the evapo-transpiration deficit, which indicates the capacity of the vegetation community to reject heat by other means than evaporation, or to survive high temperatures by any of a number of adaptive mechanisms. During some months precipitation may be much higher than necessary to meet the needs of the community (for example, in winter), so a surplus in water may result even when potential evapo-transpiration exceeds actual evapo-transpiration for the year.
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