Open-air average temperature measured in open air space is higher than that under banana canopy

Abstract

The need for rapid assessments of cropping systems using crop growth model has increased due to the potential negative impact of climate change on productivity of crops. Most of these models are driven by average air temperature taken from open air space. However, for some crops like bananas, a proportion of their life cycle is spent growing under the canopy of the mother plant. It is not clear to what extent the average air temperature measured under open air can be valid for plants growing under the banana canopy. This study was conducted to assess the variation in average air temperature measured under open space and that measured under banana canopy. One ordinary thermometer was set up on a stand in the open space away from the banana canopy at a height of about 1.5 m above the ground. The ground was covered with dead grass mulch to minimize air temperature fluctuations due to heat fluxes from the ground and also to mimic the ground surface cover under the banana canopy. On the same stand, an HTC-2 device was fixed with its sensor at the same height from the ground as the ordinary thermometer bulb. This constituted the open-air instrumentation for recording air temperature data away from the banana canopy. A second ordinary thermometer was fixed on a stand at about 1.5 m above the ground under the canopy of a randomly selected banana grove within a 5-m radius of the open space where the first set up of ordinary thermometer and HTC-2 device were installed. Next to this thermometer was also fixed the second HTC-2 device as done for the open space set up. This constituted the banana canopy instrumentation for recording air temperature under the banana canopy. Two sets of data were collected daily for 44 days between 20th November 2021 and 20th March 2022. One set of air temperature were recorded at any time of the day. The instruments were read and their respective air temperatures recorded at the material time of reading. The time of reading the instruments was randomly selected and spread from early

morning hours between midnight and 3:00 am so as to get low air temperatures, through mid- morning between 9 am and midday so as to get moderate air temperatures and also between 2:00

pm and 5 pm so as to get relatively high air temperatures. This constituted dataset 1. A second set of air temperature data were recorded at 6 pm every day for 27 days between 25th January and 28th February 2022. This contained the minimum and maximum air temperature recorded on the HTC-2 device between midnight and the time of recording on the same date. This constituted dataset 2. The findings of the study were the air temperature measured under the banana canopy is significantly lower than the air temperature measured away from the banana canopy. For every

x

°C rise in air temperature above 3.48 °C measured away from the banana canopy using an HTC- 2 device, the air temperature under the banana canopy rises by only 0.83 °C. For every °C

increase in air temperature measured in open air space above 4.86 °C using an ordinary thermometer, the air temperature rises by only 0.79 °C. The minimum and maximum air temperatures measured under the banana canopy are likewise significantly different than the corresponding values measured from open air space away from the banana canopy. The air temperature data used in this study was collected over a short duration of time. The small size of the dataset warrants further studies to be conducted for a more robust conversion of air temperature values from the open air to the corresponding values under the banana canopy. Further studies should be conducted on the relative humidity measured under open air conditions vs. that under the banana canopy since time and resources did not permit this variable to be investigated under this study. Key words: Crop growth modelling, evaporation, microenvironment, transpiration, water balance