[Analysis of paddy field evapotranspiration in North China and calculation of crop coefficient]

The potential evapotranspiration (ET0) values from 1951 to 2002 for the lower reaches of Liaohe Plain, North China, were estimated by Penman-Monteith equation, and the annual change in ET0 was analyzed. By using non-weighting lysimeters at Shenyang Experimental Station of Ecology (CAS) in North China, the daily evapotranspiration of flooded rice (Oryza sativa L.) at the interval of 10-11 days was determined for eight seasons from 1993-2002 (except for 1997 and 1999). The results showed that the total evapotranspiration ranged from 581 mm to 695 mm during 1993 and 2002, averaged 634 mm. Both in dry and wet seasons, supplemental irrigation was essential in this region for the uneven distribution of rainfall and time. The average crop coefficient (Kc) for eight years from 1993 to 2002 during the whole growth period was 1.32, with a second-degree polynomial change for the values of Kc during different rice growth stages.