Preliminary estimates of potential evaporation, rainfall and crop water requirements in the Sudan: A signal for water shortage in future

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 coun try, 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, provis ion 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