Rainfall interception

Boreal zone forest stands are also part of a patchy landscape at least in terms of tree age, size, and density, which determine stand structure. More commonly, though, the stands of trees are interspersed with herbaceous and shrubby vegetation and wetlands. On average, P is found to be 0.5 mm day less than ( = 2.3 mm day ) for forests in summer. Rainfall interception has been included in this comparison because E comprised wet and dry canopy evaporation rates except for the Yakutsk study by Kelliher et al. (1997). The difference between rainfall and evaporation rates is up to 2 mm day for the two oldest studies. These studies also report by far the highest evaporation rates. These E measurements were made using the Bowen ratio technique, which is notoriously difficult to employ over forests because of the small gradients in temperature and humidity there. Except for Lalleur (1992), who also used this technique, the other measurements were made directly by eddy covariance. 

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. 

Surface runoff can begin only after (1) rainfall or snowmelt fills storage by plant interception and surface ponding and (2) the rainfall or snowmelt rate exceeds the soil infiltration rate. Excellent sources for technical details include Chow et al.,50 ASCE Manual 28,53 and Linsley et al.56 Factors affecting Q are listed in Table 25.1. 

Radioactive aerosols are intercepted by vegetation either by mechanical trapping of dry particulate matter or in the rainfall which wets the vegetation. 

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. 

For coniferous forests, the calculated v+ increases rapidly as droplet diameter increases to 10 /iva. Also, w is typically several times greater over a forest than over grassland, so the disparity is greater in terms of vt. Lovett Reiners (1986) found vt of cloud droplets to a subalpine fir forest to be 300 mm s-1, increasing possibly to 2000 mm s-1 on the lee side of gaps in the canopy. In these conditions, occult precipitation is the equivalent of 0.1 to 0.3 mm h-1 of rainfall (Lovett, 1984). Much of the intercepted water re-evaporates, but ions dissolved in the droplets remain on the leaves and are potentially damaging. 

Grover et al. ( 7) recently measured the volatilization of 2,4-D isooctyl ester after application to a wheat field at 0.5 kg/ha (acid equivalent). He reported that total vapor losses of the isooctyl ester over a 5-day sampling period were 93.5 g/ha or 20.8% of the amount applied. The crop canopy intercepted 77% of the applied ester and thus acted as the major source of vapor loss. He found that the 2,4-D ester losses from the soil surface occurred only when the soil surface was moist, i.e., after a rainfall event or in the early hours of the morning following the disposition of dew. The ester was rapidly hydrolyzed to the acid on the wheat plants and in the soil, particularly when surface soil moisture was available. 

For a multiple-crop rotation, residues that must remain (Rmin) are calculated by the same equation used for a continuous-crop rotation (Eq. 4), except the intercept and slope are functions of the 2-yr average residue levels of each residue pair. Rmin represents the amount of residue that must be left in the field each year of the rotation to ensure that rainfall erosion does not exceed T. Note that Rmin is the same for both cropping years. This follows because the C-factor was calculated on the basis of a rotation, not two independent crops. Unlike the continuous-crop rotation, however, three potential situations can arise that will affect residue quantities that can be removed. 

Interflow fraction of rainfall that infiltrates into soils and moves laterally through the upper soil horizons until intercepted by a stream channel, or until it returns to the surface at some point downslope from its point of infiltration. 

The line shown in Figure 14.9 is in fact the line of best fit. It intercepts the vertical axis at a value of 10.6 pg/100 g and it has a gradient of+6.73 (i.e. an increase in rainfall of 1 cm/week is associated with an increase in toxin concentration of 6.73 pg/lOOg). Therefore, the line corresponds to the relationship ... 

Ulrich and co-workers determined the precipitation (r) and element input (e) above a meadow (Fir and Fie) and below a tree stand (F2T and F2e) of both Norway spruce and European beech, as well as the element output by drainage and percolation below the root horizon (Far and Fse) in the Soiling mountains, Germany (Fig. 5a). With the exception of heavy fog days. Fir is always greater than F r because some water does not reach the ground. The difference in rainfall (Fir — F2r) has been called interception which is the sum of evaporation and transpiration. 

In Malaysian tropical forest Sim (1972) found that interception varied between 25 and 80% as a function of precipitation class. Near Manaus, interception of rainfall. Table 3, was as great as 84.6% for the rainfall class of > 5 mm, with a throughfall of only 15.4%, while for the rainfall class 60-70 mm interception was reduced to 25.5% and throughfall increased to 74.1%. Stem flow was negligible regardless of rainfall class (Franken et al., 1982). 

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