GRACE a witness to the Recovery of the Tigris-Euphrates Hydrologic System

Global warming is producing climatic changes across the world that affect in major ways the livelihood of major sectors of the world’s population. Over the past decade or two, an increase in the frequency and intensity of specific climatic phenomena (e.g., hurricanes, wet or dry periods, etc.) has been reported from many parts of the globe and is believed to be climate change-related. Over the past few years, the largest and most intense precipitation events were recorded over the Tigris and Euphrates watershed (TEW), a heavily engineered watershed (> 60 main dams) that is shared by Turkey, Iran, Syria, Saudi Arabia, and Iraq. Analysis of the Global Precipitation Climatology Project (GPCP) precipitation record over the past 40 year (1979-present) across the TEW revealed a prolonged dry period (2002- to 2017; Average Annual Precipitation [AAP]: 240 km³), followed by wet years (2018 to 2020; AAP: 425 km³). The recent extensive precipitation events during the wet period are reflected in GRACE and GRACE-FO data. Throughout the dry period there was a total decline in GRACE_TWS of 212 km³ (13.3 km³/yr) followed by an increase of 246 km³ (82 km³/yr) during the wet period.  In other words, in the past 2.5 years, the TEW more than recovered its losses during the previous 15 years. This recovery was enabled in part by the impoundment of surface water behind the many dams in the riparian countries and by infiltration of precipitation that recharged the TEW aquifers. Using radar altimetry we observe an increase in surface water levels by 8 m in Lake Ataturk, 13 m in Lake Karakaya, 1.5 m in Lake Van in Turkey, 5 m in Lake Assad in Syria, and 16 m in Lake Tharthar, and 24 m in Lake Mosul in Iraq.  These translate to a volume increase of 21.7 km³ in Turkey, 3.5 km³ in Syria, and 34 km³ in Iraq during the wet period. Using GRACE data and outputs of land surface models, we estimate that groundwater storage GRACE_TWS declined at a rate of -7 km³/yr during the dry period and increased at a rate of 60 km³/yr during the wet years.