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November 2015
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Natural resources and energy
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Once of the most cost effective ways of assessing water use ef-
ficiency is to use satellite imagery to build models that accurately
reflect the production and water use of the vegetation. Recent ad-
vances in the preparation of moderate resolution (1 km) (e.g. from
the MODIS programme) estimates of net above-ground primary
production (NPP) have made it possible to more confidently predict
plant production.
These models are based on the premises that the energy from the
sun that drives photosynthesis is captured by the plants, and only
that which is not utilised is reflected back to the orbiting satellite.
The sensors on the satellite record only the amount of energy re-
flected by the earth’s surface, called the fraction of photosyntheti-
cally active radiation (fPAR). This basic principle involves some
well-researched equations from physics and enables us to produce
a weekly picture of the net primary production of the entire earth.
Similarly, it is possible to model actual evapotranspiration using a
combination of satellite imagery and weather station data from the
7 000 weather stations that are run by the World Meteorological
Network. Using some standard formulae, we can convert the net
primary production into dry matter production as plants have a
remarkably universal ratio of carbon to dry matter (0,42 g C/g DM)
and evapotranspiration (ET) have enabled the preparation water use
efficiency surfaces for southern Africa from 2000 - 2014.
Using earth observation products from the MODIS programme
(MOD 17 [NPP] and ET MOD16), we prepared annual water use ef-
ficiency surfaces for southern Africa, where water use efficiency
= NPP/ET.
We had previously evaluated the quality of the MODIS NPP product
using ground surveys of annual biomass production at five sites. To
build confidence in these models of production derived from earth
observation, we collected above-ground production data for tran-
sects in the succulent Karoo, the semi-arid savannah in the southern
Kalahari, the dwarf shrublands of the Nama-Karoo and the grass-
lands of the north Eastern Cape and KwaZulu-Natal.
We compared our ground-based measurements with those predic-
ted by the remote sensing models and were very comfortable that
the values accurately reflected those measured on the ground. We
also assessed the accuracy of the MODIS ET product using data from
the eddy covariance flux tower located at Skukuza in the Kruger
National Park.
This tower has been measuring atmospheric CO
2
and water fluxes for
over 14 years, and represents an excellent record of the amount of
water used by the savannah vegetation of the Mpumalanga Lowveld.
These assessments indicated that we could use both of these prod-
ucts as a reliable reflection of primary production and evapotranspi-
ration across a range of vegetation types in southern Africa. Water
use efficiency was calculated using these annual NPP and ET prod-
ucts for each year from 2000 until 2013 and this is the first product of
this nature for South Africa.
This data, one of which is presented here as an image, provide the
first national database that can be used to evaluate the impact of
contrasting land cover types and land use strategies on water use
efficiency. The range of water use efficiency values that have been
calculated for the region are directly comparable to those derived
from the ratio of DM production to rainfall which has been applied
in other studies.
There is an inherent stability across southern Africa, although
14 years of data may not be enough to detect significant trends.
Figure 4: An aerial photograph of the southern Kalahari, showing the vegetated dunes (light brown colour) and the
interdune slacks (grey colour).
Courtesy of
Google Earth
DRYLANDS AND RANGELANDS ACROSS SOUTHERN AFRICA