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Postglacial rebound and total water storage variations in the Nelson River drainage basin: a gravity-GPS study

GPS, absolute gravity and GRACE satellite data have been combined to map postglacial rebound (glacial isostatic adjustment) and to identify large spatial-scale water storage variations in the Nelson River drainage basin. GPS data from 27 continuous sites over the period 1996-2010 with an
average time-series length of 10.6 years were combined with data from over 50 regularlysurveyed monuments over the period 1995-2011 to produce an updated vertical velocity map for the basin. Annual absolute gravity measurements at eight sites (six co-located with continuous GPS receivers) with an
average time-series length of 15.6 years over the period 1987-2011 were used to quantify the linear relationship between vertical velocity and the rate of change of gravity characteristic of postglacial rebound. The relationship yielded two important results: 1) confirmation that the ITRF2005/2008
is wellaligned to the Earth center of mass, and 2) computation of a 'virtual' gravity rate map of postglacial rebound based on the superior spatial coverage provided by GPS. Monthly GRACE data for the period 2002-2011 complete to spherical harmonic degree and order 60 were used to produce a gravity
rate map of the study area. The availability of an independent, 'virtual' gravity rate map provided a means of removing the postglacial rebound signal from the GRACE map to reveal the annual rate of change in water storage over a 90-month period (2002-2009). A major, long-term, water storage rate
anomaly stretching from Lake Winnipeg to Saskatoon was identified and confirmed by absolute gravity and geological weighing lysimeter measurements. Short-term inter-annual variations (2 - 4 yrs.) detected by GRACE and observed at individual absolute gravity sites are generally well correlated
suggesting that the source of the variations is spatially coherent over a wide area. Surface and satellite gravity data from particular sites in the Winnipeg River and Saskatchewan River subbasins correlate well with river flow rate and deep well lysimeter) observations in the two basins,
respectively, demonstrating the water-storage origin of the gravity variations. Constraints on the
spatial scale of these variations (600 -1000 km) are provided by 1) theoretical calculations, 2) an observed gravity displacement ratio for the Winnipeg River sub-basin, and 3) differences in water-mass sub-basin averages from GRACE. Two practical applications of this study are: 1) an updated
estimate of the rate of reduction in hydraulic head at the Jenpeg generating station of 0.21 m/century as a result of postglacial rebound and lake level regulations, 2) development of a method of tracking long-term changes in water storage for use in flood prediction.

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