Articles
VALIDATION OF A MODEL USED FOR IRRIGATION CONTROL OF A GREENHOUSE CROP
Article number
458_8
Pages
75 – 82
Language
Abstract
An irrigation model, developed for a long-season tomato crop and based on easily measured variables of outside solar radiation and inside saturation deficit, was validated by measuring water use of crops grown using the nutrient film technique (NFT) and the rockwool run-to-waste system.
Measurements were made throughout a growing season including a limited period when the humidity of the air was increased by fogging.
Data from April to September are presented, covering the period when the crop had a full canopy.
During April, the NFT grown crop used about 10% more water than the model prediction, the deviation increased through May and June and by July was 31%. By September crop water use exceeded the model estimate by 20%. The same trend occurred in the rockwool but the crop used less water than the NFT grown crop.
In July the rockwool crop used 21% more water than the model estimate.
Crop water use was studied in detail during a six day period in June, in which fogging was used to increase the humidity for about two and a half days.
For periods without fogging, the crop water use at night was about 50 W/m2 which was considerably in excess of the model prediction of about 10 W/m2. For the fogging period the model predicted no water use during the night.
The recorded water use during the early period of the night was about 50 W/m2 and decreased linearly throughout the night so that by dawn the crop no longer took up water.
During the day the model accurately predicted crop water use when the saturation deficit was small but water use was considerably underestimated when the saturation deficit was large.
Reasons for the discrepancies between measurement and model estimates are discussed.
The model needs to be refined and validated for an incomplete canopy before it can be used for control.
Measurements were made throughout a growing season including a limited period when the humidity of the air was increased by fogging.
Data from April to September are presented, covering the period when the crop had a full canopy.
During April, the NFT grown crop used about 10% more water than the model prediction, the deviation increased through May and June and by July was 31%. By September crop water use exceeded the model estimate by 20%. The same trend occurred in the rockwool but the crop used less water than the NFT grown crop.
In July the rockwool crop used 21% more water than the model estimate.
Crop water use was studied in detail during a six day period in June, in which fogging was used to increase the humidity for about two and a half days.
For periods without fogging, the crop water use at night was about 50 W/m2 which was considerably in excess of the model prediction of about 10 W/m2. For the fogging period the model predicted no water use during the night.
The recorded water use during the early period of the night was about 50 W/m2 and decreased linearly throughout the night so that by dawn the crop no longer took up water.
During the day the model accurately predicted crop water use when the saturation deficit was small but water use was considerably underestimated when the saturation deficit was large.
Reasons for the discrepancies between measurement and model estimates are discussed.
The model needs to be refined and validated for an incomplete canopy before it can be used for control.
Authors
Paul J.C. Hamer
Keywords
NFT, rockwool, solar radiation, saturation deficit, tomato, fogging
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