Agricultural Research Corporation (ARC) - Sudan
Proceedings of the Meetings of the National Crop Husbandry Committee - NCHC
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Preliminary estimates of potential evaporation, rainfall and crop water
requirements in the Sudan: A signal for water shortage in future
Abdel Mohsin Hassan El Nadi
Introduction:
Almost all the irrigated areas in the Sudan of about 1.68 ha (about 4
million feddan) fall in the arid and semi-arid tropical zones of the
Sudan, in which 76% of the human population live (Ayoub, 1998) (Fig 1).
The two zones are hot and dry during the long summer months, interrupted
only by short rainy periods, mainly during July-September. Average
annual rain fall within the two zones ranges between 100 and 400 mm,
with considerable variation in total amount from year to year and also
with remarkable variation in shower intensity and shower interval. The
short winter months during November-February are generally warm during
the day; cool during the night but with scorching dry winds.
As a result of these harsh weather conditions, the evaporative potential
of the atmosphere by far exceeds precipitation, not only in these two
regions, but also in the relatively more moist regions further south.
Even in the extreme southern boundaries of the country, classified as
moist sub-humid according to UNSO 1997, there is a dry spell during
December and January. Therefore, even in the very humid parts of the
Sudan there is need for supplementary irrigation for perennial crops
e.g. sugar cane, coffee, tea, etc. There is evidence, which dates back
to the mid fifties of the last century, that supplementary irrigation
for coffee trees increased the yield of coffee beans (Ferguson,
undocumented data).
It is now fully established that reliable estimates of crop water
requirements (CWR) must be based on experimental evidence in which the
three components of CWR, namely contemporary weather conditions, crop
characteristics, soil moisture storage and release properties, are all
taken into account.
However, estimates of CWR, based on the calculation method of potential
evapotranspiration (E0) and the crop factor (cf) on the scale of
climatic zones for the whole country are needed for the purposes of
irrigation policy, planning, provision and allocation of water
resources. It is, of course, realized that such estimates must
necessarily have the limitations of the confidence in mean annual
rainfall, the validity of estimated potential evapotranspiration and the
approximated values for effective soil moisture storage and its relation
to the deficit in meeting the demand of the evaporative potential for
the particular zone (E0).
Therefore, the objective of this study is to make estimates for the
deficit from potential evapotranspiration (i.e. E0 – effective rain,
assumed to be equal to moisture storage at the rooting depth of field
crops in the different soil types in the different zones). The
calculated deficits in the different zones will establish the first step
for a more accurate approximation of crop water requirements, as
calculated by the relation:
CWR = E0 × the crop factor (cf) (equation 1)
The second step is to determine values for cf for the remaining economic
crops of the Sudan. Thereafter, CWR of all economic crops in the
different climatic zones can be estimated by using equation 1. Thus,
there shall be no need to repeat experiments of CWR in the different
zones, since E0 can be calculated for any location and cf has been
determined for each crop.
Proceedings of the Meetings of the National Crop Husbandry Committee 37th (2005) pp. 92-100
Proceedings of the Meetings of the National Crop Husbandry Committee - Sudan
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