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CHAPTER 3 

 METHODS OF DETERMINING CONSUMPTIVE USE 



Limited investigations of the use of water by natural 

 vegetation have been made by various methods. Vegetative types, 

 ranging from grasses to trees, have been studied, but owing to 

 the inherent differences in aerial and root growth, different 

 methods of approach are necessary. The source of water consumed 

 by the vegetation, whether from a high water table or from rain- 

 fall and soil moisture, is an additional factor influencing the 

 selection. 



The principal methods used are: (1) by tank investiga- 

 tions; (2) soil-moisture studies; (3) stream-flow measurements ; , 

 and (4) interpretations of water-table fluctuations. 



(1) Tank investigations are conducted under artificial 

 conditions. The growth in the tanks may be the original product 

 of an undisturbed soil although more often perennial shrubs or 

 grasses are transplanted into the tanks before water measurements 

 are begun. With annuals, seed must be planted each year and new 

 root systems developed. Artificial conditions are caused by the 

 limitations of soil, size and depth of tank, or regulation of 

 water supply, and by the very important factor of environment. 

 In ck3nsequence , the resulting tank growth lives under conditions 

 somewhat different from those affecting similar plants in their 

 native habitat, and it is usually necessary to apply correction 

 factors to tank results. 



The natural vegetation most often grown in tanks is of two 

 classes: plants which grow with their roots in water, and those 

 which use capillary moisture. Although a number of tank investi- 

 gations have been made in recent years, the plants occupying the 

 tanks have been limited to a few species. Of these, cattails and 

 tules grow in water, while saltgrass, greasewood, sweetclover, and 

 red willow ( Salix laevigata ) draw moisture from a water table be- 

 low the ground surface. In tank experiments the amounts of water 



