Monday, April 5, 2010

Minima and maxima constraints on rainwater harvesting


In a previous blog, there was a mention of how it might be more prudent to establish what are the limits to up-scaling rainwater harvesting.

The issue is essentially that harvesting superimposes an artificial catchement connected to cisterns on top of a real catchment where environmental uses require a minimum of water to remain healthy.

A paper by Stephen Ngigi, «What is the limit of up-scaling rainwater harvesting in a river basin?», examine this question... and concludes that there is a clear need for additional research to understand the effects of rain harvesting up-scaling. Nonetheless, Ngigi states that:

«Increased withdrawals of water in rainfed and irrigated agriculture may have negative implications on water availability to sustain hydro-ecological ecosystem services. The expected shifts in water flows in the water balance would affect both nature and economic sectors depending on direct water withdrawals (Rockstrom et al., 2001). Upgrading rainfed agriculture through RWH that enables dry spells mitigation, would involve the addition of water, through storage of runoff, to the rainfed system. The cumulative effect of RWH may have an impact on downstream water availability within a river basin scale. The effects are bound to be site specific and need to be studied further (Rockstrom et al., 2001).»

In a more recent article, «Identifying Barriers to Widespread Implementation of Rainwater Harvesting for Urban Household Use in Ontario», Chantelle Lidi et al. identify barriers faced in implementing rainwater harvesting. The most significant barriers are as follows:

«Initial capital cost, liability for potential health risks, limitations on the end use of rainwater, the Building Code’s poor differentiation between rainwater, greywater and non-potable water, and a lack of public environmental commitment. Health risks would be a paramount concern for public health officials, but were only a moderate concern for the majority of building practitioners interviewed.»

Rainwater harvesting is certainly a solution to some water management problems and a priori a sustainable practice.

However, a familiar worrying pattern seems to appear: constraints to up-scaling are identified, but limits to development remain unknown.

Would precaution and prevention demand that limits be identified before going forward with rainwater harvesting on a large scale?

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