Transcript Introduction to hydrology

U6115: Water
Friday, June 13 2002
The early bird may get
the worm…
but the second mouse
gets the cheese.
U6115 Syllabus: Course Outline
• The water cycle part of the class is focused on basic
physical principles (evaporation, condensation,
precipitation, runoff, stream flow, percolation, and
groundwater flow), as well as environmentally relevant
applications based on case studies.
• Most specifically, students will be exposed to water
quantity and issues from global to regional scales and how
human and natural processes affect water availability in
surface and groundwater systems.
• Note: water quality issues will be mentioned but only
briefly since they will be covered more extensively in the
following course: ENVU6220 “Environmental Chemistry
and Toxicology)
U6115 Syllabus: Grading (activities)
Water (50% of grade)
 Labs: 60% (5 formal labs)
 Mostly minds-on experiments with
computers. Lab report due (make sure you
respect scientific format!)
 Exams: 40%
 Short question answers
 Essay questions (critical thinking)
Today: Water/Hydrology
• Intro to Hydrology
• Systems and Cycles
• Flux, Source/Sink, Residence time, Feedback
mechanisms…
• Thinking about science
Water for the World
The role of water is central to most natural processes
• transport
– weathering
• energy balance
– transport of heat, high heat capacity
• greenhouse gas
– ~ 80% of the atmospheric greenhouse effect is caused by
water vapor
• life
– for most terrestrial life forms, water determines where they
may live; man is exception
Hydrology
• literally "water science," encompasses the study of
the occurrence and movement of water on and
beneath the surface of the Earth
• finite though renewable resource
– finite in quantity, unlimited in supply, use rate is
limited by 'recycling times'
• hydrologic sciences have pure and applied aspects
– how the Earth works
– scientific basis for proper management of water
resources
Introduction to hydrology
use of water in 20th century has grown dramatically
Inventory of water on Earth
Water on land
3%
Lakes, soil moisture,
atmosphere, rivers
Deep groundwater
1%
(750-4000 m)
Shallow groundwater
(<750 m)
14%
11%
74%
97%
Ice caps and glaciers
Oceans
After Berner and Berner, 1987
Cycle Approach
 Some Definitions
 Transport and transformation processes within definite reservoirs: Carbon,
Rock, Water Cycles
 Reservoir: (box, compartment: M in mass units or moles) An amount of
material defined by certain physical, chemical, or biological characteristics
that can be considered homogeneous
– O2 in the atmosphere
– Carbon in living organic matter in the Ocean
– Water in the Ocean
 Flux: (F) The amount of material transferred from one reservoir to another
per unit time (M/s or M/s.L2)
– The rate of evaporation of water from the surface Ocean
– The rate of deposition of inorganic carbon (carbonates on marine
sediments
 Source: (I or Q) A flux of material into a reservoir
 Sink: (O or S) A flux of material out of a reservoir
More Definitions…
 Budget: A balance sheet of all sources and sinks of a reservoir.
If sources and sinks balance each other and do not change with
time, the reservoir is in steady-state (M does not change with
time). If steady-state prevails, then a flux that is unknown can
be estimated by its difference from the other fluxes.
for a control volume this means: dM/dt = I'-O'
 Turnover time: The ratio of the content (M) of the reservoir to
the sum of its sinks (O) or sources (I). The time it will take to
empty the reservoir if there aren’t any sources. It is also a
measure of the average time an atom/molecule spends in the
reservoir. Or:
0 = M/O (or M/I)
 Cycle: A system consisting of two or more connected
reservoir, where a large part of the material (energy) is
transferred through the system in a cyclic fashion
The Water (Hydrologic) Cycle
The Water Cycle (in detail)
 The volume (M) of water at the surface of the Earth is
enormous: 1.37 109 km3! (total reservoir) – The Oceans cover
71% of the Earth’s surface (29% for the continent masses
above sea level)
Reservoir
Biosphere
Volume (km3)
0.6 103
Rivers
Atmosphere
Lakes
Groundwater
Glacial and other land ice (?)
Oceanic water and sea ice
Total
% Total
0.00004
1.7 103
13 103
125 103
9500 103
29000 103
0.0001
0.001
0.01
0.68
2.05
1,370,000 103
97.25
1,408,640 103
Adapted from Berner & Berner (The Global Water Cycle; Prentice Hall, 1987)
100
Fluxes (F in 103 km3/yr)
 Of total yearly evaporation, 84% evaporates from the Oceans and 16%
from surface of continents.
 However, return to Earth via precipitation: 75% falls directly on the
Oceans and 25% on the continents.
 During the year, the atmosphere transports 9% of Oceans’ evaporation
to the continents!
 This water is returned via surface streams and as groundwater
Errors!
 Precipitation and
evaporation are difficult to
measure precisely over the
oceans. They are mostly
estimated from models and
satellite data.
 Groundwater reservoir
estimates bear a inherent
error in the fact that they
are indirectly determined.
 Soil moisture and
evapotranspiration rates
depend on indirect
measurements and average
soil quality and
global/regional respiration
rates
Residence Time
(years – months – weeks)
 High probability that a certain fraction of the atoms or molecules forming
the reservoir (M) will be of a certain age (mean age of the element when it
leaves the reservoir)
 The simplified residence time  turnover time
The time it would take to empty a reservoir if the sink (O or “outflow”)
remained constant while the sources were zero
0 = M/O (or M/I)
M = 0O
Residence time of water in the atmosphere
M = ?; O = ?; 0 = ?
M = 13 103 km3
S = 297(O) + 99(C) 103 km3/yr = 396 103 km3/yr
0 = 0.033 yr = 12 days!
Replacement ~30 times/year
Residence Time
(years – months – weeks)
 High probability that a certain fraction of the atoms or molecules forming
the reservoir (M) will be of a certain age (mean age of the element when it
leaves the reservoir)
 The simplified residence time  turnover time
The time it would take to empty a reservoir if the sink (O) remained constant
while the sources were zero

0
= M/O (or M/I)
M = 0O
Residence time of water in the ocean
M = ?; S = ?; 0 = ?
M = 1,370,000 103 km3
S = 334 103 km3/yr (evaporation)
0 = M/S = 4102 yrs!
Continental Mass Balance
• quantitative description  applying the principle of conservation of mass
• for continents as control volume this can be written as
dV/dt = p - rso - et = 0 (all averaged)
• on average this means: p = rso+ et
• the water budget for all land areas of the world is: p=800mm, rs = 310mm, and et
= 490mm
• the global runoff ration (rs/p) is ~39% there are lots of local and regional
variations.
System Approach…
Feedback: All closed and open systems respond to inputs
and have outputs. A feedback is a specific output that
serves as an input to the system.
Negative Feedback (stabilizing): The system’s response is
in the opposite direction as that of the output. CLOUDS!
System Approach…
Positive Feedback (destabilizing): The system’s
response is in the same direction as that of the output.
Number of bacteria
"Bacteria in a bottle"
1.4E+18
1.2E+18
1.0E+18
8.0E+17
6.0E+17
4.0E+17
Bottle half full
2.0E+17
0.0E+00
0
10
20
30
40
Time (minutes)
50
59 min
60
System Approach…
Positive Feedback (destabilizing):
CLOUDS!
Source, use and
disposition of
water in the US,
1990
Modified from http://
water.usgs.gov/watuse/
wuto.html
Surface waters
BRF
Watershed, catchment, drainage basin
“A catchment (watershed, drainage basin) 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”
Measurement techniques
=> precipitation
=> evapotranspiration
Measurement techniques
 flow depth (stage)
 discharge
BRF
Colorado River
hydrograph
Questions:
• When does discharge peak and
why?
• The hydrographs were taken at
different locations of the river,
what is the difference in the
hydrographs and why is there
one?
Colorado River
hydrograph
• Hydrographs are
variable between years
• Discharge often peaks
in late winter or
spring, snowmelt
• Reservoirs smooth out
extremes
Canada del Oro hydrograph
=> extended periods with no discharge at all!
http://water.usgs.gov
Groundwater
• Groundwater flow is
controlled by
– differences in water table
(hydraulic head)
– hydraulic conductivity
(relation between specific
discharge – vol/t – and
hydraulic gradient)
– Hydraulic conductivity
depends on both the nature
of the fluid (viscosity) and
the porosity of the material
Hornberger et al., 1998
Measurement techniques
=> Hydraulic head, conductivity
Physical flow model