Research on conversion time and reference location of canopy temperature in scaled method for variable rate irrigation

Abstract. To generate the dynamic prescription of variable rate irrigation (VRI) management based on canopy temperature, the single-time-of-day canopy temperature obtained from infrared thermometers (IRTs) mounted on center pivot irrigation system should be adjusted to the same time. To research the reasonable conversion time and reference location of canopy temperature in scaled method for winter wheat and summer maize in the North China Plain, the diurnal variation of canopy temperature in different soil available water holding capacity (AWC) management zones were analyzed. The occurrence frequency of daily minimum and maximum canopy temperature in different times were counted, and the canopy temperature distribution maps were compared when the reference temperature was obtained in different AWC management zones. Results indicated that the diurnal variations of canopy temperature were similar in different zones. The largest difference in canopy temperature among management zones was obtained when the canopy temperature was the highest during the day. The daily minimum canopy temperature of winter wheat mainly occurred from 4:00 to 6:00 with the proportion of 76%, and 95% of the maximum daily canopy temperature occurred between 11:00 and 15:00. For summer maize, the daily minimum canopy temperature occurred from 3:00 to 7:00 with the highest proportion of 68%, and 90% of the maximum daily canopy temperature occurred between 11:00 and 17:00. The canopy temperature converted by the scaled method changed with the reference location. When the reference location was in the minimum AWC zone, the canopy temperatures were overestimated in some areas, which could up to 45℃. This study suggested that the conversion time of canopy temperature of winter wheat and summer maize should be selected when the canopy temperature was the highest and the reference location should not be only selected in the zone with the minimum AWC when generating the dynamic prescription of VRI management.