Appendix 3.4
Water Budget and Predicted Water
Level Changes for
Appendix 3.4
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WATER BUDGET AND PREDICTED WATER LEVEL CHANGES FOR SAN TIN / LOK
MA CHAU STATION COMPLEX MARSHLAND |
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SOURCE: HONG KONG
OBSERVATORY DATA REFER TO KING'S PARK MONITORING STATION, KOWLOON. |
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EASTERN MARSH WATER BUDGET BASED ON MEAN MONTHLY RAINFALL OVER
1990-1999 |
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*Cumulative balance is calculated by setting the water level to
0 at the end of the wet season |
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and at the end of the period when soil is at field capacity (i.e
after which monthly balances are in continual deficit for the dry season) |
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Water levels refer to above ground open water levels |
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Assumptions |
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Rainfall in at Lok Ma Chau
is 65% of that at King's Park (according to Hong Kong Observatory isohyet
data for 1961-90). |
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Evaporation rates do not
significantly differ between King's Park Monitoring Station and Lok Ma Chau |
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Under wetland conditions
evapotranspiration rates from open water, bare soil/mud and wetland
vegetation are approximately equal and equal to observed monthly evaporation
rates. |
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Predicted balances are
based on potential evaporation rates and do not take into account reduced
evaporation when soil water tables are low. |
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Wetland area is constant |
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Seepage rates are minimal |
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Storage pond is full at the
end of the wet season |
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Water level changes below
the soil surface have negligable effects (therefore specific yields when
water levels drop below ground level are not taken into account) |
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RA Area of roof supplying
runoff water = |
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2.4 |
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MA. Area of marsh
vegetation (excluding western marsh & water treatment reedbed) (ha) = |
2.46 |
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RC Runoff coefficient for
station roof |
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0.95 |
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Storage ditch area (ha) |
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0.22 |
Approx |
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Storage max water level set
by sluice (mPD) |
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3.00 |
Approx |
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Storage ditch volume (when
full) (l) |
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2,200,000 |
Approx |
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Storage ditch base level
(mPD) |
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2.00 |
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Targets relative to max
operating water level (mPD) |
3.00 |
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A
Marshland requirements |
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A. |
B. |
C |
D |
E |
F |
G |
H |
I |
J |
K |
L |
M |
N |
O |
P |
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King's Park |
Corrected |
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Potential |
Requirement |
Requirement |
Requirement |
Target level |
Yield from |
Requirement |
Requirement |
Requirement |
Requirement |
Target pond |
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Month |
Rainfall |
Rainfall |
Evaporation |
Balance |
cumulative |
to maintain level |
to maintain level |
to maintain level |
(below max) |
change |
for target |
for target |
for target |
for target |
water level |
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mm |
mm |
mm |
mm |
balance mm |
l |
l/day |
l/sec |
mm |
mm |
mm |
l |
l/day |
l/sec |
mPD |
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Jul |
437 |
284 |
151 |
134 |
0 |
- |
- |
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-50 |
-100 |
0 |
- |
- |
- |
2.950 |
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Aug |
466
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303 |
147 |
156 |
156 |
- |
- |
- |
0 |
-50 |
0 |
- |
- |
- |
3.000 |
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Sep |
329 |
214 |
135 |
79 |
235 |
- |
- |
- |
0 |
0 |
0 |
- |
- |
- |
3.000 |
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Oct |
136
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89 |
144 |
-55 |
179 |
1,361,462 |
45,382 |
0.53 |
-50 |
50 |
5 |
131,462 |
4,382 |
0.05 |
2.950
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Nov |
25 |
16 |
110 |
-94 |
86 |
2,305,008 |
74,355 |
0.86 |
-100 |
50 |
44 |
1,075,008 |
34,678 |
0.40 |
2.900 |
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Dec |
28 |
18 |
69 |
-50 |
35 |
1,238,684 |
39,958 |
0.46 |
-150 |
50 |
0 |
8,684 |
280 |
0.00 |
2.850 |
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Jan |
25 |
16 |
59 |
-42 |
-7 |
1,040,814 |
37,172 |
0.43 |
-200 |
50 |
0 |
- |
- |
- |
2.800
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Feb |
73 |
47 |
65 |
-17 |
-24 |
421,669 |
13,602 |
0.16 |
-250 |
50 |
0 |
- |
- |
- |
2.750 |
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Mar |
59 |
38 |
68 |
-30 |
-54 |
726,586 |
24,220
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0.28 |
-275 |
25 |
5 |
111,586 |
3,720 |
0.04 |
2.725 |
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Apr |
168 |
109 |
66 |
44 |
-10 |
- |
- |
- |
-300 |
25 |
0 |
- |
- |
- |
2.700 |
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May |
240 |
156 |
111 |
45 |
35 |
- |
- |
- |
-300 |
0 |
0 |
- |
- |
- |
2.700 |
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Jun |
477 |
310 |
97 |
213 |
248 |
- |
- |
- |
-150 |
-150 |
0 |
- |
- |
- |
2.850
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Total |
2463.6 |
1601.3 |
1219.6 |
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7,094,222 |
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54 |
1,326,740 |
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