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Operation and Importance of Hydrological Cycle :thunder_cloud_and_rain:…
Operation and Importance of Hydrological Cycle :thunder_cloud_and_rain:
Hydrological Cycle at Global Scale
Global Hydrological Cycle
Water budget
Stores
Groundwater
Fossil Water
1.1%
Soil Moisture
Green Water
0.01%
Atmospheric Moisture
0.001%
Rivers and Lakes
Blue Water
0.01%
Glaciers / Ice caps
1.9%
Ocean
97%
all water
Store size changes over time for human + Physical reasons.
Ice age
Warm periods (3mill y'ago)
Climate Warming - Antarctica + Greenland
Water storage reservoirs
Only 2.5% freshwater
only 1% easily accessible.
Drivers
Solar energy
Gravitational energy
Spheres
Cryosphere
Water stored in solid form (ice)
Hydrosphere
Water stored in liquid form (e.g. seas, lakes, rivers etc).
Mesosphere
middle of atmosphere
Biosphere
Vegetation + Soil moisture
Systems
Open
Inputs
+
outputs
of
energy
+
matter
Closed
Input
,
transfer
+
output
of
energy
but
not matter
Isolated
No
input
or
output
of
energy
or
matter
e.g. The Universe
Fluxes
Residence Times
Strong link between residence times + water pollution
Rates of flow between stores
Flows
Evaporation
Transpiration
Precipitation
Sublimation
Snow turning to water vapour / gas
Vapour Transport
Processes
Percolation
Groundwater Flow
Interception
Infiltration
Throughflow
River/ Channel Flow
Surface Run-off
Global Distribution of Water
Aquifers
Store 30% of all accessible freshwater
Underground reservoirs of water
Distribution
Drainage Basin as an Open System
Inputs
Main: Precipitation
Type of Precipitation
Convectional Rainfall
Land warms up
Air heated above it
Air expands + rises
Air cools + condenses around condensation nuclei = precipitation
Cyclonic Rainfall
When 2 masses of air of diff. temp, humid. + dens. meet.
Rainfall occurs at front of cyclone
Orographic Rainfall
Rain Shadow
Leeward region sheltered from prevailing rain-baring winds by hill range
Moist air lifted over mountain range
Air cools + condenses as it rises
Orographic clouds form = precipitation
Characteristics
influencing Impact on DB
Amount
water in DB + fluxes within
Form
Rain, Snow, Hail
Seasonality
Intensity
^ intensity = ^ risk of flooding
Variability
Secular
Long Term e.g. Climate Change
Periodic
Annual/ Seasonal/ Monthly/ Diurnal context
Stochastic
Random factors
e.g. Localisation of thunderstorm within DB
Distribution
e.g. in the Nile DB, tributaries start in different
climate zones.
Outputs
Evaporation
Moisture lost directly into
atmosphere
from water surfaces + soil.
Evap. increases in warm, windy + dry conditions.
From effects of sun's heat + air movement
Factors affecting rate of evap:
Climatic factors
Temperature
(most important factor)
Hours of
sunshine
Humidity
Wind speed
Other factors
Depth of water
Water quality
Type of vegetation cover
Colour of surface (
Albedo
)
Size of water body
Transpiration
Water lost via
stomata
+ transferred to atmosphere.
Factors affecting rate of transp.
Time of year
Type + Amount of vegetation cover
Deg. of availability of moisture in atmos.
Length of growing season
Evapotranspiration
(EVT)
Combined effect of
evap. + transp
.
Most important aspect of water loss to atmosphere.
Accounts for removal of n. 100% annual
precip
. in
arid
+
semi-arid
areas. 75% in
humid
.
No
evapotranspiration
in ice fields/ desert areas/ water surfaces/ bare rock slopes. Just evap.
Potential Evapotranspiration
(PEVT)
Water loss that would occur if = unlimited supply of water in soil - for use by veg.
Diff. btwn PEVT + EVT greater in arid areas than humid areas.
Physical Factors influencing DBHC
Climate
Type and amount of precipitation + amount of evaporation
Vegetation type
Soils
Amount of infiltration, throughflow + type of vegetation (indir.)
Geology
Rates of percolation + groundwater flow.
Indir. = soil formation
Relief
Precipitation totals
Surface run-off
Vegetation
Rates of interception, infiltration overland flow + transpiration
Human Factors influencing DBHC
River Management
Artificial reservoirs
Hold back river flows
^ evap. potential
Domestic + Industrial use
Reduces river flows
Aral Sea
Fed by
Amu Darya
+
Syr Darya
Diverted
by
Soviet Union
for irrigation +
agriculture
,
1960's
.
Now
25% of original size
+
Holds just 10%
of original
volume
.
Decreases river capacity
Channelisation
Decreases evap. + EVT + capacity
Changing land use
Urbanisation
Surfaces e.g. tarmac, cement etc.
Increase
surface run-off
Reduces
infiltration + percolation
Agriculture
Arable--> pastoral
Compaction
of
land
by
livestock
Increases
overland flow
Reduced interception
Without good drainage =
water-logging + salinisation
.
Installing drainage mitigates this.
Pastoral--> arable
Ploughing ^ infiltration = loosens + aerates soil
Deforestation
Decreases EVT+ interception + lag time.
Increases surface run-off
Increases flooding potential
^ soil compaction by rain
Amazonia
Over 20% forest destroyed
Cattle
ranching
Commercial agriculture
for
biofuels
+
soya
Legal + illegal
logging
Increased CO2 emissions
Within forests: = 75%
intercepted water returned to atmos. via EVT.
Cleared forest = 25%
only.
Afforestation
Increases interception + infiltration + lag time
Decreases surface run-off
Trees act as carbon sinks
Infiltration 5x greater in forest comp. to grassland.
Groundwater abstraction
For Irrigation
Lowers water table
Cloud seeding
Silver iodide pellets (ammonium nitrate) = condensation nuclei.
Increases precipitation in drought-stricken areas
(Results variable.)
Processes
Interception
H2O stored in plants.
Throughfall
Wet leaves shedding excess water onto ground surface.
They are larger drops = more erosive power.
Erosive power reduced if leaves = closer to ground.
Stem Flow
Flow of intercepted water down stem of plant.
Transfers precip. + nutrients from canopy to soil.
Interception loss
The interception, storage + subsequent loss of precip. via evaporation from canopies.
Infiltration
Water absorbed by soil
Factors affecting
Infiltration capacity
Time
Capacity decreases with time during rainfall
Antecedent Soil Moisture
^ infiltration in dryer soil. (low ASM)
Permeability of soil
Porosity: Sand/ Silt/ Clay
Vegetation Cover
Type + Amount + Season.
Forest vs. Bare Earth
Density of soil
Compact vs. loose
Slope angle
Shallow = infiltration Steep = surface run-off
Soil porosity
Raindrop size
Flows
Percolation
Transfer
of water into
permeable
rocks
with joints
Pervious
rocks e.g. carboniferous limestone
with pores
Porous
e.g. chalk + sandstone
Most likely in
humid
climates w/ vegetated slopes.
Throughflow
via
Percolines
lines of concentrated water flow btwn soil horizons to river channel
and natural pipes
The sporadic
horizontal flow
of water within the soil layer.
Groundwater flow
Slow transfer of percolated water through pervious/ porous rocks
Maintains steady level of channel flow in var. weather condits.
Surface runoff
(Overland Flow)
Precipitation
intensity exceeds
infiltration
rate.
Torrential Storms
Intense, long periods of rainfall
Rapid snow melt
"Baked"
unvegetated
surfaces
e.g. Arid / Semi-Arid Sahel Region
Prim. agent of
soil erosion
Rain splash
Sheet, gull + gully erosion
River / Channel flow
Flow of water in streams/ rivers
Takes place after water reaches river via OF, TF or GF.
Saturated overland flow
upward movement of
water table
into
evaporation zone
e.g. winter UK 2015, WT rises to surface in depressions + at base of hill sides.
Area of land drained by river + tributaries
Features of DB
Catchment
Watershed
Water budgets + River systems
Water Budgets
*Annual
balance between
precipitation
,
evapotranspiration
+
runoff*
Useful indication of
available water supplies
On a
local
scale, WB's show annual balance between inputs (precip.) + outputs (EVT).
Water Balance
Positive / negative
Precipitation = Evapotranspiration + Runoff ± Storage
Available soil water
Water stored in soil available for growing crops.
River Regimes
The annual variation in the discharge or flow of a river at a point. (m. in cumecs)
RR Influencing factors
Size of river
Where in course m. taken.
Amount, seasonality + intensity of precip.
Temperatures
^ evap. in summer
Geology + soils
Permeable? Porous?
Type of vegetation cover
Human abstraction
Storm Hydrographs
Flashy
Human
Deforestation
= fewer natural pipes
Agriculture
Compacts soil + removes veg.
Urban surfaces
Cement, tarmac etc.
Impermeable = increase surface run-off
Channelisation
Decreases river capacity Increases flood level
Drains
Reduce travel distance, channel water directly
Construction
Compacts soil + removes vegetation
^ surface run-off
Physical
Small basin size
Lack of vegetation
Impermeable rocks e.g. granite
No percolation
Clay soil (low infiltration)
High relief
Circular basin
^ Drainage Density
High water table/ soil saturated with H2O
Subdued
Physical
Dense vegetation
Large basin size
Elongated DB
Permeable rocks e.g. limestone
Sandy soil
Low relief
Low drainage density
Dry basin
Human
River widening
Not
urbanised
^ infiltration
Key features
Rising Limb
Falling Limb
Peak Discharge
Lag Time
Base flow
Peak Rainfall
Surface run-off increases as infiltration decreases.
Surface run-off is not a store!
Surplus