Hydrologic impacts of engineering projects on the Tigris-Euphrates System and its marshlands.

Summary Rising demands for fresh water supplies are leading to water management practices that are altering natural flow systems world-wide. One of the most devastated of these natural systems is the Tigris–Euphrates watershed that over the past three decades has witnessed the construction of over 60 engineering projects that eliminated seasonal flooding, reduced natural flow and dramatically reduced the areal extent (1966: 8000 km 2 ; 2002: 750 km 2 ) of the Mesopotamian Marshes downstream. We constructed a catchment-based continuous (1964–1998) rainfall runoff model for the watershed (area: 10 6  km 2 ) using the Soil Water Assessment Tool (SWAT) model to understand the dynamics of the natural flow system, and to investigate the impacts of reduced overall flow and the related land cover and landuse change downstream in the marshes. The model was calibrated (1964–1970) and validated (1971–1998) against stream flow gauge data. Using the calibrated model we calculated the temporal variations in the average monthly flow rate (AMFR), the average monthly peak flow rate (AMPFR), and annual flow volume (AFV) of the Tigris and Euphrates into the marshes at a location near Al-Basrah city (31°N, 47.5°E) throughout the modeled period. Model results indicate that the AMPFR (6301 m 3 /s) and average annual flow volume (AAFV: 80 × 10 9  m 3 /yr) for period A (10/1/1965–09/30/1973), preceding the construction of the major dams is progressively diminished in periods B1 (10/1/1973–09/30/1989; AMPFR: 3073 m 3 /s; AAFV: 55 × 10 9  m 3 /yr) and B2 (10/1/1989–09/30/1998; AMPFR, 2319 m 3 /s; AAFV: 50 × 10 9  m 3 /yr) that witnessed the construction of the major dams (B1: Keban, Tabqa, Hamrin, Haditha, Mosul, Karakaya; B2: Ataturk) due to the combined effects of filling artificial lakes, evaporation and infiltration of impounded water and its utilization for irrigation purposes. To investigate the impacts of reduced flow on the areal extent of the marshes, we examined the variation in marsh size extracted from temporal satellite data (1966, 1975, 1976, 1977, 1984, 1985, 1986, 1987) acquired around the same approximate time period (July to September) of the year versus simulated AFV for the period preceding the onset (1987) of major local engineering projects (e.g., Crown of Battles River, Loyalty to the Leader Canal, Mother of Battles River) in and around the investigated marshes. Results indicate that the areal extent of the Central and Al-Hammar marshes (e.g., 1966: 7970 km 2 , 1977: 6680 km 2 , 1984: 5270 km 2 ) decreases with a decrease in AFV (e.g., 1966: 60.8 × 10 9  m 3 , 1977: 56.9 × 10 9  m 3 , 1984: 37.6 × 10 9  m 3 ). Using a relationship that describes the impact of reduced AFV on the areal extent of the marshes, we evaluated the impact of additional reductions in flow that will result from the implementation of the planned engineering projects on the Tigris–Euphrates system over the next few years. Upon completion of the ongoing South Eastern Anatolia project, with projected reductions in AFV exceeding 5 × 10 9  m 3 /yr, the sustainable marshes in the Central and Al-Hammar area will be reduced by at least an additional 550 km 2 .