1918] RURAL ENGINEERING. 387 



now composing the floor of the valley. The changes necessary to remove the 

 silicic acid and lime from the mountnin waters are simple. The small concre- 

 tions of calcic carbonate met with in the sand from the strata passed through 

 at 550 ft.- indicate simple precipitation as the method of removing the lime. 

 . , . Evaporation alone is considered adequate to account for the concen- 

 tration of the sodic carbonate that we find in this section. Evaporation at the 

 present time is sufficient to add 145,500,000 lbs. of sodic carbonate t© this 

 section of the valley yearly. This is on the supposition that the mountain 

 water carries 2.5 grains of sodic carbonate in each imperial gallon, or 10 lbs. of 

 water evaporated. 



" The present agricultural condition of this section of the valley is due to the 

 accumulation of this salt, black alkali, rather than to an excess of water. Lo- 

 cal surface drainage is necessary in many small localities. The evaporation from 

 the area involved is equivalent to an inflow of 2,000 sec.-ft. throughout the 

 year. This is probably a larger amount than this section of the valley actually 

 receives, except for a very short period in the spring of the year when the 

 direct overland inflow may equal or possibly exceed this amount. 



" The San Luis Lake water is peculiar in its composition and unlike either 

 the river, ground, or artesian waters. The deposit of sodic carbonate east of the 

 San Luis Lake is probably derived from the evaporation of the brown artesian 

 water, and has no connection with the lake. The conditions which have deter- 

 mined the character of the brown artesian waters are still active in determin- 

 ing the agricultural features and questions of this section of the valley. The 

 question of black alkali in this section is in places further involved by the oc- 

 currence of nitrates. The conditions which obtain and are inimical to vegetation 

 can be ameliorated by rational irrigation, chemical treatment of the soil, and 

 surface drainage where needed." 



Run-off from the drained prairie lands of southern Louisiana, C. W. Okey 

 {V. S. Dept. Agr., Jour. Agr. Research, 11 {1911), No. 6, pp. 247-279, figs. 5).— 

 Investigations on the relations between the rainfall and the amount of water 

 that it is necessary to pump from 10 typical drainage districts in order to 

 secure a degree of drainage that will allow the grovnng of ordinary field crops 

 are reported. The districts vary in area from 647 to 7,500 acres and the details 

 of soil, crops, surface slope, and character of the drainage channels and levees 

 are different on each district. A brief description of each district is given 

 together with a brief summary of conditions prevailing during each year cov- 

 ered by the records. The results of the investigations are graphically reported. 



" It is evident that the effect on the run-off of a change in the capacity of 

 the pumping plant can not all be measured by the change of level of the water 

 in the main reservoir channels, as greater slope and velocity of water occur 

 in the drainage channels when a larger pumping plant is installed. This effect 

 extends even to the small field ditches. 



" In estimating the run-off likely to result from an assumed storm on a given 

 district the curve used should be that for the district which resembles the 

 given district in area, pumping-plant capacity, reservoir capacity, and general 

 conditions. The curves may be considered only as representing general ten- 

 dencies and not definite values. They should be of service, however, in making 

 the proper adjustment between reservoir and pumping-plant capacity. . . . 

 It is believed that, in general, after a capacity of about 0.5 in. of water over the 

 area drained has been provided in the main reservoir channels between the 

 surface and a level 5 ft. below it will be cheaper to obtain increased capacity 

 to handle storms by enlarging the pumping plant rather than the reservoir. 



45967°— 18 7 



