A Rainwater Harvesting System based Blueprint for Excess Runoff Management on Caribbean Small Island Developing States

Executive Summary Freshwater is essential to public health, food security, livelihoods, and healthy and resilient ecosystems. However, water scarcity is amongst the major problems faced by societies today. Drinkwater supply on islands is an even more complex matter as all naturally occurring freshwater on SIDS originates from precipitation. Many SIDS are located in semi-arid areas north and south of the equator, these experience high losses of surface water through evaporation. These high evaporation losses make groundwater the prime freshwater source in many SIDS. Insufficient recharge of groundwater will result in a shrinking freshwater buffer and cause water stress. Due to its position relative to the sea, groundwater storage is difficult. Anthropogenic forces, such as an increase in potable water demand, triggered by prosperity growth and tourism, will put an even greater pressure on its freshwater supplies. Therefore, RWH can be used to collect rainwater from artificial surfaces and sometimes natural surfaces as a resource to be used for potable supply of non-drinking purposes.

The Caribbean island of Curaçao is a typical example of a small island where a combination of natural and anthropogenic factors leads to insufficient groundwater recharge. Within Curacao’s NDP 2015- 2030 a long-term vision for the change of Curaçao has been established. The objective is to create a liveable and sustainable Curaçao. In the context of "water", this translates to the use of water for the development of the island, while ensuring the protection of water as a natural resource. During this research it was investigated how MRWH can serve to buffer storm water runoff to minimize flooding and erosion. The stored water can additionally be reused for domestic nonpotable purposes. The research was in fact a pilot to see whether MRWH offers a solution against both the flood-related problems during the wet season and the water shortage-related problems during the dry season.

The CN Method has been used to determine how much water is available as excess runoff from intense rainfall events. The outcome was compared with CER’s annual water demand, after which it was determined what percentage of their water consumption can be covered. Based on the comparison, it has been determined that the total capacity of the MRWH system to be implemented must be 5000 cubic meters. Wherein, four reservoirs, of different sizes (300, 700, 1000 and 3000 cubic meters), will be implemented alongside the access road. An average of 45% of CER's water consumption can be covered annually, where it serves non-potable purposes. This results in annual cost savings of approximately EUR 32,000.

In addition, it has been calculated how much excess runoff can be buffered, so that flood-related problems are reduced. A reduction of at least 50% of runoff by road is ensured, which can increase to a 100% runoff reduction in case of smaller rainfall events. Considering CER’s total catchment area, at least 8% of runoff can be buffered, which can increase to 20% when smaller events occur. Despite these percentages, it can be stated that the implementation of this MRWH system is not economically feasible and profitable due to its long payback period. However, this does not include feasibility regarding flood reduction. By including the costs of damage suffered and the costs incurred as a result of the excess runoff in the consideration, a different conclusion may be stated. This could be determined by further in-depth research.

Opportunities exist for MRWH in other parts of the Caribbean because these systems are less costly than concrete and metal storage systems; have an enormous storage capacity; and by means of its multifunctionality, a larger market can be reached through large-scale application. However, these large scale systems are not yet feasible due to their current price structure. It is possible that if used on a smaller scale, feasibility will increase.

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