Transcript PowerPoint - Agricultural and Biological Engineering

AOM 4643
Principles and Issues in
Environmental Hydrology
Definitions

Hydrologic Science
• Focuses on the global hydrologic cycle and the processes involved
in the land phase of that cycle.
• Predicts spatial and temporal distribution of water in the terrestrial,
oceanic and atmospheric compartments of the global water system
• Predicts movement of water on and under earth’s land surfaces and
the physical/ chemical/ biological processes that accompany,
conduct or affect movement

Applied or Engineering hydrology
• uses this understanding to design and operate flood control, water
supply, irrigation and drainage, pollution abatement, wildlife
protection systems. i.e., for planning and management of water
resources.
The Hydrologic Cycle

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Describes the continuous circulation of water from land
and sea to the atmosphere and back again.
Concept is based on mass balance and is simply that water
changes state and is transported in a closed system which
extends approximately 1 km down into the earth’s crust
and about 15 km up into the atmosphere.
The cycle is only closed earth-wide, not on a watershed or
continental scale. Thus practicing hydrologists are
typically faced with an open system.
The energy required to keep the hydrologic cycle going is
provided by the sun.
The Hydrologic Cycle
Definitions
precipitation
movement of water from the atmosphere to the
earth as rain or snow
 throughfall
precipitation that reaches the soil surface (i.e. not
intercepted by vegetation)
 infiltration
precipitated water “absorbed” by soil surface

Definitions
evapotranspiration
combined consumptive evaporative process by
which water is released to the atmosphere through
vegetation, soil, and water bodies
 exfiltration
rising of soil moisture due to tension and capillary
forces
 percolation
water movement into deep aquifers (recharge)

Definitions
overland flow
precipitated water which moves over the land
surface ultimately infiltrating into the ground or
discharging into streams as surface runoff.
 interflow
water flowing horizontally at shallow depths
within soil structure
 sublimation
release of water from snowpack and icecaps
directly to the atmosphere as vapor

Global Hydrology
Ocean activity dominates the global hydrologic
cycle -- receiving 79% of the earth’s rainfall and
contributing 88% of the evapotranspiration.
 More rainfall falls on the oceans than land
(because of larger surface area).
 Land receives more water than it evaporates while
oceans evaporate more water than receive as
precipitation.
 Excess water on land returns to ocean as surface
and groundwater outflow to balance the system.

Distribution of water throughout
the earth
 oceans
and saline groundwater: 97.5%
 fresh water: 2.5%
•
•
•
•
•
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ice caps 1.7%
groundwater 0.7%
lakes, rivers, streams, etc. 0.02%
atmosphere 0.001%
soil moisture 0.001%
biological water 0.0001%
Global Stores and Fluxes of
Water
Reservoir Volume
Source
Sink
(km3)
(km3/yr)
(km3/yr)
Ocean
1338x106
Rain: 458x103
Runoff: 47x103
Atmosphere
0.013x106
Land Evap: 73x103
Rain: 577x103
Ocean Evap: 505x103
48x106
Land
(surface and
groundwater)
Rain: 119x103 km3
Evap: 505x103
Evap: 72x103
Runoff: 47x103
Residence Times
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The residence time of a water molecule in a component of
the system equals average volume of water stored a
component of the system divided by the volumetric flow
rate through the system, i.e Tr = Storage /Flow Rate
e.g. for atmosphere: Tr = 12,900 km3/ 577,000 km3/yr
= 0.22 yr= 8.2 days
This very short residence time indicates that atmospheric
moisture is replaced approximately 40 times per year.
 part of what makes weather prediction so difficult.
Atmospheric portion of the hydrologic cycle is very active
and is driving force for surface hydrology
Residence Times
Atmosphere
8.2 days
0.001%
P
Surface Water
17.3 days
0.02%
Ice cap
1.7%
ET
I
Groundwater
4790 yr
0.7%
Ocean
2650 yr
97.5%
History of Hydrology
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Greeks - philosophy
• 1000 BC -Science of hydrology began with concept of
the hydrologic cycle (Plato, Aristotle, Hippocrates)
• 1BC Vitruvius postulated that mountains received
precipitation which gave rise to streamflow
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Romans 500BC-500AD - practical application
• Romans had extensive practical knowledge of
hydrology and hydraulics (aqueducts). Theory sketchy,
based on Greek philosophy
History of Hydrology
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16th-18th Century - observations, empiricism,
qualitative description
• 1500 AD - Da Vinci (Itlay) and Palissy (France) asserted on the
basis of field measurements that water in rivers came from
precipitation
• 1600 AD - Perrault proved by measurement that rain could account
for streamflow
• 1700 AD - Halley quantified hydrologic cycled by estimating
amounts of water in ocean-atmosphere-river-ocean cycle of
Mediterranean
• 18th Century- advances in hydraulics and fluid mechanics by
Bernoulli, Chezy, Pitot, and Euler
History of Hydrology
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19th-20th Century- beginning of theoretical
quantitative approaches
• Dalton quantifies the nature of evaporation and established the
present concept of the global hydrologic cycle
• Experimental work on groundwater flow by Darcy and Dupuit
• Manning, Reynolds, Stokes, Poiseuille quantify surface hydraulics
• Green-Ampt, Richards quantify flow in the unsaturated zone
• Thornthwaite, Penman quantify evapotranspiration
• Horton quantifies infiltration
• Sherman quantifies surface runoff
Current Challenges in Hydrology
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Understanding the basic physics of individual
hydrologic processes which occur instantaneously
at a point does not always extrapolate easily to an
understanding of hydrologic processes over larger
space and time scales, due to
• non-linearity of many hydrologic processes
• high degree of spatiotemporal variability in natural
systems
• difficulty and expense in obtaining data to characterize
variability.
Goals for this Class

We will cover basic physical principals which govern the
major hydrologic processes: Precipitation,
Evapotranspiration, Infiltration, Groundwater Flow,
Overland Flow, Streamflow.
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We will focus on relatively simple quantitative
representations of these processes in order to develop a
sound intuitive sense of the way water moves through the
land based portion of the water cycle.
Goals for this Class
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These physical principals are powerful tools which
constitute the foundation of hydrologic science. However,
the degree of knowledge that can be obtained with the
tools we will discuss is limited primarily by the availability
and quality of field data and sometimes because they are
conceptually inappropriate.