Interception

• storage of water above the ground surface, mostly in vegetation
• where vegetation is present, precipitation consists of
  
  
  • gross rainfall measured above the canopy or in openings in a forest
  • throughfall: water that falls between plants,
  • drippage: of water from the plants to the grounds, and
  • stemflow: flow of water down stems and trunks
  
  
• therefore, interception significantly reduces precipitation intensity as water is first temporarily sorted and much is lost

interception loss

that part of the precipitation on the canopy that doesn't reach the ground, because it evaporates from the canopy (canopy interception loss) and from near-ground plants and leaf litter (litter interception loss) or, to a lesser extent, is absorbed by plants

Controls on interception rate, capacity and loss

vegetation characteristics

• interception increases exponentially during a storm until the interception capacity is achieved and the weight of more rain overcomes the surface tension holding the water on the plants
• also snow periodically sluffs off plants when the capacity to intercept snow is achieved
• interception capacity is a function of

1. growth form: trees, shrubs, grasses, forbs
   
   
   • coniferous trees intercept 25-35% of annual precipitation
   • deciduous trees intercept 15-25% of annual precipitation, but just as much as coniferous trees during the growing season
   • trees also have greater interception capacity because they project above ground and into the wind, creating turbulence which drives water on the lee side and into the interior of the tree
   • grasses and forbs have high interception capacity during the growing but then either die (annual plants) or loose mass (perennial plants); also they are grazed and harvested (spring wheat intercepts 11-19% of precipitation before harvest)
   
   
2. plant density
   
   
   • biomass data (mass/unit area) are a poor indication of interception capacity, rather the extent of ground cover and canopy closure are the important aspect of density
   
   
3. plant structure: number, size, flexibility, strength and pattern of branches; texture, surface area and orientation of leaves
   
   
   • trees native to regions of heavy snowfall have flexible branches and trunks to support and shed heavy snow loads (10-20 kg/m2 for wet snow)
   • thus forest mensuration data (tree heights, diameters and volumes) are a poor indication of interception capacity since they don't convey tree structure
   
   
4. plant community structure
   
   
   • secondary interception occurs in stratified forest communities where water drips from the canopy and is intercepted by lower plants
   • in short vegetation, interception storage merges with surface storage, especially if the plants are flexible and bed under the weight of water (e.g., the lodging of crops, which can substantially reduce yields)
   • snowcover on shrubby vegetation and tall grasses is very irregular with large void spaces representing up to 40% of the snowpack

meteorological factors

1. precipitation intensity
   
   
   • water can be delivered too quickly for the plants to accommodate
   • a larger proportion of low intensity precipitation will be intercepted as the storage capacity is created by drippage and stemflow
   
   
2. precipitation duration
   
   
   • absolute interception storage increases with increasing storm duration
   • but, because interception decreases exponentially, a larger proportion of short duration precipitation is intercepted than is the water from a long storm which is shed once a steady state (interception capacity) is achieved
   • nearly all the precipitation from a very short storm can be intercepted, i.e., there is no drippage or stemflow
   
   
3. wind speed
   
   
   • promotes interception loss by evaporation
   • inhibits interception until an initial layer of water or snow forms to support further storage
   • increases interception by blowing water into the interior of plants and plastering wet snow against trees and shrubs
   • thus the influence of wind is complex and depends on wind speed and type of precipitation
   
   
4. type of rainfall: rain versus snow
   
   
   • liquid water has high surface tension and forms an initial layer (sooner than snow) to which subsequent rain coheres
   • at temperatures around 0oC, rain can freeze to plants
   • snow is more easily blown off or away from plants, but once it sticks, snowflakes (depending on their size, shape and liquid water content) can bridge that gap between leaves, stems and branches; thus the interception of wet snow can be considerable
   
   
5. precipitation frequency
   
   
   • a very important factor as in wet vegetation part of the interception capacity is already occupied before a storm
   • therefore, maximum interception capacity occurs with short duration precipitation events that are spaced sufficiently far apart that vegetation dries out
   • however, infrequent precipitation is not conducive to plant growth, so there may be less vegetation under these climatic conditions

horizontal interception

• fog drip and rime, where water is filtered out of fog as it passes through vegetation
• concentrated along the windward edge of coastal forests, where precipitation may be 2-3 times higher than measured by rain gauges
• all precipitation is drippage (fog drip) and stemflow, since the precipitation is induced by the vegetation and otherwise would not occur

Significance of Interception and Interception Loss

• not much because interception loss is offset by decreased transpiration
  • the amount of solar energy for evaporation and transpiration is constant for any time and place, so evaporation of intercepted moisture simply replaces the evapotranspiration that would have occurred in the absence of precipitation
  • experiments have shown decreased transpiration from wet foliage, thus
• interception loss represents a net loss of water
  • the rate of evaporation of intercepted water exceeds rates of transpiration, because transpiration is limited by soil moisture conditions and rate at which water is transferred to leaves and interception loss can occur from dead (non-transpiring) vegetation
  • evaporation of intercepted water cools the plant, suppressing transpiration and causes a heat flux from the air which contributes to further evaporation
• horizontal interception represents an additional input of water to coastal forests
  • thus interception loss from live vegetation usually represents a net loss to the basin hydrological cycle, so logging tends to increase runoff by reducing interception loss and transpiration

Summary - importance of interception

• usually results in a net loss of water available to the basin hydrological cycle
• lowers the intensity of precipitation
• washes solid particles and dissolved carbon from leaves affecting soil and water chemistry and weathering processes