Sheet n°297 - May 2008
w
A new satellite remote sensing tool for
improving agricultural land use observation
rrigated agriculture
makes a substantial contribution to the
food security of many
countries. FAO (Food
and Agriculture Organization of the United
Nations) figures indicate that irrigation involves just under 20%
of cultivated land and
supplies 40% of world
agricultural production. An estimated 8
billion human beings
will be living on Earth
in 2030, and it thus
becomes crucial to optimize water resources
and improve agricultural yields. An IRD team,
with support from the
CNES1, used high-resolution images taken
by the Taiwanese remote sensing satellite
FORMOSAT-2 to make
an extensive study of
two semi-arid farming
regions – one in Morocco and the other in
Mexico. This new observation technique
was used to trace the
changes and developments in vegetation
growth, evapotranspiration from the plant
cover and irrigated cereal crop yields. The results obtained showed
the numerous potential
functions high spatial
and temporal resolution
satellite imagery offers.
They should, in the long
term, lead to improved
management of water
and plant resources
in agricultural areas.
©IRD / Benoît Duchemin
I
Irrigated wheat crop in the plain of Haouz at the foot of the Moroccan Atlas.
FAO (Food and Agriculture Organization of the United Nations) data indicate
that annually 2500 km3 of freshwater
are used for agricultural production,
which amounts to 70% of the water resources the whole of humanity consumes in a year. With the global population
continuing to grow at a high pace, it is
essential to optimize the use of water resources and to increase agricultural production in view of the prospect of having
to feed 8 billion humans in 2030. Scientists have for many years been using
remote-sensing satellite observations to
improve water balance and farming yield
assessment on large geographical scales (at the level of irrigated agriculture
areas, catchment basins and so on). Until
quite recently, scientists had two different
observation methods available for doing
that : wide field-of-view sensors (TERRAMODIS or SPOT-VEGETATION), which
allowed daily observation of the entire
globe but with a resolution on the kilometric scale, generally extending far over
that of one parcel of crops, or decametrescale-resolution sensors (SPOT, Landsat,
ASTER), which can yield only one or two
observations per month. Since 2004, the
Taiwanese satellite FORMOSTAT-2 has
been in operation, combining the functional features of these two observation
techniques, albeit without providing an
exhaustive cover of the continents. It gives the possibility for daily observation of
small areas of around 500 km² at a spatial resolution of about 8 metres.
Research conducted by an IRD team at
the Centre d’Etudes Spatiales de la Biosphère at Toulouse, using images taken
by FORMOSAT-2, gave the opportunity
to study two agricultural areas where
farmers make extensive use of irrigation : the Tensift Plain around Marrakech
in the centre of Morocco, and the Yaqui
Valley in the State of Sonora in NorthWest Mexico. In these agricultural areas,
irrigated cultivation of cereals, fruit trees
and vegetables is practised over several
thousand square kilometres.
This activity draws on limited water resources, mainly coming from precipitation received by the nearby mountain
ranges : the Moroccan High-Atlas in the
case of the Tensift Plain, the western
Sierra Madre for the Yaqui Valley. Both
regions have an arid climate : average
rainfall is 200 mm per year. But the wa-
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For further information
BENOÎT DUCHEMIN
Unité mixte de recherche :
Université Paul Sabatier
- CNRS - CNES - IRD
Centre d’études spatiales de
la biosphère (CESBIO)
Address : CESBIO
18, avenue Edouard Belin
BP 2801
31401 Toulouse cedex 9
France
Tel : +33 (0)5 61 55 85 01
Email : duchemin@ird.fr
REFERENCES :
DUCHEMIN B., HAGOLLE O.,
MOUGENOT B., SIMONNEAUX V.,
BENHADJ I., HADRIA R. ET AL.
Agrometerological study
of semi-arid areas : an experiment for analysing the
potential of FORMOSAT-2
time series of images in the
Marrakech plain. International Journal of Remote Sensing, in press.
DUCHEMIN B., MAISONGRANDE
P., BOULET G., BENHADJ I. A
simple algorithm for yield
estimates : Evaluation for
semi-arid irrigated winter
wheat monitored with green
leaf area index, Environmental Modelling and Software,
2008, 23, 7: 876-892
Doi : 10.1016/
j.envsoft.2007.10.003
SIMONNEAUX V., DUCHEMIN B.,
HELSON D., ER-RAKI S., OLIOSO
A., CHEHBOUNI A.G, The use
of high-resolution image
time series for crop classification and evapotranspiration estimation over
an irrigated area in central
Morocco, International Journal of Remote Sensing, 2007,
29,1: 95-116
Doi : 10.1080/014311607012
50390
KEY WORDS :
ter demand is seven times as high (the
potential evapotranspiration of the plant
cover is about 1500 mm/year). It is therefore essential to portion off the water
as equitably as possible, according to the
needs of the different types of crops that
make up the agricultural landscape.
For these two areas, the study demonstrated all the potential capabilities of the
new imagery technique with its high spatio-temporal resolution. With the support
of CNES, the satellite FORMOSAT-2 was
used to obtain time series of images, from
November to May of the following year,
that is throughout one entire farming
season, at the rate of one shot every 5
days. Processing of the resulting satellite
data first helped compile land use maps
(including crop rotation and succession
patterns) with an excellent degree of discrimination between the different crop types. The availability of a large set of observations also allowed detailed monitoring
of the plant cover with time, achieved by
determination of changes in reflectance,
the proportion of the light reflected by
the land surface. These measurements
and associated vegetation indices gave
the bases for determining the variables
that describe the state of the soil-plant
system such as the degree of land plant
cover or the green leaf area index : a reliable index for vegetation, a figure below
0.15 corresponding to bare soil whereas
one of over 0.70 characterizes a crop
that is on the point of ripening (see the
series of satellite images). This detailed
and practically continuous description of
the plant cover made it possible to improve yield assessments and modelling
of water transfer between soil, vegetation
and atmosphere. The investigations run
in Morocco on parcels of wheat in particular showed that evapotranspiration
from the plant cover, the principal factor
in water loss, could be evaluated with a
margin of error of between 10 and 20%.
Irrigated agriculture, arid
environment,
satellite observation
Moreover, the plant biomass (dry above-ground phytomass) and grain yields
could be estimated to an accuracy of
about 25% at parcel scale. In conclusion,
this observation technique opens up the
possibility to track the various successive
operations (sowing, ploughing, irrigation,
harvest and so on) effected as an agricultural season progresses. Making sets
of such data available for local agencies
responsible for agro-environmental management should facilitate the reorientation of irrigation rapidly towards the
crop parcels where it is most needed.
In addition, it also makes it possible
to adapt and improve all the information and advice issued to farmers and
thus enable them to make significant
improvements to their cultivation
practices.
Grégory Fléchet - DIC
Translation - Nicholas Flay
1. This research work was conducted as part of the SudMed and MedMex programmes (IRD-CESBIO,
Toulouse), in conjunction with the
Faculté des Sciences Semlalia of
Marrakech, the Office Régional de
Mise en Valeur Agricole of Haouz
(Morocco) and the University and
Technological Institute of Sonora
(Mexico). It was financed by the
European Commission, Coopération Universitaire Franco-Marocaine
and French national space research
programmes (INSU and CNES). The
FORMOSAT-2 image series were
made available by the NSPO and
processed in the framework of an
agreement between the CNES, the
NSPO and SPOT-IMAGE.
Seasonal changes in plant cover demonstrated by FOMOSAT-2 satellite images
Dense
Vegetation
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Sheet n°297 - May 2008
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