Soils and Water, Spring 2008
The Hydrological Cycle
Water vapor helps warm up the earth
Evaporation+Transpiration
Chemical Spill
Runoff
To Oceans
Hydrological Cycle
Water potential, atmosphere
Atmospheric pool
Transpiration, T
Evaporation, E
Interception
Precipitation, P
E
Infiltration
Surface storage
Water potential, soil
SOIL
Groundwater level
Soil storage, SS
Runoff, R
Capillary Rise, CR Percolation, Pc Groundwater storage
BEDROCK
Water Balance:
P + CR = R + SS + ET + Pc
Definition of Terms
• Runoff: Part of the precipitation, snow melt, or irrigation water that
appears in surface streams, rivers, drains or sewers.
• Evapotranspiration (evaporation + transpiration): the process of
liquid water becoming water vapor, including vaporization from
water surfaces, land surfaces, snow fields and leaf surfaces.
• Infiltration: flow of water from the land surface into the subsurface.
• Percolation: The movement of water through the openings in rock or
soil that contributes to ground water replenishment.
• Capillary action: the means by which liquid moves through the
porous spaces in soils due to the forces of adhesion, cohesion, and
surface tension.
• Aquifer: a geologic formation that stores and/or transmits water,
such as to wells and springs. Use of the term is usually restricted to
those water-bearing formations capable of yielding water in sufficient
quantity to constitute a usable supply for people's uses.
Lecture 4, The Hydrological Cycle
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Soils and Water, Spring 2008
Distribution of World Water
3,800 ml
114 ml (3%)
~ 1 ml
~ 0.5 ml or 1 drop
Very dynamic property!
Monthly Soil Water Index (SWI) for
the years 1992-2000
Source: www.ipf.tuwien.ac.at/radar/index.php?go=ascat
Soils and the Hydrosphere
Volume
(%)
Equivalent
depth (m)
Residence
time
94
2500
~4000 y
Lakes, and
swamps
<0.01
0.26
~10 y
Rivers
<0.01
0.003
~2 weeks
Soil moisture
<0.01
0.13
Up to 1 y
Groundwater
4
120
Up to 10,000 y
Pool
Oceans
Ice
2
60
Up to 10,000 y
Atmosphere
<0.01
0.025
~ 10 days
Biosphere
<0.01
0.001
~ 1 week
Estimates of the water balance of the world (modified from Freeze and Cherry, 1979)
Lecture 4, The Hydrological Cycle
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Soils and Water, Spring 2008
Soil Water (Moisture)
• Small proportion of water in soil, but relatively
large residence time:
– Enough time for chemical and biological processes to
take place.
– Seasonal influence of soil moisture on climate
(memory of the land).
• Significant effort to measure and model soil
moisture at the global scale:
– Important for agricultural production
– There appears to be areas in the world where soil
water content is closely linked to precipitation.
humid
semiarid
arid
• The soil is modeled as bucket
that stores water until is filled.
The excess typically moves
downward (percolation).
• During the growing season the
‘bucket’ is emptied by water
uptake (evapotranspiration).
• Climate and soil properties
(water retention) determines
the amount of water in the
‘bucket’ over the course of the
year.
Figure 6.16 Generalized curves for precipitation and evapotranspiration for three temperate zone regions: (a)
a humid region, (b) a semiarid region, and (c) an irrigated arid region. Note the absence of percolation through
the soil in the semiarid region. In each case water is stored in the soil. This water is released later when
evapotranspiration demands exceed the precipitation. In the semiarid region evapotranspiration would likely be
much higher if ample soil moisture were available. In the irrigated arid region soil, the very high
evapotranspiration needs are supplied by irrigation. Soil moisture stored in the spring is utilized by later
summer growth and lost through evaporation during the late fall and winter.
Lecture 4, The Hydrological Cycle
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Soils and Water, Spring 2008
Figure 6.13 Partitioning of liquid water losses (discharge) and vapor losses (evaporation and transpiration) in
regions varying from low (arid) to high (humid) levels of annual precipitation. The example shown assumes that
temperatures are constant across the regions of differing rainfall. Potential evapotranspiration (PET) is
somewhat higher in the low-rainfall zones because the lower relative humidity there increases the vapor
pressure gradient at a given temperature. Evaporation (E) represents a much greater proportion of total vapor
losses (ET) in the drier regions due to sparse plant cover caused by interplant competition for water. The
greater the gap between PET and ET, the greater the deficit and the more serious the water stress to which
plants are subject. (Diagram courtesy of R. Weil)
Percolation: Function of Climate
and Soil
Figure 6.17 Percentage of the water entering the soil that is lost by downward percolation and by
evapotranspiration. Representative figures are shown for different climatic regions.
Soil Water Potential and Plants
Water potential, atmosphere
Water potential, soil
Lecture 4, The Hydrological Cycle
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Soils and Water, Spring 2008
?
3.5 m
Plants do not have a
heart
Saturated Conductivity and Trees?
Lecture 4, The Hydrological Cycle
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Soils and Water, Spring 2008
Rooting Depth
Source: Canadell, J., R.B. Jackson, J.R. Ehleringer, H.A. Mooney, O.E. Sala, and E.-D. Schulze. 1996.
Maximum rooting depth of vegetation types at the global scale. Oecologia 108: 583-595.
Effect of Compaction on Root
Growth
Competing Water Uses
Lecture 4, The Hydrological Cycle
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Soils and Water, Spring 2008
Soil Climate Limitations to
Agriculture
Climate driven
Soil driven
Soil driven
Source: Linking Land Quality, Agricultural Productivity, and Food Security. 2003. K. Wiebe.
Resource Economics Division, Economic Research Service, U.S. Department of Agriculture.
Agricultural Economic Report No. 823.
Food Production and Water
Requirement
A diet of 2,800 calories/person/day requires about 1,000 m3 of water.
Acreages of Irrigated Land
Lecture 4, The Hydrological Cycle
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Soils and Water, Spring 2008
Surface Water Withdrawals
Withdrawal: water removed from a ground- or surface-water source for use.
Groundwater Withdrawals
Fresh Water in Aquifers: a
Renewable Resource?
• About 1,500,000 km3 of
fresh water held in aquifers.
• Much of ground water is
“fossil water” (no-active
exchange with the earth
surface).
Source: Water in a Changing World. 2001. Issues in Ecology # 9.
Lecture 4, The Hydrological Cycle
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Soils and Water, Spring 2008
Main Aquifer Types in the USA
Sandstone
Carbonate Rocks
Sandstone and Carbonate Rocks
Igneous and Metamorphic Rocks
Source: http://capp.water.usgs.gov/aquiferBasics/
Unconsolidated or
Semiconsolidated Aquifers
Sand and gravel aquifers of alluvial and
glacial origin are north of the line of
continental glaciation.
Semiconsolidated sand and
gravel aquifers.
Unconsolidated sand and gravel aquifers at or near the land surface.
Source: http://capp.water.usgs.gov/aquiferBasics/
Subsidence by Groundwater
Pumping
Lecture 4, The Hydrological Cycle
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Soils and Water, Spring 2008
Regional Effects of Groundwater
Pumping
• Depletion of groundwater can cause :
– General subsidence of the surface and
compaction, which reduces the storage
capacity of the aquifer (ex, San Joaquin
Valley).
• Excess irrigation in may induce the
formation of sinkholes and other
alterations of the landscape.
Subsidence
Approximate location of maximum subsidence in the
United States. Signs on pole show approximate altitude of
land surface in 1925, 1955, and 1977. The site is in the
San Joaquin Valley southwest of Mendota, California.
Crack in the ground near an irrigation canal
Subsidence
Sinkhole produced by irrigation
Lecture 4, The Hydrological Cycle
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Soils and Water, Spring 2008
Figure 6.19 Relationship of the water table and groundwater to water movement into and out of the soil.
Precipitation and irrigation water move down the soil profile under the influence of gravity (gravitational water),
ultimately reaching the water table and underlying shallow groundwater. The unsaturated zone above the water
table is known as the vadose zone (upper right). As water is removed from the soil by evapotranspiration,
groundwater moves up from the water table by capillarity in what is termed the capillary fringe. Groundwater
also moves horizontally down the slope toward a nearby stream, carrying with it chemicals that have leached
through the soil, including essential plant nutrients (N, P, Ca, etc.) as well as pesticides and other pollutants
from domestic, industrial, and agricultural wastes. Groundwaters are major sources of water for wells, the
shallow ones removing water from the groundwater near the surface, while deep wells exploit deep and usually
large groundwater reserves. Two plumes of pollution are shown, one originating from landfill leachate, the other
from a chemical spill. The former appears to be contaminating the shallow well.
Water Pollution
• Agriculture, industry and urban areas are
the main sources of water pollution.
• Common water contaminants are:
– Sediments,
– Phosphorus, nitrates and sulfates,
– Pesticides and herbicides.
Postcard developed by the Bureau of Land and Water Quality of the State of
Maine as part of a campaign to prevent soil erosion.
Lecture 4, The Hydrological Cycle
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