Digital Signal Processing Reference
In-Depth Information
& Mather 1955). For simplified calculations, evapotranspiration is divided into two components:
evaporation from open water surface of lake and fisheries and the evapotranspiration from lake
vegetation.
a)
From water body and fisheries
The lake system has a total approximate area of 124 km
2
and includes the lake water body, the
fisheries and the vegetation. These areas are calculated from the developed ARC-GIS digital maps of
the study catchment and are divided as follows:
Area of water body = 19 km
2
Area of fisheries = 66.4 km
2
Area
of vegetation = 38.7 km
2
The rate of evaporation depends on many factors, including temperature, the amount of solar radiation,
vapour pressure, and wind speed. Lake evaporation is often estimated from regional pan-evaporation
data by applying a pan coefficient, defined as the ratio of the theoretical free-water surface evaporation
to pan evaporation (Farnsworth and others, 1982).
Table (6-1)
shows the average actual daily and
monthly evaporation rates in the Western Delta region
Table (6-1): Average daily
evaporation rates in Western Delta region
Mean
Jan
Feb
Mar
Apr
May
Jun
July
Aug
Sep
Oct
Nov
Dec
Daily (mm)
4.1
4.6
5.6
5.6
5.8
5.8
5.6
5.6
5.9
5.8
4.4
3.8
Monthly (mm)
127
129
174
168
180
174
174
174
177
180
132
118
Monthly volumes
*10
6
(m
3
)
2.41
2.45
3.30
3.19
3.42
3.31
3.30
3.30
3.36
3.42
2.51
2.24
b)
From lake surrounding vegetation
Evapotranspiration (ET) refers to water lost to the atmosphere before leaving the lake via deep seepage
or surface outflow. Much of this is attributable to the water vegetation surrounding the water body. In
order to estimate this loss, two pieces of information are necessary: 1) the area of vegetation drawing
water more or less directly from the lake, and 2) the rate at which water is drawn by this vegetation.
ET from vegetation may be slightly higher than evaporation from water body because: 1) interception
of solar radiation exceeds that expected based on the area of vegetation which if not shaded receives
extra solar radiation when the sun is low in the sky; enhancing evaporation from leaf surfaces (Hansen
1984). In order to be conservative in the calculation of the water loss from the system,
a factor of 1.2
can be used to estimate the evapotranspiration from the lake vegetation
based on values
Based on the simplifying assumptions made in calculating some of the parameters, the following
potential sources of the difference in storage (
)
have been identified;
Groundwater loss and gain to the lake were assumed to be negligible. This should however, be
verified with field measurements or through a review of the geology in the area, and referring
to other calculations done for similar lakes in the area, since the ground water table in the
Delta region is common to them all.
Inflow and outflow measurements were discrete measured values at specific times during field
work; their integration this should be verified by further fixed temporal field measurements.
The estimated water budget shows that the difference between the input and output discharges is
2.22*10
6
m
3
/day. This value is assumed to be mainly subsurface exchange with groundwater.
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