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,
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,
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).
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, seepage lakes are often more susceptible to acidification from
rain because of their low buffering
has an important impact on the quality and quantity of water entering
a lake. As Figure 8 shows, the stormwater
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.
as water flows over roads, parking lots and rooftops, it accumulates
nutrients and contaminants in both dissolved and particulate form.
3. Phosphorus export coefficients
(from Reckhow and Simpson, 1980).
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
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.
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.