Vegetation: Transport Processes Inside and Outside of Plants - Transport in the Atmosphere-Vegetation-Soil Continuum - page 249

During wetting up, the increase of intercepted amount P i is described by:

∂
∂ =−−

P
t

P
S

( ) −

i

1 rrP

i

E (6.47)

t ean mean

where P mean is the mean rainfall rate (mm d -1 ), and E mean is the mean evaporation rate
of intercepted water when the canopy is saturated (mm d -1 ). Integration of Eq. ( 6.47 )
yields the amount of rainfall that saturates the canopy, P s (mm):

 

PS
E

E

( )

P =−

mean

ln 1 −

mean

with EP ≤ 1−− rr

    (6.48)

( )

s mean me an t

P 1 −− r r

 

mean mean t

Question 6.12: Derive Eq. ( 6.48 ).

For small storms ( P ≤ P s ) the interception can be calculated from:

( )

P =−− 1 r rP (6.49)

i t

For large storms ( P > P s ) the interception follows from:

E
P

( ) +

− ( )

P =−− 1 r rP

mean

PP

(6.50) i t s s

mean

Figure 6.27 shows the Gash interception as function of rainfall amounts for typi-
cal values of a pine forest. The slope ∂ P i /∂ P before saturation of the canopy equals
(1 - r - r t ); after saturation of the canopy this slope equals E mean / P mean .
In forests, the evaporation of intercepted water occurs at rates several times larger
than for transpiration under identical meteorological conditions of a dry canopy (see
section 7.2.4 ) . Thus intercepted water disappears quickly and interception loss replaces
tree transpiration only for short periods. Therefore accurate calculation of forest inter-
ception amounts is required to determine the total amount of evapotranspiration.
At short vegetation, atmospheric conductances are much lower than at forests,
and interception loss occurs at rates comparable to transpiration. Thus for grasses
and common agricultural crops, interception loss is to a large extent compensated
by reduction in transpiration and makes little difference to cumulative evapotrans-
piration (Dingman, 2002 ). Therefore in the case of short vegetation a lower accu-
racy of interception amounts is required. Also, for short vegetation the distinction
between canopy drainage and stem low is less relevant. Von Hoyningen-Hüne ( 1983 )
and Braden ( 1985 ) performed a large number of lysimeter experiments to determine