| Summary: | Small reservoirs have become increasingly widespread across semi-arid regions, due to their ability to reduce transport of eroded soil and harvest scarce and unreliable rainfall for local users. The scale and geographical dispersion of these multiple hydro-social systems restrict their investigation, leading to difficulties in assessing their agricultural potential, their cumulative influence on runoff, and in identifying strategies to support riparian farmers. This research sought to develop a multi-scalar interdisciplinary approach to assess water availability across multiple small reservoirs and understand hydrological and wider drivers’ influence on associated agricultural practices. An Ensemble Kalman Filter approach was developed to combine 30 m Landsat flooded surface area observations with a daily hydrological (GR4J + water balance) model on 7 gauged reservoirs. Data assimilation, providing near-real time corrections, reduced runoff uncertainties generated by highly variable and localised rainfall intensities and lowered daily volume root mean square errors (RMSE) by 50% compared to the initial rainfall-runoff model forecast. Compensating for Landsat’s reduced temporal resolution and correcting outliers, the method correctly reproduced flood dynamics of 5 ha lakes (R²=0.9). Validated against extensive field data over 1999-2014, the method notably establishes Landsat imagery’s ability to assess annual water availability on ungauged reservoirs as small as 1 ha (RMSE circa 25%). Applied to 48 small reservoirs and 546 Landsat 5-8 images, the treatment chain identified the significant water scarcity and unreliability that impedes agricultural development on 80% of lakes in the Merguellil upper catchment (Central Tunisia). In parallel, rapid surveys, quantitative questionnaires and semi-directed interviews confirmed minimal withdrawals, yet highlighted the diversification of practices and the peripheral benefits accompanying small reservoir development. Many farmers lack the capabilities to increase their withdrawals and suffer physical and economic water access difficulties, mismanagement, compounded through limited and short-term government assistance. Individual successes resulted from farmers’ economic resilience and means to secure alternate water supplies during dry spells. Faced with limited storage capacities and prolonged droughts, small reservoirs must in this climatic context retain their supplementary irrigation focus and not strive to support widespread intensification of irrigated practices.
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