Lecture 6

• water inputs equal water outputs plus or minus changes in water storage in a catchment
• precipitation is measured with gauges and radar/micro wave (radar more accurate but microwave higher resolution)
• global data from radar and microwave satellites but must be validated with ground data

  • NASA Global precipitation measurement mission
  • 6 km pixels
  • can get precipitation every 30 minutes
  • misses the high intensity events (need rain gauges for that)
• soil water content is measured by loss of weight of a soil sample when heated (to 105 degrees C over 24 hours)
• indirect methods for mapping soil surface moisture content include using airborne or satellite based radar
• groundwater monitored by changing levels in wells and volume derived from estimates of the bedrock's effective porosity (e.g. chalk aquifers)
• infiltration

  whether falling liquid precipitation infiltrates or runs off depends on:

  • rainfall rate
  • soil permeability which depends on: soil porosity; degree of soil saturation; whether ground is frozen
• infiltration-excess overland flow

  precipitation falling at too fast a rate to be able to percolate/drain into the soil
• saturation-excess overland flow

  soil already full of water (saturated) and thus no more space to accommodate inputs
• flood return period:

  • used to determine how often floods occur
  • uses the record of each year's biggest flow
  • quite sensitive to the length of the timeseries
• urban catchments have peakier ('flashier') discharges with shorter lag times
• rural areas:

  • water interception (more vegetation)
  • water infiltrates into the soil/groundwater
  • water therefore takes longer to reach water courses
• atmometer


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


  measures potential evapotranspiration (PE) by monitoring weight of an isolated vegetated area of soil with an unlimited water supply
• equation for river discharge

  Q = A x V

  discharge = area x velocity
• current-meter discharge measurements are made by determining the discharge in each subsection of a channel cross-section and summing the subsection discharges to obtain a total discharge