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UNESCO-IHE Institute for Water Education, Delft (2015)

Impacts of small private irrigation along the White Volta basin in Northern Ghana

Alhassan A.-R.M.

Titre : Impacts of small private irrigation along the White Volta basin in Northern Ghana

Auteur : Alhassan A.-R.M. 

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

Grade : Master of Science (MS) 2015

Résumé
Small private irrigation systems in Ghana are usually initiated by entrepreneurial farmers because of economic and market factors. They are usually termed as non-conventional irrigation because of theirdifference with the conventional public schemes which are usually government initiated. Small private irrigation is sometimes categorised in Ghana based on the source of water used including small dug out and reservoir systems, ground water systems and lake and river lifting systems. Even though small private irrigation will double the effort of meeting growing food demand as result of urbanisation and population increase, there is the danger of adverse consequences due to increased pressure on land and water resources. This is envisaged due to the unplanned and unregulated nature of small private irrigation development. However assessing the present performance of small private irrigation compared to the conventional systems is a first step to regulating the sector and to monitor the progress and impacts for sustainable development. This study assessed impacts of small private pump irrigation systems on agricultural productivity, environmental degradation and multiple water users in the Nawuni catchment of the White Volta basin. Four indicators were used to assess the agricultural productivity including, Output per unit "cropped area", output per unit "command area", output per unit "irrigation supply" and output per unit "water consumed"by evapotranspiration (ET). Yield figures were obtained through farmer interviews while market prices were also obtained from farmers and relevant institutions. Additional two indictors were used to assess the adequacy or excesses of water supply. Adequacy or excesses of water supply was determined by "Relative water Supply" and "Relative Irrigation Supply". Water diverted was estimated by flow measurements, length of cropping season and irrigation frequency. Water consumed by ET was estimated with FAO Cropwat 8.0. Two indicators were used to assess the impact of small private irrigation on the environment. This was achieved by assessing the quality of irrigation water by analysing the salinity of irrigation water and also by analysing the contribution of irrigation to water pollution by analysing Biochemical oxygen demand of the surrounding water source. The impacts of small private irrigation on multiple water users were assessed by analysing water demands by various water users within the catchment. This was compared to three years flow into the catchment. The study found out that, small private irrigators within the study area cultivated an area of 65 ha while the government scheme cultivated 938 ha in two seasons in the year 2013. Farmers in small private irrigation sites cultivate once in a year i.e. during the dry season between October and March while farmers in the Bontanga scheme (government scheme) cultivate twice a year i.e. in the dry and wet seasons. For the years 2011, 2012 and 2013, average output per unit cultivated area for small Private irrigation was 2,570 US$/hawhile that of Bontanga irrigation scheme was 676 US$/ha. Average output per unit command area indicated that small private irrigation was 2,570 US$/ha while that of Bontanga scheme was 1,113 US$/ha. Output per unit irrigation supply was 0.33 US$/m3and 0.08US$/m3for small private irrigation sites and Bontanga irrigation scheme respectively. Output per unit water consumed by ET also indicated 0.60 US$/m3for small private irrigation sites and 0.06 US$/m3for the government scheme. It was shown from the study that Relative Water Supply was 2.11 and 1.14 respectively for small private irrigation site and government irrigation scheme. Relative Irrigation Supply also indicated that Small private irrigation achieved a ratio of 1.86 while government scheme achieved a ratio of 1.45. The study also showed that salinity of three sites where small private irrigation is carried out along the White Volta river were in the range of 0.054 dS/m and 0.087 dS/m while that of the Bontanga reservoir was 0.063 dS/m. Biological oxygen demands (BOD) of the White Volta River at three small private irrigation sites were 2.3mg/l, 1.0 mg/l and 0.65 mg/l, while the Bontanga reservoir recorded average BOD of 2.3 mg/l. Five main categories of water withdrawals were identified in the Nawuni catchment and these include ; Rural domestic water users, livestock water use, urban water supply, small private irrigation and commercial private irrigation. Total withdrawal by all these sectors amounted to 17 million cubic meters (Mm3) annually. Theminimum flow of the White Volta River at Nawuni between year 2011 and 2013 was 78 Mm3. Small private irrigation contributed 3% of the total annual withdrawal from the river between 2011 and 2013. There was significant difference in land productivity between the small private irrigation and the government scheme. Output per unit cultivated area was four times higher in the small private irrigation sites than in the government scheme within the study years (2011-2013) while output (US$/ha) per unit command area was two times higher in the private scheme than in the government scheme. This is as a result of cultivation of vegetables by all small private irrigators which have relatively higher prices in the market than rice which is the main crop cultivated by government irrigators. Water productivity for the government scheme was relatively very low as a result of high cultivation of rice in the scheme especially in the dry season. There were excesses of water supply experienced in both schemes but the government scheme performs better than small private irrigation in terms of avoiding excess water supply. High water productivity by small private irrigators was therefore influenced by the crops cultivated i.e. crops with low crop water requirement. This confirms the first hypothesis of the studies which indicates that small private irrigation perform better in terms of overall land and water productivity of the schemes and adequacy of water delivery than government schemes, however in this study they perform less than the government schemes in terms of avoiding excess irrigation. However water productivity by various crops could not be compared because it was not possible to show how much water was diverted for only vegetables in the government scheme. Small private irrigation in this study does not significantly affect soil quality and water quality in the Nawuni catchment because of the good quality of water from the White Volta River and relatively smaller areas under cultivation. This contradicts the hypothesis that small private irrigation in the Nawuni catchment has degraded the environment, however intensifying irrigation i.e. both small private and commercial private irrigation in future could have effect on the quality of the water and soil in the catchment. Finally, contrary to the third hypothesis of this study, present consumption of water by small private irrigation had no significant effect on the availability of water for other users in Nawuni catchment of the White Volta River. Although there was significant increase in withdrawal due to small private irrigation in the dry season, this had no significant effect on availability for other water users because of high flows in the White Volta River. Future scenarios indicate that 25% of the available suitable area for agriculture if used for small private irrigation will result in critical shortages of water in the river during the dry season especially if efficiencies of irrigation water use by farmers are 40% or below. It also showed that 25% of the area can be used for vegetable production under small private irrigation if irrigation efficiency is improved to 70%. This indicates that irrigation of 25% of the total agriculture area will result in very high competition for water by the various sectors using water from the river if irrigation efficiency is below 70%. Furthermore, scenario of cultivating 50% of the total agricultural area indicates that there will be significant shortages even under improved irrigation water use efficiencies to 70%. However these scenarios are subject to the assumption that the future hydrology of the river remains similar as the present.

Sujets  : irrigation systems ; private systems ; government ; performance evaluation ; river basins ; Ghana

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Page publiée le 29 décembre 2016, mise à jour le 11 novembre 2019