76 



usually ranging from 30 to 45 per cent, of the total, but much 

 depends on the character of the rainfall, and on the species of tree. 

 In a district of heavy annual rainfall a smaller proportion of the pre- 

 cipitations is caught by and evaporated from the trees than where the 

 rainfall is light. Similarly in the rase of heavy and long-continued 

 rain, as contrasted with gentle showers ; in the latter case, in fact,, 

 but little of the water reaches the ground through the leafy canopy 

 of a dense forest Then again much depends on the kind of tree. 

 evergreens intercepting more water throughout a year than deciduous 

 trees, and a larger proportion of the rainfall is evaporated from the 

 leaves and branches in summer than in winter. 



But although less rain-water reaches the soil of a wood than finds 

 its way to the gmund in the open country, the moisture in the soil is 

 much better conserved in the former than in the latter case. Long- 

 continued observations have shown that more water drains from a 

 wooded area than from one devoid of trees. The greater abundance 

 of water in forest soil, in spite of the trees intercepting a large pro- 

 portion of the rain-fall, is due partly to the reduction of evaporation 

 owing to the exclusion of the sun's rays by the foliage, partly to the 

 reduction of evaporation owing to the exclusion of the sun's rays by 

 the foliage, partly to the air in a forest being more humid, and thus 

 better fitted to discourage evaporation, and partly to the absorbent 

 and retentive character of the decaying vegetable matter that covers 

 the ground of a dense and well managed wood. The lace-work of tree 

 roots, too, that occupy the soil of a forest, offers mechanical resistance 

 to the rapid surface-flow of water. It is also to be noted that roots 

 penetrate to great depths, and when they die they leave holes through 

 which water readily penetrates from the surface. The friable condi- 

 tion of the soil of a wood, too, permits ready percolation of water, 

 whereas in the open country the denser character of the surface of the 

 ground is less favourable to the entrance of water. The consequence 

 is that streams in a wooded country are not so subject to rapid rises 

 and talis, the flow being maintained more equably throughout the 

 year. Where water supply for domestic or industrial purposes is con- 

 cerned, the avoidance of violent freshets on the one hand, and scanty 

 flow on the other, is alike desirable. Not only may the water of 

 sudden and heavy floods be lost owing to the incapacity of the reser- 

 voir to contain it, but such floods have also the disadvantage of carry- 

 ing much mud and similar material in suspension, and this gradually 

 silts up reservoirs, besides entailing increased expenditure in filtering 



It may be pointed out that the water of a reservoir surrounded 

 by well stocked woodland is not subjected to the same amount ot 

 violent agitation during gales as is the case when such sheltering 

 agency is absent. The mud and silt deposited on the bottom, and 

 especially along the margin, is consequently left comparatively undis- 

 turbed, with corresponding advantages in the matter of purity. 



When a catchment area is covered with trees, and with the vege- 

 table matter that accumulates on the surface of the ground, the water 

 that reaches the soil as rain is impeded in its flow, and its evaporation 

 is hindered, so that the general effect is equivalent to an increase in 

 the size of the reservoir. It is also important to note that snow 

 melts more slowly underneath trees than in the open country, 



