The Hydrologic Cycle - Climate Dynamics - page 148

THE HYDROLOGIC CYCLE
The global hydrologic cycle links the climate system components ( Figs. 1.1 and
1.2) , with water constantly moving through the world's oceans, the cryosphere,
the lithosphere (in lakes, rivers, and soils), the atmosphere, and the biosphere.
Figure 9.1 provides a schematic overview of these interactions and some of the
processes that must be considered to understand the hydrologic cycle.
The hydrologic cycle can be evaluated by constructing budgets based on the
concept of conservation of (water) mass, similar to the heat balance equations
written in chapter 5 . Budgets for an atmospheric column and a land surface
volume are derived in the following sections. In general, observations of global
and regional hydrology are not sufficiently accurate to allow us to quantify
individual terms in the water budgets beyond fairly rough estimates, such as
those shown in Figure 2.24 .
9.1 ATMOSPHERIC WATER BALANCE
Although the mass of water in the atmosphere is tiny compared with the other
reservoirs of water in the climate system (see Table 2.1) , its radiative properties
( chapter 4 ) make atmospheric water vapor a primary determining factor of
climate on all space and time scales.
Consider a rectangular column of air with unit cross-sectional area extend-
ing from the surface to the top of the atmosphere ( Fig. 9.2) . Water vapor in-
creases in the volume when water evaporates ( E ), and decreases when water
vapor condenses and precipitation ( P ) removes water from the atmospheric
column. The water vapor content can also change when the atmospheric cir-
culation converges water vapor into the column ( Fig. 9.2a) or diverges water
vapor out of the column ( Fig. 9.2b) . This simple water vapor budget neglects
ice formation and sublimation, and assumes that all liquid water falls to the
surface. Note that because the physical surface is excluded from the atmo-
spheric column, the redistribution of water on the surface by runoff, subsurface
flow, or ocean currents is not included in the atmospheric budget.
To develop an equation that governs conservation of water vapor in an
atmospheric column, we write a mathematical expression for the time rate of
change of the amount of water vapor in the volume and set it equal to the sum
of the sources and sinks of water for the volume. The total amount of water va-
por in the column per unit cross-sectional area, W (kg- water/m 2 ), is calculated
by integrating the elemental water vapor mass, dW , over the volume. If r V is
the density of water vapor in the volume, then