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Irrigation programmes are usually based on technical and economic considerations without substantial involvement of the target farmers. This study aimed to investigate farmers' discourse, practices and perceptions, and to emphasize progress, limit and synergy of farmers' representation in the modernization programme undertaken by engineers in the Fatnassa oasis in southern Tunisia. Analyses of constraints were based on the modernization programmes and on farmers' interviews. Causal relationships were represented separately as problem trees according to farmers' and engineers' perceptions. The technical solutions implemented by engineers did not achieve the expected enhancement because they disregarded inappropriate community management of the oasian system. Farmers were aware of constraints related to waterlogging and hot irrigation water since they can link causes and effects. The low frequency of delivery water turns was cited as the main constraint in spite of the fact that the farmers were themselves accountable for some inefficient irrigation practices and continuous extension of date palm plantation. Farmers failed to mention soil degradation and salinity, which were only perceived indirectly, or because they applied corrective measures. This case study allowed a critical reflection on the limitations of each approach and for the combination of experts and farmers' knowledge. Copyright agriculteurs sont eux-mêmes responsables de la faible efficience des irrigations et de l'extension continue des plantations de palmier-dattiers. Les agriculteurs n'ont pas mentionné la dégradation des sols, indirectement perçue ou parce qu'ils appliquent des mesures correctives. Cette étude de cas soutient une réflexion critique sur les limitations de chaque approche, et les avantages d'une combinaison des connaissances des experts et des agriculteurs.

Community-managed irrigation systems are commonly faced with problems of low system-level performance. A comprehensive survey was undertaken to assess and quantify the causes of the low irrigation performance in a community-managed oasis in southern Tunisia. At the irrigation system level, results showed that irrigation intervals were double those that had been expected (19-21 days). The main cause was inappropriate farmers' practices at field level where the irrigation duration was almost twice the expected 10 h ha -1. The extension of date palm plantation and technical dysfunctions played a less important role as they were counterbalanced by cancellation of irrigation in some plots. At the field scale, excessively long irrigation duration resulted from the joint effects of small fields, poor maintenance of field-level water delivery systems, uncontrolled night irrigation events and low farmer commitment. This situation emphasizes the question of efficiency and sustainability of irrigation systems when farmers subvert collective rules. Further, the situation raises the question of equity among farmers which hinders their support for the water users' association and the strict application of water allocation rules. These findings should help identify an appropriate approach and lead to positive changes in the management of community irrigation systems.

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