A flexible plant based irrigation control for greenhouse crops
Simba, Farai Malvern
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The project sought to minimize water use, improve water use efficiency and improve crop productivity for greenhouse crops. A flexible plant based automated irrigation system was designed and implemented for a tomato crop in a greenhouse at the Biological Science Department, University of Zimbabwe. The system used stem heat balance sap flow gauges to measure the transpiration rate and the information was sent to a computer program via a K8000 Velleman interface card. The computer program used this information to calculate the daily crop water requirements and time to replace the lost water for the greenhouse crops. The performance of the automated system was evaluated by comparison of the leaf temperatures and amount of water used against an existing scheduling technique in which the crop water requirements were calculated using the Penman-Monteith equation. In order to find typical values for water lost during the day under different conditions, a ten-day monitoring of sap flow rates on the tomato crop was done and typical daily sap flow rates were varying from 0.11 to 0.98 L m-2 d-1 with an average sap flow rate of 0.53 L m-2 d-1. Corresponding external daily total solar radiation ranged from 9.19MJm-2d-1 to 26.56 MJm-2d-1, with an average of 20.05 MJ m-2 d-1, while air temperatures ranged from 27.25 Degrees Celsius to 14.73 Degrees Celsius, with an average of 20.99 Degrees Celsius. In addition, a control treatment was established, in which the plants were subjected to drought stress by withholding water for a number of days and the transpiration rates again measured using sap flow gauges as before in order to find typical values for water lost during the day under different drought stress conditions. Typical values were ranging from 0.1 L m-2 d-1 to 0.45 L m-2 d-1. In a separate six-day monitoring period, the average ETo values were 2.16 mm d-1 and 2.15 mm d-1 for the Penman-Monteith based treatment and the automated treatment, respectively. Total water replaced over the six days by the automated system was 88.03 L and by the Penman Monteith based method was 171.84 L and the total irrigation time used was 73.5 minutes and 180 minutes, respectively. For the six consecutive days, the automated irrigation system supplied the least amount of water daily and thus avoided under irrigating or over irrigating, cut on total working hours of the irrigation system, labour and energy costs and had good response time and was flexible. Leaf temperature ranged from 12.55 Degrees Celsius to 31.94 Degrees Celsius with an averaged value of 22.28 Degrees Celsius. We can conclude, therefore that the physiological response of the crop was not negatively affected by this reduction in water through the use of the automated scheduling.