WATER BUDGET AND PREDICTED WATER
LEVEL CHANGES FOR SAN TIN / LOK MA CHAU STATION COMPLEX MARSHLAND |
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SOURCE: |
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EASTERN MARSH WATER BUDGET BASED ON MEAN MONTHLY RAINFALL DURING
DRIEST YEAR BETWEEN 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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