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
Outputs
Flows
Stores
Infiltration
Throughflow
Overland flow
Base flow
Vegetation
Aquifers
Cryosphere
Evaporation
Transpiration
Precipitation of varying
types and intensity
Movement of water over the
land, downslope toward
a surface water body. (Occurs
when precipitation exceeds
infiltration or soil is saturated)
The movement of water from
the surface to below the ground
(Not to be confused with percolation)
Flow of water below the
ground, through saturated
soil
Part of the river discharge that
is provided by groundwater
seeping into the bed of the river
(relatively constant)
Process in which a liquid
changes into a gas (affected
by reflectivity (albedo) of water,
temperature and surrounding
humidity)
Process by which plants lose
lose water, primarily through
the leaves. Evapotranspiration
is the main driver of outputs
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
Soil
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
Water stored in snow and ice
(up to 66% of freshwater storage)
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
Channel characteristics
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.
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
Velocity
Roughness
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
Caused by irregularities on the river bed
(e.g. materials, sediments, etc)
Formula for river roughness
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
River processes
Erosion
Deposition
Transportation
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 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
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)
Defined as: Volume of water flow at a given time
Formation of different river landforms 🚫
Flooding and flood mitigation
Big question: How does physical and
human factors exacerbate and mitigate
flood risk for different places?
Physical factors
Human factors that cause
higher flood risk
Flood mitigation measures
Seasonality: Affects the river regime
of the river (using data gathered
throughout the year)
Urbanisation
Deforestation
Structural
Planning
Levee strengthening
Personal insurance
Water scarcity and water quality
Big question: What is the varying
power of different actors in relation
to water management issues
Water scarcity
Physical
Economic
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.
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.
Leads to:
Poor plant growth because of
damage to the roots by high salt
content
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
Water quantity vs water quality
Water quantity
Water quality
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
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
Where a country physically has sufficient
water to meet its needs but requires additional
storage and transport facilities to ensure
adequate water supply.
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?
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
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)
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
Channel modification
Afforestation
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)
High performance turf reinforcement mat (HPTR)
technique can be used for levee strengthening/
making it higher to decrease bank-full discharge
Increase the amount of interception and
reduce the amount of overland flow.
Flood preparation and
warning technology
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
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.
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
Snowfall
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