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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
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.
Page publiée le 22 novembre 2021