In Romania, arid regions occupy a large area especially in the south-eastern and southern regions eg the Romanian Danube Plain and Dobrogea, where most of the crops need to be irrigated during summer time. The method recommended by FAO uses the Penman-Monteith reference evapotranspiration (PM-ETo) to determine irrigation water requirements (IWRs) and crop evapotranspiration (ETc). Unfortunately, there are places in this country and most probably in the neighbouring countries with similar climate, where long-term records of solar radiation, wind speed and air humidity are missing. The purpose of this paper is to document the range of an aridity index, called De Martonne's aridity index (Iar-DM) as it applies across the country of Romania, and to determine its relationship with irrigation water requirements of representative crops of Romania. Iar-DM is the ratio between the mean values of precipitation (P) and temperature (T) plus 10°C, annually or monthly. The goal is to help preserve water resources through use of the index. Iar-DM was calculated and averaged for a period of about 100 years and was plotted as maps by using the kriging interpolation method. Iar-DM was correlated with both crop evapotranspiration and irrigation water requirements of the crops in this country. Inverse and strong regression equations were found between the correlated parameters. These relationships have a regional character and might help irrigation system designers and planners estimate the order of magnitude of IWRs and ETc by using Iar-DM, and evaluate environmental water resources in temperate climate regions like areas in Romania and neighbouring countries, where only measurements of temperature and precipitation exist. Because of competition for water among various consumers, this study might help hydrologists and planners make proper decisions concerning how to use scarce water resources during summertime. The procedures used in the paper might be applied in many regions in Europe, and thus have widespread application.