Addressing Intermittent Water Supply in Mexico City through Optimal Household Water Portfolios

Less than 30% of the approximately 20 million inhabitants of the Mexico City Metropolitan Area receive continuous (24-hour) water service. 14 million people are subject to inconsistent water supply and nearly 20% (4 million people) do not receive daily water delivery. Households in Mexico City have adapted by installing water storage tanks at their homes to store water during periods when city-supplied water is unavailable. These tanks give residents other water supply options, such as purchasing additional bulk water from water trucks (pipas) and capturing rainwater. The reliability of water supply at the household level is therefore dependent not only on the availability of the municipal water supply, but also each household's decisions on managing their storage and water supply portfolios. Modeling these household decisions (or control policies) is challenging due to their dependence on unique types of input variables (e.g. intermittent water supply schedule, season, day of week). Universal approximators provide a solution to this challenge by allowing for flexible shaping of these control polices.

This study uses Radial Basis Functions to determine optimal household water management decisions which maximize reliability of water supply while minimizing costs. This model is then used to explore the effects of changing conditions such as water supply schedule, water storage tank size, and rainfall patterns, on a household's cost and reliability of water delivery. Pareto optimal solution sets are then explored to understand what household-level investments (e.g. additional storage, rainwater collection) are most cost effective for improving the reliability of access to water. Preliminary results have shown that investing in larger storage tanks can lead to lower water costs overall, especially in households with very low access to municipal water supply, up to a tank size of around 1500-2500 liters depending on the availability of municipal water. Results have also shown that rainwater harvesting systems are a good investment under all municipal water supply scenarios and may provide households with annual savings of up to $500 USD per year. Study results will provide important insights into the question of how Mexico City as a whole may equitably improve the water security of its residents.