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Fig. 3.19 Sketch illustrating
the estimation of the
interception storage
capacity S ic ,from
the lower envelope
of the data points of
total versus net
precipitation,
under conditions of
negligible or
minimal
evaporation, so
that L i =
4
PD
2
S ic
S ic .
2
4
(1 c)PD (mm)
3.4.4 Empirical equations
In the past many attempts have been made to relate interception empirically to the cumu-
lative precipitation ( PD ) by linear regression equations (Helvey and Patric, 1965; Jackson,
1975), which can produce useful results in certain applications (Gash, 1979). Comparison
of Equation (3.19) (or (3.14)) with (3.20) (or (3.16)) suggests that this may be a reliable
approach for the early stages of a storm, but not later on, after the canopy is saturated. Once
the canopy is saturated, Equation (3.20) indicates that the duration of the storm may be a
better predictor than the amount of precipitation. The issue has been discussed by Horton
(1919), who felt that expressing interception in terms of shower duration would be more
logical than in terms of amount of precipitation; nevertheless he proposed a linear regression
equation in terms of cumulative precipitation, after he found that this is close to linearly
related with shower duration.
As a rough estimate for interception over longer time periods, Equation (3.6) suggests
that the following may be useful with standard hourly rainfall data
L i = n ( S ic + cE po D )
(3.21)
where D is the average duration of the n precipitation events during the period, and E po the
average rate of evaporation from a wet surface during the same events. Detailed calculations
with more complex formulations have shown that in the growing season on average E po is
relatively invariant over a wide range of climatic conditions (Gash et al ., 1980; Lloyd et al .,
1988; Valente et al ., 1997) and that good results can be obtained with values mostly around
0.2 mm h 1 and ranging only between 0.15 and 0.30 mm h 1 .For c and S ic the typical
values can be used that are mentioned above. The vegetation cover fraction c is usually
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