In this study, the FLUKE Mini IR62 infrared device was used to measure the canopy temperature (Tc). One of the characteristics of all infrared devices is a special feature called field of view. The field of view is the maximum angle between the rays coming from the object being measured (the leaves of the black gram in the present study) received by the device. The larger the field of view, the larger the image size measured by the device. Furthermore, the greater the distance of the device to the target being measured the greater the field of view. Therefore, the field of view of the device is expressed as D:S which is the ratio of the object diameter (point size or object diameter) to the distance from the device. The D:S ratio of the infrared thermometer in the present study was 10:1.

Tc of black gram was not measured from planting to 10th July due to its small size and field of view of the device12. Tc was measured from four geographical directions for each treatment (with three replications) when it was sunny and cloudless6,28. Measurements were made on different leaves of black gram at an angle of 30° to 45° to the horizon and an average Tc was obtained from an average 12 readings for each treatment28. Generally, for each treatment in a single day, 84 Tc readings were obtained in 7 h (8:50 to 14:50). To obtain the lower baseline equation of the method of Idso et al.10 (Eq. (2)), Tc of black gram was measured for control treatment (I3) from 8:50 to 14:50 in the post-irrigation days6. Meantime, in a bid to determine the experimental CWSI (Eq. (5)) and the calculation of (Tc − Ta)m, Tc of black gram was measured from 11:50 to 14:50 in the pre-irrigation days for all three treatments (I1, I2 and I3). CWSI values were calculated for the four growth stages of black gram including floral induction-flowering, pod formation, seed and pod filling and physiological maturity.