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How good is measurement of water underground? I feel like we simply haven't drilled enough holes to know.

by th0ma5 7y ago

_Microft 7y ago
You don't have to drill holes to monitor ground water. Here's an example for remote measurements: https://en.wikipedia.org/wiki/GRACE_and_GRACE-FO#Oceanography,_hydrology,_and_ice_sheets https://en.wikipedia.org/wiki/GRACE_and_GRACE-FO#Oceanograph...
- mturmon 7y ago
  Yes: Gravimetric measurements from GRACE have been a breakthrough. The press release referenced within your wiki citation is well worth reading in this context: https://www.jpl.nasa.gov/news/news.php?feature=4626 https://www.jpl.nasa.gov/news/news.php?feature=4626 Besides in situ wells, other helpful measurement techniques are InSAR (interferometric SAR) and post-processed GPS. Both let you measure surface subsidence to better than 1cm accuracy over broad areas, mostly due to groundwater withdrawals. This subsidence (which is in the multi-meter range in parts of CA, https://www.usgs.gov/centers/ca-water-ls https://www.usgs.gov/centers/ca-water-ls) is of concern in and of itself, due to disruption of infrastructure like highways and pipelines caused by subsidence. But, subsidence is indirect as a measurement of groundwater withdrawals, because you don't know the relationship between height lost at the surface and volume withdrawn underneath. This depends on a compaction parameter which is very hard to know. Property owners can stymie the use of wells as a groundwater monitoring tool, because it is often not in their direct interest to supply information about local groundwater depletion. For this and other reasons, it's rather hard to use well data, even in well-instrumented areas like California.
jubjubbird 7y ago
In the U.S. it varies widely by state. The USGS has a map of "recent" groundwater levels: https://groundwaterwatch.usgs.gov/default.asp https://groundwaterwatch.usgs.gov/default.asp With some obvious large gaps. There are more collected by state agencies, consultants, etc. In most places there are probably more wells than you'd think. In addition to measuring gravity from space to monitor groundwater it can be measured at the land surface, with better spatial resolution and worse accuracy. A 1x10-8 m/s^2 change in gravity is 2.4 cm of groundwater! https://pubs.usgs.gov/fs/2018/3032/fs20183032.pdf https://pubs.usgs.gov/fs/2018/3032/fs20183032.pdf

How good is measurement of water underground? I feel like we simply haven't drilled enough holes to know.

by th0ma5 7y ago

How good is measurement of water underground? I feel like we simply haven't drilled enough holes to know.

You don't have to drill holes to monitor ground water. Here's an example for remote measurements: https://en.wikipedia.org/wiki/GRACE_and_GRACE-FO#Oceanography,_hydrology,_and_ice_sheets https://en.wikipedia.org/wiki/GRACE_and_GRACE-FO#Oceanograph...

mturmon 7y ago
Yes: Gravimetric measurements from GRACE have been a breakthrough. The press release referenced within your wiki citation is well worth reading in this context: https://www.jpl.nasa.gov/news/news.php?feature=4626 https://www.jpl.nasa.gov/news/news.php?feature=4626 Besides in situ wells, other helpful measurement techniques are InSAR (interferometric SAR) and post-processed GPS. Both let you measure surface subsidence to better than 1cm accuracy over broad areas, mostly due to groundwater withdrawals. This subsidence (which is in the multi-meter range in parts of CA, https://www.usgs.gov/centers/ca-water-ls https://www.usgs.gov/centers/ca-water-ls) is of concern in and of itself, due to disruption of infrastructure like highways and pipelines caused by subsidence. But, subsidence is indirect as a measurement of groundwater withdrawals, because you don't know the relationship between height lost at the surface and volume withdrawn underneath. This depends on a compaction parameter which is very hard to know. Property owners can stymie the use of wells as a groundwater monitoring tool, because it is often not in their direct interest to supply information about local groundwater depletion. For this and other reasons, it's rather hard to use well data, even in well-instrumented areas like California.

jubjubbird 7y ago

In the U.S. it varies widely by state. The USGS has a map of "recent" groundwater levels: https://groundwaterwatch.usgs.gov/default.asp https://groundwaterwatch.usgs.gov/default.asp With some obvious large gaps. There are more collected by state agencies, consultants, etc. In most places there are probably more wells than you'd think. In addition to measuring gravity from space to monitor groundwater it can be measured at the land surface, with better spatial resolution and worse accuracy. A 1x10-8 m/s^2 change in gravity is 2.4 cm of groundwater! https://pubs.usgs.gov/fs/2018/3032/fs20183032.pdf https://pubs.usgs.gov/fs/2018/3032/fs20183032.pdf