In this paper a simplified methodology for the estimation of drought impacts on crop yield in rainfed agriculture is proposed. The methodology is based on empirical models for the estimation of actual to potential evapotranspiration ratio (ET/PET). This ratio is related to the Reconnaissance Drought Index (RDI), an expression of cumulative precipitation to cumulative potential evapotranspiration. The actual to potential evapotranspiration ratio is the basic variable in the simple linear regression functions used for estimating the relative crop yield (actual/potential yield). A case study from Thessaly (Greece) is presented in which a large number of years were analysed to assess production losses.