Sprinkler irrigation scheduling of winter wheat in the North China Plain using a 20 cm standard pan

Based on evaporation from a 20 cm diameter pan placed above the crop canopy, sprinkler irrigation scheduling of winter wheat was studied in the North China Plain (NCP) in the 2001-2004 winter wheat seasons. Results showed that pan evaporation (E-pan,E-C) was closely related to actual evapotranspiration (ET) measured using weighing lysimeters. The combined pan-crop coefficient (K-c,K-pan), the ratio of ET to E-pan,E-C, was closely related to leaf area index (LAI) and plant height. Data from the 2002-2003 season were used to establish the relationships between K-c,K-pan and LAI (method A) or plant height (method B), and used to determine the crop coefficient (method C). ET computed by the three methods was compared with measured ET using lysimeters in the 2001-2002 and 2003-2004 seasons. Mean relative error of estimated daily ET by the three methods ranged from 20 to 30%, and the relative error in cumulative ET in the experimental periods ranged from 1 to 19%. Among the three methods, results from methods A and B were not significantly different from each other (P > 0.01), and were closer to the lysimeter data than results from method C (P < 0.001). Method B, being easier to measure, was recommended for ET estimation in NCP.