Environmental Engineering Reference
In-Depth Information
separated from each other by ridges, hills, or mountains called a drainage divide. In flatland
areas, drainage divides are not so obvious. During precipitation, water infiltrates the ground at
different depths. Some water remains in shallow soil layers, and the rest goes much deeper and
end up recharging aquifers. Water in shallow soil slowly moves downhill through the soil and
eventually seeps out to form streams than ultimately become creeks, rivers, and so on. When
the soil becomes saturated, or when the soil has poor permeability, water runs on the surface
of the land becoming what is called precipitation runoff. If the runoff occurs on bare soil, like
in areas that suffered deforestation, sediments are removed by the water runoff and transported
to creeks and rivers, which sometimes diminishes their water quality and cause damage to
water reservoirs. Sediments are not always bad though. Sometimes, they are part of the natural
cycles that enrich river banks and improve farm land.
Interactions of surface water with groundwater
Surface water and groundwater are treated as separate groups, but that does not mean
they are independent. Both surface water and groundwater are connected through the
hydrologic cycle.
The driving force of the hydrologic cycle is evaporation, which occurs from water surfaces
(i.e., ocean, rivers, and lakes), the soil, snowfields, and vegetation. Large vegetation, such as
trees, can develop deep roots capable of reaching the saturation zone and pulling water that is
then transpired through the leaves.
Streams interact with groundwater in three different ways:
1. Gaining stream: streams gain water from groundwater through the streambed (Fig. 9.2A).
2. Losing stream: streams lose water to groundwater by outflow through the streambed
(Fig. 9.2B).
3.
They do both, gain water in some reaches and lose in others (Winter et al., 1998).
Streams that are connected to shallow aquifers gain or lose water depending on water
pressure at the streambed and the aquifer in the vicinity of the stream. When the pressure
exerted by the stream is higher than the pressure in the aquifer then the stream loses water and
recharges the aquifer. In a gaining stream situation, pressure exercised by the aquifer is higher
than the water pressure at the surface of the stream.
Also, a stream can interact with an aquifer without been physically connected. Water can
seep out from an aquifer that is located at a higher level than the surface of the stream and
discharge water in the form of a spring. In other situations, the aquifer is located underneath
the stream, but they are isolated from each other through the unsaturated zone. Still the stream
can lose water to the unsaturated soil. In a third scenario, the stream can disappear into a sink-
hole that is connected to the aquifer.
Natural lakes interact with groundwater as they do with streams. They can receive water
from the ground throughout their entire bed, give water to the ground, or have both mecha-
nisms present at the same time by receiving water in certain parts and seeping out water in
other areas. This is the most frequent mechanism found and is responsible for making lakes
keep an almost constant level year round.
Wetlands interact with groundwater as they do with streams and lakes, but the main differ-
ence is that wetlands are not necessarily located at low points in the terrain. They are also be
on slopes and drainage divides, and many wetlands are in the banks of rivers and coastal areas.
In cases in which wetlands are located in uplands (e.g., bogs) water that feeds the wetland
comes almost exclusively from precipitation (Winter et al., 1998).
