Principles of terrestrial ecosystem ecology.pdf

4
Terrestrial Water and Energy Balance
The hydrologic cycle is the matrix in which all other biogeochemical cycles function.
This chapter describes the controls over the hydrologic cycle and ecosystem
energy balance, which drives the hydrologic cycle.
Introduction
Water and solar energy are essential for the
functioning of the Earth System. Since neither
is distributed evenly around the globe, the
mechanisms by which they are redistributed
(the global hydrologic cycle and energy budget)
are important (see Chapter 2). These processes
are so tightly intertwined that they cannot
be treated separately (Box 4.1). Solar energy
drives the hydrologic cycle through the vertical
transfer of water from Earth to the atmosphere
via evapotranspiration, the sum of evaporation
from surfaces and transpiration, which is the
water loss from plants. Conversely, evapotranspiration
accounts for 75% of the turbulent
energy transfer from Earth to the atmosphere
and is therefore a key process in Earth’s energy
budget (see Fig. 2.2). The hydrologic cycle
also controls Earth’s biogeochemical cycles
by influencing all biotic processes, dissolving
nutrients, and transferring them within and
among ecosystems. These nutrients provide the
resources that support growth of organisms.
The movement of materials that are dissolved
and suspended in water links ecosystems within
a landscape.
The importance of the hydrologic cycle raises
concerns about the extent to which it has been
modified by human activities. Humans cur-
rently use half of Earth’s readily available fresh
water, which is about half of the annual mean
runoff in regions accessible to people. Water
use is projected to increase to 70% by 2050
(Postel et al. 1996). This human use of fresh
water affects land and water management;
the movement of pollutants among ecosystems;
and, indirectly, ecosystem processes in unmanaged
ecosystems. Land use changes have
altered terrestrial water and energy budgets
sufficiently to change regional and global
climate (Chase et al. 2000). Finally, human
activities alter the capacity of the atmosphere
to hold water vapor. Water vapor is the major
greenhouse gas. It is transparent to solar radiation
but absorbs longwave radiation from Earth
(see Fig. 2.1) and thus provides an insulative
thermal blanket. Climate warming caused by
emissions of other greenhouse gases increases
the quantity of water vapor in the atmosphere
and therefore the efficiency with which the
atmosphere traps longwave radiation. This
water vapor feedback explains why climate
responds so sensitively to emissions of other
greenhouse gases (see Chapter 2). Warming
accelerates the hydrologic cycle, increasing
evaporation and rainfall at the global scale (see
Chapter 15). Warming also causes the sea level
to rise, due primarily to the thermal expansion
of the oceans and secondarily to melting of
71