330 AGRICULTURAL REPORT. 



PROPORTION OF RAINFALL THROV»^N OFF IN SURFACE-DRAINAGE. 



The actual proportion of the rain falling on any district, thrown off 

 in the ordinary surface-drainage, has been calculated with care in 

 many cases as a basis for the construction of works for the supply 

 of water to cities. The results are variously stated at from 50 to 75 per 

 cent, of the annual fall, in average districts of the Atlantic States. 

 Ellet calculated the discharge of a stream called Anthony^s Creek, 

 a tributary of the Greenbrier lliver of Virginia, by daily measure- 

 ments for one year, to be 70 per cent, of the quantity falling in that 

 year, and 05 per cent, of the average fiill for a period of years. 

 In all these localities of the Eastern States a fair proportion of 

 woodland, and also of loose soil and cultivated surface, would be in- 

 cluded, thus affording conditions much more favorable to the retention 

 of moisture, and its absorption into the soil, than those existing on the 

 general surface of the plains. If the rain-fall of Maryland, at 44 inches, 

 should average to throw off GO per cent, as a basis of available drainage 

 for water-supply to reservoirs, there would be 26^- inches of waste, or of 

 ordinary tlow into rivers, and 17^ inches permanently diverted by ab- 

 sorption and evaporation. And of the quantity thus retained and 

 re-sui^plied to the air some portion would inevitably be re-deposited in 

 rain; and thus the primary rain-fall, as it maybe called, may at the 

 season of showers receive considerable augmentation. 'And this increase 

 once established, it becomes permanent so long as the surface continues 

 favorable, increasing the humidity of the local atmosphere and the cul- 

 tivable capacity of the soil. 



Applying this analogy to the plains, a quantity of 20 inches now found 

 falling on the dry surface, and of which 70 to 90 per cent, is immedi- 

 ately thrown off' in the streams and rivers, may, by covering the surface 

 with forests in i)art, and by breaking up the hardened turf in cultiva- 

 tion, diminish the waste from 80 to CO, or even 50, per cent., retaining, 

 say, G or 8 inches of this quantity in some form, and at least preventing 

 the immediate waste from which no secondary benefits can now be de- 

 rived. The practical value of the primary Avater-deposit cannot fail 

 under such change of surface to be equivalent to an addition of 10 per 

 cent, to such original quantity; sufficient in many cases to secure im- 

 portant results, and to obviate deficiencies that are now decisively ad- 

 verse to whole classes of crops. A further measure may be suggested 

 here, in the construction of what may be called temporary reservoirs, 

 in which the surplus of the profuse showers falling in spring and early 

 summer may be retained to be distributed by channels of irrigation, or 

 to secure general benefits hy simple retention. Tlie cost of such works 

 need not be great, nor need they be more than such temporary obstruc- 

 tion of the smaller drainage-channels as is within the power of a few 

 settlers at any locality to construct at any time. For these, as for all 

 preliminary works of the kind, the agency and means of the railroad 

 companies may be easily and effectively emi)loyed. Whatever may be 

 found practicable in this respect, it is clear that 8 to 10 inches of rain- 

 fall in the three or four months of most rapid growth can be utilized to 

 a much greater extent than to permit 80 per cent, of the quantity to 

 run off at the moment it falls, affording no useful result, and only flood- 

 ing the valleys of the Kansas and its tributaries at a time when no sur- 

 plus is wanted in these valleys. Travelers and surveying parties have 

 frequently found these ri\^ers flooded in May and June from the rain-fall 

 of the plains alone, and not from the melting of mountain snows. The 

 Platte has generally a rise in June, due to its mountain supplies, but 



