How Far Can Meteorological Stations Provide Reliable Data for Reference Evapotranspiration Estimation?

ABSTRACT

Accurate estimation of reference evapotranspiration (RET) is essential for water resource management in water‐stressed regions such as the Urmia Lake basin, northwestern Iran. While RET can be readily estimated from station records, the spatial extent over which a single station's data remain reliable is seldom quantified. This study addresses this gap by developing a spatial accuracy‐radius framework that defines, for each of 24 synoptic meteorological stations in the Urmia Lake basin, the geographic extent within which its FAO Penman‐Monteith RET estimates remain reliable, using ERA5‐Land gridded data (463 points, 9 km resolution, 2010–2020) as a spatial reference. Station accuracy was assessed using the normalized root mean square error (nRMSE), and station‐specific accuracy radii were delineated based on the station with the minimum nRMSE at each point. The results show that station accuracy decreases with increasing distance from the station, and that stations located in urban and industrial areas exhibit lower reliability, consistent with the urban heat island effect and other local environmental influences. Based on this analysis, 18 of the 24 stations were identified as valid and used to define the accuracy‐radius coverage of the basin. Compared with the conventional nearest‐station approach, the proposed accuracy‐radius method increased the mean basin area with nRMSE below 0.2 from 70% to 91%, and reduced the mean basin‐wide nRMSE from 0.197 to 0.168. These findings indicate that accounting for station‐specific spatial reliability, rather than proximity alone, provides a more robust basis for meteorological station selection in hydrological modeling, agricultural planning, and sustainable water resource management.