From the UNIVERSITY OF TEXAS AT AUSTIN and the “not so amazing GRACE” department.
Discrepancies between satellite and global model estimates of land water storage
Research led by The University of Texas at Austin has found that calculations of water storage in many river basins from commonly used global computer models differ markedly from independent storage estimates from GRACE satellites.
The findings, published in the Proceedings of the National Academy of Sciences on Jan. 22, raise questions about global models that have been used in recent years to help assess water resources and potentially influence management decisions.
The study used measurements from GRACE satellites from 2002 to 2014 to determine water storage changes in 186 river basins around the world and compared the results with simulations made by seven commonly used models.
The GRACE satellites, operated by NASA and the German Aerospace Center, measure changes in the force of gravity across the Earth, a value influenced by changes in water storage in an area. The computer models used by government agencies and universities were developed to assess historical and/or scenario-based fluxes in the hydrological cycle, such as stream flow, evapotranspiration and storage changes, including soil moisture and groundwater.
“People are depending more and more on global models to determine projections of the impacts of human water use and climate on water resources,” said lead author Bridget Scanlon, a senior research scientist at the university’s Bureau of Economic Geology. “We are now able to evaluate water storage changes from models with GRACE data, which suggests that the models may underestimate large water storage changes, both large declining and rising storage trends.”
For example, in the Amazon River basin, GRACE data indicate that water storage increased by 41 to 43 cubic kilometers during the study period — the largest increase in water storage of any basin in the world. But most of the models projected huge declines in water storage, with one simulating a loss of 70 cubic kilometers. The model that most closely matched the GRACE data calculated an increase of only 11 cubic kilometers.
In the Ganges River basin, GRACE showed a loss of 12 to 17 cubic kilometers of water per year over the 12-year period — the biggest decrease in water storage measured by the study. The models range between a loss of 7 cubic kilometers and an increase of 7 cubic kilometers.
Overall, the model results calculated a decline in global water storage during the study period, while GRACE data indicate it was on the rise. However, the study notes that while the climate increased water storage globally, humans caused significant declines in certain regions. The study area covered about 63 percent of global land area and excluded Greenland and Antarctica because most of the water in those areas is trapped in glaciers or ice sheets.
The study shows that there are regions where global models need to be improved, said co-author Hannes Müller Schmied, a senior research scientist at Goethe University Frankfurt. The global perspective of the research can help scientists understand why models are performing better in some areas than others, and zero-in on the areas where they’re not matching up with the data.
“Based on the comprehensive assessment of water storage changes, we have important additional information and can now focus on challenging regions and analyze more in-depth which processes need to be included, for example floodplain dynamics and backwater effects in the Amazon, or to be improved upon, like human water extractions in the Ganges River basin,” Müller Schmied said.
The study also notes that scientists should work on improving regional assessments.
“GRACE is great because it highlights the global picture of what’s happening with global water storage, and at a coarse grid-scale it’s really nice to see what’s happening,” Scanlon said. “But ensuring water availability for human consumption and agriculture is in many cases a regional to local issue, and we should put increased emphasis on analyses at this scale by, for example, integrating local data. The specific situation could be much better investigated than with global-scale studies only.”
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computer models predict what the model builders subconsciously believe to be true. otherwise the builders would judge the models to be in error and change them.
in any system where there is more than one possible outcome mother nature is free to chose and no computer yet built can predict the actual outcome.
even a simple system like a coin toss is beyond the ability of the most powerful computer to reliably predict the next toss.
Same theme again,the measurement method is fuzzy,imprecise, we NEED BETTER instruments.Some wonderful speculation can be inferred, but honestly cannot say much using tools we have.Must build better instruments.
Being the conclusion of the scientists and engineers involved.
Political decision,money best spent elsewhere.
Same politicians insist Climate Change TM is priority one, direct threat to civilization.
Frustrated government scientists are pressured to draw policy conclusions based on insufficient data.
Policy based evidence manufacturing results.
Wow! I just cannot fathom this piece of dribble! Comparing a satellite reporting system, with 7 models ( and we know how accurate climate models are!), with no actual comparison to real data? What? Are these people stupid or something? Unless you can back up your findings with falsifiable data, you are just pi$$ing in the wind! Unless I’m missing something about the supposed accuracy of GRACE…………………………!
Ah… Amazing GRACE. Just hope she can avoid Es-GRASP
The ability of GRACE to detect a modest gravity anomaly such as is posited for water storage may be checked by at least two ways. Nether is expensive or requires any additional data collection.
As an afterthought a third option is suggested. It involves fielding a gravimeter, which can be expensive unless slave labor willingly supplied by starving grad students is used.
First, examine GRACE passes over the Bay of Fundy at low tide and high tide and compare the profiles. Because of different orbital passes they are probably not exact duplicates, so one is forced to comparing profiles extracted from fields. DamnifIknow what a GRACE data ‘point’ looks like. Hopefully somewhere in there is some data that might be used to see if GRACE can detect a the change in a body of water that is more or less comparable with the gravity anomaly expected from the stored water in an aquifer. If a difference between the low tide and high tide observations can be seen then there might be hope. If one could take other observations and successfully detect other high and low tide times then there are grounds for even more hope.
The second way is simple. It can be done by anyone with access to modest PC based 2.5 D gravity modeling software. Determine the gravitational effect of a prism of water 100 meters thick, 5 km long and 2 km wide. Add that to a regional gravity profile as observed by GRACE somewhere along the Gulf Coast away from any river draining into the GOM. Calculate profiles of the ‘observed’ field with and without the water prism at increasing heights above the model. Intuition suggests the observed differences will fade into the noise level well below GRACE’s orbital height. However, I am quite willing to be surprised at the answers.both
As I am tapping away here it occurs to me that the UT Geophysics department could unearth one of their Worden gravimeters and head to the field with some fancy new GPS positioning equipment over the aquifer. Very precise location information, especially elevation, is needed to make the corrections necessary to determine the actual gravity value at each station’s location. A series of profiles or better yet a grid of data should be collected during the dry season and the wet season. If one really wanted to tip the budget a series of holes could be drilled and logged with a bore hole gravimeter. After the observations are reduced one could see if any changes are observable in a uncharged vs fully charged aquifer. Intuition and some experience with gravity data collection, reduction, and modelling suggest the effect, if observable at all, will be quite small.
But doing that would mean leaving the comfort of the air condition offices and doing real boots in the mud, feeding the bugs and being very mindful of the cotton mouths field work. I love watching Ph.Ds ankle deep in swampy waters reading instruction manuals while swatting mosquitos.
Not quite sure those Teasips are up for that.
Mosquito bites to itch, real data to reformat and reduce…. Meaningful measurements of something we think we might be able to see. That’s how science moves forward.
Naw, lets just stay in the office and model the number trail GRACE gives us and call it good. Oh, and write up another grant proposal real quick now.
Finally, I remember something called the Eotvos effect as being a real bear to deal with while reducing gravity data collected from a moving object, like a ship or airplane. Would someone who knows more about these things please chime in?
Thanks, Tom Bakewell, sorta-retired geophysicist.