Plant Water Stress Index Research Articles

The Afternoon/Morning Ratio of Tower-Based Solar-Induced Chlorophyll Fluorescence Can Be Used to Monitor Drought in a Chinese Cork Oak Plantation

Monitoring drought stress is crucial for estimating productivity and assessing the health status of forest ecosystems under global climate change. Solar-induced chlorophyll fluorescence (SIF) is mechanistically coupled to photosynthesis and has advantages over greenness-based vegetation indices in detecting drought. In recent years, SIF has commonly been used in monitoring drought stress in crop ecosystems. However, the response of tower-based SIF to drought stress in forest ecosystems remains unclear. In this study, we investigated the potential of tower-based SIF to monitor drought, which was quantified using the plant water stress index (PWSI) in a Chinese cork oak plantation. The results show the negative effect of drought on SIF, and afternoon depression of SIF emission under drought stress was observed. Canopy SIF (F) exhibited a nonlinear relationship with PWSI, while the quantum yield of SIF (ΦF) exhibited a significant linear relationship with PWSI at 687 nm and 760 nm (ΦF687: R2 = 0.90; ΦF760: R2 = 0.85). Incident radiation (PAR) and canopy structure affected the response of SIF to drought stress, with PAR as the main factor causing the nonlinear relationship between F and PWSI. Afternoon depression was described as the afternoon/morning ratio (AMR). AMRF and AMRΦF exhibited a negative linear response to PWSI. AMRF was less affected than F by PAR and canopy structures, and AMRΦF was more physiologically representative than ΦF. Moreover, AMRΦF was sensitive to VPD and REW, and it might be a good indicator of drought. Red SIF was more sensitive to drought than far-red SIF, as the R2 of PWSI with AMRΦF687 (R2 = 0.89) was higher than that with AMRΦF687 (R2 = 0.84). These results highlight the potential of tower-based SIF, especially red SIF, for drought monitoring in a plantation, and consideration of the physiological diurnal variation in SIF under drought stress is crucial for improving the accuracy of drought stress monitoring.

Comparison of Plant Water Stress Index (CWSI) and Water Deficit Index (WDI) in Wheat Using Remote Sensing (RS)

Comparison of Plant Water Stress Index (CWSI) and Water Deficit Index (WDI) in Wheat Using Remote Sensing (RS)

Comparing Different Light Use Efficiency Models to Estimate the Gross Primary Productivity of a Cork Oak Plantation in Northern China

Light use efficiency (LUE) models have been widely used to estimate terrestrial gross primary production (GPP). However, the estimation of GPP still has large uncertainties owing to an insufficient understanding of the complex relationship between water availability and photosynthesis. The plant water stress index (PWSI), which is based on canopy temperature, is very sensitive to the plant stomatal opening and has been regarded as a good indicator for monitoring plant water status at the regional scale. In this study, we selected a cork oak plantation in northern China with an obvious seasonal drought as the research object. Using the ground-observed data, we evaluated the applicability of the LUE models with typical water stress scalars (MOD17, MODTEM, EC-LUE, ECM-LUE, SM-LUE, GLO-PEM, and Wang) in a GPP simulation of the cork oak plantation and explored whether the model’s accuracy can be improved by applying PWSI to modify the above models. The results showed that among the seven LUE models, the water stress scalar had a greater impact on the model’s performance than the temperature stress scalar. On sunny days, the daily GPP simulated by the seven LUE models was poorly matched with the measured GPP, and all models explained only 23–52% of the GPP variation in the cork oak plantation. The modified LUE models can significantly improve the prediction accuracy of the GPP and explain 49–65% of the variation in the daily GPP. On cloudy days, the performance of the modified LUE models did not improve, and the evaporative fraction was more suitable for defining the water stress scalar in the LUE models. The ECM-LUE and the modified GLO-PEM based on PWSI had optimal model structures for simulating the GPP of the cork oak plantation under cloudy and sunny days, respectively. This study provides a reference for the accurate prediction of GPP in terrestrial ecosystems in the future.

Carbon–water coupling and its relationship with environmental and biological factors in a planted Caragana liouana shrub community in desert steppe, northwest China

Abstract The carbon and water cycle, an important biophysical process of terrestrial ecosystems, is changed by anthropogenic revegetation in arid and semiarid areas. However, there is still a lack of understanding of the mechanisms of carbon and water coupling in intrinsic ecosystems in the context of human activities. Based on the CO2 and H2O flux measurements of the desert steppe with the planted shrub Caragana liouana, this study explored the carbon and water flux coupling of the ecosystem by analyzing the variations in gross primary productivity (GPP), evapotranspiration (ET) and water use efficiency (WUE) and discussing the driving mechanisms of biological factors. The seasonal variation in climate factors induced a periodic variation pattern of biophysical traits and carbon and water fluxes. The GPP and ET fluctuated in seasons, but the WUE was relatively stable in the growing season. The GPP, ET and WUE were significantly driven by global radiation (Rg), temperature (Ta and Ts), water vapor pressure deficit, leaf area index and plant water stress index (PWSI). However, Rg, temperature and PWSI were the most important factors regulating WUE. Rg and temperature directly affected WUE with a positive effect but indirectly inhibited WUE by rising PWSI. Plant water stress inhibited photosynthesis and transpiration of the planted shrub community in the desert steppe. When the plant water stress exceeded a threshold (PWSI >0.54), the WUE would decrease since the GPP responded more quickly to the plant water stress than ET. Our findings suggest that policies related to large-scale carbon sequestration initiatives under afforestation must first fully consider the status of water consumption and WUE.

Determination of Irrigation Time by Utilizing Plant Water Stress Index (CWSI) Values of II. Crop Sesame Genotype in Siirt Conditions

Determination of Irrigation Time by Utilizing Plant Water Stress Index (CWSI) Values of II. Crop Sesame Genotype in Siirt Conditions

Determination of Irrigation Time Using Plant Water Stress Index Values of Second Crop Sunflower in Semi-Arid Climate Conditions

This study was carried out in 2020 to determine crop water stress index (CWSI) by using infrared thermometer (IRT) data calculated by leaf canopy temperature measurements of the second crop sunflower genotype in semi-arid climate conditions, and to determine the relationships between irrigation time, seed yield of sunflower plant and CWSI by using these index values. Irrigation program consisted of a full irrigation and 2 different levels of stress, which were 100% (I100), 70% (I70), 35% (I35) of water losses within the effective root depth of 90 cm every 7 days. A total of 644 mm of irrigation water was applied to I100 (control) irrigation. The water consumption for full irrigation was 721 mm and the yield was 3516.00 kg/ha. Lower limit (LL) value without water stress required to determine plant water stress index was Tc-Ta=-2.528×VPD +0.749 (R2=0.814) and upper limit (UL) value, where the plant is completely under water stress, was determined as +3.27℃. Crop water stress index value threshold at which sunflower seed yield started to decrease was calculated as 0.33 using the infrared thermometer measurements at the time of irrigation. In addition, a negative correlation was obtained between sunflower seed yield and CWSI values. The results revealed that the yield tends to decrease as the CWSI increases.

Pulse drip irrigation and fertigation water depths in the water relations of coriander

ABSTRACT This study was installed to evaluate the effect of pulse drip irrigation under different water depth levels (fertigation) in the main water relations of coriander crop (Coriandrum sativum). The experiment was installed under protected environment conditions, at UFRPE, Recife, Brazil. The experimental design was randomized blocks in a 2x5 factorial scheme, with three replications. Treatments consisted of two application types (continuous and pulsed irrigation), and five water depths with replacement of 40, 60, 80, 100 and 120% of crop evapotranspiration (ETc). Water demand was estimated through water balance using a drainage lysimetry. Six irrigation pulses were defined with a 60-minute-rest interval between two successive applications. Plants irrigated by pulses showed higher water productivity values (fresh and dry mass), higher water content (shoot and root), higher shoot mass production index and lower root mass production index and root-shoot ratio. The reduction of applied fertigation water depths increased plant water stress index. The irrigation pulses used ensured water content able to maintain the crop’s marketable quality. Pulse application in irrigation system increased shoot mass production index and reduced root mass production index.

Water stress controls on carbon flux and water use efficiency in a warm-temperate mixed plantation

Water stress can cause the structural and physiological changes of plants to prevent excessive water loss according to species-specific water use strategies. CO2 and water vapor fluxes were measured by the eddy covariance in a mixed plantation during a 4–year period. The potential evapotranspiration (PET) was obtained by the Shuttleworth–Wallace model. Evapotranspiration (ET) versus PET was used to determine the plant water stress index (PWSI). The objective of this study is to quantify the effect of PWSI on net ecosystem productivity (NEP), gross primary productivity (GPP) and water use efficiency (WUE). The relationship between PWSI and soil water content in sunny and cloudy skies was examined. Mean seasonal PWSI values varied substantially among years. The average annual value of PWSI was 0.49 ± 0.07. PWSI was strongly correlated with vapor pressure deficit, illustrating that the moisture limitation through high evaporative demand was more remarkable than water supply limitation. Water stress can substantially reduce canopy conductance and leaf area index, and hence decreasing photosynthesis. Water stress suppressed GPP while having little influence on ecosystem respiration, resulting in the increase of net carbon uptake. NEP decreased as a reaction to water stress when PWSI was more than 0.4, whereas WUE was enhanced under moderate and severe water stress (PWSI > 0.6). Compared to NEP, the difference of ET between cloudy and sunny sky conditions was more evident at the same PWSI value. Therefore, cloudy sky conditions were beneficial to enhancing WUE. Water stress restricted the mixed plantation in the period of spring-early summer. At the annual scale, the mixed plantation was exposed to moderate water stress. The PWSI approach is a promising tool quantifying water stress in forest ecosystems.

Identification of Water Usage Efficiency for Corn (Zea mays L.) Lines Irrigated with Drip Irrigation Under Green House Conditions as Per Plant Water Stress Index Evaluations

The present study was conducted as a greenhouse study under Çukurova conditions during the years 2011-2012. Two different dent corn lines (5A2-B and 22K) were used as the plant material of the experiments. Crop water stress index (CWSI) values determined through leaf canopy temperature measurements performed ahead of irrigations were used to find out water use efficiencies. Experiments were conducted in randomized blocks-split plots experimental design with three replications. Drip irrigation was used for irrigations and two different irrigation programs were created as of I100 (supplying 100% of depleted water in every seven days) and I75 (applying 75% of depleted water. The amount of irrigation water applied for in the interaction of I100 irrigation and corn lines (5A2-B and 22K) in the first and second year were respectively varied between 253.0-274.0 and between 238.0-261.0 mm, seasonal plant water consumptions varied between 294.0-305.0 and between 284.6-302.6 mm, kernel yields varied between 2950.0-2990.0 and between 3130.0-3186.0 kg ha-1 . With regard to (in the interaction irrigation treatments x corn lines), the lowest and the highest CWSI values were observed in the interaction full irrigation (I100) and of the line 22K as 0.22 and in deficit irrigation treatment (I75) of the line 5A2-B as 0.41 in the first year; in full irrigation treatment (I100) of the line 22K as 0.20 and in deficit irrigation treatment (I75) of the line 5A2-B as 0.36. The greatest and the lowest chlorophyll contents (in the interaction irrigation treatments x corn lines) were respectively observed in I100 treatment of the line 22K as 58.3 spad and in I75 treatment of the line 5A2-B as 50.9 spad in the first year; in I100 treatment of the line 22K as 59.1 SPAD and in I75 treatment of the line 5A2-B as 55.1 spad. While the effects of irrigation treatments on average dry matter contents of the lines were not found to be significant, significant differences were observed in water use efficiency and kernel yield of dent corn lines (p<0.01). Current findings revealed that 22K dent corn line was prominent with regard to CWSI and chlorophyll content and the relevant line was also able to use available water holding capacity with an optimum efficiency. Therefore, it was concluded that the dent corn line (22K) could be used in further researches to improve water efficiencies.

Parameter Variability and Drought Models: A Study Using the Agricultural Reference Index for Drought (ARID)

Water stress is one of the crucial factors that determine crop yield losses. The Agricultural Reference Index for Drought (ARID) is a generic plant water stress index that estimates the level of water stress as the ratio of actual evapotranspiration to reference evapotranspiration, on a daily time step, considering a hypothetical grass reference crop. The objective of this study was to determine the effect of soil inter‐ and intraclass variability, plant characteristics, and climate locations on ARID output using global sensitivity and uncertainty analysis. Root zone depth, maximum uptake factor, the wilting point (θwp), field capacity (θfc), drainage coefficient, and runoff curve number were the ARID model parameters, while inputs consisted of rainfall and reference evapotranspiration. Global sensitivity analysis was performed for five locations in the southeastern United States and four soil types, sandy loam, silt loam, sandy clay loam, and silty clay, using three methods, the method of Sobol’, partial correlation coefficients, and partial rank correlation coefficients. Sensitivity analyses results indicated the monotonic and linear nature of ARID. The effect of location on the results of this study was minimal. Root zone depth was found to be the most influential model parameter, followed by soil hydraulic properties (i.e., θfc and θwp). Uncertainty in ARID outputs varied from soil to soil (intersoil variability), with soils having similar sand content showing similar results. Considerable uncertainty was seen in the sandier soils, indicating the need for accurate estimation of soil hydraulic properties in such soils.

Non-invasive water status detection in grapevine (Vitis vinifera L.) by thermography

Grapevines are preferentially grown under mild to moderate water stress conditions to achieve the best compromise between wine quality and quantity. Water status detection for advanced irrigation scheduling is frequently done by predawn leaf water potential (ΨPD) or leaf stomata conductance (gL) measurements. However, these measurements are time and labor consuming. Therefore, the use of infrared thermography (IRT) opens up the possibility to study large population of leaves and to give an overview on the stomatal variation and their dynamics. In the present study IRT was used to identify water stress of potted grapevines. In order to define the sensitivity of IRT measurements to water stress, the IRT-based water status information were compared with simultaneously measured ΨPD and gL data. Correlations between IRT-based CWSI data on the one hand and gL and ΨPD on the other showed the potential of IRT for water stress detection. However, the CWSI calculation procedure is laborious and the sensitivity of CWSI for water stress detection still needs to be improved. Therefore, further improvements are necessary in order to apply remote IRT-based systems for irrigation scheduling in the field. Keywords: infrared thermography, leaf temperature, stomata conductance, leaf water potential, plant water stress and crop water stress index (CWSI) DOI: 10.3965/j.issn.1934-6344.2009.0 4 .0 46 -0 54 Citation: Shamaila Zia, Klaus Spohrer, Nikolaus Merkt, Du Wenyong, Xiongkui He , Joachim Muller. Non-invasive water status detection in grapevine ( Vitis vinifera L.) by thermography. Int J Agric & Biol Eng, 2009; 2( 4 ): 46 <span style=font-size: 9.5pt; line-height: 140%; font-family:

The potential information in the temperature difference between shadow and sunlit of surfaces and a new way of retrieving the soil moisture

The thermal inertia and plant water stress index are often adopted to estimate soil moisture available for crops or plants. However, it is not very easy to obtain two temporal temperatures for thermal inertia model and air temperature for the plant water stress mode. Shadows of ground objects are often referred to noise on visible and near infrared remote sensing. But the difference of temperature between shadows and sunlit contains rich information concerning with heat-water status for soil. This paper presented a new way to excavate just by temperature difference usually between shadow and sunlit surface. Experiments validated the ideal. We can adopt thermal camera to measure the differences in the field measurements. However, we must use inversion based on multianglar thermal infrared remote sensing data in airborne and spaceborne. An inverting model was also presented by using Monte-Carlo and the least square method. Results show that this way is feasible.

Thermal Infrared Radiation for Assessing Crop Water Stress in Wheat

AbstractField studies on differentially irrigated wheat (Triticum aestivum L.) crops were conducted at Pantnagar for two years to evaluate the use of thermal infrared radiation to detect crop water stress. Data show that the stress‐degree‐day index (mid day canopy‐air temperature difference) is influenced by environmental variability other than soil moisture. Improvement in the stress‐degree‐day (SDD) index was achieved by including the measurements of vapor pressure deficit and approach was termed as plant water stress index (PWSI). Observations indicated that daily variation in SDD values due to meteorological changes was removed through PWSI. Better correlation was found between soil‐induced leaf water potential and plant water stress index than between total leaf water potential and plant water stress index. It is concluded that remote sensing of thermal infrared radiation offers a promising technique which can be incorporated into irrigation management programme.

Atmospheric Carbon Dioxide Enrichment Effects on Cotton Midday Foliage Temperature: Implications for Plant Water Use and Crop Yield1

AbstractIn an experiment designed to determine the likely consequences of the steadily rising carbon dioxide (CO2) concentration of Earth's atmosphere for the foliage temperature, water use, and yield of cotton (Gossypium hirsutum L. var. Deltapine‐61) plants, cotton was grown out‐of‐doors at Phoenix, AZ, in open‐top, clear‐polyethylene‐wall, CO2‐enrichment chambers for three summers under mean daylight CO2 concentrations of 340, 500 and 640 μmol CO2 mol−1 air on an Avondale clay loam soil [fine‐loamy, mixed (calcareous), hyperthermic Anthropic Torrifluvent]. Infrared thermometer measurements of the cotton foliage temperature (TF) indicated that a 330 to 660 μmol CO2 mol−1 air doubling of the atmospheric CO2 content results in a midday TF, increase of 1.1 °C for well‐watered cotton at Phoenix in the summer. This temperature increase was predicted to produce a 9% reduction in per‐unit‐leaf‐area plant transpiration rate and an 84% increase in crop biomass production, which compared favorably with the measured crop biomass increase of 82% for such a doubling of the air's CO2 content. These findings, together with similar findings for a second plant species—water hyacinth [Eichhornia crassipes (Mart.) Solms]—allowed us to develop a technique for assessing the effects of a 330 μmol CO2 mol−1 air CO2 concentration increase on the percentage yield increase (Y) of a crop via infrared thermometry by means of the equation Y = 7.6% × (IJ)−1, where IJ represents the Idso‐Jackson plant water stress index. If this equation holds up under further scrutiny, it could provide a rapid and efficient means for assessing the yield response of crops to atmospheric CO2 enrichment.

Foliage temperature: Effects of environmental factors with implications for plant water stress assessment and the CO2/climate connection

Throughout the summer and fall of 1985, several day‐long sets of foliage temperature measurements were obtained for healthy and potentially transpiring water hyacinth, cotton, and alfalfa plants growing in a sealed and unventilated greenhouse at Phoenix, Arizona, along with concurrent measurements of air temperature, vapor pressure and net radiation, plus, in the case of the water hyacinths, leaf diffusion resistance measurements. Some data for these plants were additionally obtained out of doors under natural conditions, while dead, nontranspiring stands of alfalfa and water hyacinth were also monitored, both out of doors and within the greenhouse. Analyses of the data revealed that plant nonwater‐stressed baselines, i.e., plots of foliage‐air temperature differential versus air vapor pressure deficit for potentially transpiring vegetation, were (1) curvilinear, as opposed to the straight lines which have so often appeared to be the case with much smaller and restricted data sets, and (2) that these baselines are accurately described by basic theory, utilizing independently measured values of plant foliage and aerodynamic resistances to water vapor transport. These findings lead to some slight adjustments in the procedure for calculating the Idso‐Jackson plant water stress index and they suggest that plants can adequately respond to much greater atmospheric demands for evaporation than what has been believed possible in the past. In addition, they demonstrate that the likely net radiation enhancement due to a doubling of the atmospheric carbon dioxide concentration will have little direct effect on vegetation temperatures, but that the antitranspirant effect of atmospheric CO2 enrichment on foliage temperature may be substantial.

An analysis of observed linear correlations between net photosynthesis and a canopy-temperature-based plant water stress index

Recent observations have shown significant linear correlations between leaf net photosynthesis and crop water stress index (CWSI), which essentially normalizes the stress-degree-day parameter for environmental variability. Resistance equations for canopy net photosynthesis ( N c) and evaporation are combined to express N c in terms of CWSI. Calculations for cotton showed that the relationship between N c and CWSI is somewhat curvilinear, when carbon dioxide (CO 2) concentration within the stomatal cavities ( C i) is taken to be a constant fraction of the ambient CO 2 concentration. However, when C i is allowed to decrease with CWSI, as per observations, the curvilinearity between N c and CWSI decreased to such an extent that the relationship between these two parameters becomes essentially linear. The present analysis suggests that the effect of soil water stress on canopy net photosynthesis under high radiation conditions could be inferred using infrared radiometric observations of canopy temperature, in addition to ambient air temperature and humidity.

Enclosing leaves for water potential measurement and its effect on interpreting soil-induced water stress

Accurate measurement of leaf xylem pressure potential ( Ψ x exp) is needed to interpret plant water deficit effects in the field. Recent work has shown that values of Ψ x exp obtained from exposed leaves differ markedly from leaves enclosed immediately prior to excision. The present study was conducted to compare the effect of using values from exposed ( Ψ x exp) and enclosed ( Ψ x enc) leaves in the calculation of the soil-induced component of plant water stress. The pressure chamber technique was modified by using a sealing compound around the petiole and a laminated aluminium foil and polyethylene envelope to enclose a leaf beginning a few seconds prior to excision until the reading was completed one or two minutes later. The data showed that, as the air vapor pressure deficit (AVPD) increased, the absolute difference between Ψ x exp and Ψ x enc increased. When AVPD = 4 kPa, this difference was 0.8 MPa. The soil-induced component of the leaf pressure potential of cotton ( Gossypium hirsutum L.) can be underestimated in absolute magnitude by up to 44% if Ψ x exp values are used in the calculation. Removing this error by using Ψ x enc values subsequently improves both the discriminating ability of the plant water stress index proposed by Idso et al. (1982) and the accuracy of the estimate of the soil-induced component of leaf xylem pressure potential.

Quantifying effects of atmospheric CO 2 enrichment on stomatal conductance and evapotranspiration of water hyacinth via infrared thermometry

Measurements of stomatal conductance and evaporative water loss from two tanks of water hyacinths growing at Phoenix, AZ, one under ambient conditions and one considerably enriched in atmospheric CO 2, are reported. Stomatal conductances of plants in the CO 2-enriched treatment were reduced to values half as great as those of plants in the ambient treatment at a mean mid-day CO 2 concentration of 550 ppm, which resulted in a 22% decrease in total evaporative water loss; while in going from an ambient CO 2 concentration of 310 ppm to a doubled concentration of 620 ppm there was a 27% decrease in evaporative water loss. Both of these physiological responses were well characterized by the Idso—Jackson plant water stress index. Additionally, it was found that the stomatal response to increasing atmospheric CO 2 was identical to that induced by removing water from the plant roots, and that the reduction in evaporative water loss with increasing atmospheric CO 2 was an inverse linear function of the plant water stress index — both of which phenomena had previously been theorized but never before experimentally verified.

Stomatal conductance and photosynthesis in water hyacinth: Effects of removing water from roots as quantified by a foliage-temperature-based plant water stress index

Stomatal conductance and net photosynthesis measurements were made over a period of a week in mid-October on two stands of water hyacinths floating in sunken metal stock tanks at Phoenix, AZ. On the second day of the experiment, all free water in one of the tanks was removed. Foliage temperature measurements were subsequently used to quantify the water stress experienced by the water-robbed plants; and a plant water stress index derived from the foliage temperature and air vapor pressure deficit data was used to study the effects of developing water stress on the plant physiological parameters being measured. The results obtained were practically identical to those derived from two independent season-long studies of water stress effects in cotton: net photosynthesis decreased linearly to become negative at a plant water stress index of 0.9 (where 1.0 represents the maximum possible stress), while a parameter related to plant water use efficiency first increased with increasing stress to reach a maximum at a plant water stress index of 0.6, after which it dropped off rapidly to zero with additional stress.

Effect of Wind on the Crop Water Stress Index Derived by Infrared Thermometry1

AbstractA new plant or crop water stress index (CWSI) that depends on interpreting crop or foliage‐to‐air temperature differential (Tf‐Ta) shows promise in both research and irrigation water management. The method relies on the unique relation between Tf‐Ta, and atmospheric water vapor pressure deficit (VPD). However, the method of estimating the upper limit of Tf‐Ta, a critical step in CWSI calculation, was questionable and lacked field validation. We measured the upper limit of Tf‐Ta, and attendant micrometeorological variables on severely water stressed sorghum (Sorghum bicolor L.), corn (Zea mays L.), and bean (Phaseolus vulgaris L.) grown on a Typic Xerotherent, and cotton (Gossypium hirsutum L.) grown on a Typic Torriorthent. Actual measured values of the upper limit of Tf‐Ta ranged from 2.5 to 8.5 °C across crops. The poor agreement between actual and calculated values was analyzed with respect to net radiation, VPD, and windspeed. Windspeed was found to be the primary factor causing erroneous estimation of the upper limit of Tf‐ Ta, and hence CWSI values. Crop water stress index values measured at low windspeed overestimated the level of water stress while those measured at high windspeed underestimated it. Crop specific changes in the boundary layer resistance‐windspeed relationship partially explain the discrepancy between measured and calculated Tf‐Ta, upper limit values. Modification of the CWSI to account for the influence of windspeed is discussed.