Optimised Selection of Water Supply and Irrigation Sources—A Case Study on Surface and Underground Water, Desalination, and Wastewater Reuse in a Sahelian Coastal Arid Region

António Freire Diogo,Ricardo Alves Resende,António Luís Oliveira

doi:10.3390/su132212696

Abstract

This paper presents a general conceptual procedure and an economic, sustainable, resilient, and integrated solution for the water supply and landscape irrigation of a large touristic complex projected in an arid to semi-arid southwest coastal region of the island of Santiago in Cape Verde, in the Western African Sahel. It is assumed that underground water can be explored, but at a minimum level to avoid aquifers depletion or any eventual saline intrusion, and that surface water can be mobilised with the construction of a dam. Other important sources of water that were identified, assessed, and compared include treated wastewater produced in the complex, which can be reused for irrigation after undergoing tertiary treatment, and desalinated sea water, which requires the construction of a desalination plant. The devised procedure includes the definition of all feasible alternatives by a deterministic complete enumeration, and the selection of the optimised solution that may be determined by a single objective cost function to be minimised, as well as by a multi-criteria decision-making process that considers a conventional 1–5 score to be maximised. Beyond the cost, four additional criteria are considered and scored for each solution, namely, Environmental Sustainability, Water Quality, Resilience, and Local Integration, and for the relative weighting between criteria a weighted sum model is assumed. For the parameters and cost models used, the best solution that was found considers that the landscape irrigation is performed with surface water and reused wastewater, and that the water distribution network is supplied with underground and desalinated water. Other solutions more or less close can be selected based on different scores or different decision maker weighting preferences.

Highlights

The greater part of our planet is covered with large masses of salty water, and both the planet surface and the salty water are subject to the natural hydrological cycle
If effective measures are not taken, disequilibrium between demands for water—as a condition of human activities—and the natural recharge of existing bodies of fresh water may lead to the depletion of water resources, from the surface or from underground aquifers, which may result in possible desertification in the medium to long term
The African tropical Atlantic island of Santiago has a total area of 991 km2, a current population of approximately 274,000 inhabitants, and is the largest and most populated island of the ten main islands of the Cape Verde archipelago

Summary

Introduction

The greater part of our planet is covered with large masses of salty water, and both the planet surface and the salty water are subject to the natural hydrological cycle. Water is a renewable good that may be considered practically inexhaustible on earth. With respect to fresh water, due to various factors, there are huge asymmetries on earth, either in time as in space, and this vital good may be extremely scarce in many places for long periods of time. If effective measures are not taken, disequilibrium between demands for water—as a condition of human activities—and the natural recharge of existing bodies of fresh water may lead to the depletion of water resources, from the surface or from underground aquifers, which may result in possible desertification in the medium to long term.

Methods

Results

Conclusion