A New Hybrid Method for Estimating Hydrologically Induced Vertical Deformation From GRACE and a Hydrological Model: An Example From Central North America

Hydrologically induced deformation of Earth's surface can be measured with high precision geodetic techniques, which in turn can be used to study the underlying hydrologic process. For geodetic study of other Earth processes such as tectonic and volcanic deformation, or coastal subsidence and its relation to relative sea level rise and flood risk, hydrological loading may be a source of systematic error, requiring accurate correction. Accurate estimation of the hydrologic loading deformation may require consideration of local as well as regional loading effects. We present a new hybrid approach to this problem, providing a mathematical basis for combining local (near field) and regional to global (far field) loading data with different accuracies and spatial resolutions. We use a high-resolution hydrological model (WGHM) for the near field and GRACE data for the far field. The near field is defined as a spherical cap and its contribution is calculated using numerical evaluation of Green's functions. The far field covers the entire Earth, excluding only the near-field cap. The far-field contribution is calculated using a modified spherical harmonic approach. We test our method with a large GPS data set from central North America. Our new hybrid approach improves fits to GPS-measured vertical displacements, with 25% and 35% average improvement relative to GRACE-only or WGHM-only spherical harmonic solutions. Our hybrid approach can be applied to a wide variety of environmental surface loading problems.