-8-
13
information is available to establish the relationship between
the rainfall and the runoff from the ground, and the only data
which were available were obtained during the gauging of the
Shẳng Mun River for the years 1927 to 1935, which have been
adopted for our salculations.
39. This relationship varies for different rivers,
depending largely upon the amount of permeable material present
on the surface which absorbs rain during wet weather, to be
given out in the shape of springs and seepages of water in dry
weather, thus augmenting the flow of the stream during that
period. This effect, however, is unimportant during the monsoon
period when the ground becomes saturated and no more rain is
absorbed.
40. From the few gaugings which are available, it would
appear that the flow of the Tai Lam Chung is not so great per
unit of catchment area as the Shing Mun River during the dry
period, but in adopting the relationship established for the
latter river, we are of opinion that no serious error is involved.
Evaporation from the Water Surface.
41.
Another factor which effects the yield of a reservoir
is the allowance to be made for evaporation from the water
surface, which has been assumed to be the same as at the Tytam
Tuk Reservoir.
Yield of the Reservoir.
42. The rainfall observations at the Royal Observatory
which extend over a period of more than 50 years, show that
the Colony has passed through two periods of deficient rainfall,
viz: the years 1898 to 1901 inclusive, when for four years in
succession the annual rainfall was much below the average, being
only 67% of that average in 1898 and 65.6% in 1901; and the
years 1895 and 1896 when the rainfall for the year 1895 was only
54.3% of the average.