estimation of reference crop evapotranspiration using the least meteorological data (case study: khorasan razavi province)

abstract one of the losses decrease's ways in fields, is the proper irrigation management, which its base is the accurate estimation of crop water requirement. equations which are used to calculate the reference evapotranspiration (eto), do not use the same climatic parameters and due to their empirical base, are not match to all climatic situations. so it is needed to clarify proper methods for each region. in this investigation, lysimeteric data, which are taken from the climatologic station of mashhad university in a 6-month period tests, ware compared with the f.a.o-penman-montieth (f.p.m), evaporation pan, monthly and yearly adjusting coefficient. also the f.p.m's calculated eto for all the synoptic stations of razavi khorasan province were regression with the air temperature, radiation and psychrometeric coefficient to suggest a simple equation. the best results were obtained from the f.p.m equation with the monthly (r2=0.99) and yearly (r2=0.92) adjusting coefficient, respectively. so it is advised to assess the reference situation of the station and use adjusting coefficients for the non-reference ones.