Modelling water-harvesting systems in the arid south of Tunisia using SWAT

In many arid countries, runoff water-harvesting systems support the livelihood of the rural population. Little is known, however, about the effect of these systems on the water balance components of arid watersheds. The objective of this study was to adapt and evaluate the GIS-based watershed model SWAT (Soil Water Assessment Tool) for simulating the main hydrologic processes in arid environments. The model was applied to the 270-km(2) watershed of wadi Koutine in southeast Tunisia, which receives about 200 mm annual rain. The main adjustment for adapting the model to this dry Mediterranean environment was the inclusion of water-harvesting systems, which capture and use surface runoff for crop production in upstream subbasins, and a modification of the crop growth processes. The adjusted version of the model was named SWAT-WH. Model evaluation was performed based on 38 runoff events recorded at the Koutine station between 1973 and 1985. The model predicted that the average annual watershed rainfall of the 12-year evaluation period (209 mm) was split into ET (72%), groundwater recharge (22%) and outflow (6%). The evaluation coefficients for calibration and validation were, respectively, R-2 (coefficient of determination) 0.77 and 0.44; E (Nash-Sutcliffe coefficient) 0.73 and 0.43; and MAE (Mean Absolute Error) 2.6 mm and 3.0 mm, indicating that the model could reproduce the observed events reasonably well. However, the runoff record was dominated by two extreme events, which had a strong effect on the evaluation criteria. Discrepancies remained mainly due to uncertainties in the observed daily rainfall and runoff data. Recommendations for future research include the installation of additional rainfall and runoff gauges with continuous data logging and the collection of more field data to represent the soils and land use. In addition, crop growth and yield monitoring is needed for a proper evaluation of crop production, to allow an economic assessment of the different water uses in the watershed

Apport des données Landsat Thematic Mapper pour la cartographie des sols dans la région de Menzel Habib

Un essai de cartographie des sols a été réalisé au niveau de la zone de Menzel Habib, située en Tunisie présaharienne dans la région naturelle des basses plaines méridionales. Cette zone fait partie d’un Réseau d'Observatoires de Surveillance Écologique à Long Terme (ROSELT) mise en place par l’Observatoire du Sahel et du Sahara(OSS), dans le cadre d’un programme de suivi environnemental de la désertification. Ainsi, conformément aux objectifs généraux de ce programme insistant sur l’intérêt de la valorisation et l’exploitation des données anciennes pertinentes en relation avec le thème recherché par la présente étude. On a adopté une approche méthodologique reposant principalement sur la classification multispectrale d’une image satellite en ayant recours aux cartes des ressources en sols et pédologiques existantes. Il en ressort une carte pédologique couvrant toute la zone de Menzel Habib répartis en huit classes.Mots-clés : sols, cartographie, classification multispectrale, image satellite

Object-based assessment of tree attributes of Acacia tortilis in Bou-Hedma, Tunisia

Acacia tortilis subsp. raddiana represents the most important woody species in the pre-Saharan zone. It is the only forest tree persisting on the edge of the desert. Due to tree/environment interactions, canopy sub-habitats arise, enabling an increased storage of soil water, soil nutrients and soil oxygen. Depending on their density, they can also reduce erosion and reverse desertification. Soil erosion and desertification are the main problems faced by the UNESCO Biosphere Reserve in South-Tunisia (Bou-Hedma National Park). The restoration of the original woodland cover to combat desertification (particularly) by afforestation and reforestation of Acacia tortilis goes hand in hand with a climate change in the Biosphere Reserve, also influencing rural population outside the Biosphere Reserve. In order to study the different effects of woodland restoration in Bou-Hedma, the number of Acacia trees and their attributes have to be known. High resolution satellite imagery (GeoEye-1), was used with a GEOBIA approach. Field measurement of bole diameter, crown diameter and tree height were collected at > 400 locations. After segmentation, correlations with > 200 object features and tree attributes were calculated. For crown diameter and tree height, high correlations were observed with the features area and GLCM Entropy Layer 4 (90 degrees). Relations between these features and measured tree attributes were modeled, resulting in RMSE values of resp. 1.47 m and 1.62 m for crown diameter estimation and 0.92 m for tree height. The results show that a GEOBIA working strategy is suitable for estimating tree attributes in open forests in semi-arid regions

Replicable Participatory Water Harvesting Selection Methodology

The developed methodology for the selection workshop can be summarized as shown in table 1. The developed methodology ensures that the stakeholder workshop are executed in similar fashion in all 4 study sites, which will make it easier to compare workshop results between study sites. It should be noted that study site partners do need to ensure that the methodology that is followed fits into the local context and circumstances; therefore the developed methodology may be adapted on details to better suit local conditions

An interdisciplinary methodology to design integrated and innovative MAR systems in arid and semi-arid regions. Two case studies in Algeria and in Tunisia

Drought,  desertification, Managed Aquifer Recharge (MAR), Tunisia, Algeri