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November 2015
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TONY PALMER,
specialist research: ARC-Animal Production Institute, Grahamstown
FOCUS
Natural resources and energy
Special
DRYLANDS AND RANGELANDS ACROSS
SOUTHERN AFRICA:
Using earth observation
to define the most water efficient regions
W
ater use efficiency is one of several unifying concepts
for comparing different rangeland condition classes and
land use options. Throughout Southern Africa there are
examples of places that are perceived to be degraded
due to the application of incorrect farming practices including over-
grazing and incorrect ploughing.
With increasing levels of atmospheric CO
2
, there is an urgency to
improve the efficiency of getting CO
2
out of the atmosphere, and
to understand under what land use conditions we are achieving in-
creased efficiency of CO
2
uptake.
In over-grazed rangelands, the predominance of short green grass
does not necessarily mean that an ecosystem is less efficient, as the
high numbers of livestock may simply be eating the grass production
immediately after it has grown. It is therefore important to develop
indices that reflect the functionality of the ecosystem before we
pronounce on the desirability of one or other vegetation state.
Water use efficiency has been developed over many years in the
irrigation sector, but has seldom been applied to producers in the
dryland cultivation and rangeland/grazing community. The concept
essentially embodies the amount of water that it is necessary for a
plant to evaporate in order to capture carbon.
Figure 1: The water use efficiency of Southern Africa during 2006. This was a relatively dry year in the region and this enables us to
see some of the contrasting values more clearly. This image shows that Lesotho, which is regarded by many as severely degraded,
displays a much lower water use efficiency value than the adjacent part of South Africa.
In order for plants to capture CO
2
from the atmosphere, they must
open their stomata, and that results in water loss.
It is therefore essential that plants control water loss by opening the
stomata for just long enough to effect carbon dioxide uptake. If the
plant leaves its stomata open for too long, it can lose too much water
through evapotranspiration and become desiccated.
Similarly, if the stomata are closed too much in the wet season, then
the opportunity to capture CO
2
is lost, and the plant starves.
Thus, achieving a balance between water loss and CO
2
uptake is cru-
cial to the success of crops and veld grasses. Although there are
several forms of the water use efficiency index, for our purposes we
define it as the weight of dry matter (kg) collected per mm of evapo-
transpiration (kg DM/ha/mm/year).
This also needs to be defined on an area basis (in this case hectares)
and time (year). It has been suggested that lower than expected wa-
ter use efficiency may reflect landscape-scale degradation or dis-
functionality of a landscape.
It also provides for an assessment of the efficiency of dryland pro-
duction for rainfed agriculture.