W1 - Intro Flashcards
What is a river catchment?
Delimited by the watershed which is the boundary separating land draining to one river/stream from land draining to adjacent rivers (Gregory, 2000)
What is the water balance equation?
P = Q + E +∆ (I + M + G + S)
What is a hyetograph?
The plot of rainfall on a hydrograph
What are the 3 key areas to be concerned with in rivers and river catchments?
Water quantity, Water quality, Land use and management
What issues are we concerned with in rivers and river catchments? (Water quality)
- Pollutants: nutrients and metals
- Sources
- Upland vs lowland
- Material flux
- Physical and chemical biological impacts
What issues are we concerned with in rivers and river catchments? (Water quantity)
- Flow pathways
- Low flow – drought
- Flooding
- Water resources
What issues are we concerned with in rivers and river catchments? (land use and management)
- Agricultural intensification
- Channel straightening
- Artificial waterbodies, urbanisation
- Deforestation
6 key hydrological processes
- Precipitation
- Evaporation
- Interception
- Soil Water
- Infiltration
- Groundwater
What is precipitation?
“The release of water from the atmosphere to reach the surface of the earth” (Davie and Quinn, 2019, p19)
What conditions are needed to form precipitation?
- Cooling of atmosphere
a. Warm air can hold more moisture than cool air
b. When atmosphere cools, the moisture falls from the sky - Condensation nuclei
a. Salt, smoke, dust etc.
b. Something that our water can condense onto - Growth of cloud droplet/ice crystal
a. Need to be heavy enough to combat wind etc.
b. Two smaller droplets come together or more condensation - Moisture supply
a. Need addition of moisture to the atmosphere for precipitation to continue
What are the main types of precipitation?
rain, drizzle, sleet, snow (grains / pellets), hail, dew and frost, mist and fog
How are dew and frost formed?
- Water vapour condenses into liquid water (dew) after coming into contact with a cold surface
- Common in humid/temperate countries in autumn – night air is warm but surfaces i.e. vegetation are cooler
- Can form frost when <0oC
How are mist and fog formed?
- Cooling air and condensation of dew causes a shallow layer of supersaturated air at the surface
o Band of condensed air at ground level - Condensation nuclei enable mist to form which can then become deeper and thicker, to form a fog
- In arid environments these can be significant water sources
How is precipitation distributed globally?
- Rainfall at equator where evaporation is high, and air is rising -> high energy
- Rainfall higher in mountainous regions – altitude + aspect – rain shadow effect (side of slope facing the prevailing weather receives more precipitation than the one facing away)
- Precipitation decreases with distance from sea
————->Most of the water vapour for precipitation over land is supplied by evaporation from oceans - Values decline from equator to poles
————> Equatorial regions – converging trade wind systems and monsoon rainfall regions
How is precipitation distributed regionally?
more rainfall on the west as that’s where the prevailing wind comes from
Altitude (Orographic precipitation)
Why is precipitation important for water quantity?
- Precipitation intensity controls amount of runoff during a storm event
—————>Dictates flow pathways - Antecedent (previous time period) conditions impact flow pathways
- Annual rainfall distribution controls need for irrigation and other water resources
Why is precipitation important for water quantity?
- Dilution
- Airborne pollutants
————>Wet deposition – precipitation can pick up pollutants
What is evaporation?
“The transferral of liquid water into a gaseous state and it’s diffusion into the atmosphere” (Davie and Quinn, 2019, p49)
What does evaporation need?
- Presence of liquid water
- Available energy from sun/atmosphere
- Atmosphere that can absorb further moisture (not saturated)
(All of these depend on the climate of the area)
What is evaporation’s water supply?
Directly from a water surface
————>Lake, pond, puddle, droplets etc.
Soil water evaporates from near the surface
————>Leads to soil moisture gradient that draws water from deeper in the soil towards the surface
————>Water brought to surface by plants using osmosis in their rooting system
Water supply from soil less than that from open water
What is evaporation’s main source of energy?
Main source of energy from the sun
o Heats up the atmosphere, air, and water itself
Also heat stored in buildings from domestic heating systems
o Makes surrounding air warmer
o Urban environment in winter: main heat source could be from buildings and not sun
Advective energy – originates from elsewhere and is transported to the evaporative surface i.e. latent energy that arrives in cyclonic storm systems
What is atmospheric mixing in terms of evaporation?
- Once water is transformed to vapour it must be absorbed into the atmosphere (diffusion)
- Needs unsaturated atmosphere
- Parcel of air next to evaporative surface must move away, to be replaced by a drier parcel of air
- Atmospheric mixing – how well a parcel of air is able to diffuse into surrounding atmosphere
————>Affected by windspeed
————>Windier areas, parcels of air will be mixing regularly
————>If you put washing out on a windy day it get dry quicker due to atmospheric
What is evapotranspiration?
o Evaporation from the soil matrix
o Transpiration from plants – evaporation from leaves though stomata (tiny hole via which gaseous water will leave the plant into the atmosphere)
What is evapotranspiration rate controlled by?
Rate controlled by opening/ closing of leaf stomata:
Soil water availability
Wet soil = faster transpiration
Plant species
Plant’s ability to transfer water from the soil to its leaves
Plant’s ability of regulate its stomata
Ability of the atmosphere to absorb the transpired water
Humidity
Atmospheric mixing – wind speed
Importance of evapotranspiration on water quantity
Determines water availability
Impacts the amount of water reaching the stream during high rainfall
—-> Rainfall event but high evaporation = lower discharge
Impacts the amount of water able to infiltrate into the soil and into ground water
Influences quantity and timing of water flowing down a river
importance of evapotranspiration on water quality
- Through impurities left behind after water has evaporated i.e. salination of soils (build up of salts)
- In hot environments, agricultural river water runoff contains a lot of minerals
What is interception?
“The process whereby the fall of precipitation is intercepted/ interrupted by being caught by vegetation” (Davie and Quinn, 2019, p70)
what are the components of interception?
Throughfall
Stemflow
Interception loss
What is the LAI?
Leaf Area Index (LAI): the total one-sided green leaf area per unit of ground surface
What are the two types of throughfall?
Direct and indirect throughfall
What is direct throughfall?
falls through the ground directly through gaps in the canopy
Controlled by canopy coverage (LAI)
LAI of less than 1
Higher LAI = leafier vegetation and higher coverage
What is indirect throughfall and what is it controlled by?
drips off leaves, stems or branches
Controlled by LAI and canopy storage capacity
How much water can the leaves hold
Although water is reaching the ground, it is delayed
The importance of climate on throughfall
o Rainfall characteristics - how quickly the canopy storage capacity is reached
o Wind – blowing leaves around and encouraging water to fall off them
o Evaporative demand
What is stemflow and what is it controlled by?
The rainfall that is intercepted by stems and branches and flows down the tree trunk into the soil
2-10% of canopy rainfall reaches the ground in this way
o Higher rates where trees have smoother bark
Funnels water from a large area of the canopy to a small area of the soil
o Water rapidly enters the soil at the base of a tree through flow along roots
Determined by smoothness of the bark
What is interception loss?
water sitting on the canopy is directly evaporated
What controls interception loss?
Rainfall characteristics – rainfall frequency
o Need time for the catchment to dry out in between rainfall events
Aerodynamic roughness of the canopy – turbulent flow
o Mixing of air important for evaporation
Canopy structure
o What is the LAI?
o Angle of leaves in the structure, flat to hold water or sloped?
Temperature, wind
Species
o Broadleaf plants hold water well
o needled plants hold less per leaf
Seasonal changes – deciduous forests
o LAI – bigger seasonal changes
What is soilwater storage?
Soil is saturated when all the air spaces are filled with water as opposed to air
The particle component of soils determines the overall hydrological characteristics of a soil
Can go up or down
Can influence catchment discharge as when it is unsaturated, it can prevent water from reaching the river
Holden, 2017
What effects a soil’s ability to absorb, retain and release water?
Grain size distribution (texture) – what proportion of the soil is made up of particles of different sizes
What does soil formation depend on?
- Parent material (geology)
- Topography
————->Steeper slopes = thinner soils - Climate (rainfall, temperature)
- Action of organisms
- Time > variations in soil type
- Smaller scale - variation of soils downslope (a catena)
What is soil structure?
The orientation of soil particles, or their grouping into larger formations
What is a soil matrix?
dominant structure of soil particles – preferential flow pathways?
What are examples of macropores?
cracks, cavities, animal burrows, old roots
What is the equation for soil porosity?
Porosity (n) = Vp/Vt
o Vp = The volume of the pores
o Vt = the total volume of the rock or soil
What does soil porosity depend on?
Geological parent material
Location in the soil profile
Soil lower down more compact
Location-specific history of soil forming processes
Land management practices
How is soilwater important in terms of water quality and quantity?
Quality: Ions dissolve into soilwater i.e., washes away fertilisers
Quantity:
Influences production of OLF (Overland flow)
Saturated soils force water to move across the soil surface
Controls vegetation response > transpiration
What is infiltration?
“The process whereby rainfall or ponded water penetrates the surface of the soil and become soil moisture” (Davie and Quinn, 2019, p123)
How is soilwater moved during infiltration?
Driven by gravity
Dry soils exert suction that draws the infiltrating water towards the drier area, laterally and vertically
Infiltrating water moves down the soil profile in a wetting front
Ability of soil to transmit water dependent on pore sizes and connections between pores
Soils with large pore spaces and well connected will allow more infiltration
Bodman and Colamn (1943)
What is infiltration rate limited by
the rate of supply of water
the capacity of the surface to absorb water:
What is infiltration rate?
how much water enters the ground during a certain period.
What is infiltration capacity?
the maximum rate of at which water is absorbed by the soil
Relationship between infiltration rate and capacity
If infiltration rate > infiltration capacity, there will be overland flow
Balance between infiltration capacity and rainfall intensity determines how much rain will infiltrate, and how much will become OLF
Relationship between infiltration rate and capacity
If infiltration rate > infiltration capacity, there will be overland flow
Balance between infiltration capacity and rainfall intensity determines how much rain will infiltrate, and how much will become OLF
How does infiltration capacity change over the course of a storm?
Compaction
Rain splash impact – compaction
Swelling
Clays can swell – losing pore space
Clogging
Fine particles can be washed into the pores
Importance of infiltration for water quantity and quality
Water quantity
* Determines proportion of surface flow vs subsurface flow
* Controls vegetation response –> transpiration
* Lots of infiltration –> more groundwater flow
* High overland flow –> faster peak
Water quality
* Implications for sediment
* Little infiltration + high overland flow = soil erosion
What does bedrock porosity depend on?
rock formation and subsequent processes
—- burial, percolation of mineral rich water - precipitates out in pore spaces
—- Igneous rocks also more prone to fracturing and faulting – space in which you can store water
What is effective porosity?
the proportion of spaces that are connected – how permeable the rock is, or it’s ability to allow water to flow through
Need pore spaces to be joined for water to move through it
Types of groundwater storage
– Aquifer: a body of rock or unconsolidated sediment that can store water
a. Mostly sedimentary rock and unconsolidated material
– Aquitard: saturated material of intermediate water holding capacity or permeability
a. may slow the flow of water but not exclude it
– Aquiclude: saturated but relatively impermeable material that does not yield much water
– Aquifuge: a relatively impermeable material that does not contain or transmit water
a. Unweathered igneous and metamorphic rock
What are the temporal variations in groundwater?
Recharge – rainfall infiltrates into the vadose (soil) zone, then percolates into bedrock to become groundwater
o Relies on the rainfall being able to infiltrate into the soil
In seasonal climates:
o higher rainfalls in the wet season
o more stored groundwater
o rise in the regional water table
Importance of groundwater on water quantity and quality
Water quantity
* Speed of groundwater flow – implications for discharge timing
* Flow through groundwater is very slow
* Groundwater dominated catchments tend to be far more stable
* Flows in most streams are sustained by groundwater flow
* Without groundwater flow, our rivers would dry up
Water quality
* Ions from bedrock dissolve into groundwater i.e., limestone aquifers > hard water
* Dissolution + solute concentrations
