Climate and water

The hydrological cycle

Take away ideas and understandings

The water cycle is the fundamental concept in hydrology, it can be characterized by reservoirs, fluxes, and residence times.
97% of water on Earth is in the oceans.
Many problems in hydrology can be expressed as a water balance equation.

fundamental concept in hydrology
largest circulation of matter within the Earth-atmosphere system
solar energy drives the hydrological cycle
potential energy; gravitational flows are more than an order of magnitude lower than evaporational exchanges
within the various compartments of the hydrological cycle, water can be stored in any one of three separate phases or states: gas (vapor), liquid, or solid.
gradual degassing of the Earth's mantle formed hydrosphere, most likely; rate ~1 km³/y; this source still contains 15 times as much water than presently free in the hydrosphere
pathway of a water molecule, mechanisms of water movement in the hydrological cycle (Fig) or (Fig. 1.3)

94-97.5% of the water is in the oceans (Fig)
quantitative description of the hydrological cycle by applying the principle of conservation of mass (also: water balance or water budget)
for a control volume this means: dM/dt = I'-O'

^-1

(note: sometimes the term 'flux' is used insead of 'flow', however, a flux is a reat per area, to be exact, e.g. [MMT^-1L³]
in case the density is constant this can be replaced by dV/dt = I-O (density r: [M L^-3], the mass per unit volume of a substance, defined at a point)
for continents as control volume this can be written as

dV/dt

r_si + r_gi - r_so - r_so

on average this means: p = r_so+et (dimensions [L³T^-1] or [LT^-1])
the water budget for all land areas of the world is: p=800mm, r_s = 310mm, and et = 490mm
the global runoff ration (rs/p) is 39% there are lots of local and regional variations (Table 1.2).
to quantify the global hydrological cycle we can examine the relative sizes of the various storage compartments and the magnitudes of the various flows to and from these compartments
residence time: Tr = V/I [T], a measure of the average time a molecule of water spends in a reservoir. The residence time defined for steady-state systems is equal to the reservoir volume divided by the inflow or outflow rate.
example: residence time of water in toilet @ 1.6 gal/flush
reservoir, flows (Fig) (Fig)
those numbers have fairly large uncertainties (Fig)
balances are continuously shifting, particularly between the oceans and terrestrial ice

at maximum glaciation, sea level was 125m lower, 3.5 times as much water as today locked into ice
today's sea level rise: approximately 1mm/year

flux=434*10³km³-> residence time=3230y

excercise: calculate residence times of components of the hydrologic cycle (Fig)
residence times of water in the compartments of the hydrologic cycle (Table 1.1).

ice sheets and glaciers contain 85% of the freshwater resources
polar glaciers, frozen to their beds; alpine glaciers tend to maintain a lubricating layer of meltwater -> shorter residence time
icemelt from alpine glaciers contributes 1/200th of world river runoff
zone of net accumulation and ablation divided by equilibrium snowline (Fig)
75% of moisture in atmosphere forms ice and snow, but only 5% of world precipitation is ice and snow

hold the least amount of water
not the quantity in store is important but the quantity that passes through the system
lakes: 50-60 times more storage, but annual river flow is equivalent to 4-5 lake volumes
annual river discharge is ~4 times the renewable annual yield from active groundwater

a catchment is an area of land in which water flowing across the land surface drains into a particular stream or river and ultimately flows through a single point or outlet on that stream or river (Fig 1.5)
the boundary of a catchment area is the divide (example: continental divide, Fig 1.1)
determination of a water balance for the James River Basin above Scottsville, Virginia (refer to in homework, section 1.4.2)