116 Water 



alone since the saturation deficit provides a measure of the capacity 

 of the air to take up additional moisture. An increase in the satura- 

 tion deficit produces a rise in evaporation rate. 



An increase in temperature similarly speeds up the evaporation 

 process. A good indication of the magnitude of this influence is ob- 

 tained from the amounts by which reservoirs are lowered in regions 

 of different temperatures. During one year the water level in a 

 reservoir in Ontario dropped 38 cm, a reservoir in California lost 2.4 

 m, and one in Egypt was lowered 3.6 m through evaporation alone. 

 The distribution of plants often reveals the influence of temperature 

 on direct and indirect water loss. On mountain slopes vegetation 

 zones are found at different altitudes according to the exposure to the 

 heat of the sun. Moisture-demanding species are restricted to 

 higher levels on the side of the mountain toward the equator than they 

 are on the cooler opposite side. The variation in the amount of rain 

 required for a good growth of short grass furnishes another illustra- 

 tion of the effect of temperature. In Montana a precipitation of 

 about 35 cm is sufficient, but in northwestern Texas a rainfall of 53 

 cm is needed to produce the same amount of grass. The explanation 

 of this fact is easily apparent when it is realized that evaporation in 

 the 6 summer months in Montana amounts to 84 cm, whereas in 

 Texas it averages 137 cm (Weaver and Clements, 1938). 



The wind velocity also exerts a major effect on the loss of water. 

 With a gentle zephyr of only 8 km per hr the transpiration of plants 

 is increased 20 per cent over that which they exhibited in still air. A 

 wind velocity of 16 km per hr increases the transpiration by 35 per 

 cent and one of 24 km per hr increases it by 50 per cent. The desic- 

 cating action of warm dry winds sometimes prevents the invasion of 

 windward mountain slopes by vegetation that grows perfectly well on 

 the leeward sides of the same slopes where water loss is much re- 

 duced. The harmful effect of excessive evaporation caused by the 

 wind may also be seen in one's own garden. Here winter killing is 

 occasionally due not to very low temperatures, but to a combination 

 of wind and high temperatures! If a warm, dry wind blows for too 

 long a period when the ground is frozen, the plants may lose water 

 faster than their roots can obtain it and they will succumb as a result. 



Our discussion of moisture in the soil and in the air has indicated 

 that living organisms— particularly the higher plants— may exert a pro- 

 found reciprocal influence on the water conditions in the terrestrial 

 environment. Water is a highly modifiable factor in land habitats, 

 and living elements may act either to augment or to diminish the 

 amount of moisture present. Vegetation tends to catch the rain, so 



