Session 2 : Assessing water harvesting suitability
184
environmental factors, and represents a very important data source. The procedure outlined in
Figure 2 is necessary in order to be able to match the LURs of WH LUTs to the LQs of the LUs
delineated in the survey. Note that Figure 2 represents the structure of the assessment
methodology for every LQ, but that the content in this case is only for LQA: ability to make
sufficient water available, for the LUT under consideration [crop/tree/grass.]
Assessing LQ Ai: ability to generate runoff
As can be seen from Figure 2, LQ Ai can be assessed in a qualitative and/or semi-quantitative
manner as a first attempt, depending on the initial degree of knowledge about the area, the
objectives of the survey, and the resources available [see Figs. 1 and 2].
Suitability Filter Level 2: Qualitative asssessment of runoff potential
There are many simple, visual indices of erosion status and/or hazard which include tips on
recognizing telltale signs of runoff which assign qualitative rankings or values for attributing -in
combination with other factors - erosion classes. One of the more systematic guides and
specifically developed for semi arid areas [rangeland in Australia] is the CSIRO Rangeland Soil
Condition Assessment Manual (Tongway, 1994). Local people can also be an excellent fount of
knowledge about soils.
Suitability Filter Level 2: Semi-quantitative assessment of runoff potential: ‘Catalogues’ of
rainfall-runoff relationships
If working in the Sahel, one has access to the brilliant ‘Catalogue des états de surface’ (Casenave
and Valentin, 1989). The Catalogue, from a user’s perspective, is essentially a collection of
photographs of typical surface types found in the Sahel, with associated diagnostic features, and
for each surface type an equation is given which allows one to calculate runoff solely from
rainfall depth and antecedent moisture; the non meterological parameters are implicit because
each equation is an empirical relationship tied to a particular surface type.
In practice, Puech (1994) found that in many areas the catalogue gave runoff values up to
300% different from rainfall simulations he carried out in situ, within the zone of validity. The
Catalogue is perhaps overambitious in hoping to provide models valid for a large part of the
Sahel. It would be preferable to apply the Catalogue concept, but on a local scale, using Rsim in
situ; indeed this is a necessity in semi-arid areas outside the Catalogue’s ‘zone of validity’.
Assessing LQ Aii: ability to store runoff
The assessment of suitability as runon areas is similar to assessing suitability for rainfed and
particularly irrigated crops. The key hydrological Quality, ability to store runoff and denoted Aii
in this case, is essentially the available water capacity [AWC], ie the quantity of water held
between field capacity [FC] and the permanent wilting point [PWP]. AWC is basically a function
of soil texture and depth [with a volume reduction for stoniness], and although PWP is set at an
‘average’, somewhat random value, it should be crop specific. The most applicable methods by
which to determine crop water requirements and yield reduction as a function of water shortage
are outlined in FAO (1986). These have been applied to WH planning by Critchley and Siegert
(1991), who also uses them as a basis to calculate runoff : runon area:
184
environmental factors, and represents a very important data source. The procedure outlined in
Figure 2 is necessary in order to be able to match the LURs of WH LUTs to the LQs of the LUs
delineated in the survey. Note that Figure 2 represents the structure of the assessment
methodology for every LQ, but that the content in this case is only for LQA: ability to make
sufficient water available, for the LUT under consideration [crop/tree/grass.]
Assessing LQ Ai: ability to generate runoff
As can be seen from Figure 2, LQ Ai can be assessed in a qualitative and/or semi-quantitative
manner as a first attempt, depending on the initial degree of knowledge about the area, the
objectives of the survey, and the resources available [see Figs. 1 and 2].
Suitability Filter Level 2: Qualitative asssessment of runoff potential
There are many simple, visual indices of erosion status and/or hazard which include tips on
recognizing telltale signs of runoff which assign qualitative rankings or values for attributing -in
combination with other factors - erosion classes. One of the more systematic guides and
specifically developed for semi arid areas [rangeland in Australia] is the CSIRO Rangeland Soil
Condition Assessment Manual (Tongway, 1994). Local people can also be an excellent fount of
knowledge about soils.
Suitability Filter Level 2: Semi-quantitative assessment of runoff potential: ‘Catalogues’ of
rainfall-runoff relationships
If working in the Sahel, one has access to the brilliant ‘Catalogue des états de surface’ (Casenave
and Valentin, 1989). The Catalogue, from a user’s perspective, is essentially a collection of
photographs of typical surface types found in the Sahel, with associated diagnostic features, and
for each surface type an equation is given which allows one to calculate runoff solely from
rainfall depth and antecedent moisture; the non meterological parameters are implicit because
each equation is an empirical relationship tied to a particular surface type.
In practice, Puech (1994) found that in many areas the catalogue gave runoff values up to
300% different from rainfall simulations he carried out in situ, within the zone of validity. The
Catalogue is perhaps overambitious in hoping to provide models valid for a large part of the
Sahel. It would be preferable to apply the Catalogue concept, but on a local scale, using Rsim in
situ; indeed this is a necessity in semi-arid areas outside the Catalogue’s ‘zone of validity’.
Assessing LQ Aii: ability to store runoff
The assessment of suitability as runon areas is similar to assessing suitability for rainfed and
particularly irrigated crops. The key hydrological Quality, ability to store runoff and denoted Aii
in this case, is essentially the available water capacity [AWC], ie the quantity of water held
between field capacity [FC] and the permanent wilting point [PWP]. AWC is basically a function
of soil texture and depth [with a volume reduction for stoniness], and although PWP is set at an
‘average’, somewhat random value, it should be crop specific. The most applicable methods by
which to determine crop water requirements and yield reduction as a function of water shortage
are outlined in FAO (1986). These have been applied to WH planning by Critchley and Siegert
(1991), who also uses them as a basis to calculate runoff : runon area: