The article evaluates changes in weather and hydrological variables in the upper Hirmand basin over the past century using global climate data and a rainfall-runoff model. By comparing climate data from ground weather stations in Afghanistan and simulating runoff with the SWAT model, the study found a decrease in annual water volume entering the Kajaki dam. The data showed a shift in precipitation and temperature patterns before and after the early 1940s, resulting in lower rainfall and higher temperatures. This research methodology can be effectively applied to simulate watersheds in Iran and Afghanistan.