WATER LOSSES FROM WET AREAS 



51 



use of water by salt grass for comparison with nse of water by other 

 wild growths in order to determine the net draft on the ground water 

 supply by native vegetation. 



In experiments with salt grass grown in tanks at the Santa Ana 

 station, distribution of moisture above the water table was determined 

 by means of soil-moisture samples. Depth to water in the different 

 tanks ranged from 1 to 5 feet. Reference to Table 10 shows the average 

 soil moisture in the top foot of soil, when the water table was 1 foot 

 from the ground surface, to be 27.1 per cent and that it decreased 

 rapidly until the water table was 3 feet in depth, or near the limit of 

 the capillary rise. With the water table below 3 feet there was little 

 difference in the moisture content of the top foot of soil. 



It is evident, then, that there was greater soil evaporation in the 

 tanks having the highest water tables. In the same tanks there was 

 also greater transpiration because of a heavier crop of grass, due, in 

 turn, to a higher moisture content. As consumptive use of water 

 includes both soil evaporation and plant transpiration, it follows that 

 plant growth in tanks having the highest water tables has the higher 



TABLE 10 



RELATION OF SOIL MOISTURE TO DEPTH OF WATER TABLE IN TANKS 



AT SANTA ANA STATION 



consumptive use. This is indicated in Plate V, which shows a compari- 

 son of monthly use of water by each set of salt grass tanlvs. The set 

 having the highest water table, shown by the solid bar, used the most 

 water. It will be noted that bars representing tanks of disturbed soil 

 show a greater use of water than do tanks of undisturbed soil, when 

 both have the same depth to water table. 



The draft of salt grass on the ground water supply depends upon 

 the depth to the water table and the soil structure. This may be 

 estimated by comparison with results of tank experiments made dur- 

 ing this investigation. When an irrigated area is extended to include 

 uncultivated lands, the resulting net draft on the ground water supply 

 is the difference between the amount of water required by cultivated 

 crops and by the indigenous growth which the cultivation replaces. 

 If the cultivated crop uses no more water than the native growth in 

 the same area, no depletion results because of the change. Practical 

 experience and scientifie experiments have indicated the necessary 

 water requirements of crops, but little information is available as to 

 the consumptive use of water by noneconomic plant life. 



