Lecture 6: Catchment Hydrology and Hydrological Change

Water Balance in a Catchment Area

• water inputs equal water outputs +/- changes in storage

• P = Q + E + Δ (I + M + G + S)

• P = precipitation

• Q = river discharge 

• E = evapotranspiration

• Δ = change of

• I = interception and biological water storage

• M = soil water storage 

• G = groundwater storage

• S = channel and surface storage (e.g. as lakes)

• inputs, storage, throughputs, or outputs within the system

Measuring River Flow

• Q = P - E - Δ (I + M + G + S)

  • river flow = precipitation minus evapotranspiration minus the changes in interception, soil water, groundwater, and surface storage

Precipitation Measurement

• point measurements using rain gauges used historically to measure precipitation

  • Thiessen polygons with geospatial analysis

• now a combination of gauges and radar/microwave used

  • radar more accurate but microwave higher resolution

  • must be validated with ground data

  • NASA Global precipitation measurement mission

    • 6 km pixels

    • precipitation measured every 30 minutes

    • misses high intensity events, needs rain gauges for that

Evaporation and Evapotranspiration Measurement

• Atmometer measures evaporation by monitoring loss of water from a porous surface

• Lysimeter measures potential evapotranspiration by monitoring weight of an isolated vegetated area of soil with an unlimited water supply

• PE can also be estimated indirectly by measuring variable (e.g. solar radiation, temperature, humidity, wind speed, surface roughness), values applied to an empirical equation

  • Penman-Monteith and Shuttleworth equations

River Discharge Measurement

• measured in cumecs

• Q = A x V

  • discharge = cross-sectional area x velocity

• need several velocities

• done in the field by dividing the channel into equal widths and recording the depth of each subsection to find areas, then summing subsection discharges

• Acoustic Doppler Velocimetry

  • uses acoustic beams to measure fluid velocity

  • incredibly detailed velocity information

• more efficient way

  • measure discharge at different river ‘stages’

  • establish what how each discharge corresponds to water level at each gauging station

  • estimate the discharge by plotting water level along a curve

• also V-Notch weirs, etc.

Soil and Groundwater Measurement

• soil water content measured by loss of weight of a soil sample when heated to 105 C over 24 hours

• indirect methods for mapping soil surface moisture content include using airborne or satellite based radar

  • GRACE gravity satellite can sense terrestrial water storage changes

• groundwater monitored by changing levels in wells and volume derived from estimates of bedrock’s effective porosity

Elements of Hillslope Hydrology (4)

• infiltration

  • whether or not it infiltrates or runs off depends on

    • rainfall rate

    • soil permeability

      • soil porosity

      • degree of saturation

      • whether or not the ground is frozen

• overland flow

• throughflow

  • matrix flow

  • macropore flow

  • pipeflow

• groundwater flow

Overland Flow Types

• infiltration-excess OF -> rain too fast

  • also called Hortonian overland flow 

  • Horton’s curve/Infiltration capacity curve

• saturation excess OF -> sail already saturated

  • water table intersects with ground surface

Flood Hydrographs

• characteristic asymmetric shape

• precipitation, discharge, time, lag time, rising limb, falling limb, storm/baseflow

• duration of a flood will be influenced by shape and area of a catchment and storm characteristics

• rate of flow increase on ising limb typically faster than rate of decrease

• urban catchments have peakier discharges, more flooding, shorter lag times, longer falling limb

  • controls on infiltration and runoff change with increasing urbanisation

Flood Return Period

• used to determine how often floods occur, recurrence intervals

• uses the record of each years biggest flow

• highly sensitive to length of time series

• ‘100 year flood’ does NOT mean guaranteed to happen once every 100 years

Impact of Climate Change on Floods

• increase in river floods in North-Western Europe

• decrease in river floods in Southern Europe, Eastern Europe

• largely driven by rainfall, also other factors such as land use, flood capacity (e.g. sediment), temperature