33 
able to accommodate our- expected tourist crowds with this 
essential fluid, much less furnish it in sufficient and permanent 
quantities for the domestic needs of the resident population. 
The available supply of water is most certainly going to decrease 
unless necessary and radical steps are taken now to do every- 
thing possible to maintain and preserve the forest on the city 
watershed. Not only must the injury done to the forest in the 
past be repaired by artificial and natural means, but every step 
must be taken to head off and prevent further damage to the 
forest from whatever cause. 
My predecessor, after 10 years' acquaintance with the native 
forest, was convinced that "to get the best results from the 
water bearing forests in Hawaii in the way of steady and con- 
tinuous yields, it is in many localities essential that both animals 
and men be vigorously excluded. 
Influence of Forest on the Runoff. 
To everyone who has stopped to consider the benefits derived 
from a forest cover, it is plainly evident that the forest is one 
of the best means of preventing not only undesirable erosion but 
also of storing the water that falls from the skies. The force 
of the rain is broken by the trees, the underbrush, and the litter 
on the ground so that it does not beat directly upon the soil. 
Much of the precipitation reaches the earth by running down 
the twigs and branches. In a heavy rain the water drips down 
so quietly as to have practically no beating effect upon the soil 
and there is no perceptible surface runoff until large quantities 
of rain have fallen. The forest cover tends to convert the 
surface runoff into underground runoff or percolation, and the 
influence of a satisfactory forest cover in regulating the stream- 
flow is based on the principle that rain waters penetrate more 
readily a forest-covered soil than one that is bared of trees. The 
foliage, together with the loose litter of the forest floor, also 
reduces the compacting effects of the raindrops and the drying 
effect of sun and wind thus keeping the soil granular so that the 
water can easily percolate. The mechanical obstruction which 
the trunks, underbrush, litter, and ground cover offer to the 
rapid surface drainage of waters also lengthens the time during 
which this percolation may take place. The network, also, of 
deeply penetrating roots offers additional channels for a change 
of surface drainage into sub-drainage. 
In all of these operations the condition of the forest cover 
has much to do with the degree of its effectiveness and the con- 
dition of the forest floor is of even more moment than that of 
the leaf canopy. The existence of a forest on our city water- 
shed, therefore, enables a large part of the rainfall to percolate 
into the soil, there to be collected in the natural underground 
reservoirs and thence to be fed out gradually to the springs and 
