55 



winter months, evaporation from the soil and water in the tank 

 continues in small amounts. Monthly records of consumptive use 

 are given in Table 8. It is again emphasized that none of the 

 tule tank records is applicable to field conditions without ad- 

 justment . 



The excessive use of water by aquatic plants growing 

 otherwise than in their native habitat led to investigations in 

 the Mojave Valley to determine the difference in consumptive use 

 by tules growing naturally in swamp areas and other tules trans- 

 planted into exposed tanks removed from the swamp influence, and 

 to establish a relation between consumptive use by natural swamp 

 growth and evaporation from water. Both objectives are important 

 if tank data are to have value, particularly if estimates of con- 

 sumptive use are desired in other nearby localities where only 

 evaporation data exist. With the relation once established, it 

 is possible to apply it elsewhere within the same climatic area. 

 Discussion of the Mojave Valley investigation appears on page 59. 



Brush . —/--In arid and semiarid regions practically all 

 moisture from precipitation is held in the top few feet of soil 

 where it is available for use by plants. This is the condition 

 in the foothill area of the Santa Ana Valley. Vegetation on out- 

 wash slopes may be divided roughly into two groups: perennials 

 having a woody structure, such as brush and shrubs; and annuals, 

 as weeds and grasses. In this region precipitation occurs during 

 the winter months, and the summers are long and dry. Contribu- 

 tion to underground water is limited to periods of heavy rainfall 

 when the soil is moistened to field capacity below the root zone 

 (field capacity being the amount of water retained in the soil 

 after excess mobile water has drained away and the rate of down- 

 ward movement has materially decreased following an application 



17 Field investigations with dry-land brush and grass and weeds 

 in the Santa Ana River Valley and with tules in the Mojave River 

 Valley were conducted by Colin A. Taylor, Associate Irrigation 

 Engineer, Division of Irrigation, in cooperation with the Division 

 of Water Resources, Department of Public Works, State of Califor- 

 nia. 



