1 Spatial Modelling of the Terrestrial Environment - Georeferencial

Near Real-Time Modelling of Regional Scale Soil Erosion 163
model analyses of meteorological parameters. This process for warm cloud precipitation
estimation takes into account surface wind direction, relative humidity and terrain. The
combined technique incorporates rainfall from both the convective and the stratiform cloud
types, producing a final estimate of total accumulated precipitation.
The main problem of implementing the SCS model with the FEWS rainfall data is that
the model is only applicable for rainfall events whereas the rainfall data are on a decadal
timestep. To overcome this, the model was adjusted to run on a decadal basis where OF(i)
is calculated in the following manner (Zhang et al., 2002):
OF(i) =
n∑
rOFpJ (6)
i=1
OFp is overland flow per n different classes of rainfall intensity, i = 1, ...,n,
r = (r max − I a )
(7)
n
r max is the maximum rainfall per rain day which is assumed to be 500 mm, and J is the rain
day frequency density function which is assumed to be:
J = J 0
e − r i
r 0 (8)
r 0
where J 0 is the total number of rain days per month, r i is the rainfall per rain day and
r 0 is the mean rainfall per rain day (mm). This methodology allows the use of FEWS
data in the overland flow model; however, a map of the number of rain days per decad is
required. We produced this by interpolating the daily GTS raingauge data using indicator
Kriging (Symeonakis, 2001). Indicator Kriging maps the probability of a pixel being wet.
By defining a probability level of 50% this field was transformed to a binary one, with ones
where rain occurred and zeros where it did not for each day of the month. These maps were
then summed into decads and used to implement the overland flow model.
8.2.2 Sediment Transport and Routing
The process of soil erosion by water includes soil detachment, transport and deposition. So
far we have only modelled soil detachment and availability for transport but not transport
itself or when and where it will be deposited. To estimate sediment input into the lake we
have to consider these factors.
Eroded soil can be divided into two types: (i) sediment deposited when the sediment
concentration is higher than the transport capacity or (ii) sediment transported into the lake
when the concentration of sediment in the overland flow is less than the transport capacity.
Sediment transport capacity can be calculated using models such as the European Soil-
Erosion Model (EUROSEM), Water Erosion Prediction Project (WEPP) and the sediment
continuity equation (Kothyari et al., 1997). However, such models require numerous input
parameters, many of which cannot be readily derived at the regional scale. Therefore, such
models are not currently suitable for regional scale modelling, particularly in Africa where
input data are relatively sparse.
An alternative way to model deposition is to estimate the sediment delivery ratio (Dr),
the ratio of sediment yield to erosion. The delivery ratio of each cell in a catchment can