also called the drainage basin, is all of the land and water areas that
drain toward a particular river or lake. Thus, a watershed is defined
in terms of the selected lake (or river). There can be subwatersheds
within watersheds. For example, a tributary
to a lake has its own watershed, which is part of the larger total drainage
area to the lake.
A lake is
a reflection of its watershed. More specifically, a lake reflects the
watershed's size, topography, geology,
landuse, soil fertility and erodibility,
and vegetation. The impact of the watershed is evident in the relation
of nutrient loading to the watershed:lake
surface area ratio (Figure 7). See also the section on conductivity.
Typically, water quality decreases with an increasing ratio of watershed area to
lake area. This is obvious when one considers that as the watershed to lake area increases there are additional sources (and volumes) of runoff to the lake. In larger watersheds, there is also a greater opportunity
for water from precipitation to contact the soil and leach
minerals before discharging into the lake. Lakes with very small watersheds that are maintained primarily by groundwater flow are known as seepage
lakes. In contrast, lakes fed primarily by inflowing streams or rivers are known as drainage lakes. In
keeping with the watershed:lake area relationship, seepage lakes tend to have good water quality compared with drainage lakes. However, lakes are often more susceptible to acidification
rain because of their low buffering
Landuse has an important impact on the quality and quantity of water entering a lake. As Figure 8 shows,
discharge to a lake differs greatly among landuses. In urban areas,
the high proportion of impervious surfaces
prevents absorbance of rainwater into the soil and increases the rate
of surface water flow to the lake. The high flushing
rates from urban areas can increase erosion of stream banks and
provide sufficient force to carry large particles (i.e., soil) to the
lake. Thus, water quantity affects water quality.
Additionally, as water flows over roads, parking lots and rooftops, it accumulates nutrients and contaminants in both dissolved and particulate form.
Table 3. Phosphorus export coefficients
(from Reckhow and Simpson, 1980).
Table 3 gives representative values
of export rates
of phosphorus from various landuses and other sources. Phosphorus is
particularly important because its availability often controls the amount
of algae and the overall productivity
of a lake. These values are in units of kg/km2/yr (mass of
phosphorus per unit area per year). Not included here, but also important,
is the influence of soil type and slope. Finer particles and steeper
slopes mean higher export rates.
To clarify the relative landuse impacts, we can compare annual loads from 10 hectare (24 acre) plots of the selected landuses using the high export coefficients in Table 3.
|4.5 kg phosphorus
|30.0 kg phosphorus
|50.0 kg phosphorus
see that, all other things being equal, converting a forest into a city
can increase the phosphorus export to a lake more than ten times. Another
way to look at these numbers is that almost seven years of phosphorus loading from a forested area can be deposited within one year by
mixed agriculture areas and almost eleven years of phosphorus loading
from a forested area can be deposited within a year from urbanized areas.
A greater loading rate puts a greater
strain on the system to assimilate the nutrients.