Abstract: Effect of different ngstrm-Prescott coefficients on the reference crop evapotranspiration (ET0) was studied,using data series in last 50 years from 3 meteorological stations in the middle reach of Yellow River. The results showed that 56% and 93% variation of the ngstrm-Prescott coefficients(as& bs )in this region resulted in 0.6% -26.2% monthly variation, and 18.2-31.9%(173.1-197.4mm)annual variation for ET0. The effect of ngstrm-Prescott coefficients on ET0 was spatial dependent. It increased with the increasing of altitude and sunshine hours. It suggested that ngstrm-Prescott coefficients should be calibrated in regions with high altitude and intensive radiation. Though the ngstrm-Prescott coefficients were recommended by the FAO 56 PM to apply to the world wide, they could not been applied to in China directly according to this result. It seemed necessary to be validated and calibrated further to these coefficients.

Key words: ngstrm-Prescott, ngstrm-Prescott, Reference crop evapotranspiration, Penman-Monteith