Role of temporal correlations in the uncertainties of the GRACE hydrological trend

Abstract

This study examines hydrological trend estimations derived from GRACE (Gravity Recovery and Climate Experiment) mascon solutions at the GSFC (Goddard Space Flight Center) in terms of the Equivalent Water Thicknesses (EWT) for the world's major river basins. The estimation of hydrological trends in mascon time series involves two steps: deterministic modeling and stochastic modeling. A deterministic model is characterized as a harmonic regression that incorporates trend and seasonal signals. For stochastic modeling, the EWT observables are typically considered to be equally weighted and uncorrelated, hence the term "white noise-only model". This interpretation, however, is misleading because typical discrete geodetic time series have temporal correlations, which generate colored noise. To explain the role of temporal correlations, we employ two different methodologies. Applying the least squares variance component estimation (LS-VCE) to time series for a combination of colored noise (i.e., flicker noise) and white noise is one of them. The second methodology uses autoregressive noise models to model the time series. Finally, the white noise-only model is compared to stochastic modeling incorporating temporal correlation. The findings show that a white noise-only model in a GRACE time series underestimates the uncertainty of the hydrological trend.