Session 2 : Assessing water harvesting suitability
188
satellite imagery using newly developed processing techniques (Ichoku et al., 1996), by direct
measurement or, if available, from digitized topo maps.
The current state of physical research in the pilot study area
The development of a methodology for assessing the physical potential for WH has two goals: a)
to develop or test the utility of existing inexpensive field based tests and to compare their
accuracy to more expensive procedures and b) to characterize LUs in terms of runon and runoff
potential in order to determine which types of LUs and which characteristics of the LUs are
discernible from optical remote sensing. Some of the tests have been described above. The rest
will only be listed.
At Level 2 of Figure 2, qualitative and semi-quantitative assessment, 56 sample sites have been
characterized by:
--Hand texture analysis; consistence--hand plasticity and stickiness, strength using penetrometer
at three shallow depths; pH using colorimetric method [can be >1 unit different from glass
electrode (Stocking, 1987) but sufficient for survey purposes (Landon, 1991)]; carbonates
[10%HCL]; average aggregate size; pore abundance and type with hand magnifier; visual
evidence of water movement and erosion; micro and macro topography; basal and aerial cover;
soil colour (Munsell, dry); hand sprayer, with and without crust; surface stoniness; GPS location;
hand spectoradiometry; evidence of crust--depth and nature; cracking; photograph.
At Level 3 of Figure 2, quantitative assessment, at 18 sites where Rsim carried out, the above
plus the following.
--Bulk Density; Field Capacity; Air Filled Porosity; Total Porosity; Backpack Sprayer; Single
Ring--and dig under to describe wetted area and to check effective depth, stoniness and roots;
Drainage under undug Ring after 24 hours; Rainfall Simulation--photographing suface changes
[especially aggregate breakdown, closing of cracks and incipient ponding], digging under for
wetting depth, returning after 24 hours to determine if change in strength.
In summary the physical tests, mostly selected for their rapidity, simplicity and low cost, have
proven to be satisfactory to Level 2 accuracy requirements when validated against published lab
data; they allow for a semi-quantitative ranking of LU’s. In addition, they have proven to be
practicable under tough field conditions and manageable for field assistants. In terms of
limitations, fertility, salinity/sodicity, and Permanent Wilting Point in particular are important
and cannot quickly and/or cheaply and/or accurately be determined outside the lab; a cost benefit
analysis should be used to ascertain at Level 3 whether lab work is justified.
Mapping WH potential from remote sensing
From the sky down: techniques relevant for assessing MACRO WH suitability
Assuming that it has been possible to set some class limits for LCs, and therefore that LUs have
been defined in some semi-quantitative manner and matched to WH LURs for potential runoff
and runon areas, the question is now how to extrapolate the results from the few sampled ideal
types assumed to be representative of the study area as a whole. This constitutes the last box in
Figure 2. There have been many attempts to take advantage of RS to predict runoff from
ungauged catchments. The most common approach in semi-arid regions has probably been to use
188
satellite imagery using newly developed processing techniques (Ichoku et al., 1996), by direct
measurement or, if available, from digitized topo maps.
The current state of physical research in the pilot study area
The development of a methodology for assessing the physical potential for WH has two goals: a)
to develop or test the utility of existing inexpensive field based tests and to compare their
accuracy to more expensive procedures and b) to characterize LUs in terms of runon and runoff
potential in order to determine which types of LUs and which characteristics of the LUs are
discernible from optical remote sensing. Some of the tests have been described above. The rest
will only be listed.
At Level 2 of Figure 2, qualitative and semi-quantitative assessment, 56 sample sites have been
characterized by:
--Hand texture analysis; consistence--hand plasticity and stickiness, strength using penetrometer
at three shallow depths; pH using colorimetric method [can be >1 unit different from glass
electrode (Stocking, 1987) but sufficient for survey purposes (Landon, 1991)]; carbonates
[10%HCL]; average aggregate size; pore abundance and type with hand magnifier; visual
evidence of water movement and erosion; micro and macro topography; basal and aerial cover;
soil colour (Munsell, dry); hand sprayer, with and without crust; surface stoniness; GPS location;
hand spectoradiometry; evidence of crust--depth and nature; cracking; photograph.
At Level 3 of Figure 2, quantitative assessment, at 18 sites where Rsim carried out, the above
plus the following.
--Bulk Density; Field Capacity; Air Filled Porosity; Total Porosity; Backpack Sprayer; Single
Ring--and dig under to describe wetted area and to check effective depth, stoniness and roots;
Drainage under undug Ring after 24 hours; Rainfall Simulation--photographing suface changes
[especially aggregate breakdown, closing of cracks and incipient ponding], digging under for
wetting depth, returning after 24 hours to determine if change in strength.
In summary the physical tests, mostly selected for their rapidity, simplicity and low cost, have
proven to be satisfactory to Level 2 accuracy requirements when validated against published lab
data; they allow for a semi-quantitative ranking of LU’s. In addition, they have proven to be
practicable under tough field conditions and manageable for field assistants. In terms of
limitations, fertility, salinity/sodicity, and Permanent Wilting Point in particular are important
and cannot quickly and/or cheaply and/or accurately be determined outside the lab; a cost benefit
analysis should be used to ascertain at Level 3 whether lab work is justified.
Mapping WH potential from remote sensing
From the sky down: techniques relevant for assessing MACRO WH suitability
Assuming that it has been possible to set some class limits for LCs, and therefore that LUs have
been defined in some semi-quantitative manner and matched to WH LURs for potential runoff
and runon areas, the question is now how to extrapolate the results from the few sampled ideal
types assumed to be representative of the study area as a whole. This constitutes the last box in
Figure 2. There have been many attempts to take advantage of RS to predict runoff from
ungauged catchments. The most common approach in semi-arid regions has probably been to use