Rain interception

In forested areas, rain intercepted by the tree canopy can have a different isotopic composition than rain in open areas (Saxena, 1986 Kendall, 1993 DeWalle and Swistock, 1994). On average, through fall during individual storms at a site in... 

The contribution of atmospheric dust to surface dust depends on the dust falling to the earth. This occurs either as dry dust fall or wet washout with rain, snow or hail (1-6,8-10). Dry dust fall occurs by s imentation, impaction, interception or diffusion. Sedimentation, the fall under gravity, may be estimated using Stoke s law which relates the density and diameter of particles to their falling velocity. A particle of density 1.0 g cm"3 and diameter around 0.1 pm would fall with a velocity of around 9 x 10" cm s" ... 

Wheat stubble may physically intercept up to half of the atrazine as it is applied (Ghadiri et al., 1984). Subsequent rains wash atrazine off the stubble and bring it into contact with the soil. In Nebraska, suppression of summer annual weeds in wheat stubble was proportional to the amount of stubble. Crutchfield et al. (1985) showed that even though heavier wheat stubble intercepted more metolachlor applied in spring prior to ecofallow com planting and reduced the amount of herbicide actually reaching the soil, weed suppression by the heavier stubble more than offset the interception effect. 

Triazine herbicides such as atrazine and cyanazine are not tightly adsorbed to surface crop residue, allowing rainfall to wash intercepted herbicide into the soil. Low vapor pressures also avoid excessive vapor losses of residue-intercepted triazine herbicides. When atrazine was applied to corn-stalk residue, 52% of the herbicide washed off the stalk residue by the first 0.5 cm of simulated rainfall (Martin et al., 1978). After 3.5 cm of rain, 89% of the intercepted atrazine had washed off the residue. Similarly, in another study (Baker and Shiers, 1989), 75% of applied cyanazine washed off com-stalk residue with 0.7 cm of simulated rain, and an additional 11% was recovered from the residue. 

The interception by foliage of activity in rain has been studied experimentally by Hoffman et al. (1989). Polystyrene microspheres, of diameter 3, 9 and 25 pm were labelled with 141Ce, 95Nb and 85Sr respectively. Artificial rain, with the particles in suspension, was applied to plots of clover, fescue and mixed herbage, and the fractional interception of the activity by the foliage was measured. 

The greatest amounts of fallout from Chernobyl were found in those areas where heavy rain fell as the plume arrived, and the fractional interception by foliage appears to have been low in these circumstances. Near Aberdeen the fallout of 137Cs was 2.7 kBq m-2, mostly concentrated in a heavy rainstorm on 3 May 1986 (Martin et al., 1988). Activity of 137Cs in grass, extrapolated back to the time of deposition, was 800... 

Table 2.15. Interception by foliage of particulate activity deposited in rain (analysis of results of Hoffman et al., 1989)...

Hoffman, F.O. et al. (1989) Pasture grass interception and retention of1311, 7Be and insoluble microspheres deposited in rain. Oak Ridge National Laboratory Report ORNL 6542, Springfield, Va. N.T.I.S. 

For a consideration of wet deposition mechanism it is useful to make a distinction between processes transferring material to cloud droplets before they begin their descent as a raindrop, known collectively as rain-out and processes transferring material to falling raindrops known as washout, There are five mechanisms [8] by which particulate and gaseous compounds may be captured by cloud or rain drop diffusiophoresis, brownian diffusion, impact and interception, solution and oxidation of gaseous species (notably SO2 and NO2) and the cloud condensation nuclei (CCN) pathway. 

More recent work has not confirmed the large proportion of stem flow. Work by Franken et al. (1982), carried out in typical Amazon high forest near Manaus, used a system of rain gauges in cleared areas and in the forest canopy, with an extensive series of stem flow collectors. They found that 77.7% of average precipitation reached the soil surface as throughfall, 22% of the rain was intercepted by the canopy, while stem flow only represented 0.3% of the total. 

This equation, known as the global meteoric waterline (GMWL), is based on precipitation data from locations around the globe. The slope and intercept of the local meteoric waterline (LMWL) for rain from a specific catchment or basin can be different from the GMWL. The deuterium excess (d excess, or d) parameter has been defined to describe these different meteoric waterlines (MWLs), such that... 

An example of a highly efficient interception system is the canopy of a coniferous forest. The large specific surface area of pine and spruce trees exceeds that of broadleaved deciduous trees, giving them a high scavenging efficiency for radioactive aerosol particles. Pine needles have been acknowledged as useful monitors of atmospheric pollution (Eriksson et al., 1989). However, radionuclides will be redistributed in the forest ecosystem due to various removal mechanisms such as rain... 

Heavy rains leach out a portion of the bases in the mull layer and below, most of which are intercepted by the tree roots and returned to the surface again later with the annual leaf fall. Since the leachates are usually nearly neutral in reaction, they extract less mineral matter than where the percolating water is high in organic and nitric acids. This continuous cycling of nutrients accounts for stabilization of the soil with little tendency toward the distinct stratification and impoverishment that is characteristic of true podzols. 

Foresters and hydrologists name the interception of rain drops by leaves and needles of trees rainfall interception (or sometimes canopy-interception however, this term is not to separate from cloud and fog interception, and should therefore not be used). Because of evaporation, interception of liquid water leads to loss of that precipitation for the drainage basin. 

Meanwhile later emissions affected central Europe, Germany, Austria and the western Balkans, giving further widespread heavy depositions whenever the plume was intercepted by rain. 

The most common type of flood results from the runoff of rain or of melted snow and ice, or a combination of these, towards the site. Runoff occurs when the amount of water from precipitation falling or melting in a given period exceeds the losses of water by evaporation, transpiration, interception (such as by the leaves of trees), infiltration into the ground and storage in depressions in the ground. 

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