131
X.
Some Additional Notes on Essex Watersheds.1
By Prof. G. S. Boulger, F.L.S., F.G.S., F.R.G.S., &c.
[Read December 15th, 1883.]
Of the numerous practical applications of the exact obser-
vations of Science few are of more universally recognised
importance than those bearing on the question of water-
supply. Without referring to the limitless waters of the
ocean that wash the eastern shores of our county and may
soon be utilised as a tidal motive force of matchless power
and economy, we have in our county rainfall and its natural
and artificial distribution, a topic of general concern. The
rainfall of Essex averages about 22 inches per annum, though
locally, as at Epping, it exceeds 25 inches. It is estimated
that the greater part of this is returned to the air by evapo-
ration to fall again; and, from the small general slope, cold
impervious soils, and proximity to the sea, the rate of evapo-
ration in Essex is probably comparatively high. Similarly,
while the general ratio of the outfall of rivers to rainfall is
either as one to three or four, in the stiff clays and loams of
Essex it is probably the first rather than the second, the
amount soaking into the ground being relatively small. The
chalk districts round Walden and Purfleet are exceptional in
this respect in the extent to which their drainage is sub-
terranean ; in other parts of the county streams are nume-
rous, but small, depending upon surface drainage rather than
upon large or deep-seated springs. More than a quarter of
the county drains into the Thames, either directly or through
its tributaries, of which the chief are the Lea, the Roding,
1 [Supplemental to the paper "On the River Basins of Essex as
Natural History Provinces," Trans. E. F. C., vol. ii. pp. 79-87.—Ed.]