2^^ Illinois State Dairymen's Association. 



Run-off. 



The amount of washing depends upon the amount of surface 

 run-off. The amount of run-off and the effect it has will be modi- 

 fied to a large extent by the character of the soil and subsoil, by 

 the length and steepness of the slope, by the tillage practiced, 

 and by the vegetation produced. It is very difficult to compute 

 the amount of run-off because of these many conditions causing 

 variation. Considerable work has been done, however, w^hich 

 forms the basis for a fair estimate. 



Mr. F. H. Newell, of the United States Geological Survey, 

 has determined the amount of water running off from different 

 catchment basins in various parts of the United States, and in 

 this way has been able to estimate the amount of run-off as com- 

 pared with the rainfall per annum. His work shows that for 

 eight years, from 1884 to 1891, 48.9 per cent of the rain falling 

 in the basin of the Savannah river found its way to the sea. In 

 the Connecticut river basin an average for thirteen years gives 

 56.5 per cent, and in that of the Potomac for six years, from 1886 

 to 1891, 53 per cent ran off. In general, Mr. Newell says that 

 where the mean annual rainfall on mountain topography is 40 

 inches the run-off approaches 30 inches, where the rainfall is 25 

 inches the run-off is about 12 inches, and where the rainfall is 15 

 inches the run-off is less than. five inches. Where the surface is 

 made up of broad valleys and gentle slopes in open country, a 

 mean annual rainfall of 50 inches gives an annual run-off of about 

 25 inches, or 50 per cent of the rainfall; where the rainfall is 40 

 inches the run-off is about 15 inches, or 37 J4 per cent, and where 

 the rainfall is 30 inches about 8 inches, or 27 per cent, will run 

 off. Neither of the above represents the rolling and hilly -land of 

 this state, which is about a mean between the two forms of 

 topography. Greenleaf estimates the average run-off for the Illi- . 

 nois river basin at about 24 per cent of the total rainfall of that 

 catchment area. Leverett estimates the run-off for the entire 

 state at about 21 per cent of the rainfall. These estimates are too 

 low for the hilly and broken land of the state, but if we take the 

 mean of the mountain topography and broad valleys as given by 

 Newell we get an amount that comes near the truth. This ap- 

 proximate mean gives the run-off as shown in the following 

 table : 



