275 



cayed, offers additional channels for a change of surface drain- 

 age into sub-drainage. 



In these operations the condition of the forest cover has much 

 to do with the degree of its effectiveness and, the condition of 

 the forest floor is of more moment than that of the leaf canopy. 

 Although on a forested area the tree growth may be left intact, 

 yet, if the loose litter and underbrush have been burned off 

 and the soil has been compacted by the trampling of sheep or 

 cattle, the effectiveness in regulating streamflow is much im- 

 paired. 



The forest co\er, therefore, tends to convert the surface runoff 

 into underground runoff or percolation. This is desirable, be- 

 cause the former is likely to do injury by eroding the soil, while 

 the latter is generally beneficial to vegetation in the formation of 

 sprmgs and in raising the water level in the soil. It is particularly 

 desirable in these islands wherever there are artesian basins which 

 <'ire drawn upon by pumping and which must perforce be re- 

 plenished with water by this natural method. 



A concrete example of this is offered on the Island of Oahu, 

 which is roughly 600 square miles in area, and on which it is 

 estimated there falls annually enough rain to cover the island to 

 a depth of five feet. Of this total precipitation, it is estimated 

 that there reaches the sea or is lost through evaporation, the 

 equivalent of a depth of 3^ feet, leaving lyl feet of water over 

 the 600 square miles to be the flow of the artesian wells and sur- 

 face springs together. Anything that will tend to increase this 

 proportion is evidently most desirable. The rainfall in these 

 islands is comparatively heavy and the catchment areas are rela- 

 tively small, the stream gradients very high and the runoff from 

 deforested and barren slopes very rapid with resulting erosion and 

 damage to agricultural soils. The only satisfactory way to 

 prolong this runoff is by means of the cover afforded by pro- 

 tection forests which will retain at least a portion of the rainfall 

 and feed it gradually to the surface and underground water 

 sources, thus serving as a regulator to decrease floods, and to 

 increase the dry season discharge. 



In these islands, under tropical conditions, the virgin forest 

 is much more dense than in other places and the beneficial or 

 destructive effects of a forest cover or the lack of it is proportion- 

 ately greater than elsewhere. There is probably no other part 

 of United States territory where the relations betweeen available 

 waters and forest cover are more intimate and more deHcate or 

 where the natural balance is more easily disturbed with dis- 

 astrous results. 



This distribution of the water, which lengthens the time during 

 which the atmospheric precipitation can be employed, and which 

 under circumstances in some regions may lengthen the supply for 

 years, the water reaching the river or the artesian basin a long 

 time after it fell on the mountain top. renders springs and ar- 

 tesian basins independent of wet and dry seasons, and equalizes 



