Abstract:

Aims Crop coefficients are the key parameters in calculating crop evapotranspiration. There are few reference values for crop coefficients of natural grass in FAO irrigation and drainage papers. Our aim was to define crop coefficients under different water conditions for typical steppe in Inner Mongolia based on actual observation data so as to accurately calculate the water demand and actual evapotranspiration.
Methods Data were collected from field experiments at Xilinhot (44°08′03 N, 116°19′43 E, elevation 990 m), Inner Mongolia in 2008, and meteorological data from 26 years were obtained from four stations. Using the water balance equation, we calculated crop coefficients of typical steppe using statistical analyses and developed the standard crop coefficients of typical steppe and simulated equations through comparison and validation with recommend values by FAO. We established a climatic correction method for the standard crop coefficients and a correction method of crop coefficients under water stress.
Important findings Standard crop coefficient values of typical steppe were fitted with trinomial using days after return of the green period of grass and accumulated temperature (≥0 °C), with correlation coefficients >0.94. Crop coefficients of typical steppe were described as an exponential equation with relative leaf area index (LAI) and relative plant cover. Average value of standard crop coefficient of typical steppe for the growing season was 0.60, and the maximum value was 1.02. Typical values of crop coefficients in different growing stages were 0.40 for initial stage, 0.93 for medium stage and 0.80 for later stage, and threshold ranges of three growing stages were 0.35-0.45 for initial stage, 0.85-1.00 for medium stage and 0.70-0.90 for later stage. Compared with measured data from a lysimeter, the simulated values for 10 days of evapotranspiration indicated an average relative error of 20%-24% for whole growing season, and <10% for the active growing stage. The method and results defined here are applicable to typical steppe in Inner Mongolia.

Key words: correction of crop coefficient with leaf area index, correction of crop coefficient with wetness index, crop coefficient, lysimeter, water balance equation