Télédétection et ressources en eau/Remote sensing and water resources
183
ASSESSING THE ENVIRONMENTAL SUITABILITY OF WH
The main concern on the environmental side of this research has been to operationalize the
prediction of the key physical factor in WH, runoff. This is being attempted by linking rainfall
simulation [RSim] to remote sensing of surface characteristics in order to be able to identify and
characterize the range of runoff generating units in the study area and then extrapolate these to a
nearby test zone to validate them.
Simultaneously, attempts are being made to reduce costs and increase practical usability by
testing simpler means of, at least qualitatively, ranking LUs’ runoff potential as well as by
working with local people to manually classify hardcopy imagery. The relevant characterisitics of
runon zones and their amenability to detection by RS is also being explored; the runon zones have
LURs similar to that for irrigated agriculture, except that drainage is even more important in the
case of WH because of the significant depth of ponding after large storms. Finally, assessment of
environmental suitability has been systematized conceptually by fitting it into the Framework
structure and in practice by using ALES and IDRISI [see figs. 1 and 2]. The system is conceived
as hierarchical, though more in terms of cost of data collection than scale, though the two are
related. Further and more precise but expensive modes of data collection are carried out only if
and to the degree that cheaper, indicative results prove promising [see fig. 1 and in particular fig.
2 {note ‘Suitability Filters’ 1-3}].
Assessing LQA: sufficiency of water
Working through Figure 2 as an example of options for and levels of assessment of LQ’s
As WH systems are composed of runoff and runon areas irrespective of the particular
proportions and degree of complexity of a given system, the basic principle for suitability
assessment remains the same across systems. Nonetheless, there is a fundamental difference
between Macro and Micro WH in that the former is a) more amenable to RS because it involves
larger surfaces and b) entails more complex hydrology because it likely involves several or even
many types of runoff contributing constituent surfaces. Furthermore, Macro WH likely involves a
mixture of overland and channel flow; and probably requires the estimation of the partial area
contribution (PAC) which, in semi-arid areas (Tauer and Humborg, 1992) is due to the short
duration of the convective rainshowers, meaning that most runoff is lost before it reaches the
drainage system.
The runoff area is assessed in terms of its ability to generate runoff, and the runon area
principally but by no means exclusively in terms of its ability to store [and make available] the
runoff generated. Fertility is usually the second most important limitation in the semi-arid areas in
which WH is normally applied (Critchley et al., 1991). Furthermore, WH can potentially leach
what nutrients are available (Stroosnidjer, pers. com., 1995), an often overlooked risk in WH. On
the other hand, WH makes investment in nutrients [manure or inorganic] more worthwhile, as the
fertilized crops are more likely to receive water (Tabor, 1995).
Figure 2 outlines the stages and options for defining LUs, assessing LQs and--if possible-setting class limits. Setting class limits for the LCs which constitute LQs implies a quantification
of the effects of LCs on yield, for which data may not be available. The FAO originally hoped to
specify class limit for LCs when creating the Guidelines for LE for Rainfed Agriculture, but this
did not prove possible for this very reason (FAO, 1983). Now, however, FAO has produced
Ecocrop 2, which is effectively a computerized database of effects on yield of a range of
183
ASSESSING THE ENVIRONMENTAL SUITABILITY OF WH
The main concern on the environmental side of this research has been to operationalize the
prediction of the key physical factor in WH, runoff. This is being attempted by linking rainfall
simulation [RSim] to remote sensing of surface characteristics in order to be able to identify and
characterize the range of runoff generating units in the study area and then extrapolate these to a
nearby test zone to validate them.
Simultaneously, attempts are being made to reduce costs and increase practical usability by
testing simpler means of, at least qualitatively, ranking LUs’ runoff potential as well as by
working with local people to manually classify hardcopy imagery. The relevant characterisitics of
runon zones and their amenability to detection by RS is also being explored; the runon zones have
LURs similar to that for irrigated agriculture, except that drainage is even more important in the
case of WH because of the significant depth of ponding after large storms. Finally, assessment of
environmental suitability has been systematized conceptually by fitting it into the Framework
structure and in practice by using ALES and IDRISI [see figs. 1 and 2]. The system is conceived
as hierarchical, though more in terms of cost of data collection than scale, though the two are
related. Further and more precise but expensive modes of data collection are carried out only if
and to the degree that cheaper, indicative results prove promising [see fig. 1 and in particular fig.
2 {note ‘Suitability Filters’ 1-3}].
Assessing LQA: sufficiency of water
Working through Figure 2 as an example of options for and levels of assessment of LQ’s
As WH systems are composed of runoff and runon areas irrespective of the particular
proportions and degree of complexity of a given system, the basic principle for suitability
assessment remains the same across systems. Nonetheless, there is a fundamental difference
between Macro and Micro WH in that the former is a) more amenable to RS because it involves
larger surfaces and b) entails more complex hydrology because it likely involves several or even
many types of runoff contributing constituent surfaces. Furthermore, Macro WH likely involves a
mixture of overland and channel flow; and probably requires the estimation of the partial area
contribution (PAC) which, in semi-arid areas (Tauer and Humborg, 1992) is due to the short
duration of the convective rainshowers, meaning that most runoff is lost before it reaches the
drainage system.
The runoff area is assessed in terms of its ability to generate runoff, and the runon area
principally but by no means exclusively in terms of its ability to store [and make available] the
runoff generated. Fertility is usually the second most important limitation in the semi-arid areas in
which WH is normally applied (Critchley et al., 1991). Furthermore, WH can potentially leach
what nutrients are available (Stroosnidjer, pers. com., 1995), an often overlooked risk in WH. On
the other hand, WH makes investment in nutrients [manure or inorganic] more worthwhile, as the
fertilized crops are more likely to receive water (Tabor, 1995).
Figure 2 outlines the stages and options for defining LUs, assessing LQs and--if possible-setting class limits. Setting class limits for the LCs which constitute LQs implies a quantification
of the effects of LCs on yield, for which data may not be available. The FAO originally hoped to
specify class limit for LCs when creating the Guidelines for LE for Rainfed Agriculture, but this
did not prove possible for this very reason (FAO, 1983). Now, however, FAO has produced
Ecocrop 2, which is effectively a computerized database of effects on yield of a range of