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.