under unnatural conditions, without adjustment. 



Because of the high rate of consumptive use in exposed 

 tank's, other investigations were undertaken at King Island in the 

 Delta, with similar growth in tanks set in natural swamp areas. 

 Under these conditions, recorded consumptive use more nearly 

 approximates actual swamp use without the necessity of applying 

 a modifying factor. Data from this investigation, shown in 

 Table 20, while not as complete as in the previous table, indi- 

 cate that actual swamp use of water by aquatic growth in this 

 region is about 46 per cent of amounts indicated by exposed tank 

 data. This indication closely agrees with the average of approx- 

 imately 40 per cent resulting from the southern California inves- 

 tigations. From incomplete data, Lee (18) has estimated that a 

 factor of 50 per cent applied to consumptive use by tules and 

 cattails grown in exposed tanks will closely approximate actual 

 swamp consumptive use of water. It seems that a value between 

 40 or 50 per cent may be used with safety. It will be seen that 

 aquatic growth uses large quantities of water. 



Weeds . --The measure of encroachment of weeds on the water 

 supplies of irrigated crops has also been given consideration. 

 The results warrant the statement that weeds are likely to use 

 more water, in proportion to the ground actually occupied, than 

 the general run of crops. A heavy stand of what is known locally 

 in the Delta as smartweed ( Polygonum acre ) is likely to use two to 

 three times as much water as is required for the proper irrigation 

 of alfalfa. Curly dock may use as much as 100 inches in depth 

 during a single season where there is an ample water supply v/ithin 

 easy reach of the plant roots. Results of weed investigations in 

 the high ground-water Delta area are shown in Table 21. According 

 to Stout (30) nearly 300,000 acre-feet of water, or 24 per cent of 

 the annual consumptive use in the Delta, goes to sustain plants 

 serving little or no useful purpose. He estimates that about five 

 parts go to crops and such weeds as grow in the field with them, 

 and two parts to noncrop plants of all kinds which grow apart from 



