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UNESCO-IHE Institute for Water Education, Delft (2014)

Reservoir water use optimization for crop production in semi-arid regions : case of Mushandike irrigation scheme : Zimbabwe

Fitshane D.

Titre : Reservoir water use optimization for crop production in semi-arid regions : case of Mushandike irrigation scheme : Zimbabwe

Auteur : Fitshane D.

Université de soutenance : UNESCO-IHE Institute for Water Education, Delft

Grade : Master of Science 2014

Résumé
There is an urgent need to save dwindling water resources in Zimbabwe especially in the agriculture sector since it is taking about 70% of the available fresh water. This can be possible by increasing crop water productivity, increasing irrigation efficiencies through rehabilitation and timely maintenance of irrigation structures. This research was to investigate the amount of water available for irrigation from Mushandike reservoir, determine the causes of water shortages in the once famous irrigation scheme and to come upwith remedial measures to rescue this situation. The materials used in this research include the study area of Mushandike which is located between latitude 20.2002o S and longitude 30.6888oE, climatic, soils and geological conditions of the area, the irrigation scheme establishment and institutions responsible for running the scheme. Various secondary and field data were collected and analyzed by Microsoft excel, FAOCROPWAT (ver. 8) and AquaCrop (ver.4) models. Spearman’s rank test for the absence of trend was used for climatic trend analysis. The results showed that there is intra-annual variability in rainfall and evaporation but without any significant trends in the long term ; however temperature data showed some significant upward trend. The conveyance efficiency was derived from long term data of water releases compared against the actual water delivered at every block of farms. The other efficiencies were derived from secondary data from Mushandike Irrigation scheme design document. The overall irrigation efficiency was found to be in theorder of ± 42%. According to the Hydrological branch of Energy, Water Resources and development (1977), Mushandike reservoir has a firm yield of 9.35 Mm3 at 10% risk. This will mean that the maximum hactrage (under irrigation) this reservoir can sustain should not be more than 780 ha (12*103 m3/ha/a) as per the Zimbabwean guide on water application. Over the years the reservoir had not spilled except during cyclone year like in 2000 when there were Cyclone Eline induced floods. Regressing reservoir filling against the annual rainfall shows the reservoir filling heavilydepends on annual rainfall. Therefore increasing the area under irrigation from 624 ha to 997 ha has exerted much pressure on the reservoir and this was demonstrated here by various scenario models of Area versus firm yield. A scenario of increasing the irrigation efficiency to 60% was modelled and showed that the reservoir can meet the current irrigation water demands. This lowered significantly the Reservoir water abstraction from about 12.3 Mm3at 42% efficiency to 8.64 Mm3at 60% efficiency. Another scenario where a shift in the cropping calendar was modelled, avoiding cropping in a period ofAugust to October, which are the hottest and driestmonths and concentrate the cropping from November to July where there is relatively higher effective rainfall to compliment irrigation. This resulted saving up to 3.6 Mm3/a of reservoir water from a potential of 12.3 Mm3/a. The final scenario was to model a case where farmers would reduce the area under winter wheat cultivation by half [ 500 ha] and reserve water for summer crops. The results of the model showed that winter reservoir water abstraction would be about 3 Mm3which will leave an amount of over 6 Mm3for summer crops and sum surplus to cover for uncertainties. The causes of water shortage at this scheme were found to be mainly the extension of the scheme from 624 ha to 997 ha and low irrigation efficiencies [± 42%]. Reservoir siltation has little or no effect on causing water shortage at this scheme. Given all these scenarios, the optimum condition for a sustainable reservoir water use will be by combining the increase of the overall scheme irrigation efficiency and a shift in the cropping calendar. However a quick fix non-structural solution is needed, that will be a combination of shifting the cropping calendar and reducing winter wheat cultivation. This will strikea balance between cash and food crop production with a high level of reliability.

Mots clés  : water availability ; irrigation scheduling ; crop production ; semi-arid areas ; case studies ; irrigation schemes ; Zimbabwe

Présentation

Page publiée le 26 février 2015, mise à jour le 13 octobre 2018