Transcript AOM 4643 - 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
• 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.
Definitions
• precipitation - movement of water from the atmosphere to
the earth as rain or snow
• evapotranspiration - combined consumptive evaporative
process by which water is released to the atmosphere
through vegetation and soil
• throughfall - water not intercepted by vegetation
• interflow - water at shallow depths within soil structure
• overland flow - precipitated water which moves over the
land surface ultimately infiltrating into the ground or
discharging into streams as surface runoff.
Definitions
• infiltration - precipitated water “absorbed” by soil
surface
• percolation - water movement into deep aquifers
(recharge)
• exfiltration - rising of soil moisture due to tension
and capillary forces
• 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%
–
–
–
–
–
–
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
• 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
• Tr = Storage = 12,900 km3
= 8.2 days
Flow Rate
577,000 km3/yr
• Thus 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
0.001%
8.2 days
97%
2650 yr
GW
0.6%
4790 yr
History of Hydrology
Challenges in Hydrology
• 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.
• 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.
• 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.