Articles
Impact of measurement errors in the calculation of CWSI as an indicator of water status in crop species
Article number
1409_38
Pages
291 – 296
Language
English
Abstract
Thermal information is usually proposed to evaluate the water status of crops, and the crop water stress index (CWSI) is the most commonly used of the temperature-derived indices.
For the calculation of CWSI, the difference between air and canopy temperature (Ta and Tc, respectively) is normalized by the lower and upper limits.
The two limits correspond to the Tc-Ta value of a crop transpiring at its maximum and that of a crop where the transpiration is completely halted.
In the empirical approach described by Idso and Jackson in 1981, the two limits are derived from the relationship between the Tc-Ta value of a well-watered canopy and the vapour pressure deficit, known as the non-water stress baseline (NWSB). Nevertheless, the effects caused by errors in these inputs used for calculating CWSI require further analysis.
This work quantifies the errors caused by the accuracy of the input data in the calculation of CWSI, such as air and canopy temperature and relative humidity.
The assessment was carried out assuming measurement errors of ±1°C for Ta, ±10% for the relative humidity and errors in Tc ranging from 0.25 to 2°C. We demonstrate that the magnitude of the effect of these errors on the resulting CWSI is crop-specific and depends on the slope of the NWSB. This study presents the results obtained after analysing input data for 26 crops.
Those crops with less steep slopes of the NWSB (mainly corresponding to tree crops) showed more significant effects on CWSI due to input errors.
In orange, the ranges of errors considered in this study in Ta and RH may result in a CWSI deviation between 87 and 28%, as the VPD value increased from 2 to 6.6 kPa, respectively.
Meanwhile, herbaceous crops displayed steeper slopes, and the errors’ relevance was of less magnitude.
For the same range of errors, the deviation of CWSI in broccoli was less than 7% for the whole range of VPD considered.
The relevance of the coupling of the different crops with the atmosphere on the errors obtained when calculating CWSI are discussed.
For the calculation of CWSI, the difference between air and canopy temperature (Ta and Tc, respectively) is normalized by the lower and upper limits.
The two limits correspond to the Tc-Ta value of a crop transpiring at its maximum and that of a crop where the transpiration is completely halted.
In the empirical approach described by Idso and Jackson in 1981, the two limits are derived from the relationship between the Tc-Ta value of a well-watered canopy and the vapour pressure deficit, known as the non-water stress baseline (NWSB). Nevertheless, the effects caused by errors in these inputs used for calculating CWSI require further analysis.
This work quantifies the errors caused by the accuracy of the input data in the calculation of CWSI, such as air and canopy temperature and relative humidity.
The assessment was carried out assuming measurement errors of ±1°C for Ta, ±10% for the relative humidity and errors in Tc ranging from 0.25 to 2°C. We demonstrate that the magnitude of the effect of these errors on the resulting CWSI is crop-specific and depends on the slope of the NWSB. This study presents the results obtained after analysing input data for 26 crops.
Those crops with less steep slopes of the NWSB (mainly corresponding to tree crops) showed more significant effects on CWSI due to input errors.
In orange, the ranges of errors considered in this study in Ta and RH may result in a CWSI deviation between 87 and 28%, as the VPD value increased from 2 to 6.6 kPa, respectively.
Meanwhile, herbaceous crops displayed steeper slopes, and the errors’ relevance was of less magnitude.
For the same range of errors, the deviation of CWSI in broccoli was less than 7% for the whole range of VPD considered.
The relevance of the coupling of the different crops with the atmosphere on the errors obtained when calculating CWSI are discussed.
Authors
V. Gonzalez-Dugo, P.J. Zarco-Tejada
Keywords
weather station, meteorological data, CWSI, NWSB, slope
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