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Freshwater systems (KWH 2001) (Water scarcity and water quality Big…
Freshwater systems
(KWH 2001)
Drainage basin hydrology and geomorphology
Big Question: How does
physical processes
influence drainage system basin systems and
landforms?
A drainage system is an
open system with:
Inputs
Precipitation of varying
types and intensity
Snowfall
Outputs
Evaporation
Process in which a liquid
changes into a gas (
affected
by reflectivity (albedo) of water,
temperature and surrounding
humidity
)
Transpiration
Process by which plants lose
lose water, primarily through
the leaves.
Evapotranspiration
is the main driver
of outputs
Flows
Infiltration
The movement of water from
the surface to below the ground
(Not to be confused with percolation)
Throughflow
Flow of water below the
ground, through saturated
soil
Overland flow
Movement of water over the
land, downslope toward
a surface water body. (Occurs
when
precipitation exceeds
infiltration
or
soil is saturated
)
Base flow
Part of the river discharge that
is
provided by groundwater
seeping into the bed of the river
(relatively constant)
Stores
Vegetation
Made up of three processes, which include:
Interception
--> Water that is retained by
plant examples
Throughfall
--> Water that fall through gaps
in vegetation
-
Stem flow
--> Water that trickles down stems
and twigs before flowing down the trunk
Aquifers
Aquifers
Commonly known as groundwater storage
Made up of a
permanently saturated
phreatic zone
and a
water table
that
has a water level that
varies seasonally
Recharge:
Refilling of water
in the pores
that have dried up or extracted by humans
Refill by:
1)
Infiltration of precipitation
from surface
2)
Seepage
through the banks and bed of water bodies
3)
Groundwater leakage
from surrounding rocks
4)
Artificial recharge
from reservoirs and irrigation
Cryosphere
Water stored in snow and ice
(up to
66% of freshwater storage
)
Soil
Soil moisture as subsurface water in soil.
Field moisture --> Amount of water for near
saturation
Wilting point --> Amount of water in soil
until plants permanently wilt
River discharge
Stream flow
Subjected to two main forces:
Gravity
Friction against the banks
Categorized into two types
1) Laminar flow
Shallow channels
Low velocity
Smooth and straight channels
2) Turbulent flow
Complex channel shapes (such
as meandering)
High velocities
Cavitation in which pockets of
air burst under high pressure
Channel characteristics
Velocity
Significantly
affects sediment erosion,
deposition and transportation
.
Affected by the following factors:
Channel shape
Volume of water
Roughness of bed
Gradient of stream
Width and depth of river
Roughness
Caused by irregularities on the river bed
(e.g. materials, sediments, etc)
Formula for river roughness
Hydraulic radius
Measure of efficiency by considering
the
ratio of cross-sectional area and
wetted perimeter
.
A
higher hydraulic
radius
means that lower friction is
experienced and a
higher efficiency is
observed
.
Defined as: Volume of water flow at a given time
River processes
Erosion
Hydraulic action
:
Compression of air gaps
in river with water
Abrasion
: Collision of sediments with the
river bed
Attrition
: Rubbing of sediments
against each other
Solution
:
Dissolved substances
in the water are transported
Deposition
Deposition occurs when the sediments
do not have enough energy to move.
Occurs when
1) The gradient becomes shallower
2) River velocity is slow
3) Friction between sediments and bed is high
4) Sediments are heavy
Transportation
Traction
: The
rolling of large sediments
on the river bed
Saltation
: The
bouncing of sediments
near the river bed
Solution
Dissolved substances
are transported in the water
Suspension
:
Floating of the lightest sediments
on the surface of the river
Can be affected by:
Channel characteristics (see above)
Seasonality
Different seasons
affect river flow
,
e.g. Dry vs wet season, the four seasons
that will affect rate of flow of water, etc.
Different changes in seasonality is
measured by a
river regime
(not to be
confused with hydrograph which
measures over a shorter period of time)
Formation of different river landforms :forbidden:
Flooding and flood mitigation
Big question: How does physical and
human factors
exacerbate and mitigate
flood risk for different places?
Physical factors
Hydrograph characteristics
Seasonality: Affects the
river regime
of the river (using data gathered
throughout the year
)
Other factors affecting flood risk:
1) Precipitation types and intensity (causes flooding
with
high intensity rainfall/rapid melting of snow
)
2) Temperature and evapotranspiration (causes flooding
as
warmer air contains more moisture
and lead to higher
rainfall
3) Antecedent moisture (causes flooding when ground is
overly saturated
that causes a
higher overland flow
4) Drainage basin size and shape (causes flooding when
drainage basin is small and lag time is short
)
5) Drainage density (causes flooding when
it is very dense
Also causes
local relief
but
aggravate flood problems downstream
)
6) Permeability of soils and rocks (causes flooding when
rocks are impermeable
and overland flow is large and fast
7) Slopes (causes flooding when
slopes are steep
and
overland flow is large)
8) Vegetation type (causes flooding when vegetation have
thin leaves with low interception
)
9) Land use (causes flooding when
too much land is used
for urbanisation
and therefore impermeable)
Human factors that cause
higher flood risk
Urbanisation
More impermeable surfaces in urban areas and
more drainage channels. Urban hydrographs have:
1) Shorter lag time
2) Steeper rising limb
3) Higher peak flow
4) Steeper recession limb
Deforestation
Similar to urbanisation with higher flood risk
due to
higher peak flow and shorter lag time
1) More often than urbanisation due to
clearing
for other land uses
too (e.g. agriculture)
2) More overland flow leading to
higher erosion
(loss of topsoil can
cause desertification
)
3) Rivers transporting more sediment (
pollution
)
4) Can also lead to
eutrophication due to nutrient
rich top soil
5) Reduced evapotranspiration
Flood mitigation measures
Structural
Levee strengthening
High performance turf reinforcement mat (HPTR)
technique can be used for levee strengthening/
making it higher to
decrease bank-full discharge
Channel modification
1) Flood embankment with sluice gates (may
raise flood levels downstream
)
2) Channel enlargement (may become clogged
with weed and therefore
may not be used completely
)
3) Flood relief channel (appropriate if
original
channel cannot be modified
--> reduce costs)
4) Intercepting channels (diverts part of the flow
away and
allow water flow to towns and for agriculture
)
5) Removal of settlements (
may not be widely accepted
but is used as a last resort)
6) Increasing capacity of water body through
dredging
7) Straightening of river channel (but will
increase the flood risk downstream
)
Afforestation
Increase the amount of interception and
reduce the amount of overland flow.
Dams case study
Planning
Personal insurance
Not very common as typical home insurance
do not cover floods (including from hurricanes)
Home owner is required to buy a separate policy
that is specifically to cover flooding
Flood preparation and
warning technology
Most widely used measure to reduce damages
from flooding.
e.g. Bangladesh has
more than 72 hours of flood
prediction and warning
due to flooding from snow
melt in the Himalayas (that is far away from the city)
However, there is significantly lesser flood warning
time (
less than 6 hours
) if the flood does not originate
from snow melt.
Two contrasting and detailed examples of flood mitigation
Water scarcity and water quality
Big question: What is the
varying
power of different actors
in relation
to water management issues
Water scarcity
Physical
Droughts: Extended periods of dry weather
leading to conditions of extreme dryness
Absolute droughts (dry spells): Less than
0.2 mm of rain
total in 15 consecutive days
Partial droughts: Less than 0.2mm of rain
daily in
29 consecutive days
Causes:
1) Climate change (caused the incidence
of droughts to increase and effect of el nino
to increase in intensity
2) Incidence of el nino event
Economic
Where a country physically has
sufficient
water to meet its needs
but requires
additional
storage and transport facilities
to ensure
adequate water supply.
Water quantity vs water quality
Water quantity
Depends on:
1) Rates of rainfall
2) Evapotranspiration
3) Rivers and groundwater flows
1)
Less than 1% of freshwater
is available for use
2) Available freshwater supply is
not distributed evenly
Affected by
season
About
three quarters of rainfall
occurs in
less than a
third
of the world population
3) Water stress occurs when per capita water supply is
less than 1,700 cubic meters
per year.
4) Water use has
increased 6 times
since 1922 and by 2025,
half the world's population
will live under
severe water stress
Water quality
Adequate water quality is needed
for consumption. This is affected by:
1) Water availability
2) Water infrastructure
3) Cost of water (People
not connected
to the water system
often need to pay
at least 5 times
more cost for water
4) Presence of salinisation and eutrophication
Environmental
consequences of agriculture
Irrigation (salinisation)
Frequent irrigation leads to increase
in amount of salt in the soil.
(Water
evaporates and leaves salt behind)
Groundwater level that is too close
to the surface
may rise to the surface
by capillary action
and evaporate, causing
salt levels to increase
.
Leads to:
Poor plant growth because of
damage to the roots
by high salt
content
Pollution (eutrophication)
Runoff from cropland (
fertiliser
)
and
improper disposal of animal
feaces
causes an algae bloom in
the water due to
increased nitrogen
content
Leads to:
1)
Decreased sunlight
in lower parts
of the basin
2)
Decreased amount of oxygen
in
the basin (more O2 being used by algae)
3)
Increase in carbon dioxide levels
in the basin due to increased decay.
Case study of one international water resource
and the role of diff stakeholders to find solution
Detailed example to illustrate
role of diff stakeholders to
maintain water quality
Growing human pressures on
lakes and aquifers
1) Due to an increase in population, amount
of
water consumption per capita
has increased
2) Agriculture is largest user of water from lakes,
with a rise of
more than 60% of water needed
to grow enough food
3) Economic development has also increased the
amount of water used for
industrialisation
4)
Less interception of water
by vegetation and
filtration of water into aquifers
(lower water table)
5)
Pollution of water
due to economic growth (eutrophication)
that led to
70% of China's rivers
being polluted
Water management features
Big question: What are the
future possibilities
for
management intervention
in drainage basins?
Community-level responses
to water management
Water saving: Changing lifestyle choices
1) Smaller capacity baths
2) Water efficient showerheads
3) Fixing dripping taps
Customer pricing: Paying according
to amount of water used
Saving water
Measured through the
use of meters
Affordable water
Choose the water company that supplies
water and
encourage competition
to
lower prices.
Subsidised cost of water especially in
agricultural societies
New technologies:
Nanotechnology infiltration
(lowering costs
of
desalination
) --> Use of
silver ions and
ceramic membranes
for cheaper desalination
Biomimicry
--> Use of research into
biological
processes
to reduce water purification costs
Smart monitoring technologies
-->
Detection
of leaks in pipes
more accurately and quickly
Precision irrigation systems
--> Reduces water
usage by
watering at specific times and volumes
Sludge digesters
-->
Removes nutrients to be
used in fertilisers
while lowering costs of water treatment
Zoning
--> Identification of
nitrate vulnerable zones
and encourage farmers against putting more fertiliser
to reduce risk of eutrophication.
Rainwater harvesting: Making use of available water before
it evaporates.
Use of water butt (container) to collect rainwater
Irrigation of individual plants rather than whole field
Covering expenses of water with plastic to reduce evaporation
Storage of water underground in gravel- filled reservoirs
Dam building
Integrated basin management scheme
Wetlands and the Ramsar convention
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