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 Lag time
200
Hydrograph of Single Storm Event
Rising limb
Falling limb
100
Storm run off
Base Flow
Base Flow
0
0
10
20
30
40
50
Hours
Figure 1.2.9 Theoretical hydrograph showing the major features of stream flow through
a storm event
stream. A large quantity of water (and water-born materials) may enter a lake via streamflow without
detection if the brief period of the storm hydrograph passes unobserved. Moreover, the concentration
of materials is rarely uniform throughout these flow changes. Often the maximum concentration will
occur early in the hydrograph, perhaps before maximum flow is attained. For this reason, measuring
loading to lakes can be very intensive work - especially if there are numerous significant inflows.
1.2.7.2 Important Chemical Elements
Assuming that a lake already contains materials in the water, there are important processes which
influence their distribution and form. The materials of greatest interest in reservoirs are Oxygen (O),
Carbon (C), Nitrogen (N), Phosphorus (P) (these are four macronutrients) and the metals, Iron (Fe )
and Manganese (M n) (sometimes Mg, Na, K, Ca and others especially if toxic materials are
suspected). Of all of these, three (Oxygen, Carbon and Nitrogen) exist in both gaseous and chemically-
combined solid forms. And to better understand these three elements, laws governing gases must be
understood. Technically, hydrogen (H) is also a major element because it is one of the two elements in
water and also exists in biological tissues along with carbon and oxygen in the form of biochemicals
such as carbohydrates. However, hydrogen does not normally exist as a free gas and is almost always
combined with other elements.
The ideal gas laws apply approximately to the major components of air, Nitrogen, Oxygen and
Argon. Under most circumstances, argon is ignored because it composes less than 1% of the gaseous
content and because it is an inert gas. Because it does not enter into complex reactions, it is a
conservative element .
1.2-21
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