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yournamehere · 04-26-2009, 09:14 AM

I think that's the benefit (or lack of) of rainwater harvesting that drove the O.P., right, Will?

If you take a fairly well vegetated piece of undeveloped land, it has a runoff coefficient of between .2 and .5; depending on soil type, vegetation, and slope. That means that during an "average" rain event, 20% to 50% of the rain will drain off the property.

Once you develop that land, and cover it with buildings, parking lots, sidewalks, and patios, you're changing the runoff coefficient of much of that property to around .8 or .9; meaning that up to 90% of the rain will now drain offsite. This has a significant accumulative effect on not only downstream properties, but also on the stability and sedimentation of the watercourses that deliver that runoff.

On one hand, it can cause severe erosion and flooding; on the other hand, it can supply a free source of water for the operation of water treatment plants, or - as is the case in Phoenix - water for the cooling towers of a nuclear power plant. Or - it can simply find its way hundreds of miles downstream to a reservoir or river, where it will eventually be collected by the powers that be, and sold as a commodity.

It is economics - nobody wants their customers to get their product for free. On a residential site, approximately half of all water use is for outdoors, where rainwater could fulfill the needs even better than what comes out of the meter. How do you think Walmart would feel if half their products were stacked out in the parking lot; free for the taking?

04-26-2009, 09:14 AM	#16 (permalink)
yournamehere Invisible Location: tentative, at best	I think that's the benefit (or lack of) of rainwater harvesting that drove the O.P., right, Will? If you take a fairly well vegetated piece of undeveloped land, it has a runoff coefficient of between .2 and .5; depending on soil type, vegetation, and slope. That means that during an "average" rain event, 20% to 50% of the rain will drain off the property. Once you develop that land, and cover it with buildings, parking lots, sidewalks, and patios, you're changing the runoff coefficient of much of that property to around .8 or .9; meaning that up to 90% of the rain will now drain offsite. This has a significant accumulative effect on not only downstream properties, but also on the stability and sedimentation of the watercourses that deliver that runoff. On one hand, it can cause severe erosion and flooding; on the other hand, it can supply a free source of water for the operation of water treatment plants, or - as is the case in Phoenix - water for the cooling towers of a nuclear power plant. Or - it can simply find its way hundreds of miles downstream to a reservoir or river, where it will eventually be collected by the powers that be, and sold as a commodity. It is economics - nobody wants their customers to get their product for free. On a residential site, approximately half of all water use is for outdoors, where rainwater could fulfill the needs even better than what comes out of the meter. How do you think Walmart would feel if half their products were stacked out in the parking lot; free for the taking? __________________ If you want to avoid 95% of internet spelling errors: "If your ridiculous pants are too loose, you're definitely going to lose them. Tell your two loser friends over there that they're going to lose theirs, too." It won't hurt your fashion sense, either.