MV ESTF: Water: Feedback
Mark Gilkey
"Only 1% of water used in MV is sourced in MV, enough to meet 40% of our daily needs."
Marn-Yee tried to clarify this by saying that we COULD meet 40% of our daily needs by pumping groundwater, but she wasn't sure whether this was sustainable.
I did a back-of-the-envelope calculation that suggests this would not be sustainable. Assuming 14 inches of rain per year (the figure Steve Bishop cited during the meeting), approximately 100% of the rainwater that falls on Mountain View would have to go into the groundwater. But of course much of the rain water runs off, or evaporates, or is absorbed by plants and then "breathed back out".
Basing the calculation strictly on what falls as rain within our city limits may not be correct. Some water that falls in the Santa Cruz Mountains may seep into our groundwater (if Los Altos doesn't pump it first), so I can't be sure that we couldn't get 40% of our water from groundwater, but I think it's unlikely. Unless we are darn sure that we can meet 40% of our needs sustainably, I would not tell people that we can meet 40% of our needs from groundwater.
2) You were "criticized" for saying that a 90% reduction in snowpack might mean a 90% reduction in available water because some water that would fall as snow now will fall as rain in the future, and thus X% reduction in snowpack does not mean X% reduction in available water. This is true. However, it is also true that a 90% reduction in precipitation (water and snow combined) can mean a MORE than 90% reduction in runoff, since the amount of water absorbed into the ground and evaporated typically rises less than linearly with the amount of precipitation. (To oversimplify, it's possible for the first 4 inches to all be absorbed, the next 3 inches to evaporate, and the rest to be runoff. In that situation, a reduction from 14 inches to 7 inches means a 100% reduction in runoff, not a 50% reduction. This is an oversimplification because of course timing matters a lot. If future storms are less frequent but more intense, then the ground may absorb less water over a year, and runoff may fall less than proportionally to the reduction in precipitation. As you can see, this is complex, but it's not safe to say (as your "critics" said) that a 50-90% reduction in snowpack means that the actual reduction in runoff will be less than 50-90%.)
3) The water in "purple pipes" should be at a lower pressure than the pressure of water in potable water pipes. That way, even if both the potable water pipe and the graywater/purple pipe break, it's unlikely that "dirty" water will seep into the "clean" water pipes. This is not something that needs to be in your top 10 recommendations; it's appropriate for the fine print in the appendix.
4) Someone said that we have a multi-layer aquifer, and that one layer is at least 600 feet below the land surface. That would put it at least 500 feet below sea level, which suggests that unless we very reliably pump at less than the recharge rate, we risk not only soil compaction, but also saltwater (sea/bay water) intrusion.
--mgilkey
Mark Gilkey
MV ESTF
at-large member (SQoL and Baseline)
and Water Workgroup Watcher
Elizabeth Sarmiento
Hello Water Team:
Have you listen, watch the latest regarding water conservation?
KRON TV interviewed Director Kamei regarding yesterday’s board decision asking staff to work with retailers and develop a mandatory conservation program. Meenakshi did interviews with KCBS and KLIV radio, while Marty Grimes (PR Dir.) was interviewed by KDTV 14 Univision regarding the mandatory conservation plan.
The messages were: Because of two successive dry years and reduced supplies through the Delta, our demand has exceeded the annual water supply and we are reaching into our reserves to make up for the difference. In June 2008, the board had called for a 10% voluntary cutback in water use, but we have seen only 3%-4% reduction. In view of the declining reserves, reduced supply through the Delta and a possibility of a third dry year, the board has asked staff to work with retailers and develop a mandatory conservation program and bring it to the board in August. The message for the public is to seriously step up conservation to reach the 10% voluntary conservation mark and that the water district can help people reach that goal, through various conservation programs, incentives and rebates. Urged public to contact district conservation hotline or visit our website to find more (www.valleywater.org). In addition, Marty also explained that any new conservation rules and fines would be imposed by the cities, not the district. In the past, there have been restrictions such as prohibitions on daytime watering, car washing with hoses and washing down patios with water.
Regards,
Elizabeth Sarmiento
Water Use Efficiency Unit
5750 Almaden Expwy
San Jose, CA 95118-3686
(408) 265-2607 x 3140
Email: esarm...@valleywater.org