Temporal And Spatial Trends Of Reference Evapotranspiration In Agro-Ecological Regions Of India : Due to economic and environmental constraints on new water resources developments, and increasing municipal and industrial needs, proper planning and management of water resources is of immense importance to increase agricultural production. Therefore, one of the basic components of the hydrologic cycle, evapotranspiration (ET), needs to be estimated accurately. However, ET is a complex and non-linear phenomenon as it depends on several interacting factors such as temperature, humidity, wind speed, radiation, and type and growth stage of crop. Due to wide application of ET data, a number of indirect methods for estimation of reference ET (ETo) from easily available meteorological data have been developed which span the entire spectrum from empirical to physically based. However, not only data requirement varies from method to method, but also each method’s performance varies with climatic condition. Therefore, it becomes impractical for many users to select the best ETo estimation method for the available data and climatic condition. To overcome this problem, an existing decision support system (DSS_ET) was modified to include additional ETo estimation methods, generalized data input format, features to estimate missing data, and to improve its graphical user interface for enhancing its applicability as a research and teaching tool. Monthly meteorological data for 32 years (1971-2002) were procured from India Meteorological Department (IMD), Pune, for 133 selected stations evenly distributed over 19 out of 21 agro-ecological regions (AERs) of India. Different methods for estimation of missing solar radiation (Rs) from air temperature extremes were compared since both Rs and sunshine hour data, either of which is mandatory for better performing radiation and combination methods, were missing for most of the stations. The original Hargreaves (1994) method was found to be the best suited for Indian conditions.