No irrational exuberance? What's with the title - no free lunch? Perhaps I should have said, 'No soup for you!' Read on...
Hot off the electronic press in Water Resources Research: Hydrologic Implications of GRACE Satellite Data in the Colorado River Basin, by Bridget R. Scanlon, Zizhan Zhang, Robert C. Reedy, Donald R. Pool, Himanshu Save, Di Long, Jianli Chen, David M. Wolock, Brian D. Conway, and Daniel Winester.
More GRACE in the Colorado Basin! Anything new to report? You bet! But first - the Abstract and Conclusions. You can scroll down past the Conclusions to get to the heart of the matter.
Abstract
Use of GRACE (Gravity Recovery and Climate Experiment) satellites for assessing global waterresources is rapidly expanding. Here we advance application of GRACE satellites by reconstructing long-term total water storage (TWS) changes from ground-based monitoring and modeling data. We applied the approach to the Colorado River Basin which has experienced multiyear intense droughts at decadal intervals. Estimated TWS declined by 94 km3 during 1986–1990 and by 102 km3 during 1998–2004, similar to the TWS depletion recorded by GRACE (47 km3) during 2010–2013. Our analysis indicates that TWS depletion is dominated by reductions in surface reservoir and soil moisture storage in the upper Colorado basin with additional reductions in groundwater storage in the lower basin. Groundwater storage changes are controlled mostly by natural responses to wet and dry cycles and irrigation pumping outside of Colorado River delivery zones based on ground-based water level and gravity data. Water storage changes are controlled primarily by variable water inputs in response to wet and dry cycles rather than increasing water use. Surface reservoir storage buffers supply variability with current reservoir storage representing ~2.5 years of available water use. This study can be used as a template showing how to extend short-term GRACE TWS records and using all available data on storage components of TWS to interpret GRACE data, especially within the context of droughts. [italics mine]
Wonderful - now here are the conclusions:
Conclusions
The Upper and Lower Colorado River basins are hydrologically distinct with 80% of runoff generated in the UCRB supplying reservoir storage primarily in Lake Powell and much greater water use in the LCRB and exports to California. The Basin has been subjected to multiyear intense droughts at approximately decadal intervals in the late 1970s, around 1990, early 2000s, and 2010s with wet periods mostly in the 1980s and 1990s as shown by PDSI. TWS was estimated (TWSe) back to 1980 by summing SnWS, RESS, and SMS in theUCRB plus GWS in the LCRB. In the UCRB TWSe declined by 31 km3 from 1986 to 1990 and by 42 km3 in1998 to 2004 droughts. TWSe depletions are dominated by SMS and RESS. In the LCRB TWSe declined by ~60 km3 for the 1990s and 2000s droughts and is dominated by GWS and SMS in the late 1980s and by GWS followed by RESS and SMS in the 2000s drought. GRACE data show variable trends in TWS throughoutthe 2000s followed by depletion of 27 km3in 2011–2013 in the UCRB and 20 km3 in 2010–2013 in theLCRB. Depletion in the UCRB can be explained mostly by RESS and SMS declines. In the LCRB subtraction ofSMS and RESS components from TWS results in a residual of 15 km3that is attributed to GWS and is similarto GWS declines derived from GW level monitoring data (14 km3). Uncertainties in the residual are large, ranging from 5 to 31 km3 based on different combinations of GRACE products and SMS from various LSMs. Ground-based gravity data show increases in water storage of 2.4 km3 in the LCRB (2002–2009) in the Phoenix Active Management Area and by 2.4 km3 in the Pinal AMA further south consistent with GW level monitoring data and increases in TWS derived from GRACE data during this time. Regional analysis of GW level data indicate that GWS changes in the LCRB are dominated by variations in precipitation during wet and dry periods and irrigation pumpage in areas that do not receive water from the Colorado River. The CRB is dominated by variable water supplies in response to wet and dry periods whereas water use has been relatively stable. Reservoir storage is used to buffer variability in supplies with an estimated ~2.5 years of storage remaining based on current levels of water use. Water storage has expanded from surface reservoirs to aquifer storage through managed aquifer recharge within the past two decades. This study emphasizes the importance of placing GRACE TWS changes in context of longer term hydroclimatic records and using modeling and ground-based monitoring data to isolate different components of TWS from GRACE. [italics mine].
So now you're weondering: didn't you previously post a paper on the use GRACE data in the CRB? Yes, I did - about 17 months ago:
Groundwater Depletion During Drought Threatens Future Water Security of the Colorado River Basin, by Stephanie L. Castle, Brian F. Thomas, John T. Reager, Matthew Rodell, Sean C. Swenson, and James 'Jay' S. Famiglietti
The title of the blog post was indicative of my opinion of the work:
Castle and her colleagues reported large declines in the CRB's groundwater storage over a nine-year period - about 4.5 MAF per year. Scanlon et al. use some ground-truthing to poke a hole in the Castle et al. approach: ~2600 wells (water levels), groundwater pumping records, and ~200 ground-based gravity stations.
My colleague, friend and CRB expert John Fleck, who directed me to the Scanlon et al. paper, said it best:
Scanlon concluded that in the Colorado River’s Upper Basin, the loss of water identified by GRACE came in part from a reduction in reservoir storage, with the rest of the loss of water attributable to the natural loss of soil moisture that occurs across large areas during drought – not groundwater pumping.
In the lower basin, the new paper concludes that aquifer levels are stable or rising beneath the most populated parts of the state – the Phoenix-Tucson corridor – and that the groundwater losses seen by the satellite data are primarily in outlying areas where groundwater is less well regulated (something that is a hot topic of discussion in Arizona right now).
In places like the CRB, where groundwater pumping is relatively small compared to places like the High Plains Aquifer System (HPAS), supporting data are needed to extract groundwater storage changes from GRACE-based total storage change. Scanlon et al. did that, and as they said, their approach is a template for future studies.
This issue points out something the late James V. Taranik, an early proponent and practitioner of remotely-sensed and satellite-based data analysis in the earth sciences, told me over 30 years ago when I was at DRI: (words to this effect):
"There is no free lunch with remotely-sensed data; without ground-truthing, you don't have the full story." - James V. Taranik, c. 1985
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