290 THE JRRIGATION AGE, 



The differences in the amounts of water given off by the outdoor 

 evaporator on different days bear some relation of the differences in 

 the temperature and humidity of the air. They are too great, how- 

 ever, to be accounted for by those factors alone; they were, in fact, 

 largely due to different velocities of the wind. By way of improving 

 this, we make use of the data recorded during the month of Novem- 

 ber. During the first ten days of that month the average daily evap- 

 oration, under the constant action of the northeast trade wind, was 

 33.7 per cent. During the following eight days, when the wind direc- 

 tion was south and the air was almost still, the average evaporation 

 was only 13.2 per cent. During these eighteen days the maximum 

 evaporation under a very high wind reached 41. 2 per cent, while upon 

 another day, no motion of the air being observed, the evaporation 

 was only 8.1 per cent. In the course of these twenty day the temper- 

 ature variations were very small. 



From the determinations that have been recorded it may be seen 

 that the monument of the air is the paramount factor in controlling 

 the rate of evaporation from water and soil surfaces. Soil whose sur- 

 faces are exposed to the actions of strong driving winds will give up 

 more moisture, and.will therefore need more- water, than areas in shel- 

 tered locations. Water surfaces exposed to the sweep of the wind lose 

 heavily by evaporation. Economy of water therefore, dictates that 

 reservoirs be built so as to have the greatest depth and the least sur- 

 face, and that they be located so as to be sheltered from the direct 

 action of pervailing strong winds. 



TRANSPIRATION OF MOTRTURE BY VEGETATION. 



The volume of water evaporated from the soil and the volume 

 transpired by the plant during its growth are the controlling factors 

 in determining the total water required in the production of a crop, 

 and therefore the quantity of water to be supplied by irrigation. 



Water enters very largely into the structure of all living organ- 

 isms. It is not only the agent which makes possible the mobility of 

 other constituents of the plant, conveying them from one location to 

 another, but it enters in large proportion into the structure of the 

 organism itself. Consequently plants and trees at all times hold a 

 great volume of water, the supply of which is constantly replenished 

 by the water taken up by the roots and as constantly depleted by the 

 moisture given off into the air by means of transpirtation. It is these 

 quantities that we require to know something definite about. 



The waters of the Hawaiian Islands are of excellent quality, if 

 they do not come in contact with the sea inflow or with soils having 

 high contents of salt, due to the overflow of the sea at some earlier 

 period. In some localities, however, contamination by sea water has 



