How big is your water tank?

How big is your water tank?

I usually ask neighbors this question to figure out the correct size of the water cistern for a house in the area. Most times, few people remember the exact size and respond “the bigger, the better”. Evidently, there is no “one size fits all”, so we took it upon ourselves to find out.

One of the most overlooked factors during architectural projects in the Riviera Maya is rainwater harvesting. Probably because the entire peninsula is irrigated by countless underground rivers that are few feet below our feet.

Due to the construction guidelines in Los Arboles, and in order to limit the exploitation of the subsoil and avoid contamination of aquifers, dwellings here must be able – as far as possible – to get their supply from the rains. In addition, rainwater contains barely any  minerals compared to well water or that distributed by CAPA.

This doesn’t only mean saving costs in water consumption on the short run, but also in the maintenance of equipment and facilities.

We are going to see the 4 factors that must be considered for a correct sizing of a rain collection system:

Provision. In Tulum, the average temperature suffers few variations throughout the calendar but it isn’t so in the case of rainfall. It is usual to accumulate a similar amount in October alone as to what you gather in the entire period from January to April. Especially unpredictable and constantly changing, the weather is a vital factor to consider. Below is an average rainfall graph in our region that will serve as the basis for our calculations.

Demand. It is generally constant, linked to the number of inhabitants in the house and their daily habits of hygiene, cleanliness and consumption. For example, in Mexico, domestic daily water consumption per person according to the OECD in 2016 was around 185 liters (48.9 Gallons), while in the United States it rose to 433 liters (114.4 Gallons). We must also consider the passive consumption generated by the evaporation of water in swimming pools or ponds as well as the irrigation of plants and gardens.

Collection surface. The rains in this region tend to be torrential so you can collect a lot of water in a few minutes. The more square meters of catching area it has, the more water it will have access to. There are also collection systems such as geomembranes that can expand the collection area around the entire construction.

Storage capacity. Due to the variability of the provision, this factor is of paramount importance for the proper calculation of the system. It is necessary to be able to store enough water when the supply exceeds demand to avoid emptying the tanks completely during the dry season. The bigger, the better? Yes, but due to the high cost of digging the soil, it is advisable to calculate properly so as not to waste every penny in the budget.

If we properly understand these 4 factors, we have the basic tools to enter data into the calculation of our system. However, there are other more complex elements that directly affect the accuracy of our calculations, such as the efficiency of the system, the variation of the demand throughout the seasons, the specific rainfall variations or the implementation of water saving systems in our facilities.

After much searching I found a tool that has been a reference for my planning process. It is a spreadsheet made by SueloSolar.com based on the online calculator (which is no longer working) of the Technological Development Unit of the School of Structural Engineering at the University of Warwick (https://warwick.ac.uk/fac/sci/eng/research/grouplist/structural/dtu/rwh/) and that we have translated for the community here.

Water cistern during construction
We decided to build a water cistern with a capacity of up to 40,000 liters to provide water to a family of 4 living full time and without external supply. Time and climate will tell if we are wrong or right! (And we will let you know here)

We hope this tool solves all your questions and helps you plan your system efficiently. If you have any questions using the spreadsheet or want to tell us how your experience with water collection has been at Los Árboles, you can tell us in the comments below.