The Region of Waterloo has taken to organizing Rain Barrel Distribution Events every April. Over the course of eight years, they've sold 34,000 rain barrels to interested citizens at a cost of $30 each. Overall, it appears to be a fairly productive project for the municipality to undertake.
Source: Region of Waterloo
Most rainwater harvesting in Canada appears to make similar use of the roof catchment systems used by the rain barrels being sold here in the region. The barrels are used for the collection and storage of rainwater from the roof, mainly for gardening. However, there is so much potential within the realm of this basic technology that should be explored beyond simple suburban lawn maintenance. The good news is it is being explored on a larger scale, with many different end purposes in mind.
In case you were unaware, the vast majority of our buildings are designed with roofs. In climates where it can be a problem, polling of precipitation is avoided by sloping roofs; furthermore, most buildings feature gutters that exist for the purpose of moving water from the rooftop to a preferable location away from the building.
Incidentally, the roof catchment rain harvesting system is nearly identical to these systems as they already exist: the addition of a tank for storage of the rainwater makes the most basic household water harvesting system possible. Additions to this design are predicated on the intended purpose of the system- for example, outdoor rain barrels can feature a filter to prevent leaves and debris from entering the barrel, which could lead to increased maintenance requirements. It also prevents mosquitoes from using the standing water as a breeding ground, which eliminates the possibility of disease being spread.
In terms of larger scale rainwater harvesting systems, we have several options for exploration. Based in British Columbia, The Rainwater Connection is a company that specializes in the development of personalized rainwater collection systems for homes that only have access to wells with high contaminant levels.
An example of the systems they create is the house on Galiano Island, which is fitted with leaf traps, ultraviolet filters, and a 15,000 gallon storage cistern.
Source: Rainwater Connection
This allows the household to use solely rainwater for all tasks, including toilet flushing, which accounts for 30% of the water used in Canadian households. Having a wet waste disposal system is extremely beneficial, for obvious reasons.
This system is expensive, as you would expect. However, cheaper alternatives exist- the Global Water Challenge has installed several roof collection systems on schools in Tanzania for hand washing at far cheaper cost. While the filtration systems used are not as intricate as those of The Rainwater Connection, the fact is that with little investment, these systems work.
Rainwater and Subsistence
This second story rooftop garden is watered by sub-irrigation with a rain barrel watering system. The rainwater falling on the roof is diverted into a series of several rain barrels in the basement of the building where it is stored until the water is required, at which point a pump is activated to bring the water onto the roof. This is a relatively small-scale system costing around $700, which is admittedly not incredibly cheap, but the value of the system is the opportunities it holds for urban agriculture- being able to grow the food you eat within the city, defying any reliance upon external farms and shipping fees, is vital to the concept of subsistence.
Furthermore, consider lavatories running entirely on rainwater collected on roof and fed downwards. The Rainwater Connection has altered at least one set of public washrooms to completely make use of rain harvesters. An larger system isn't inconceivable, as long as the waste has a destination.
There would be technical issues- preventing debris and leaves from entering the is a main issue that must be avoided, and filtration and purification systems may also be a requirement, but these issues are not particularly difficult to address.
Feasibility should also be touched upon: Peru wouldn't benefit, but there are areas that could: Kowloon Walled City, as an old example in a region that receives 2500-2800 mm of rain per year. The city was approximately 6.5 acres in size, so assuming a reasonable amount of rain- 2600 mm- approximately 68 million liters of water falls on this patch of land every year (The average Chinese individual uses 86 liters of water daily, though subsistence conditions would anticipate less.) Not all locations receive as much rain as Hong Kong, obviously, but even in locations that receive 200 to 300 mm of rain a year could make use of rainwater harvesting to supplement further water sources.
The simple fact is that the systems are simple, easy enough to construct, and extremely beneficial for obtaining water for many purposes without having to rely on traditional infrastructure for access. In a subsistence urbanity, this is a vital building building block.