GRACE Response to Climate Change over Engineered and Nonengineered Basins

The Gravity Recovery and Climate Experiment (GRACE) and GRACE-Follow On (GRACE-FO) have enabled understanding and monitoring of the impacts of climate change on many of the major world’s watersheds. The Tigris Euphrates watershed covers an area of 1×106 km2 with two main rivers, the Euphrates River (length: 2800 km) and the Tigris River (length: 1900 km) originate from the highlands of Turkey, Iran, and Syria and flow downstream towards Iraq. Our analysis of multiple satellite missions (e.g., GRACE, GRACE Follow-On, Landsat 5,7,8, and satellite radar altimetry) and global land surface models over the highly engineered Tigris Euphrates watershed (30 dams) showed an impressive recovery of the system following a prolonged drought (2007–2018; Average Annual Precipitation [AAP]: ~400 km3) by an extreme precipitation event in 2019 (726 km3) with no parallels in the past 100 years. This recovery (113±11 km3) compensated for 50% of the losses endured during drought by impounding a large portion of the runoff within the reservoirs (capacity: 250 km3). In basins lacking artificial reservoirs a different response to extreme precipitation events is observed from temporal GRACE solutions.  Extreme precipitation events (2011-2022) over northern Arabia (PPT: Hail: 8.43 km3; Ad-Dahna: 2.22 km3 and Medina: 3.71 km3) and central Arabia (PPT: Riyadh: 4.66 km3 and Mecca: 0.21 km3) produced an increase in GRACETWS that lasted for a few months only. Similarly, cyclones over Oman (2011and 2015; PPT: 6 and 6.6 km3, respectively) produced a similar effect, where most of the precipitation ends up as losses from the water budget to evaporation or runoff. This is apparently the case for many of the nonengineered hydrologic systems that have no storage capacity to capture the runoff.  Our findings demonstrate the role of dams in drought mitigation and sustenance of water supplies through storage and controlled distribution, and suggest that highly engineered watersheds are better prepared to deal with the projected increase in the frequency and intensity of extreme rainfall and drought events in the 21st century.