This study aims to quantify the provisional ecosystem services of water supply in irrigated agriculture and the respective crop production, in the surrounding cropland areas of 'Lake Karla' subbasin, located in the southeastern part of Pinios river basin, Thessaly region, central Greece. The area is characterized by water scarcity and overexploitation of groundwater resources, issues that are expected to be eliminated with the full operation of Karla, an artificial multi-purpose reservoir, constructed recently on the land where the homonymous natural Lake Karla was situated before its drainage in 1962. The methodology of the work is based on the development of a dynamic GIS hydrologic and management model of the study area with SWAT (Soil and Water Assessment Tool) that simulates hydrology, reservoir operation, irrigation practices and crop production. The model is executed for a 10-y period with historic climate under two scenarios: a) with irrigation needs covered by surface water resources due to the presence of Karla reservoir and b) without Karla reservoir in operation and groundwater being the only irrigation water source. The results are compared and discussed emphasizing on the net provisional services that the local agricultural society can gain from the full operation of Karla on an annual basis, with caution on the minimum water level in the reservoir that has to be maintained to support biodiversity and other ecosystem functions. Additional benefits such as regulating (flood control) and cultural (aesthetic, educational and recreational) services are also highlighted. The study concludes that the construction of the remaining parts of the initially planned collective irrigation network around the reservoir should be of priority for the maximization of the benefits for the society and the environment by the Karla reservoir.