The hydrology section is divided into two main components, surface and groundwater. One of the most important outcomes in the water balance equation for any natural area or water body is Evapotranspiration and it is also a crucial component of the hydrologic cycle. Prediction of monthly evapotranspiration can be obtained depending on observed monthly average temperatures at a meteorological station in each year. Calculating of water balance in Iraq depending on meteorological data and Thornthwaite method was the aim of this research. Results of corrected potential evapotranspiration (PEc) obtained from applying _Thornthwaite formula_ were compared with annual and monthly rainfall in thirty two meteorological station in order to estimate actual evapotranspiration (AE). The results showed that the annual summation of rainfall increased from south west towards north east according to the increasing ratio of rainfall due to the impact of Mediterranean climate condition on Iraq. Actual evapotranspiration depends directly on water excess during calculating water balance. Water surplus contour map indicates increased values towards north-east direction of Iraq, where water surplus depends directly on both rainfall and actual evapotranspiration.