Session 2 : Assessing water harvesting suitability
186
two are potentially compatible and mutually comprehensible and an appropriate logic on which to
build a WH assessment methodology.
It should be remembered, however, that in spite of often possessing great local environmental
knowledge, local people should not be seen as some sort of ‘noble savages’ with innate knowledge
of ‘mother earth’; ISC is fundamentally practical and is limited by the experience of the
individual. Furthermore, as Ryder (1994) points out, farmers rank soils better than justified for
familiar uses. The best role for IK when assessing the potential for introducing new techniques is
to derive information about the LCs known to be critical for WH from the experiences of local
experts with WH in areas in which it is already practiced. This, then, should be the primary role
of the outside professional when assessing WH potential; as a bridge between knowledge from the
outside world - primarily in written form - and local knowledge and aspirations, primarily in oral
form.
Suitability Filter Level 2: Direct assessment of runoff storage
The most direct assessment of AWC is of course in the lab, which may be a justifiable expense at
suitability assessment level 3, for areas already selected for WH, as this data are required for
calculating C:C ratios for project implementation. When still selecting promising areas however
(i.e., level 2 assessment) there are cheaper ‘direct’ ways of observing the behaviour of a potential
runon area under simulated conditions of ponding behind a bund. This can be achieve using a
single or double ring, which although rightly criticized - as a means of assessing infiltration under
rainfall due to surface disturbance and a standing head - is perfectly suited to assessing
infiltration behind a bund, which has a standing head up to 30 cm, and over a disturbed area due
to cultivation. Although technically a double ring is preferable because, by reducing boundary
effects, it more accurately reflects the theoretical definition of infiltration as vertical movement
into the soil, in practice it is unlikely to be worth the extra water involved under difficult survey
conditions. In fact the lateral movement can be an advantage, as the shape of the wetting front is
an indication of subsoil texture and alerts the observer to the presence of less permeable layers if
the soil is dug away at the end of the test (Roose et al., 1995); which is critically important to
predict drainage problems, which would lead to waterlogging given the large inundations received
with WH.
Filter level 3: Assessing LQ Ai (Ability to Generate Runoff)
Quantitative Ways of Assessing Runoff
Moving on now in Figure 2 to the final level in terms of precision but also cost, local rainfall
simulation would be the appropriate technique at this stage in the methodology. The choice to use
Rsim would likely be made after an initial selection of promising LUs on which further,
quantified, data are required, or in the case of a small area/large scale survey being carried out
from the beginning. The goal of Rsim is to obtain a value which is a best estimate of actual R-R,
for the chosen storm intensity and duration, from which to calculate Catchment:Cultivated area
ratios for WH feasability assessment or implementation, assuming that the necessary rainfall data
are available or can be estimated.
Assessing LQ Ai using rainfall simulation in the study area
Forty-eight rainfall simulations have been carried out at 15 sites so far, at intensities of 10 to 30
mm/hr, for durations of at least one hour each. Contrary to the typical use of Rsim for erosion
186
two are potentially compatible and mutually comprehensible and an appropriate logic on which to
build a WH assessment methodology.
It should be remembered, however, that in spite of often possessing great local environmental
knowledge, local people should not be seen as some sort of ‘noble savages’ with innate knowledge
of ‘mother earth’; ISC is fundamentally practical and is limited by the experience of the
individual. Furthermore, as Ryder (1994) points out, farmers rank soils better than justified for
familiar uses. The best role for IK when assessing the potential for introducing new techniques is
to derive information about the LCs known to be critical for WH from the experiences of local
experts with WH in areas in which it is already practiced. This, then, should be the primary role
of the outside professional when assessing WH potential; as a bridge between knowledge from the
outside world - primarily in written form - and local knowledge and aspirations, primarily in oral
form.
Suitability Filter Level 2: Direct assessment of runoff storage
The most direct assessment of AWC is of course in the lab, which may be a justifiable expense at
suitability assessment level 3, for areas already selected for WH, as this data are required for
calculating C:C ratios for project implementation. When still selecting promising areas however
(i.e., level 2 assessment) there are cheaper ‘direct’ ways of observing the behaviour of a potential
runon area under simulated conditions of ponding behind a bund. This can be achieve using a
single or double ring, which although rightly criticized - as a means of assessing infiltration under
rainfall due to surface disturbance and a standing head - is perfectly suited to assessing
infiltration behind a bund, which has a standing head up to 30 cm, and over a disturbed area due
to cultivation. Although technically a double ring is preferable because, by reducing boundary
effects, it more accurately reflects the theoretical definition of infiltration as vertical movement
into the soil, in practice it is unlikely to be worth the extra water involved under difficult survey
conditions. In fact the lateral movement can be an advantage, as the shape of the wetting front is
an indication of subsoil texture and alerts the observer to the presence of less permeable layers if
the soil is dug away at the end of the test (Roose et al., 1995); which is critically important to
predict drainage problems, which would lead to waterlogging given the large inundations received
with WH.
Filter level 3: Assessing LQ Ai (Ability to Generate Runoff)
Quantitative Ways of Assessing Runoff
Moving on now in Figure 2 to the final level in terms of precision but also cost, local rainfall
simulation would be the appropriate technique at this stage in the methodology. The choice to use
Rsim would likely be made after an initial selection of promising LUs on which further,
quantified, data are required, or in the case of a small area/large scale survey being carried out
from the beginning. The goal of Rsim is to obtain a value which is a best estimate of actual R-R,
for the chosen storm intensity and duration, from which to calculate Catchment:Cultivated area
ratios for WH feasability assessment or implementation, assuming that the necessary rainfall data
are available or can be estimated.
Assessing LQ Ai using rainfall simulation in the study area
Forty-eight rainfall simulations have been carried out at 15 sites so far, at intensities of 10 to 30
mm/hr, for durations of at least one hour each. Contrary to the typical use of Rsim for erosion