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Conséquences du développement des cultures de contre-saison sur l'irrigation et la dynamique de la nappe phréatique à l'Office du Niger (Mali).

Bréhima Tangara 1
1 IER Niono, Mali
IER - Niono - Institut d'Economie Rurale du Mali - CRRA Niono
Abstract : During the last 10 years, the surface area cropped during dry season has considerably increased in Office du Niger while new irrigated areas were equipped. It noticeably increased irrigation water requirement especially during low water period of Niger River. These new operational conditions combined with the poor network design and the weak irrigation management increased the amount of water losses that are percolating towards water table. That consequently contributes to produce the current shallow water table, which is endangering agriculture development. The research goal aims at better characterizing and quantifying conditions of dry season cropping system development which is commonly considered to create high water losses and soil degradation conditions. The scientific purpose of the research aim at modeling the hydraulic system in order to assess the inter-annual impact of dry season cropping system development on irrigation performance and ground water table behavior. The developed approach consisted in (i) collecting and analyzing available secondary data, (ii) conducting on site survey and, (iii) setting up a monitoring device so as to get data necessary to a modeling process able to assess irrigation performance and to predict the increase of the ground water table. The socio economic secondary data analysis shows a significant increase of surface areas cropped during dry season that led to remove the initially designed land use. It brings farmers by developing these new farming practices endeavor to improve their household food self sufficiency and incomes. The Water management monitoring data analysis shows that dry season cropping development has increased water withdrawal rate from Niger River but conversely has cut the gross water requirement of the irrigated area by half because of the improvement of water conveyance efficiency as the cropped surface area is increasing. The monitoring of a sample of irrigation canals supplying rice fields show that the soil type of the fields is the main explanatory factor of the variability of their water intake (mean value 15 000 m3/ha). During dry season the irrigation efficiency at irrigation and plot levels is higher than during rainy season although it remains weak (65%); it improves during the cropping period from 20 à 85% as rice transplanting is implemented. Water losses conveyed by the drainage network are commonly low during dry season. At irrigation canal/plot scale, the weak irrigation efficiency is mainly caused by an over irrigation of the field before transplanting. In case of an irrigation canal supplying rice fields, these water losses provide around 25% of the amount water (200 mm), which percolates towards the groundwater table. Improved irrigation practices should reduce the ground water recharge due to rice cropping during dry season. Irrigation performances of market gardening are much weaker: the mean volume of water supplied per hectare at tertiary canal intake (14 500 m3/ha). This value is not statistically dissimilar to the rice one although rice water requirement is twice as high. The monitoring data (Kala inférieur) of the regional ground water level between 1995 and 2006 confirm the connection between the surface water table and the alluvial/continental one. It also demonstrate significant relationships between the increase of the water table volume, the surface area cropped during dry and rainy season and, the groundwater level under the irrigated area in spite of it decreases outside the irrigated area. These observations have allowed to develop groundwater modeling based on water balance likely to predict the volume of the alluvial/continental water table in proportion to the combined effect of the surface area cropped during dry and rainy season. The outputs of the simulation show that dry season irrigation supplies only 25% of the increase of the regional groundwater table volume between 2001 and 2006; the major part of it has to be ascribed to the rainy season cropping. The increased volume of the regional groundwater table and its connection with the surface one decrease the natural drainability of the cropped areas mostly during the dry season. It causes the low fluctuation of the phreatic level and results in a quasi continuous water logging of the plot soil, which restrains agricultural potentialities and increases the risk of soil alkalinization. Considering the approach based on a simplified water balance of the water table and representativeness assumptions, the identified explanatory physical processes and trends require to be corroborated. It assumes to carry on new periodic surveys of the phreatic level, and improve the monitoring device, to refine characterization of aquifer properties. These supplementary data would permit to develop a comprehensive groundwater modeling likely to not only identify management modes to prevent the agricultural constraints and environmental risks caused by the water table rise but also to use the groundwater stored as an opportunity for the further development of dry season cropping.
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Bréhima Tangara. Conséquences du développement des cultures de contre-saison sur l'irrigation et la dynamique de la nappe phréatique à l'Office du Niger (Mali).. Milieux et Changements globaux. Université de Bamako, 2011. Français. ⟨tel-00862014⟩

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