50 DIVISION OF WATER RESOURCES 



In order to liave a coniparisou of evaporations from both disturbed 

 and undisturbed soils, further tests were carried on with soil looselj^ 

 settled in water as the tanks were filled. Diflt'erences due to the methods 

 of filling the tanks became apparent immediately at the outset. Although 

 both sets of tanks had water tables at the same depth, the moisture 

 content in the loosely filled soil was enough to keep the soil surface 

 moist, Avhile the surface in tanks of undisturbed soil was dry. 



For comparison of monthly records of soil evaporation under the 

 two conditions of soil structure, further reference is made to Tables 

 13 and 14, which show the monthly use of water by all tanks. For the 

 same six -month winter period of 1929-80, during which evaporation 

 from undisturbed soil having a 2-foot depth to the water table was 

 1.775 acre-inches per acre, the disturbed soil with the same water table 

 evaporated 6.889 acre-inches per acre, or nearly four times as much. 



In applying the results of these experiments to field conditions, 

 it is obvious that only data secured from experiments with undisturbed 

 soil should be used, and that measurements of evaporation from dis- 

 turbed soil do not represent a true criterion for natural soil moisture 

 losses. The large difference in the rate of evaporation for disturbed 

 and undisturbed soils, indicates the advisability of further experimental 

 work of this character. It also seems to cast doubt on the accuracy of 

 results of some previous experiments along the same line. 



Use of Water by Salt Grass 



Salt grass {Distichlis spicata) is most often found in somewhat 

 alkalied areas of shallow water table, the limit of depth from which the 

 roots may draw moisture depending upon the soil type. In various 

 investigations the limiting depth to water at which it has been found 

 has varied. In one small locality with heavy soil, in the lower Santa 

 Ana River basin, it was observed where the ground water was at a depth 

 of 11 to 12 feet, but in general the limiting depth in lighter soils is 

 about 6 feet. Moreover, it was found to exist only in those areas that 

 were classified by the United States Bureau of Soils as containing some 

 alkali. 



The growth of the plant is spread by means of a thick creeping 

 root stalk within the upper few inches of soil from which finer roots 

 extend downward in search of moisture. The stifle, light green leaves 

 rise from each joint of the root stalk and sometimes spread to form a 

 considerable sod. The grass has a distinctly salty taste and, although 

 it is sometimes used for pasture, stock do not thrive on it. The grow- 

 ing period in southern California is from February to December and 

 although the grass dies or becomes dormant in the Avinter, there is some 

 discharge from the water table throughout the winter months. Salt 

 grass is not an excessive user of water. Its habit of growth in alkali 

 soils has caused the plant to protect itself against toxic effects of alkali 

 salts by a decreased rate of transpiration. 



Until recent years, the area devoted to salt grass in the lower Santa 

 Ana River basin has been considerable, but the advent of drainage 

 systems and extension of the cultivated area has crowded out the salt 

 grass from a number of districts. Various salt-grass areas still exist, 

 however, especially where the ground water continues to remain near 

 the surface. This investigation was begun to measure the consumptive 



