Rainfall-runoff processes

• rainfall-runoff processes determine flow routes of water
• interception: storage of precipitation water on leaves, stems and branches of vegetation

  • throughfall: excess that cannot be stored
  • stemflow: movement of water down stems and branches to soil
• interception capacity: maximum volume of water that vegetation can intercept and store

  • depends on type of vegetation and its seasonal growth patterns, intensity, duration, frequency and type of precipitation
• humans can change vegetation characteristics and climate change can change precipitation characteristics
• infiltration: downward movement of water from soil surface through soil and determines distribution of precipitation between (1) surface storage and surface runoff and (2) subsurface storage and subsurface runoff
• infiltration speed: speed at which water moves into soil defined as volume flux density (mm/hour)
• infiltration capacity: maximum possible infiltration speed at certain moment

  • precipitation intensity below infiltration capacity = all rain will infiltrate
  • precipitation intensity above infiltration capacity = some rain will not infiltrate and stay on surface
• infiltration capacity depends on land cover, soil type and soil wetness
• soil compaction: situation in which small soil particles are transported to and deposited on soil surface, closing pores and hampering infiltration
• infiltration capacity decreases during rainfall event

  • topsoil becomes wetter, leaving less space for new water
• transportation capacity of lower layer increases with soil wetness
• hydrophobic (water resistant): infiltration capacity is small and most rain water will not infiltrate
• soil consists of soil matrix and pore space between soil particles which can be filled with water and air
• porosity/pore fraction: ration between pore space to total volume
• groundwater table divided into saturated (below groundwater table) and unsaturated zone (above groundwater table)
• groundwater: water in saturated zone
• soil moisture: water in unsaturated zone
• capillary fringe: thin zone just above groundwater table which is completely saturated
• percolation: infiltrated water moves slowly downward through unsaturated zone towards saturated zone caused by gravity
• capillary rise: opposite of percolation (water moves upwards)
• several causes of surface water storage

  • land surface is impermeable
  • infiltration excess
  • saturation excess
  • seepage
• infiltration excess: precipitation intensity exceeds infiltration capacity

  saturation excess: soil is already saturated and further storage is no longer possible
• depression storage (ponding): storage of surface water in low-lying parts of landscape
• hydrological processes will not occur until snowmelt (when surface water is stored as snow) resulting in delay in water flow towards catchment outlet
• surface runoff/overland flow: water which flows over land surface during and just after rain shower which will eventually end up in catchment channels
• infiltration excess overland flow: surface runoff caused by high precipitation intensity

  saturation excess overland flow: surface runoff caused by soil already being saturated
• interflow: when water infiltrates but cannot reach groundwater due to layer of low permeability and thus water follows slope of this layer

  • only occurs during and just after rain shower
• drainage: groundwater flowing towards surface water
• upward seepage: water enters an area via deeper soil layers and moves upward towards soil surface
• downward seepage (infiltration): water infiltrates soil and transported to another area via groundwater flow