Occurrence of Water in Land Environment 109 



Occurrence of Water in Land Environment 



The only ultimate source of water for the terrestrial environment 

 is condensation— chiefly in the form of rain. If precipitation w^ere 

 evenly distributed, it would cover the earth to a uniform depth of 

 1 m each year. Quite the contrary is the case. A glance at Fig. 4.7 

 will reveal the irregularity in the geographical distribution of rainfall. 

 In the desert areas the rainfall is less than 25 cm per year and is in- 

 adequate for most organisms. In the black areas of the chart an 

 excessive rainfall of 200 cm or more is reported each year. The mean 

 annual rainfall on the south side of the Himalayas is recorded as 1232 

 cm. At the other extreme is Iquique, Chile, with an average precipi- 

 tation of 0.125 cm per year. Among the Olympic Mountains in the 

 state of Washington the annual rainfall totals as much as 380 cm on 

 the windward side of some ridges and as little as 25 cm on the lee- 

 ward side. 



Seasonal variations in the distribution of rain may be of even 

 greater importance than the geographical aspect. In many regions 

 the division of the year into a rainy season and a dry season is of more 

 ecological significance than the change in temperature between sum- 

 mer and winter. In the climatographs shown in Fig. 4.8 two situations 

 are contrasted. In the Chicago region representing a temperate cli- 

 mate, an average of 5 to 9 cm of rain falls in every month of the year, 

 but the average monthly temperature varies from about — 4°C in 

 the winter to about 22°C in the summer. In the tropical climate of 

 Barro Colorado Island the average monthly temperature changes by 

 not more than 1 or 2°C throughout the year, whereas the precipitation 

 drops to less than 3 cm per month in the dry season and rises to more 

 than 40 cm per month in the wet season. 



Wherever the amount of precipitation varies greatly during the 

 course of the year, the time of occurrence of rain in relation to the 

 temperature cycle has great effect on the vegetation. Only those 

 species will thrive whose varying needs for water in the different life 

 stages are satisfied by the seasonal distribution of available moisture. 

 In many of the grass-covered or forested areas of the temperate zone 

 the major portion of rain occurs in the summer, but in other regions, 

 of which southern California is an example, most of the precipitation 

 occurs in the winter. In the latter situation we find a special type 

 of vegetation with broad evergreen leaves known as the sclerophyllous 

 forest. In the prairie provinces of Canada the average annual precipi- 

 tation of 50 cm would not be sufficient to support agriculture if the 



