Session 2 : Assessing water harvesting suitability
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the concentration of rainfall runoff from a runoff zone to a runon zone, effectively
multiplying rainfall. It is argued that WH has great potential because it mitigates the
primary source of risk in drylands, the spatial and temporal variability of rainfall, but that
this potential remains underexploited due to a failure to take social factors and local
knowledge into account. A distinction is drawn between Macro and Micro WH, the former
being easier to assess from remote sensing [RS] but having more demanding social
implications in terms of collective action and land tenure. A lack of environmental data
also hampers successful design. The need for practical and operational guidelines for
assessing WH suitability is articulated.
The FAO Framework for Land Evaluation is explained and the logic adapted for WH
assessment. A computerized expert system based on the Framework, ALES, is described
and the application to WH discussed. ALES allows data input from local experts,
facilitated by Participatory Rural Appraisal [PRA], and output in commonly understood
map form by linking to a Geographical Information System [GIS]; both encourage
participation by local resource managers. It is suggested that PRA be used to stratify the
local population into homogenous ‘Social Response Units’ [SRU] which have common
resources and/or anticipated impacts with respect to WH introduction; furthermore, GIS
should be used to build scenarios with each SRU in order to assess differential costs and
benefits of WH..
An example of one environmental requirement to be assessed, sufficiency of water, is
outlined in order to illustrate the logic of the methodology. A nest of ‘filters’ is proposed
which correspond to degrees of precision and commensurate costs; land is evaluated at a
level which corresponds to its emerging potential for WH while the assessment procedure
runs in an iterative manner, and as a function of objectives and resources. A range of
assessment tools are discussed, each associated to a certain level of investigation. The
methodology proposes separate tools for runoff and runon areas. For the former rainfall
simulation [Rsim] is emphasized, and for the latter a variety of tests based on the single
ring infiltrometer. The relevance of indigenous knowledge [IK] to assessment of runoff
and runon is examined. The problem of upscaling from 1m
2 Rsim plots is considered, and
in particular the Partial Area Contribution phenomenon. The ways in which ground
investigations can be linked to remote sensing are explored and the prospect of using RS
to extend the spatial validity of IK suggested. It is argued that RS and IK are
fundamentally compatible because both deal with surface, visual characteristics of the land
and the latter can add a temporal dimension missing from the former.
INTRODUCTION AND BACKGROUND TO WATER HARVESTING
It has been estimated that 200 - 500 million cubic meters of rainfall is lost in the form of runoff in
the Sahel every year, which could potentially irrigate up to 40 000 hectares (Ben-Asher and
Berliner, 1994). Some of this runoff could be utilized by arresting the runoff at suitable points in
the landscapes--pockets of deeper, more fertile soils--by means of soil and water conservation
structures. This paper is concerned with the potential of one form of soil and water conservation
technique, known as water harvesting [WH] and proposes ways of assessing the potential of WH
in any semi-arid area under consideration . WH is of particular relevance to semi-arid areas, as
the emphasis is on the collection and storage of water.
There are many types of WH and many applications of the water which is collected. This
paper is concerned only with WH in which water is stored in the soil, and for crop, tree or range
applications. WH can also be used for the provision of water for people or animals, and in these
cases stored in some form of reservoir and often collected from rooftops. WH basically consists
176
the concentration of rainfall runoff from a runoff zone to a runon zone, effectively
multiplying rainfall. It is argued that WH has great potential because it mitigates the
primary source of risk in drylands, the spatial and temporal variability of rainfall, but that
this potential remains underexploited due to a failure to take social factors and local
knowledge into account. A distinction is drawn between Macro and Micro WH, the former
being easier to assess from remote sensing [RS] but having more demanding social
implications in terms of collective action and land tenure. A lack of environmental data
also hampers successful design. The need for practical and operational guidelines for
assessing WH suitability is articulated.
The FAO Framework for Land Evaluation is explained and the logic adapted for WH
assessment. A computerized expert system based on the Framework, ALES, is described
and the application to WH discussed. ALES allows data input from local experts,
facilitated by Participatory Rural Appraisal [PRA], and output in commonly understood
map form by linking to a Geographical Information System [GIS]; both encourage
participation by local resource managers. It is suggested that PRA be used to stratify the
local population into homogenous ‘Social Response Units’ [SRU] which have common
resources and/or anticipated impacts with respect to WH introduction; furthermore, GIS
should be used to build scenarios with each SRU in order to assess differential costs and
benefits of WH..
An example of one environmental requirement to be assessed, sufficiency of water, is
outlined in order to illustrate the logic of the methodology. A nest of ‘filters’ is proposed
which correspond to degrees of precision and commensurate costs; land is evaluated at a
level which corresponds to its emerging potential for WH while the assessment procedure
runs in an iterative manner, and as a function of objectives and resources. A range of
assessment tools are discussed, each associated to a certain level of investigation. The
methodology proposes separate tools for runoff and runon areas. For the former rainfall
simulation [Rsim] is emphasized, and for the latter a variety of tests based on the single
ring infiltrometer. The relevance of indigenous knowledge [IK] to assessment of runoff
and runon is examined. The problem of upscaling from 1m
2 Rsim plots is considered, and
in particular the Partial Area Contribution phenomenon. The ways in which ground
investigations can be linked to remote sensing are explored and the prospect of using RS
to extend the spatial validity of IK suggested. It is argued that RS and IK are
fundamentally compatible because both deal with surface, visual characteristics of the land
and the latter can add a temporal dimension missing from the former.
INTRODUCTION AND BACKGROUND TO WATER HARVESTING
It has been estimated that 200 - 500 million cubic meters of rainfall is lost in the form of runoff in
the Sahel every year, which could potentially irrigate up to 40 000 hectares (Ben-Asher and
Berliner, 1994). Some of this runoff could be utilized by arresting the runoff at suitable points in
the landscapes--pockets of deeper, more fertile soils--by means of soil and water conservation
structures. This paper is concerned with the potential of one form of soil and water conservation
technique, known as water harvesting [WH] and proposes ways of assessing the potential of WH
in any semi-arid area under consideration . WH is of particular relevance to semi-arid areas, as
the emphasis is on the collection and storage of water.
There are many types of WH and many applications of the water which is collected. This
paper is concerned only with WH in which water is stored in the soil, and for crop, tree or range
applications. WH can also be used for the provision of water for people or animals, and in these
cases stored in some form of reservoir and often collected from rooftops. WH basically consists