Résumés
Résumé
L'article qui est proposé s'inscrit dans le cadre de la recherche de systèmes de gestion de l'eau qui soient en mesure de satisfaire une demande alimentaire croissante dans un contexte de rareté de l'eau, tout en respectant les exigences de l'environnement. Il développe la phase 1 d'une démarche méthodologique visant l'élaboration d'un plan de gestion des eaux d'un lac collinaire en zone semi-aride tunisienne pour un développement agricole durable. Cette phase correspond à l'utilisation d'un modèle de prévision du ruissellement et des sédiments par averse. Le modèle mathématique à paramètres physiques distribués KINEROS est utilisé et appliqué sur un petit bassin versant de 135,35 ha pour 9 averses enregistrées durant les deux années 1997 et 1998 en affectant aux différents paramètres du modèle des valeurs réalistes basées sur les résultats expérimentaux collectés dans la littérature.
L'application de ce type de modèle nécessite la connaissance de l'occupation du sol à un instant donné. Pour ce faire, une carte d'aménagement agricole pour l'ensemble du bassin versant a été établie correspondant à la résultante de deux cartes élaborées ; la carte culturale et la carte d'aménagement antiérosif.
L'analyse des résultats obtenus a montré que le modèle KINEROS peut être utilisé comme outil d'aide à la gestion des terres et des sols en zone semi-aride tunisienne à relief accidenté.
Mots-clés:
- Gestion des ressources,
- modèle,
- paramètres physiques distribués,
- carte agricole,
- averses,
- ruissellement,
- sédiments
Abstract
The process of erosion-sedimentation is initiated by rainfall and runoff from hill slopes and channels. Runoff is considered as a transport vector, which is related to agricultural management systems and land use. To simulate and predict the behaviour of ungauged watersheds, distributed parameter models are of great interest. Among the different developed models, conceptual and physically-based models are interesting to investigate, specifically under semi-arid conditions. As reported by many authors, these models allow reliable evaluation of the hydrologic responses to land use changes and thus can be used for elaboration of water and soil conservation plans. The accuracy of erosion and sedimentation results is related to the quality of the hydrologic component. An appropriate way of studying soil erosion is through the formulation of the fundamental transport equations of water and sediment using the kinematic wave simplification. In this context, the KINEROS model is used. This paper deals with water management systems, which are able to satisfy an increasing food supply requirement within a context of water scarcity while respecting environmental requirements. It develops the first phase of a methodological approach for the establishment of a management plan for water stored in a small dam for a sustainable agricultural management.
This phase was elaborated and applied to the M'Richet El Anze watershed in a Tunisian semi-arid area using the following approach. A study of the hydrological and sedimentological performances of the KINEROS model using a set of hydrological and sedimentological data measured at a hydrologic station for a sub-catchment of 40 ha of the M'Richet El Anze watershed during the period September 1994 - March 1996. Flow has been measured since 1994 at a hydrometric station with a rectangular channel (9.3 m T 1.2 m T 1.5 m in height, with an average slope of 2.5%). A limnigraph (OTT X) and a triangular flume, situated at the end of the channel, were used to measure the flow rate with a time step of 10 min. Sediment load was measured by taking manual samples with 1 L bottles during flow events at non-regular times. Sediment deposited in the artificial channel was weighed after the runoff event. Rainfall was measured with a time step of 5 min using two gauges situated within the watershed. Input data files were elaborated using : (1) a topographic map for watershed discretisation, slope and size planes; (2) soil sampling for soil characteristics and parameters; and (3) land surface and vegetation status maps of the watershed and photos on parcels corresponding to different periods of each year for land use and surface information parameters.
The KINEROS model was used to predict runoff and sediment loads for a small dam in the M'Richet El Anze watershed using an elaborate agricultural management map, which is the product of a crop map and a water and soil conservation map. Elaboration of the crop map was based on three criteria : soil texture, depth and rock cover, whereas the water and soil conservation management map was based on two criteria: land slope and soil rock cover. The rainfall data used for the runoff and sediment simulation were measured for the period 1997-1998 and only rainfall-generated runoff at the hydrologic station was considered. Characterization of rainfall erosivity was applied for different rainstorms, using the empirical erosivity index EI. The maximum erosivity was obtained for the 1998-09-25 rainstorm (39.4 K.J.mm/m2.h). Results showed that autumnal rainfalls were the most aggressive events affecting soil erosion in the Tunisian semi-arid region, especially those in September and October. We also noted an important rainfall erosivity index for June with an orographic rainstorm (30.1K.J.mm/m2.h). Related to these results, runoff seems to be related to rainstorm erosivity and soil moisture. Erosivity can affect soil structure while soil moisture involves an infiltration component, and both affect soil hydraulic conductivity. In this context, analyzing runoff-rainstorm events within a watershed and relating them to the rainfall erosivity index allowed runoff modeling, which can be done with more results. As developed by the author in 1988 for a small watershed in a Tunisian semi-arid region, a boundary intensity (IL) can be identified that is related to an antecedent soil moisture index (IPA). Related to the same work, runoff generation is defined when a pounded rainfall (PI) is anticipated. The difference between total and pounded rainfall correspond to excess rainfall. The PI is related to antecedent soil moisture index (IPA) and the boundary intensity (IL). These results showed the importance of the hydrologic component of the model and especially infiltration modeling.
Thus, accuracy of erosion data is related to the precision of the hydrological results. The following can be concluded from the application of the KINEROS model:
1. The KINEROS model can be used for predicting runoff from ungauged watersheds and for evaluating future land use master-plans for Tunisian semi-arid high lands ;
2. Best runoff simulations were obtained for rainstorms that lasted less than one hour, with high intensity and a total rain of more than 20 mm ;
3. The accuracy of erosion results was related to the hydrological component and precision of parameters assessment ;
4. For runoff forecasts in the small dam of M'Richet El Anze, the small amount and the irregularity of runoff prompt the use of collected water for extra irrigation of crops and vegetation tolerant to water stress ;
5. Development of annual crops would increase the efficiency of autumnal runoff ;
6. Irrigation duty duration from the small dam was related to the extent of water erosion and sediment deposition, which were related to runoff and topography ;
7. The developed methodology used for the application of the KINEROS model to predict the impact of water and soil conservation management on a Tunisian semi-arid watershed without calibration is interesting, especially because it allows the elaboration of distributed soil parameters and crop maps, which can be used as a decision support system for erosion control on hill slopes, keeping dams located downstream in good working order.
Keywords:
- Resources management,
- model,
- physically-distributed parameters,
- agricultural map,
- rainstorms,
- runoff,
- sediments