Distribution of the Run-off. 



There are two kinds of run-off, surface and seepage. The first is 

 on the whole harmful, while the latter is beneficial. On cleared land 

 the soil becomes baked and hardened by the sun, so that when the 

 rain falls on it the water runs over the surface to the nearest stream. 

 If the rain is heavy or the slope steep, the soil is washed and the brooks 

 become filled with sediment and impurities. The forest, on the other 

 hand, is provided with a floor of vegetable material, decayed leaves 

 and branches, called humus, anywhere from 2 inches to 2 feet in the 

 thickness. This humus has great absorbing powers, and acts as a 

 huge sponge, converting the surface drainage into a seepage run-off. 

 Ebermayer estimates that the water-storing capacity of humus is 

 considerably more than its own weight; while Henry, from laboratory 

 experiments, "Revue des Eaux et Forets," makes it four times its own 

 weight. The interlocked roots of the trees prevent any washing of 

 the soil. 



By converting a surface run-off into a seepage run-off, water which 

 comes in the season of excess rainfall is kept in the ground to feed the 

 springs during the time when rainfall is deficient. It is not generally 

 considered that this region has a distinct rainy season, but there is a 

 period in the year when more than the average amount of rain falls. 

 The following table shows the average monthly rainfall for the region 

 to be 3.96 inches; for the four months December to March inclusive 

 it is 4.24 inches, and during the months from June to September it 

 averages 3.50 inches a month, a loss of about 17 per cent, from the 

 winter average. 



AVERAGE MONTHLY RAINFALL (INCHES). 



