22 



BULLETIN 181, U. S. DEPARTMENT OP AGRICULTUEE. 



of the rainfall flowing off was, as previously calculated, 56 per cent 

 for this storm period. Fifty-six per cent of 3.9 inches is 2.18 inches, 

 or 15,950,000,000 cubic feet. The run-off which produced the 

 maximum rate is equal to the total run-off, represented by the area 

 maoqs, minus the above computed amount and should be equivalent 

 to the area mafanrs under the hydrograph as constructed for improved 

 conditions. 



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March April 



Fig. 4. — Discharge hydrograph of Pearl River at Jackson, Miss. 



May 



It can be seen from the figure that the area maunrs is common to 

 each of the two distinct hydrographs for actual and improved con- 

 ditions. Therefore the run-off to be provided for by the triangle afa 

 must equal the run-off in the area aoqrnu, minus the above amount 

 to be deducted for rainfall occurring subsequent to April 5. The 

 area aoqrnu equals 16.76 square units, or 46,350,000,000 cubic feet^ 

 which diminished by 15,950,000,000 equals 30,400,000,000 cubic 



