The management of water resources requires adequate information on the quantities of water supplied from the basins that outfall into a river, especially during the flood seasons. The study area located in the western part of Iraq within the administrative boundaries of the Heet district about 70 km from Haditha Dam, 45km from Ramadi in Anbar province. The study aims to evaluate the amount of surface runoff through a long-term period (1981-2019). Soil and Water Assessment Tool (SWAT) related to Geographic Information System (ArcGIS) was used for the simulation. The input data was the Digital Elevation Model (DEM) of SRTM with resolution 30m, land use/land cover map from the European Space Agency (ESA) with resolution 300m and, soil map from the Food and Agriculture Organization (FAO). The weather data used in the study were obtained from the Climate Forecast System Reanalysis (CFSR) combined with the weather data from the Surface meteorology and Solar Energy (SSE) produced by NASA. These weather data prepared using SWAT weather database software to be ready for the simulation processes. Al-Mohammedi valley was calibrated and validated using SWAT-CUP software using the available recorded discharges at Heet, Ramadi, and Al-Warar gauge stations. The calibration is based on the meteorological data for the period January 1, 2002, to December 31, 2006, and the validation was based on the data between January 1, 2007, to December 31, 2009. The model calibration and validation results based on two objective functions “Nash-Sutcliffe (NS) and coefficient of determination(R2)” showed that SWAT was successfully simulated Al-Mohammedi valley with NS = 0.72 and R2 = 0.76 for calibration, and NS = 0.63 and R2 = 0.65 for validation. According to SWAT results, the average runoff volume in the long-term period of simulation from January 1, 1981, to October 31, 2019, was 79.2 million m3 while the average runoff depth was 18.25 mm with about 17 % of rainfall becomes surface runoff.
Estimation of runoff in an ungauged watershed is a significant part in the process of the water resources management. In the Iraqi western desert, the accessibility reliable surface runoff knowledge is scarce, that affects a critical difficulty for the hydrologic engineers. Estimation of surface runoff quantity in valleys of interrupted flow is significant to mobilize the deficiency water resources and manage valleys flow accurately. The incorporation of the Soil Conservation Service Curve Number (SCS-CN) approach with the geographic information system (GIS) was applied for estimating runoff volume of Wadi Hijlan, Fahamy, and Zgadan. The amount of runoff of the maximum storm were 7388700 m3, 12750000 m3 and 9851590 m3 for Hijlan, Fahamy and Zgadan respectively. In addition, the results showed acquired via the SCS-CN technique, revealed that the runoff depth fluctuated from 12.5 mm to 20.3 mm for (48mm) the maximum storm of rainfall through 2018-2019. The present strategy can be used for planning and development other valleys in the western desert of Iraq.