100 SYLVICULTURE IN THE TROPICS n .i 



In Ceylon, the action of the Government in pro- 

 hibiting the sale of lands at an elevation of above 5000 

 ft. has already been mentioned ; but this action has 

 not been sufficient to prevent the silting up of several 

 rivers, notably the Kelaniya Ganga and the Mahaweli 

 Ganga, and the consequent obstruction to navigation 

 in dry weather and extensive floods during the monsoon. 

 Indeed, an easy object-lesson is obtained by any one 

 standing during a downpour of rain on slopes, some 

 of which are covered with forests, while others bear tea 

 estates. On the former, the streams come down almost 

 as clear as crystal, while the water which runs off the 

 surface of the tea estates, although its impetus is 

 checked by numerous catchment drains, reaches the 

 bottom of the slopes in a turbid flood. The water 

 which comes down grass-covered slopes, although not 

 as muddy as that from cleared lands, is by no means 

 as clear as that which comes from forest land. 



When rain falls on bare land, the soil at first absorbs 

 the moisture, and, when this has been absorbed to 

 saturation point, the water flows off to the nearest 

 watercourse, carrying with it, in increasing proportion, 

 according to slope and friability of the soil, the loose 

 particles of the latter. The depth to which this 

 saturation will take place also depends on the nature of 

 the soil, the gradient of the slope, and the duration of the 

 rain ; but once this is accomplished the surface water runs 

 off. On forest land, and by this I mean particularly 

 in forest protected from fire, where the soil is either 

 covered by a layer of humus, or, where this is not 

 visible, where it contains in its upper layers almost 

 invisible particles of this humus, and there is on the 

 surface deposits of various vegetable debris, the satura- 

 tion point is much higher than on bare slopes. It lias 

 been calculated that on forest-covered slopes about one- 

 third of the water is detained. This water is gradually 

 given oft' into the soil, led down by the roots of the 

 vegetation growing in it, and feeding underground 

 reservoirs, while the surplus water, checked in its impetus 



