Informations et ressources scientifiques
sur le développement des zones arides et semi-arides

Titre : Development of a 3D water flow modelling based on scarce data for arid land water resources management : case study of Ambouli and Kourtimalei watersheds in Djibouti

Auteur : Fadoumo Ali Malow

Université de soutenance : Tokyo University of Agriculture and Technology

Grade : PhD Génie Agricole 2018

Résumé
Water shortage and groundwater degradation have become two primary environmental concerns for Djibouti since the 1990s. The local aquifers in the capital city, as the dominant sources for domestic and agricultural water supply, are depleting due to groundwater abstraction and continuous drought in recent years with rapid urbanization and increasing water consumption. Therefore, understanding the hydrogeological system is fundamental for a sustainable water resources management. This work focused on two watershed speciafically (Ambouli and Kourtimalei) because of the available information. Due to semi-arid to arid climatic conditions the mean annual precipitation averages only ca.150 mm in a very erratic uneven way. The two main types of aquifer systems are encountered in the Republic of Djibouti : sedimentary aquifers (25%) and volcanic hard rock aquifers (75%). Wadi alluvial aquifers are located along the main wadis and are exploited in rural areas by means of more than 700 shallow wells and a few tubular wells for domestic, agricultural and livestock needs. Volcanic aquifers of limited extent (<2,000 km2), receive localized recharge through wadi alluvia aquifers. These local volcanic aquifers are the most intensively exploited aquifers, particularly for Djibouti city, which uses about 35,600 m3 day-1. The country, especially the capital city, has to face continuously difficulties in its water supply which is derived by 95% from groundwater. First, the arid climatic conditions in the recent years had led to a recurrent drought period. The economic development programs and population growth in Djibouti city increased groundwater demand leading to an overall overexploitation of the resources and water quality degradation. Future water requirement (2020, 2030) are estimated at three times more than the present production rate. The new phase of water resource development must inevitably be followed by management and protection of the aquifers systems and not just the search for the new groundwater resources. In order to understand the groundwater dynamics and to improve the management of water resources this research proposes a 3D water flow model to represent the groundwater level (surface also) and flow system in general. To create the objective flow model, a general-purposed numerical simulator GETFLOWS was selected. It supports multiphase, multicomponent flow, gas dissolution to water, solute and heat transport. It also traces the hydrologic mass and flow processes with fully-coupled surface and multi-phase subsurface fluids on a multidimensional finite difference models. The capability of GETFLOWS has been demonstrated in several case studies including fluid flow modelling for watershed system management. The first case study focuses on the simulation of a surface terrestrial water flow process model in the area of Kourtimalei (40.6 km2) in Djibouti using GIS, RS and GETFLOWS. A trial and error method were used to calibrate the model using observed surface water level of the pond. The manual calibration was performed until the surface water level of the pond RMSE to be 0.40 m and κ >0.8 with satellite derived pond extent comparison. The analysis showed that GETFLOWS successfully simulated the surface water flow process. We conclude that the use satellite derived datasets can help calibrate and evaluate GEFTLOWS hydrologic model for an ungauged watershed like in the present case.

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