Geology Reference
In-Depth Information
1
0.9
C = Cb Exp(-10 Cf)
R^2 = 0.75***
0.8
C = Sed. Conc. of Erosion Plot (g/l)
Cb = Sed. Conc. of Bare Plot (g/l)
0.7
0.6
0.5
0.4
Fig. 11.5
The sediment
concentration ratio
c
/
c
b
as
affected by surface contact
cover fraction. Reproduced with
permission from Paningbatan
et al
. (1995) Alley cropping for
managing soil erosion of hilly
lands in the Philippines.
Soil
Technology
8
: 193-204, Elsevier.
0.3
0.2
0.1
0
0
0.1
0.2 0.3
Contact cover fraction (Cf)
0.4
0.5
0.6
rate was closely related to rainfall rate, with much
less dependence on cumulative infiltration
amount than would be indicated by models such
as the Green-Ampt equation (Yu, 1999). The
Green-Ampt infiltration equation predicts little
sensitivity to depth of ponded water, which
implies little sensitivity to rainfall rate, which is
not the case in this extensive dataset. The out-
come of this extensive examination of alternative
infiltration models was the finding that the most
efficient and accurate of the range of alternative
models investigated (Yu
et al
., 1997c) for the
spatially-averaged infiltration rate
I
−
is that given
by the equation:
(non-linearly) with rainfall rate up to a maxi-
mum limiting value, when the entire plot is gen-
erating runoff. This behaviour in response to
increasing rainfall rate is consistent with an
increasing fraction of the plot area generating
excess rainfall, together with the remaining
unponded area experiencing increased infiltra-
tion by its rainfall acceptance. That excess rain-
fall is generated from only some fraction of the
catchment or plot area is commonly described as
the 'partial-area' concept of runoff generation
(Rose, 2004).
The infiltration equation (11.22) provides one
of the three components in a predictive model
relating the dynamics of runoff from an area to
the time-varying rainfall it receives. The other
two components are as follows:
●
In general a certain amount of rainfall needs
to fall on a given area before any runoff is
produced. This threshold amount of rainfall (or
infiltration) depends on how wet or dry the land
area is prior to the rainfall received, as illus-
trated by Yu
et al
. (2000a).
●
There is also a time lag between the generation
of excess rainfall and its appearance as runoff due
-
1
II
=--
(1
exp(
PI
/
))
(m s )
(11.22)
m
m
where the parameter
I
m
is the maximum possible
value of the spatial mean infiltration rate for the
complete plot area and
P
is the rainfall rate (m s
−1
).
This model describes in parametric fashion the
spatial variation in infiltration rate using a single
parameter.
The infiltration equation (11.22) shows that
the spatially-averaged infiltration rate increases
