CO129-588-16 Water Supply- report on the Tai Lam Chung Valley Scheme 3-1-1941 - 3-1-1941 — Page 13

-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.