14 
Arterial Drainage 
During summer months the percolation on most soils is little 
or nothing, the whole of the rainfall being absorbed hy the 
thirsty ground and the growing vegetation, or passing away 
by evaporation. In the four winter months, beginning with 
November, the tendency of percolation is to approach the amount 
of rain fallen. 
In the Thames basin it is calculated that at least 3 inches of 
continuous rainfall is required in the autumn to replace the 
evaporation of the summer months, and that no essential per- 
colation contributes to the streams until the soil has become 
thoroughly saturated. Mr. Rawlinson states that in a district in 
South Wales, after two or three dry summers, unless 2^ inches in 
depth of rain fell within six days, nothing came off to flow 
down the stream in the valley.* Beardmore, as the result of his 
observations, found that in September and October it takes 
1 inch of rain, repeated twice in a week, materially to affect the 
streams unless the country is hilly and precipitous.t In the Fen 
districts a great portion of the water sent into the rivers from 
the springs on the high lands in summer is absorbed by the 
Fen soil. The Witliam, in Lincolnshire, drains 1063 square 
miles, less than half of which is fen land. The upper portion is 
fed by several tributaries deriving their source from the oolites, 
the supply being perennial. Yet in a very dry summer, so great 
is the absorption in the Fen portion of the basin, that scarcely 
any fresh water passes away to sea, the supply from the high 
land streams being utilised for keeping up the water level in the 
fen ditches and sewers. In the dry summers, of 1864 and 1868, 
so great was the absorption that not a single drop of water 
passed during the summer and autumn out of the river down 
the haven to the sea ; and it was not until quite the end of the 
year — in the year 1864 the end of December — before the water 
in the river had risen sufficiently high to flow over the sediment 
which had collected in the haven. The circumstances of this 
river are no doubt peculiar, as the flow of the tidal water is 
arrested in its progress about 5 miles from the mouth of the 
river by a sluice placed across the channel ; and no benefit 
is derived from the semi-diurnal ebb and flow of the tides in 
maintaining the water level in the pores of the soil nor from 
the scour in keeping the channel free from deposit ; but the 
fact stated is sufficient to show the immense amount of absorp- 
tion going on in free soils when under cultivation. 
Evaporation is greatest off the surface of water, amounting in 
the course of the year to a quantity nearly equal to that falling- 
* " Kaiiifall and Evaporation," 'Trans. Instit. Civil Engineer?,' vol. xlv. 
t ' Manual of Hydrology.' By Beardmore. Waterlow and Sons, 1862. 
