B.5 - Hydrology (Convener: Matthew Rodell)

Difficulties in Assessing Reliability of Groundwater Storage Changes from GRACE Satellite Data
(Scanlon, Longuevergne, Wilson)

Increasing groundwater storage in discontinuous permafrost areas in the Lena river basin, Eurasia, detected from GRACE
(Velicogna, Tong, Zhang, Kimball)

Excessive Groundwater Depletion in Southern Murray-Darling Basin From Satellite Gravimetry and In Situ Observations
(Chen)

The Use of GRACE Data to Identify Spatial and Temporal Trends in Water Storage Across Africa
(Ahmed, Wahr, Yan, Milewski, Mohsen, Chouinard)

GRACE detection of water storage changes in the Three Gorges Reservoir of China and comparison with in situ measurements
(Wang, de Linage, Famiglietti, Zender)

Recent GRACE-Enabled Hydrologic Research and Applications
(Rodell, Famiglietti, McWilliams, Beaudoing, Li, Forman)

Comparison of a GRACE Hydrology Product to Observations
(Castle, Rodell, Swenson, Chambers, Lo, McWilliams, Famiglietti)

Independent patterns of water mass anomalies over Australia from satellite data and model
(Forootan, Awange, Kusche, Heck)

Session: B.5 - Hydrology
Title: Difficulties in Assessing Reliability of Groundwater Storage Changes from GRACE Satellite Data
First Author: Bridget Scanlon
Presenter: Bridget Scanlon
Co-Authors: Longuevergne, Laurent; Wilson, Clark

Abstract: There is increasing interest in use of GRACE satellite data to monitor changes in groundwater storage. The objective of this study was to assess the reliability of storage change estimates from GRACE using comparison with ground based data. Groundwater basins with large signals from groundwater depletion for irrigation were selected and include the High Plains and Central Valley aquifers in the US. The High Plains aquifer is the most straightforward because it is large (450,000 km2), and has only two storage components (soil moisture and groundwater). The aquifer is unconfined; therefore, converting from water levels to storage is relatively straightforward. Good correlations were found between GWS from GRACE and ground-based estimates from 2002 - 2007. The Central Valley aquifer is much more difficult to analyze because it is much smaller (52,000 km2), and has a large variety of storage components (snow, surface water, soil moisture, and groundwater) with different spatial distribution. The aquifer varies from unconfined to confined; therefore, storage coefficients vary by orders of magnitude. By processing the larger river basins that include the Central Valley increases the area to 154,000 km2. Snow data are available from SNODAS and surface water reservoirs are monitored directly. Groundwater levels are monitored; however, it is difficult to determine which wells are penetrating unconfined, semiconfined, or confined portions of the aquifer, increasing uncertainties in water storage estimates from water level data. Good correspondence was found between water storage changes from GRACE (25 - 26 km3) and ground-based estimates for the 2006 - 2009 drought (23 km3). The North China Plain has similar problems as the Central Valley, small aquifer (140,000 km2), neighboring Taihang Mountains with snow, inflow from mountains, aquifers ranging from unconfined to confined, and local groundwater depletion in municipal areas. Assessing water storage changes in the NCP is also complicated by irrigation from the Yellow River in the south, increasing groundwater to the north. Improving quantification of groundwater storage changes will require quantification of inputs and outputs to aquifers, incorporating irrigation into land surface models to provide reliable data on soil moisture storage changes, and improved estimates of groundwater storage from water level or potentiometric surface data. While some suggest that satellite data may replace ground-based monitoring, reliable estimates of groundwater storage require strong integration of satellite and ground-based data and modeling analyses.

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Session: B.5 - Hydrology
Title: Increasing groundwater storage in discontinuous permafrost areas in the Lena river basin, Eurasia, detected from GRACE
First Author: Isabella Velicogna
Presenter: Isabella Velicogna
Co-Authors: J. Tong, T. Zhang, J. Kimball

Abstract: We use monthly measurements of time-variable gravity from the GRACE (Gravity Recovery and Climate Experiment) satellite mission to determine the increase in terrestrial water storage (TWS) in the Lena river basin, Eurasia, during the period between April 2002 and September 2010. We estimate a TWS increase of 32 10 km^3/yr for the entire basin. We compare monthly TWS from GRACE with TWS from time series of precipitation (P) minus evapotranspiration (ET), from ERA-Interim re-analysis and observational river discharge (R). We find an excellent agreement between the two time series. If we account for a negative bias in P-ET of 7% of the average annual precipitation during the analyzed period, the two time series agree to within 12%, and we effectively close the terrestrial water budget. From this comparison, we attribute both the increase in R and in TWS to an increase in P. The TWS increase is dominated by a large signal in the central Lena basin centered at 120E and 61.5N, in an area of discontinuous permafrost. We attribute the observed signal to an increase in groundwater storage of 6819 km^3 or to the surface water recharging the ground water through areas not underlain by permafrost over the study area, while changes in active layer thickness have likely less impact. This will have a significant impact on the terrestrial hydrology of the region, including increased baseflow and alteration of seasonal runoff.

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Session: B.5 - Hydrology
Title: Excessive Groundwater Depletion in Southern Murray-Darling Basin From Satellite Gravimetry and In Situ Observations
First Author: Jianli Chen
Presenter: Jianli Chen
Co-Authors:

Abstract: Satellite gravity data from the Gravity Recovery and Climate Experiment (GRACE) provides quantitative measures of terrestrial water storage (TWS) change at large spatial scales. Combining GRACE TWS observations and model estimates of water storage changes in soil and snow at the surface, we show that groundwater storage in southern Murray-Darling Basin (MDB) and surrounding regions in Australia has decreased significantly, up to 18.9 ± 9 Gt/yr over the period 2003 and 2010. Comparisons with precipitation observations and model estimated soil and snow water storage change suggest that this significant groundwater depletion is primarily related to excessive groundwater pumping for agricultural and domestic consumptions. The significant groundwater depletion in southern MDB observed by GRACE is consistent with available in situ groundwater level observations from wells in that region. Analysis of in situ well groundwater level measurements indicate that the mean groundwater level in southern MDB and surrounding regions, in particular in the Victoria territory has been declining steadily since around mid 90's, causing a great constraint on the economic development in the region.

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Session: B.5 - Hydrology
Title: The Use of GRACE Data to Identify Spatial and Temporal Trends in Water Storage Across Africa
First Author: Mohamed Sultan
Presenter: Mohamed Sultan
Co-Authors: Mohamed Ahmed; John Wahr; Eugene Yan; Adam Milewski; Fadi Mohsen; Kyle Chouinard

Abstract: The Gravity Recovery and Climate Experiment (GRACE) data was used for monitoring spatial and temporal trends in water storage variability over a time period of nine years for the purpose of identifying current trends in water storage availability across the African continent and for making futuristic projections. Monthly GRACE gravity field solutions (Center of Space Research [CSR] RL04) in the form of Spherical Harmonic Coefficients (SHC's) that span the period from April 2002 through November 2010 were processed (temporal mean was removed, de-striped, smoothed [250 km; Gaussian], and converted to 0.5° x 0.5° equivalent water thicknesses) over the African continent and surroundings. Several GRACE bi-products (e.g., standard deviation, annual trend) were generated over time periods of six, seven, eight, and nine years. The GRACE bi-products were then analyzed in a GIS environment together with relevant co-registered data sets and derived products (e.g., precipitation, topography, geology, VNIR Landsat, NDVI, stream network distribution, water bodies distribution, watershed boundaries, etc). The following were observed: (1) large areas showed persistent and increasingly pronounced (in spatial extent and magnitude) statistically significant (linear fitting) trends (+ve: increasing mass; -ve: decreasing mass) in maximum annual (average for three consecutive months) GRACE gravity field solutions over periods of six, seven, eight, and nine years; (2) +ve trends were observed over the source areas for the Blue Nile basin (4.2 mm/yr), Zambezi basin (24 mm/yr) and the southern source areas of the Congo basin (7 mm/yr) and vice versa for the source areas of the White Nile basin (-2 mm/yr) and northern source areas of the Niger Basin (-1 mm/yr); (3) +ve trends over the southernmost domains affected by the Intertropical Convergence Zone (ITCZ) and over the coastal areas of central west Africa (12 mm/yr), and vice versa for areas proximal to the northern boundary of the ITCZ (-2 mm/yr); and (4) -ve trends over the African deserts (e.g., Saharan belt, Kalahari, Namibia; -3 mm/yr). We interpret these observations to indicate: (1) mass variations are related to variations in storage in one or more of these reservoirs: soil column, shallow aquifer, and deep aquifers; (2) areas showing +ve trends are here interpreted to indicate increasing water availability with time and vice versa for areas showing -ve trends; (3) the advanced methodologies using GRACE data could provide practical, and cost-effective methods for predicting water availability on local (e.g., source area) and/or regional (e.g., Sahara or Kalahari deserts); and (3) observed trends are probably caused by global warming-related rise in Sea Surface Temperature (SST) which affects the intensity of Indian and Atlantic monsoons, the position and magnitude of the ITCZ, and the precipitation patterns over Africa on decadal scales.

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Session: B.5 - Hydrology
Title: GRACE detection of water storage changes in the Three Gorges Reservoir of China and comparison with in situ measurements
First Author: Xianwei Wang
Presenter: Caroline de Linage
Co-Authors: C. de Linage; J.S. Famiglietti; C.S. Zender

Abstract: Water impoundment in the Three Gorges Reservoir (TGR) of China caused a large mass redistribution from the oceans to a concentrated land area in a short time period. We show that this mass shift is captured by the Gravity Recovery and Climate Experiment (GRACE) unconstrained global solutions of the Center for Space Research at a 400-km spatial resolution after removing correlated errors. The WaterGAP Global Hydrology Model (WGHM) is selected to isolate the TGR contribution from regional water storage changes. For the first time, this study compares the GRACE (minus WGHM)-estimated TGR volume changes with in situ measurements from April 2002 to May 2010 at a monthly time-scale. On the long run, GRACE-WGHM estimated TGR volume changes show an increasing trend consistent with the TGR in situ measurements during the study period, and lead to similar estimates of impounded water volume. GRACE-WGHM estimated total volume increase agrees within 14% (3.2 km3) with in situ data. This indicates that GRACE can retrieve the true amplitudes of large surface water storage changes in a concentrated area that is much smaller than its global harmonic solutions. The GRACE-WGHM estimated TGR monthly volume changes explain 76% (r2 =0.76) of in situ measurement monthly variability for the months of October to December and have an associated uncertainty of 4.62 km3. The agreement can be improved (r2 =0.91, error=3.24 km3) by filtering out the high frequency noise using a Singular Spectrum Analysis. Our results also indicate reservoir leakage and groundwater recharge due to TGR filling and contamination from neighboring lakes are non-negligible in the GRACE total water storage changes.

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Session: B.5 - Hydrology
Title: Recent GRACE-Enabled Hydrologic Research and Applications
First Author: Matthew Rodell
Presenter: Matthew Rodell
Co-Authors: J.S. Famiglietti; E. McWilliams; H.K. Beaudoing; B. Li; B. Forman

Abstract: This presentation will describe recent results of hydrologic research and applications projects which make use of GRACE data. These will include the following: 1) water balance estimates of evapotranspiration over several large river basins; 2) a summary of NASA's Energy and Water Cycle Study (NEWS) state of the global water budget project; 3) drought indicator products now being incorporated into the U.S. Drought Monitor; 4) new GRACE data assimilation results over several regions.

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Session: B.5 - Hydrology
Title: Comparison of a GRACE Hydrology Product to Observations
First Author: Stephanie Castle
Presenter: Stephanie Castle
Co-Authors: S. Castle, M. Rodell, S. Swenson, D. Chambers, M. Lo, E. McWilliams and J. Famiglietti

Abstract: GRACE data are currently being used on multiple spatial scales to assess changes in terrestrial water storage across the globe. However, because several processing centers release different GRACE products, there is confusion amongst the hydrologic community when selecting the appropriate product for use in a particular research problem. This has already resulted in end users arbitrarily applying products that are not optimal for their specific applications. Recently the GSTM has advocated for a 'hydrology-specific' gridded, scaled water storage dataset which removes some of the uncertainty associated with proper product selection. In this study we evaluate one of these, from S. C. Swenson, which is publicly available on the GRACE Tellus website. We compare this product to coupled land-atmosphere water balance estimates of total water storage calculated using MERRA atmospheric convergence and GRDC runoff data for several river basins.

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Session: B.5 - Hydrology
Title: Independent patterns of water mass anomalies over Australia from satellite data and mode
First Author: Ehsan Forootan
Presenter: Joseph-L Awange
Co-Authors: J.L. Awange; J. Kusche; B. Heck

Abstract: The Gravity Recovery and Climate Experiment (GRACE) satellites provide valuable information on water variations over the globe. However, the gravitational potential changes measured by GRACE represent mass change integrated over vertical columns requiring to be separated into their original sources. Such separation is vital for proper interpretation of GRACE-hydrological signals specifically in regions such as Australia that suffers from weaker hydrological signal, surrounding oceanic leakage and the propagating scheme of the annual cycle.

The principal component analysis (PCA) separation method, which has been widely used to separate the GRACE derived hydrological signals for Australia, suffers from inability to assign the hydrological signals into their provenance sources. This is due to the fact that PCA uses only the second order statistical information contained in the correlation matrix of data to explain the maximum amount of variance in the dataset, which is not well suited for separating the GRACE hydrological signals.

Independent component analysis (ICA) method that uses higher order statistics is applied here to extract Australia's GARCE-derived hydrological signals. GRACE-data are obtained from three sources: GFZ, CSR and ITG2010 (from Bonn University). To validate the results, WaterGAP Global Hydrology Model and Tropical Rainfall Measuring Mission (TRMM) rainfall dataset are employed.

Compared to the PCA, the ICA results show a remarkable improvement in separating the hydrological signals into their regional sources, what so far was unachievable by PCA. The hydrological signals are separated from the surrounding oceanic signals. The annual and inter-annual cycles are also localized into their original areas. The impact of the 2006-2007 droughts on Australian water resource is clearly identified in the southern-Australia, Murray Darling Basin and the Western Australia regions, while the recent 2010-2011 floods are also visible in eastern Australia. The oceanic annual and S2 aliased pattern are also separated into the individual modes.

Keywords: PCA, ICA, GRACE, Australian hydrological signal

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