Hydrology EESC BC 3025
The hydrological cycle
The blue planet
- Why does the Earth appear blue from the
distance
(Fig)?
- scattering of light by the atmosphere
- preferential adsorption of red light by
the
ocean
The hydrological cycle
- fundamental concept in hydrology
- largest circulation of matter within the
Earth-atmosphere
system
- solar energy drives the hydrological cycle
- 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 ~0.3 to 1 km3/y; this source still
contains 15
times
as much water than presently free in the hydrosphere
- extraterrestrial additions: total water mass added to the
terrestrial oceans in 4.5 Ga could have been as much as 4 and 22%
according to
the relative contribution of comets and water-rich carbonaceous
chondrites or asteroids (this is controversial).
- pathway of a water molecule, mechanisms
of
water
movement in the hydrological cycle (Fig)
or (Fig. 1.3)
The water budget
global
- ca 97% of the water is in the oceans (Fig)(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'
(I', O' [MT-1], mass inflow and outflow rate)
- (note: sometimes the term 'flux' is used instead of 'flow',
however, a flux
is mass (or volume) per time and area, to be exact, e.g. [MT-1L-2]
- 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 = p + rsi + rgi - rso
- rgo -et = 0 (all averaged, bar)
- on average this means: p = rso+ et
(dimensions
[L3T-1] or [LT-1])
- the water budget for all land areas (A) of the world is:
p/A=800mm/y, rs/A=
310mm/y, and et/A = 490mm/y
- 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
- example: residence time of water in the
ocean
- reservoir=(50,000+460,000+890,000)*103km3
flow=434*103km3 /y
-> 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).
Terrestrial ice/snow
- 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
Terrestrial waters
- groundwater
- soil moisture
- river channels
- 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
Catchment scale
- 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)