1. Arctic terrestrial water storage changes from GRACE satellite estimates and a land surface hydrology model Fengge Su a Douglas E. Alsdorf b, C.K. Shum b Dennis P. Lettenmaier a a University of Washington, Seattle, WA b The Ohio State University, Columbus, OH

2. Background Since 2002, the The Gravity Recovery And Climate Experiment (GRACE) satellite mission has provided a basis for estimating spatial and temporal variations of global terrestrial water storage. Evaluation of the accuracy of the GRACE terrestrial water storage is complicated by the absence of direct observations of terrestrial water storage. Land surface hydrology models, forced with observations, provide an opportunity for evaluating GRACE estimates regionally and globally.

3. Methodology GRACE provides: anomaly of monthly total water storage (TWS) GRACE-derived TWS Change (TWSC): Hydrology model: We use Eq(3) and Eq(4) calculated from output of hydrology model to compare with GRACE TWS and GRACE-derived TWSC, Eq(1). (2) (3) (4) (1)

4. GRACE Data GRACE data are available from three science processing centers: CSR, GFZ, JPL http://grace.jpl.nasa.gov/data/mass/ http://grace.jpl.nasa.gov/data/mass/ We also use the data from OSU (Ohio State University). Here we use data from at smoothing radii of 300 km. Data length: 2002.8-2008.8 GRACE resolution: 1°×1°

5. Hydrology Model VIC large scale land surface hydrology model. Study area: pan-Arctic Simulation period: 2002-2007 Model resolution: 100 km × 100 km Precipitation forcing: GPCP 1dd (http://ftp.ncdc.noaa.gov/pub/data/gpcp/1dd/doc/ )http://ftp.ncdc.noaa.gov/pub/data/gpcp/1dd/doc/ Temp and Wind: NCEP/NCAR reanalysis

6. Land mask of pan-Arctic Mackenzie Lena Yenisei Ob

7. Monthly anomalies of basin- averaged TWS estimated from GRACE and VIC GRACE VIC Lena Yenisei Ob Mackenzie The two estimates show good correspondence in seasonal and inter-annual variation, however the VIC model shows greater amplitude. Monthly anomalies of TWS R 2 =0.79 R 2 =0.93 R 2 =0.67 R 2 =0.86

8. Monthly basin- averaged total water storage change (TWSC) from VIC and GRACE. VIC GRACE Estimates of TWSC from GRACE and VIC track each other fairly well in the seasonal variation. Lena Yenisei Ob Mackenzie Monthly TWSC for the four major Arctic river basins R 2 =0.75 R 2 =0.64 R 2 =0.73 R 2 =0.58

9. Monthly TWS and TWSC over the entire pan-Arctic GRACE VIC R 2 =0.77 R 2 =0.88

10. Annual cycle of monthly basin- averaged P, E, R and TWSC. Negative TWSC in warm season is mostly influenced by E and R; positive TWSC in cold season is mostly influence by P. VIC TWSC GRACE TWSC Evap Runoff Precip

11. Spatial patterns of mean monthly TWSC from GRACE and VIC over the pan-Arctic GRACE VIC

12. Spatial fields of VIC simulated SWE Snow accumulates starting from October and SWE reaches to the maximum on April. SWE corresponds to the positive TWSC for November-April.

13. Spatial fields of VIC simulated evaporation and runoff Evaporation Runoff

14. Conclusion The difference among the existing GRACE datasets is much smaller than the difference between the GRACE estimates and the VIC model simulation. All GRACE estimates show good correspondence to the VIC model in the spatial and temporal variations of TWS/TWSC over the Arctic river basins, while the VIC model shows greater amplitude. Possible reasons:1) uncertainty from the VIC model input and the model itself, 2) the smoothing effects on the GRACE data (smoothing attenuates the real signals). Spatial patterns of TWSC from both GRACE and VIC over the pan-Arctic show dominant positive signals during winter and negative signals during summer. The spatial pattern of TWSC can be explained by the VIC model simulation: snow mostly contributes the positive TWSC signals for the months November-April; E and R deplete the TWS and contribute to negative signals of TWSC for the months May-September over the pan-Arctic.

15. Acknowledgments Qiuhong Tang (UW) Lei Wang (OSU) Jianbin Duan (OSU)