1. Introduction

[2] Water scarcity in the Middle East, and the high frequency of conflict that emerges over what few resources do exist, is well established [e.g., Amery and Wolf, 2000; Wolf and Newton, 2007a; Wolf, 1998]. The recent drought that began in 2007 has further strained the limited water resources in the region [Integrated Regional Information Networks, 2010; U.S. Department of Agriculture (USDA), 2008]. News reports detailed a dire situation in which fields lay fallow, wetland ecosystems were destroyed, and hundreds of farmers migrated to urban centers in search of employment [Michel et al., 2012; Sullivan, 2010]. Such drought typically amplifies the impact of management decisions by upstream users, as any decision to use or store water substantially influences total water availability within a river system, with potentially severe consequences for downstream users.

[3] Water management in the Tigris‐Euphrates River Basin has been historically challenging [Solomon, 2010]. The Tigris‐Euphrates is a transboundary river system (Figure 1a) that is shared among Turkey, Syria, Iraq, and, to a lesser extent, Iran. Both rivers contain extensive water management infrastructure, and the surface waters provided by the rivers are integral to the agricultural economies of the region [Food and Agriculture Organization (FAO), 2009]. Struggles between the management decisions of the upstream user—Turkey—and the downstream demands of Syria and Iraq dominate the hydropolitics of the region [Wolf and Newton, 2007a]. In particular, the Southeastern Anatolian Project (Turkey's Greater Anatolia Project (GAP)) elevated tension among the three nations as Turkey acted unilaterally to construct over 20 dams on both the Tigris and Euphrates rivers [Bayazit and Avci, 1997]. This intensive infrastructure development has significantly altered the Tigris and Euphrates Basins in many ways. Turkish, Syrian, and Iraqi water managers now dictate the river flows with timed releases from the reservoirs. In addition, a complex system of transboundary groundwater aquifers underlies this region [FAO, 2009]. Domestic and international monitoring and regulation for the groundwater aquifers is lacking, despite the fact that it is a vital resource for the region, especially where and when surface water is unavailable.

Figure 1 Open in figure viewer PowerPoint Lehner and Döll, 2004 Graham et al., 1999 Siebert et al., 2007 (a) Representation of selected study area. Thick black line with hashed fill represents the TEWI region for which GRACE data were extracted and supporting data sets compiled. All mass balance calculations were confined to this bounded region. Thin black lines represent political boundaries. Surface water bodies (light blue) were taken from the Global Lakes and Wetlands Database []. Rivers are delineated in blue, and the respective watershed boundaries in crosshatched yellow [.,]. (b) Small grid squares display percent of land under irrigation [.,]. Blue to red gradient represents intensity on a 0% to 100% scale, respectively.

[4] Two major issues complicate water management in the region. First, there are no formal water allocation rights for both surface and groundwater. At the core of this dilemma are underlying differences in the interpretation of international water law [United Nations, 1997; Weiss, 2009], including its applicability to groundwater and to surface‐groundwater interactions. These differences in interpretation severely limit the potential for any agreement for legal allocations or management policies for the Tigris and Euphrates Rivers.

[5] A second challenge is the paucity of hydrologic data for the region. Inconsistent monitoring combined with a lack of data transparency and accessibility is a problem that plagues water managers around the globe, and the Tigris‐Euphrates region is no exception. Such data scarcity and inaccessibility result in an incomplete understanding of water availability and use in this area of the Middle East. Although there have been other studies in the region [Chenoweth et al., 2011; Jones et al., 2008], Kavvas et al. [2011] showed that publicly available observations of streamflow, precipitation, and evaporation data are sparse to nonexistent, and if available, data sets are often incomplete. Classified, government‐controlled data do exist, but access to these data requires the permission and cooperation of the respective governments. Access to groundwater information is similarly constrained, with limited or no data related to water table height or annual groundwater extraction available publically. Consequently, despite its importance, there have been few basin‐wide hydrological studies using observational data for the Tigris‐Euphrates Basin in recent years.

[6] Satellite observations of time‐variable gravity from the Gravity Recovery and Climate Experiment (GRACE) satellite mission [Tapley et al., 2004] present a new and valuable tool to fill these gaps in data availability and water monitoring [Lettenmaier and Famiglietti, 2006; Rodell et al., 2009; Tiwari et al., 2009; Famiglietti et al., 2011b]. GRACE provides a record of variations in total terrestrial water storage (defined as all of the snow, surface water, soil moisture, and groundwater) across the globe [e.g., Rodell and Famiglietti, 1999; Wahr et al., 2004; Ramillien et al., 2004; Syed et al., 2008]. Recent studies have demonstrated that water storage changes can be inferred from the GRACE data with sufficient resolution and accuracy to benefit water management [Yeh et al., 2006; Rodell et al., 2007; Ramillien et al., 2008; Zaitchik et al., 2008]. For example, GRACE data have been used to estimate rates of groundwater depletion [Rodell et al., 2009; Tiwari et al., 2009; Famiglietti et al., 2011b], flood potential [Reager and Famiglietti, 2009], drought [Andersen et al., 2005; Yirdaw et al., 2008; Leblanc et al., 2009; Agboma et al., 2009; Chen et al., 2010], and reservoir storage changes [Swenson and Wahr, 2009; Wang et al., 2011].

[7] In this study, we used 84 months of GRACE data (January 2003 to December 2009) to examine the behavior of water storage in the north‐central region of the Middle East, an area that includes most of the Tigris and Euphrates River Basins and western Iran. Additional data sets, including precipitation, evapotranspiration, streamflow, reservoir levels, and soil moisture, were compiled to help characterize the causes of observed variations and emerging trends. As an area that is well known for water scarcity and tension over transboundary waters, the Tigris‐Euphrates region offers a compelling example of the power of satellite observations to provide insight into critical water resource issues in regions where hydrological observations are otherwise difficult to obtain. Wada et al. [2010] and Siebert and Döll [2008] developed methods to quantify changes in water resources in areas with limited observational data by using global hydrological and water resources models to model surface water discharge as well as groundwater recharge and to estimate water consumption based on statistics on population, gross domestic product (GDP), and irrigated areas. These methods highlight potential options when observational data are limited, and in addition to the approach followed here, in our opinion, may well provide an example of ‘best current capabilities’ in regions like the Middle East, where data access can be severely limited.