Télédétection et ressources en eau/Remote sensing and water resources
187
studies, which is interested in erosive events involving intensities greater than an erosive threshold
[generally taken to be 25 mm/hr for the tropics, but often using much higher intensities in order to
ascertain the effects of extreme events], Rsim for WH requires knowledge of the threshold depth
and intensity which produces runoff. In order to help assess the effects of the crust and of
depression storage, the experiments were repeated with the crust perforated, roughened, and
completely removed. It should be noted that there can be a ‘threshold intensity’ at which the crust
appears to break (Patrick, 1993), which may explain a ‘wave effect’ sometimes observed with
runoff readings using Rsim in the area.
The rainfall simulator design chosen for this study is a ‘drip type’, whereas the most
commonly used simulator design is a ‘spray type’ because it attains more realistic KE at lower
simulator heights as the drops emerge at velocity and/or can be sprayed upwards to increase
effective fall height. The drip type was chosen in this case because of the much lower water
consumption; to carry out two simulations of one hour each at 30 mm/hr onto a 1m
2 plot would
require approx 80 L versus approx 2 000 L for an average spray type. As water is obviously in
short supply in semi-arid areas, the drip type is more practical for remote areas, provides greater
control over water source and therefore quality, is much more portable and is much cheaper for
developing country use. A spray type typically costs $5-10 000 (Ben Asher and Berliner, 1994);
the one used in the pilot study costs less than $500 to make.
In short, rainfall simulation can be a valuable tool for runoff response when assessing WH
potential, however availability of the rainfall data required to make the exercise worthwhile by
allowing one to select a realistic design intensity, and the limitations of the particular simulator
chosen, must be borne in mind before deciding whether to assess runoff potential by this method
and when interpreting the results.
Upscaling from rain simulation results for assessing Macro WH suitability
The question of the validity of upscaling from a m
2 Rsim plot to a macro catchment which can be
thousands of times larger is obviously not straightforward. This is a massive topic which will not
be discussed here except to outline a key concept of relevance to assessment of runoff potential
for Macro WH. The partial area concept [PAC], originally developed for humid climates (Betson,
1964), denotes the differential contribution of different zones within a catchment to runoff at the
outlet. The explanation proposed for humid areas is the existence of more saturated belts of
reduced infiltration capacity around the drainage network, which therefore contribute a greater
proportion of the final runoff. The PAC phenomenon has also been observed for arid climates
(Yair et al., 1978). The explanation in this case proposed by Tauer and Humborg (1992) is that
the short duration of rain events means that rain falling on the ‘catchment’ at a distance from the
drainage network will infiltrate in situ and therefore not contribute to runoff.
Predicting runoff by additive calculation from Catalogue surface types on eight Sahelien
catchments of various sizes was found, on average, to yield double the actual runoff (Puech,
1994). Similarly, on a 114 ha rocky catchment in Mali, Ben Asher and Humborg (1992) found
that not taking PAC into account led to overestimates of up to 600% compared to straightforward
upscaling from Rsim on and Catalogue values for the surface types constituting the catchment.
Furthermore, they found that there is very little change in the size of the partial area in response
to rainfall. It should, therefore, be possible to delimit a partial area for a given catchment when
assessing runoff potential for Macro WH, within a reasonable margin. The position and slope of
the drainage network can be calculated from digital photogrammetry of scanned air photos, from
187
studies, which is interested in erosive events involving intensities greater than an erosive threshold
[generally taken to be 25 mm/hr for the tropics, but often using much higher intensities in order to
ascertain the effects of extreme events], Rsim for WH requires knowledge of the threshold depth
and intensity which produces runoff. In order to help assess the effects of the crust and of
depression storage, the experiments were repeated with the crust perforated, roughened, and
completely removed. It should be noted that there can be a ‘threshold intensity’ at which the crust
appears to break (Patrick, 1993), which may explain a ‘wave effect’ sometimes observed with
runoff readings using Rsim in the area.
The rainfall simulator design chosen for this study is a ‘drip type’, whereas the most
commonly used simulator design is a ‘spray type’ because it attains more realistic KE at lower
simulator heights as the drops emerge at velocity and/or can be sprayed upwards to increase
effective fall height. The drip type was chosen in this case because of the much lower water
consumption; to carry out two simulations of one hour each at 30 mm/hr onto a 1m
2 plot would
require approx 80 L versus approx 2 000 L for an average spray type. As water is obviously in
short supply in semi-arid areas, the drip type is more practical for remote areas, provides greater
control over water source and therefore quality, is much more portable and is much cheaper for
developing country use. A spray type typically costs $5-10 000 (Ben Asher and Berliner, 1994);
the one used in the pilot study costs less than $500 to make.
In short, rainfall simulation can be a valuable tool for runoff response when assessing WH
potential, however availability of the rainfall data required to make the exercise worthwhile by
allowing one to select a realistic design intensity, and the limitations of the particular simulator
chosen, must be borne in mind before deciding whether to assess runoff potential by this method
and when interpreting the results.
Upscaling from rain simulation results for assessing Macro WH suitability
The question of the validity of upscaling from a m
2 Rsim plot to a macro catchment which can be
thousands of times larger is obviously not straightforward. This is a massive topic which will not
be discussed here except to outline a key concept of relevance to assessment of runoff potential
for Macro WH. The partial area concept [PAC], originally developed for humid climates (Betson,
1964), denotes the differential contribution of different zones within a catchment to runoff at the
outlet. The explanation proposed for humid areas is the existence of more saturated belts of
reduced infiltration capacity around the drainage network, which therefore contribute a greater
proportion of the final runoff. The PAC phenomenon has also been observed for arid climates
(Yair et al., 1978). The explanation in this case proposed by Tauer and Humborg (1992) is that
the short duration of rain events means that rain falling on the ‘catchment’ at a distance from the
drainage network will infiltrate in situ and therefore not contribute to runoff.
Predicting runoff by additive calculation from Catalogue surface types on eight Sahelien
catchments of various sizes was found, on average, to yield double the actual runoff (Puech,
1994). Similarly, on a 114 ha rocky catchment in Mali, Ben Asher and Humborg (1992) found
that not taking PAC into account led to overestimates of up to 600% compared to straightforward
upscaling from Rsim on and Catalogue values for the surface types constituting the catchment.
Furthermore, they found that there is very little change in the size of the partial area in response
to rainfall. It should, therefore, be possible to delimit a partial area for a given catchment when
assessing runoff potential for Macro WH, within a reasonable margin. The position and slope of
the drainage network can be calculated from digital photogrammetry of scanned air photos, from