users, who accrue only limited economic benefit by being centrally located, in the
form of lower transportation costs. In between will be zones dominated by lower-
volume retailing, wholesale suppliers, manufacturers, and other users. The desire
of each set of users to pay higher land costs as a function of distance from
the CBD can be graphically illustrated via a bid-rent curve for each group. The
curve for those willing to pay the highest bid rents in the CBD is quite steep, while
the curve for those users who will only pay lower bid rent, located in the outer zone,
is almost flat. In bid-rent theory, any residential users living closer to the CBD must
cluster in high-density units (i.e., high-rise apartment buildings) to offset higher bid
rents. The resulting pattern of urban land use is a set of circular zones, emanating
from the CBD. This pattern ofurbanizationis called the Concentric Zone Model,
and is one of several generalized models of urban morphology.Biogeochemical Cycles
Along with the circulation of theatmosphereandoceansthere are energy and mass
circulating through and interacting with the various life subsystems ofEarth. The
corporate name for these translocations is biogeochemical cycles. The term is apt
in that it connotes the interplay of life and its chemical environment on, over and
under Earth’s landscapes. In toto, the magnitudes of these flows are unimaginably
vast and complicated but their components have been studied to the extent that the
basics are well known. The cycles provide needed supplies of mass energy because
these commodities are finite on our planet. In short, the biogeochemical cycles
bathe the planet in ways that provide for sustained life. Although it can be shown
that humans have had major, unintentional impacts of some of these cycles, it is
evident that there is much resilience in the way they function.
The energy cycle is usually the first to be noted because it is the fuel by which
all the other cycles act. A huge amount of solar energy is incident on the Earth sys-
tem. Instantaneously, the amount is about 174 petawatts (10^15 watts), which is bil-
lions of times the rate of electric energy generated by human devices. Assolar
energypasses through the atmosphere there are all manner of pathways as
described elsewhere.
The key energy for life is supplied by photosynthesis. Although incredibly com-
plex with many nuances yet to be understood, photosynthesis is responsible for
energizing the large bulk of biomass on the planet and, ultimately, is the source
of energy on which the human organism depends. Photosynthesis occurs in plants
and uses carbon dioxide and water commonly available in the environment. Not
all solar energy received is used. Perhaps 0.6 percent of solar energy incident28 Biogeochemical Cycles