The document discusses the hydrological cycle and water resources. It describes how the hydrological cycle involves the continuous movement of water between the atmosphere, biosphere, lithosphere and hydrosphere. It then explains key components of the hydrological cycle like evapotranspiration, infiltration, percolation, runoff and groundwater. Finally, it discusses factors that influence water retention in soil like soil texture, particle size, and organic matter content.
2. Overview
Intersection of Atmosphere, Hydrosphere,
Lithosphere and Biosphere
Hydrologic Cycle
Components
Water Budget
The supply of water available for use
Groundwater
aquifers
examples of aquifers as resources
Global distribution of water resources
Pollution
4. Hydrologic Cycle
Describes the way that water passes
between hydro-, atmo-, litho- , biospheres
In general
describes the balance between water leaving
the atmosphere (precipitation) and water re-
entering the atmosphere (evaporation, and
transpiration)
More specifically
details the processes that occur in between the
general processes described above (infiltration,
percolation, runoff, photosynthesis)
5.
6.
7. Infiltration
Water access to subsurface regions of soil moisture
storage through penetration of the soil surface
Occurs at a constant rate, measured the same way as
precipitation
e.g., inches per hour, millimeters per hour
If precipitation rate exceeds the infiltration rate, runoff
will occur.
Capillarity
Forces that cause water to move upward through the
soil
Plant root uptake (transpiration is essentially capillary action
through the plant, from root to leaf)
Evaporation of water at the soil’s surface
Common in arid regions
8. Runoff occurs when individual soil particles
can no longer hold onto infiltrated water
Dynamic tension between attractive forces
(holding soil and water particles together) and
gravity (pulling water away from soil).
Affected by the size of soil particles: smaller particles
have a greater summed surface area, and can hold onto
water more strongly
Sand, silt, clay
Hygroscopic water (or unavailable water)
held tightly by soil, not available to plants or capillary
evaporation (attractive forces much greater than gravity)
Capillary water (or available water)
soil holds water tightly enough to prevent runoff, but not
capillarity (attractive forces balanced with gravity)
Saturation
Gravitational water (runoff)
soil cannot hold onto water (gravity exceeds attractive
forces)
9. The ability of soil
to retain moisture
is a direct
consequence of
hydrogen bonding
between water and
soil particles
(colloids)
Texture of colloids
affects retention of
water as well
Gravel
Sand 2mm
Silt 50 μm
Clay 2 μm
Finer particles
retain more water
10. Gravitational water –
not held by soil,
available to plants or
Root
runoff and
percolation
Root exerts
additional Capillary
capillary force water – held
by the colloid,
but available
Colloid to plants
Hygroscopic
water – held
tightly by
the colloid
11. Finer particles create smaller pores, so there is less gravitational
water, hence better moisture retention
12. Finer particles have more surface area per volume, which allows
greater opportunity for water to attach, and causes greater amounts
of hygroscopic water as well as capillary water. As particle size
decreases, capillary water increases initially, then decreases as
hygroscopic water increases
4 2 1
1 4 4 4 4
8 8
4+4+4+4 2 4 4 4 4
4 = 16 4 4 4 4
8 8
4 4 4 4
Surface area = 16 Surface area = 32 Surface area = 64
13.
14. Field Capacity
This refers to the maximum amount of
capillary/available water a specific soil type can
hold.
Adding more water to the soil, after field capacity
is reached, results in the build up of gravitational
water, and runoff and percolation occur
Wilting Point
The amount of hygroscopic water that a specific
soil type can hold
When all available water is gone and only
hygroscopic water remains, there is no water
available to plants (hence the name)
Varies with soil particle size mixture
smaller particles hold more water
clays can hold water too strongly (less available)
Loams tend to have highest field capacity
18. Plant-Water Interaction
Plant roots exert an attractive force on
available soil water (Capillary Force)
Photosynthesis (absorbs solar energy)
6CO2 + 6H2O C6H12O6 + 6O2
Respiration (releases solar energy)
C6H12O6 + 6O2 6CO2 + 6H2O
Storing energy as sugar or starch also stores
water; plant growth and life processes
release water
Transpiration: respiratory water released as
water vapor
19. Soil-Water Budget
A mathematical way of expressing the
difference between input (precipitation) of
water into an area against its output
(evapotranspiration and runoff).
Conceptually very similar to hydrological cycle
PRECIP = ACTET + SURPL + ΔSTRG
where ACTET = POTET – DEFIC
Precipitation = Evapotranspiration +
gravitational water + available water
Varies over space and time
20. Variables
PRECIP: Precipitation
ACTET: Actual evapotranspiration. Occurs when
plants do not have sufficient water to reach their
maximum metabolic potential
POTET: Potential evapotranspiration. Represents
the maximum metabolic potential of plants, and
occurs when there is sufficient water
DEFIC: The amount of water short of achieving
POTET
SURPL: Surplus water, corresponds to
gravitational water
ΔSTRG: Change in storage. Corresponds to
capillary water (available water).
Negative if plants are utilizing the available water
Positive if available water is being recharged to field
capacity
21.
22.
23.
24. Scenarios
PRECIP > POTET
POTET = ACTET
DEFICIT = 0
SURPL + ΔSTRG > 0
Fall: ΔSTRG > 0, SURPL = 0; Available water increases
until Field Capacity is achieved (Soil Moisture Recharge)
Spring: SURPL > 0, ΔSTRG = 0 (after Field Capacity is
reached), gravitational water; runoff and percolation occur
PRECIP < POTET
DEFIC > 0
ACTET = POTET – DEFIC
SURPL = 0, no runoff or percolation
ΔSTRG < 0, available water is utilized until the
wilting point is met
