How to plan your rainwater harvesting system
Don’t pray for rain, if you can’t take care of what you get.
—R. E. Dixon (1937) Superintendent, Texas Agricultural Experiment Station, Spur, Texas
Precipitation, in its various forms including rain, is the primary source of freshwater within our planet’s hydrological cycle. It supplies to all the secondary sources of water, including groundwater and surface water in rivers, lakes and ponds. Since precipitation is naturally distilled through evaporation prior to cloud formation, it is also one of our purest sources of water. Rainwater is also considered soft as compared to much of our ground and surface water which is hard due to the presence of calcium and magnesium compounds that dissolve as water runs through or over soil. This makes rainwater excellent for (i) cooking since it improves heat conduction and prolongs the life of water heaters and cooking appliances and (ii) washing and bathing since it reduces detergent and soap requirements, and eliminates soap scum, hardness deposits, and the need for a water softener, besides being a natural hair conditioner.
Yet, unfortunately, our contemporary water management systems invest vast resources acquiring lower-quality, secondary sources of ground and surface water. On the other hand, easily accessible rainwater which should ideally be treated as our primary renewable source of freshwater, is treated as a nuisance - diverting it to the storm drain, drainage ditch or pollutant-laden street.
Transitioning to rainwater as the primary source for domestic water needs can be a simple, first step every one of us can take to contribute to the shift back to rainwater. This journal post aims at giving a very broad overview of considerations to be kept in mind while designing a rooftop rainwater harvesting system for domestic / residential daily requirements. After reading this post, you should be able to plan the broad contours of your rainwater harvesting system by assessing the rainwater harvesting potential of a rooftop based on rainfall data and rooftop area, comparing that with the estimated demand and usage patterns of the inhabitants to help arrive at an optimal storage size to meet those requirements. The journal post also discusses other important design and maintenance considerations.
Planning the rainwater harvesting system
Step 1 - Calculate the rainwater harvesting potential or the “supply” side of your water budget
(a) Identify catchments on your site - this can be a roof, or a solar panel that you may have set up or any other surface that is exposed to the open sky from where a considerable amount of rainwater can be collected. The surface should preferably be impervious.
(b) Determine the runoff coefficient (RC) of your catchments. Every surface that receives rainfall can lose a certain percentage of the water to evaporation, or even retention / absorption by the surface material itself. The more porous or rough your catchment surface, the greater the water loss to these factors. The remaining water that runs off the surface (after factoring in the water loss) is available for collection / storage.
RC = Rainfall that runs off a surface ÷ Rainfall received by that surface
Given below are the approximate RCs of various materials which you can use to estimate your catchment’s RC:
(c) Determine the area of your catchments. Note that the slope of the catchment need not be taken into account, and you can simply calculate the area based on the catchment’s footprint.
(d) Collect and study historical rainfall data of your area. This data can be sourced from the meteorological website of the government or various other sources on the internet. Forecasting the amount of rain can be an inexact science, and therefore, historical data can be a reasonable basis for planning your rainwater harvesting system. While climate change is increasingly causing changes in rainfall patterns which calls into question the reliability of historical data for planning purposes, forecasting rainfall can be even more complicated. It would be good to know the average annual rainfall of your area.
However, usually rainfall is not uniformly distributed throughout the year, and so knowing the following would also be helpful in the planning process (specifically for step 4 i.e. planning the tank size)
the average monthly rainfall patterns in your area;
the duration of the dry season on the one hand; and
on the other hand, the historical frequency and size of storm events (where large volumes of rainfall are experienced in time spans as short as a day or a few days). You could consider the highest rainfall experienced in a day in your area and double it - weather patterns are only becoming more extreme, so something totally out of the ordinary can be expected in the future.
(e) Put it all together and calculate the rainwater harvesting potential of your catchment areas. Refer below to see how that’s done:
Step 2 - Estimate your water demand
In a home, the water demand can vary depending on the habits of the inhabitants. For estimating your water demand, you could read your water meter to find out how much water your household uses on a daily basis.
On an average, a person in India uses 135 litres per day (which includes 5 litres each for drinking and cooking, 55 litres for bathing, 30 litres for flushing toilets, 20 litres for washing clothes and 10 litres each for washing utensils and the house) - you could use this as a reference template and adjust for your usage pattern.
Step 3 - Compare and analyze the “supply” and “demand” side of your rainwater budget
In this step, you check if the annual rainwater harvesting potential of your catchments meet your annual water requirements. If your rainwater catchments are unable to meet your water requirements partially, you could revisit your water requirements schedule to see if there is scope to reduce or eliminate water usage towards certain needs. A part of the reason for harvesting rainwater is to inculcate a sense of stewardship towards the precious resource that rainwater is, and also to build discipline in water usage. It would be ideal if you could limit water usage to an extent that your land alone can sustain rather than depending on external sources.
If you are still falling short of meeting your annual requirements, you would need to explore other water sources that could supplement your rainwater supply to bridge the shortfall. Potential alternative sources you could consider, for example, are open wells to meet your domestic needs and treated greywater or stormwater for irrigation.
Step 4 - Plan storage capacity
The final step in planning your rainwater harvesting system is to arrive at an appropriate and optimal storage size for the rainwater. The objective of this step is to ensure that the tank capacity:
is not so large that there is underutilization of capacity (underutilized capacity costs money and yet does not perform any function)
is not so small that it causes overflow of rainwater that could help in meeting the water demand (it is okay to let any rainwater in excess of the water demand to overflow)
Let’s try understanding tank sizing by continuing with the example we saw under step 1(e) above:
Catchment area = 300 square meters
Average annual rainfall = 900 mm
Runoff coefficient (RC) = 0.9
Average monthly rainfall pattern (which you have collated as part of your study of historical rainfall data):
Let’s also assume that your household consists of 4 people and each person uses 135 litres per day
As part of your tank sizing exercise, you could prepare a simple monthly water budget as below:
The overall annual rainwater that can be harvested by the catchment (i.e. 2,43,000 litres) is greater than the annual water demand (i.e. 1,97,100 litres) by 45,900 litres
The rainfall is not uniform and higher rainfall is harvested during the period between May and November (‘rainy months’), however, the demand is more or less uniform throughout the year. This results in a situation where we have surplus rainwater during the rainy months whereas a deficit during the remaining months.
Our storage capacity should be large enough to accumulate the surplus rainwater received during the rainy season, so that we can use that surplus to meet the deficits during the dry season. But do we really need to hold onto all the surplus (i.e. 1,04,490 litres being the aggregate surplus between May and October) by the end of the rainy season? In our example, this is not necessary since our total deficit during the dry season is only 58,590 litres and therefore, it would be sufficient to hold on to a surplus of 58,590 litres by the end of October (i.e. the end of the rainy season).
However, a storage capacity of 58,590 litres accounts for only the deficits of the dry season that have to be met. During the rainy months, you would not only be storing up water for the coming dry period, but also for the respective month’s water demand (~ 16,740 litres). So ideally, you should have a capacity of 75,330 litres (i.e. 58,590 litres + 16,740 litres). Put in other words, your storage capacity of 75,330 litres would be able to store 16,740 litres of water for use in the respective month (which you would also end up utilising in that month) and would additionally have a capacity of 58,590 litres that would fill up over the rainy season to provide for the dry season deficits.
Note - Due to various reasons (including costs and space constraints), you may be unable to install a storage capacity of 75,330 litres. In such a case, we recommend that you revisit your water demand schedule to see if there is scope to reduce or eliminate water usage towards certain needs. After you have re-estimated your water demands, you could perform this monthly water budgeting again to arrive at a lower optimal storage capacity.
Considerations and protocols for maintaining rainwater quality
Currently, in our homes, water flowing from our tap is pumped from deep ancient aquifers that are not visible to us or is delivered by massive central municipal systems that pump water from afar. We are, thus, disconnected from our source of water. We currently invest vast resources in building massive dam and canal projects to divert water away from other locations and people, just so we can meet our own demands. Diversions and competition for water are contributing to its commodification, as water shifts from a social resource belonging to all life, to an economic commodity bought and sold in bottles, cans and tankers by businesses that profit from the increasing scarcity of water.
By choosing the path of harvesting rainwater locally for our needs, we can connect with our source of water and work towards preventing and reversing commodification of water. Water is part of the Commons, to which we are all entitled. Water scarcity attracts market forces, water abundance does not - we can all begin this journey towards water abundance in our own homes by harvesting rainwater!
Acknowledgements: Rainwater Harvesting for Drylands and Beyond by Brad Lancaster, Designing Rainwater Harvesting Systems by Celeste Allen Novak and Harvesting Rainwater from Buildings by Syed Azizul Haq were invaluable resources for us in putting down this journal post.