Télédétection et ressources en eau/Remote sensing and water resources
177
of a runoff and runon zone, adjacent to one another, where runoff is delivered either as channel or
overland flow. The former can be called ‘MACRO’ WH because of the relatively large--usually
natural--catchments, and involves water spreading in the runon zone. In the latter case, called
‘MICRO’ WH because the catchment area is rarely larger than a few hectares, and runoff is
concentrated into the runon area. WH, and particularly Macro WH, has been used widely in the
Middle East for at least 2 500 years, but is not as common in Subsaharan [SSA] Africa (Pacey
and Cullis, 1986), where it is largely an introduced technology by way of development projects.
There are, however, also many SSA cases of ‘indigenous’ WH, particularly Micro WH (Reij et
al., 1996). For a general review of WH see, for example, Siegert (1994).
The underexploited potential of water harvesting
A review of promising low external input technologies for SSA by the US Office of Technology
Assessment rates WH as the top option in terms of effectiveness and potential adoptability (OTA,
1988). One of the great advantages of WH is its potential risk mitigation effect by integrating
spatially poorly distributed rainfall [in the case of Macro systems]; and by making maximum use
of the rains in spite of their poor temporal distribution, which extends the cropping season and
spreads labour demand peaks.
Yet ironically, in a systematic survey of the experience of projects introducing WH in SSA,
Reij et al. (1988) and Critchley et al. (1992) found the results to be disappointing in terms of
adoption, a central reason being an inadequate attention to social factors. Similar findings have
been reported for soil and water conservation in general (Hudson, 1991). In terms of the
environmental aspect of WH schemes, Critchley et al. (1987) noted that trial and error is the
predominant design practice, which is attributed principally to lack of rainfall-runoff data. This
can lead to bund breakage, which undermines farmer confidence, or conversely to overdesign,
which may require excessive labour. Furthermore, many of the attempts to transfer WH from a
successful area to an untried area have failed, due to the potential effects of different social
and/or physical conditions not having been taken into account.
There is clearly, therefore, a need for a generic, flexible methodology which allows planners
and users (from national to local level) to assess in a systematic, practical, and informed manner
the potential for WH for semi-arid areas, as well as against other possible development
interventions. (Patrick, 1996a) Furthermore, a critical analysis of the nature of the problem being
addressed and a careful assessment of the degree to which the proposed intervention matches the
component requirements and limitations is necessary in order to avoid wasted investment when
considering rainfall conservation schemes (FAO, 1995).
A. CONCEPTS AND RELEVANT TOOLS
The FAO Framework for Land Evaluation and Water Harvesting Assessment
An appropriate tool within which to structure such a methodology would be the FAO Land
Evaluation Framework (FAO, 1976). The Framework is a usefull conceptual planning tool
because it is explicitly structured around the concept of matching ‘demand’ and ‘supply’; a
proposed Land Use Type [LUT] and its Land Use Requirements [LUR] on the demand side, and
177
of a runoff and runon zone, adjacent to one another, where runoff is delivered either as channel or
overland flow. The former can be called ‘MACRO’ WH because of the relatively large--usually
natural--catchments, and involves water spreading in the runon zone. In the latter case, called
‘MICRO’ WH because the catchment area is rarely larger than a few hectares, and runoff is
concentrated into the runon area. WH, and particularly Macro WH, has been used widely in the
Middle East for at least 2 500 years, but is not as common in Subsaharan [SSA] Africa (Pacey
and Cullis, 1986), where it is largely an introduced technology by way of development projects.
There are, however, also many SSA cases of ‘indigenous’ WH, particularly Micro WH (Reij et
al., 1996). For a general review of WH see, for example, Siegert (1994).
The underexploited potential of water harvesting
A review of promising low external input technologies for SSA by the US Office of Technology
Assessment rates WH as the top option in terms of effectiveness and potential adoptability (OTA,
1988). One of the great advantages of WH is its potential risk mitigation effect by integrating
spatially poorly distributed rainfall [in the case of Macro systems]; and by making maximum use
of the rains in spite of their poor temporal distribution, which extends the cropping season and
spreads labour demand peaks.
Yet ironically, in a systematic survey of the experience of projects introducing WH in SSA,
Reij et al. (1988) and Critchley et al. (1992) found the results to be disappointing in terms of
adoption, a central reason being an inadequate attention to social factors. Similar findings have
been reported for soil and water conservation in general (Hudson, 1991). In terms of the
environmental aspect of WH schemes, Critchley et al. (1987) noted that trial and error is the
predominant design practice, which is attributed principally to lack of rainfall-runoff data. This
can lead to bund breakage, which undermines farmer confidence, or conversely to overdesign,
which may require excessive labour. Furthermore, many of the attempts to transfer WH from a
successful area to an untried area have failed, due to the potential effects of different social
and/or physical conditions not having been taken into account.
There is clearly, therefore, a need for a generic, flexible methodology which allows planners
and users (from national to local level) to assess in a systematic, practical, and informed manner
the potential for WH for semi-arid areas, as well as against other possible development
interventions. (Patrick, 1996a) Furthermore, a critical analysis of the nature of the problem being
addressed and a careful assessment of the degree to which the proposed intervention matches the
component requirements and limitations is necessary in order to avoid wasted investment when
considering rainfall conservation schemes (FAO, 1995).
A. CONCEPTS AND RELEVANT TOOLS
The FAO Framework for Land Evaluation and Water Harvesting Assessment
An appropriate tool within which to structure such a methodology would be the FAO Land
Evaluation Framework (FAO, 1976). The Framework is a usefull conceptual planning tool
because it is explicitly structured around the concept of matching ‘demand’ and ‘supply’; a
proposed Land Use Type [LUT] and its Land Use Requirements [LUR] on the demand side, and