GRACE-based estimates of groundwater variations over North America from 2002 to 2017

Abstract

GRACE-based estimates for groundwater storage (GWS) changes in North America substantially depend upon correction of glacial isostatic adjustment (GIA) effects, which are usually removed with GIA models. In this study, GIA effects are eliminated by employing an independent separation approach with the aid of Global Navigation Satellite System (GNSS) vertical velocity data. Our goal is to provide an independent estimate for monthly GWS changes within North America in 1-degree-grids and their trends over the whole GRACE mission lifetime from April 2002 to June 2017. This estimate is derived from the release-6 version of GRACE monthly level-2 data, GNSS data, land surface models for soil moisture and snow water equivalent, and satellite altimetric lake level data. We find a GWS anomaly in form of an increasing trend in Saskatchewan, which affects the Saskatchewan Province and the states of Montana, North Dakota and Minnesota, and 4 GWS anomalies with declining trends in Nevada, California, Arizona and Texas, respectively. The monthly changes of these GWS anomalies, except for the one in Nevada, are validated by well level data. We provide results for average monthly GWS changes and the trends for the 5 anomalies but also in separate form for the 13 affected states or provinces. The increasing trends of the Saskatchewan GWS anomaly and the affected 3 states are related to increasing precipitation and can be elucidated by the decreasing drought intensity level. On the contrary, the declining trends in GWS can be explained by weakening precipitation and are mostly supported by the increasing drought intensity level in the other 4 anomalies and the affected states, which are Nevada, California, Arizona, New Mexico, Texas, Oklahoma, Kansas, and Colorado. Our estimates of monthly GWS changes and their trends can serve as alternative and beneficial input for the sustainable management of groundwater resources in North America.