Digital Signal Processing Reference
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Since the outflow from the lake to the sea exceeds the inflow rate, this implies an inflow from
ground water.
Table (6-2): Water balance estimates for Lake Edko
Item
Discharges
Measured & Calculated
values
(Million m
3
/day) 2005
Measured & Calculated
values
(Million m
3
/day) 2007
Inputs
1
Inflow from Edko Drain
4.01
5.36
2
Inflow from Barseek Drain
0.7
0.7
Total
4.71
6.06
Outputs
1
Outflow from sea exit
6.25
6.67
2
Evaporation losses from water body
0.11
0.084
3
Evaporation losses from fisheries
0.39
0.29
4
Evapotranspiration from vegetation
0.27
0.2
Total
7.02
7.2
2.31
1.18
Groundwater is found at shallow depths. Before the implementation of tile drainage systems, it
fluctuated between 0 and 1 m below the ground surface (RIGW, 1986). Since EPADP installed tile
drainage systems throughout most of the area the groundwater table has been lowered and fluctuates
between 1 and 1.6 m below the surface. In most areas, freshwater layer overlays saline water, towards
the north, the freshwater lens becomes thinner and more saline. Waterlogging occurs locally, notably
along the continuous flowing canal sections (seepage and leakage).
This groundwater inflow/outflow should be calculated based on data for the groundwater
characteristics in the Nile Delta complemented by modelling. A detailed water budget study was
conducted for the coastal Lake Burullus in the middle Delta region. This showed groundwater inflow
to the lake (Shinnawy, 2000). For the estimation of the water balance for Lake Edko, the results could
be accepted within the described data limitations. We conclude that the calculation of groundwater
recharge to the lake system and the evapotranspiration are two main issues for water balance
calculation in shallow lakes.
6.3.
CALCULATION OF LAKE RESIDENCE TIME
The average length of time water remains within the boundaries of an aquatic system (hydraulic
residence time) has been proposed in the literature as an important parameter with which to explain a
range of water quality phenomena such as the variability in lake eutrophication processes, thermal
stratification, isotopic composition, alkalinity, dissolved organic carbon concentration, elemental
ratios of heavy metals and nutrients, mineralization rates of organic matter, and primary production
(see Monsen et al. 2002, for a list of references). The basic concepts on transport time scales and their
application to coastal environments are laid out in the works of (Zimmerman 1976) and (Dronkers and
Zimmerman 1982). There, the most commonly used terms to measure the retention of water or scalar
quantities transported in the water are carefully defined, and suitable experiments are presented to
calculate the defined time scales, with examples for coastal environments. The experiments go further,
suggesting that the hydraulic residence time is a key parameter controlling the structure of aquatic
ecosystems and the extent to which these systems are self-organized or dominated by outside
influences. The lake's size, water source, and watershed size are the primary parameters that determine
the retention time. Rapid water exchange rates allow nutrients to be flushed out of the lake quickly.
Such lakes respond best to management practices that decrease nutrient input. Impoundments, small
drainage lakes, and lakes with large volumes of groundwater inflow and stream outlets (groundwater
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