Crop Water Stress Index Values Research Articles

Crop water stress mapping for site-specific irrigation by thermal imagery and artificial reference surfaces

Variable-rate irrigation by machines or solid set systems has become technically feasible, however mapping crop water status is necessary to match irrigation quantities to site-specific crop water demands. Remote thermal sensing can provide such maps in sufficient detail and in a timely way. In a set of aerial and ground scans at the Hula Valley, Israel, digital crop water stress maps were generated using geo-referenced high-resolution thermal imagery and artificial reference surfaces. Canopy-related pixels were separated from those of the soil by upper and lower thresholds related to air temperature, and canopy temperatures were calculated from the coldest 33% of the pixel histogram. Artificial surfaces that had been wetted provided reference temperatures for the crop water stress index (CWSI) normalized to ambient conditions. Leaf water potentials of cotton were related linearly to CWSI values with R2 = 0.816. Maps of crop stress level generated from aerial scans of cotton, process tomatoes and peanut fields corresponded well with both ground-based observations by the farm operators and irrigation history. Numeric quantification of stress levels was provided to support decisions to divide fields into sections for spatially variable irrigation scheduling.

ÍNDICE DE ESTRÉS HÍDRICO DEL CULTIVO DE TOMATE DE CÁSCARA

The infrared thermometer used to measure crop water stress index (CWSI) is a reliable tool for irrigation scheduling, which, combined with efficient irrigation systems can maximize crop productivity. A study was conducted to determine the crop water stress index in husk tomato, or tomatillo, (Physalis ixocarpa Brot.) under a drip irrigation system, its relationship with irrigation depth and plastic mulch in scheduling irrigation and predicting fruit yield. The experiment design was completely randomized with three replicates Treatments consisted of five irrigation depths (replacement of 40, 60, 80, 100 and 120 % of the reference evapotranspiration estimated by the Penman-Monteith method). CWSI was estimated using infrared radiation gun measurements of canopy temperature, air temperature, and relative humidity, and water vapor pressure deficit was calculated. The equation which defines the lower limit expresses the relationship between vapor pressure deficit (VPD) and temperature difference (crop and air (Tc-Ta)). When the crop transpires, the relationship is: Tc -Ta = 1.21 - 131 DPV (r2 = 0.68, P <0.01, n = 42), and the upper limit (stressed) was 2.8 °C, when transpiration stops. Fruit yield showed a positive linear correlation with average CWSI values: Y = 52.53-69.7CWSI, (r2 = 0.65, P<0.01 and n=30). Prediction models of CWSI and means of the effect of irrigation water and plastic mulch were fit with r2 = 0.87 to 0.96, P<0.01 and n=30. The CWSI increases linearly when the soil water potential decreases.

Energy balance and crop water stress in winter maize under phenology-based irrigation scheduling

In eastern India, cultivation of winter maize is getting popular after rainy season rice and farmers practice irrigation scheduling of this crop based on critical phenological stages. In this study, crop water stress index of winter maize at different critical stages wase determined to investigate if phenology-based irrigation scheduling could be optimized further. The components of the energy budget of the crop stand were computed. The stressed and non-stressed base lines were also developed (between canopy temperature and vapor pressure deficit) and with the help of base line equation, [(Tc − Ta) = −1.102 VPD − 3.772], crop water stress index (CWSI) was determined from the canopy-air temperature data collected frequently throughout the growing season. The values of CWSI (varied between 0.42 and 0.67) were noted just before the irrigations were applied at critical phenological stages. The soil moisture depletion was also measured throughout the crop growing period and plotted with CWSI at different stages. Study revealed that at one stage (silking), CWSI was much lower (0.42–0.48) than that of recommended CWSI (0.60) for irrigation scheduling. Therefore, more research is required to further optimize the phenology-based irrigation scheduling of winter maize in the region. This method is being used now by local producers. The intercepted photosynthetically active radiation and normalized difference vegetation index over the canopy of the crop were also measured and were found to correlate better with leaf area index.

Development of crop water stress index of wheat crop for scheduling irrigation using infrared thermometry

This study was conducted to develop the relationship between canopy–air temperature difference and vapour pressure deficit for no stress condition of wheat crop (baseline equations), which was used to quantify crop water stress index (CWSI) to schedule irrigation in winter wheat crop ( Triticum aestivum L.). The randomized block design (RBD) was used to design the experimental layout with five levels of irrigation treatments based on the percentage depletion of available soil water (ASW) in the root zone. The maximum allowable depletion (MAD) of the available soil water (ASW) of 10, 40 and 60 per cent, fully wetted (no stress) and no irrigation (fully stressed) were maintained in the crop experiments. The lower (non-stressed) and upper (fully stressed) baselines were determined empirically from the canopy and ambient air temperature data obtained using infrared thermometry and vapour pressure deficit (VPD) under fully watered and maximum water stress crop, respectively. The canopy–air temperature difference and VPD resulted linear relationships and the slope ( m) and intercept ( c) for lower baseline of pre-heading and post-heading stages of wheat crop were found m = −1.7466, c = −1.2646 and m = −1.1141, c = −2.0827, respectively. The CWSI was determined by using the developed empirical equations for three irrigation schedules of different MAD of ASW. The established CWSI values can be used for monitoring plant water status and planning irrigation scheduling for wheat crop.

Non-Contacting Techniques for Plant Drought Stress Detection

Plant drought stress indicators such as crop water stress index (CWSI), plant motion in the form of covariance of top-projected canopy area (COVTPCA), leaf water content represented as equivalent water thickness (EWT), and their threshold values for drought stress detection were established from measurements. Performances of these indicators in detecting drought stress of New Guinea Impatiens plants in a controlled environment were evaluated. Analysis of variance (ANOVA) was conducted to compare the timing of drought stress detection by these indicators against the timing of incipient drought stress defined by evapotranspiration (ET) and timing of human visual detection. Statistical analysis was also performed to study the consistency of the threshold values of the indicators in different experiments. ANOVA results showed that the CWSI was the most reliable indicator for early plant drought stress detection. The timing of the drought stress detection from the earliest to the latest was CWSI, EWT, and COVTPCA. While COVTPCA and EWT were not able to detect drought stress as early as CWSI, ANOVA results indicated that these two indicators were able to detect drought stress no later than visual detection. ANOVA results also showed that there was no significant difference in threshold values of CWSI and COVTPCA in different experiments, but different cultivars used in the experiments resulted in significant differences in EWT threshold values.

Surface energy fluxes and crop water stress index in groundnut under irrigated ecosystem

Reliable estimation of surface sensible and latent heat flux is the most important process to appraise energy and mass exchanges among atmosphere, hydrosphere and biosphere. In this study the surface energy fluxes were measured over irrigated groundnut during winter (dry) season using Bowen ratio ( β) micrometeorological method in a representative groundnut growing areas of eastern India, i.e. Dhenkanal, Orissa. The crop was grown with four irrigations based on phenological stages viz., (i) branching, (ii) pegging, (iii) pod development and (iv) seed filling and assessed what the crop stress was at those times to see if irrigation scheduling cold be optimized further. Study revealed that the net radiation ( R n) varied from 393–437 to 555–612 W m −2 during two crop seasons (2004–2005 and 2005–2006). The soil heat flux ( G) was higher (37–68 W m −2) during initial and senescence growth stages as compared to peak crop growth stages (1.3–17.9 W m −2). The latent heat flux (LE) showed apparent correspondence with the growth which varied between 250 and 434 W m −2 in different growth stages. The diurnal variation of Bowen ratio ( β) revealed that there was a peak in the morning (9.00–10.00 a.m.) followed by a sharp fall with the mean values varied between 0.24 and 0.28. The intercepted photosysnthetic photon flux density or photosysntehtically active radiation (IPAR) by the crop was also measured and relationship between IPAR and leaf area index (LAI) was established with days after sowing. This relationship will be useful in developing algorithm of crop simulation model for predicting LAI or IPAR. The stressed and non-stressed base lines were also developed by establishing relationship between canopy temperature and vapour pressure deficit (VPD). With the help of base line equation, [( T c − T a) = −1.32VPD + 2.513], crop water stress index (CWSI) was derived on canopy-air temperature data collected frequently throughout the growing season. The soil moisture depletion was measured throughout the crop growing period and plotted with CWSI at different stages. The values of CWSI (varied between 0.45 and 0.64) were noted just before the irrigations were applied based on phenological stages. Study revealed that at two stages (branching and pegging), CWSI were much lower (0.46–0.49) than that of recommended CWSI (0.60) for irrigation scheduling. Therefore, more research is required to optimize the phenology based irrigation scheduling further in the region, which method is using now by local producers.

Evaluation of a crop water stress index for irrigation scheduling of bermudagrass

This study was conducted to assess crop water stress index (CWSI) of bermudagrass used widely on the recreational sites of the Mediterranean Region and to study the possibilities of utilization of infrared thermometry to schedule irrigation of bermudagrass. Four different irrigation treatments were examined: 100% ( I 1), 75% ( I 2), 50% ( I 3), and 25% ( I 4) of the evaporation measured in a Class A pan. In addition, a non-irrigated treatment was set up to determine CWSI values. The status of soil water content and pressure was monitored using a neutron probe and tensiometers. Meanwhile the canopy temperature of bermudagrass was measured with the infrared thermometry. The empirical method was used to compute the CWSI values. In this study, the visual quality of bermudagrass was monitored seasonally using a color scale. The best visual quality was obtained from I 1 and I 2 treatments. Average seasonal CWSI values were determined as 0.086, 0.102, 0.165, and 0.394 for I 1, I 2, I 3, and I 4 irrigation treatments, respectively, and 0.899 for non-irrigated plot. An empirical non-linear equation, Q ave = 1 + 6 [ 1 + ( 4.853 CWS I ave ) 2.27 ] − 0.559 , was deduced by fitting to measured data to find a relation between quality and average seasonal CWSI values. It was concluded that the CWSI could be used as a criterion for irrigation timing of bermudagrass. An acceptable color quality could be sustained seasonally if the CWSI value can be kept about 0.10.

Canopy-air temperature differential for potato under different irrigation regimes*

The main objective of this study was to determine canopy-air temperature differential, which can be used to quantify crop water stress index (CWSI) for potato. The crop was cultivated under furrow irrigation and subjected to three irrigation regimes in which irrigation was applied when 30, 50 and 70% of the available water holding capacity was consumed, and three irrigation levels (100, 50 and 0% replenishment of soil water depleted). The highest yield and water use was obtained under non-water-stressed treatments (100% replenishment of soil water depleted) for each irrigation regime. The lower (non-stressed) and upper (stressed) baselines were determined empirically from measurements of canopy temperatures, ambient air temperatures and vapour pressure deficit values and the CWSI was calculated with three irrigation levels for each irrigation regime. Trends in CWSI values were consistent with the soil water contents induced by the deficit irrigations. Average CWSI before irrigation values of 0.49 for irrigation when 30% of the available water holding capacity was consumed, 0.55 for irrigation when 50% of the available water holding capacity was consumed and 0.69 for irrigation when 70% of the available water holding capacity was consumed, produce maximum tuber yield. The yield was also directly correlated with seasonal CWSI values.

IRRIGATION SCHEDULING FOR WATERMELON WITH CROP WATER STRESS INDEX (CWSI)

This study was designed to evaluate different threshold values of crop water stress index (CWSI) to schedule irrigation for watermelon (Citrullus vulgaris) grown with drip irrigation Irrigations were started when CWSI values reached to 0.2, 0.4, 0.6, 0.8 and 1.0 (non-irrigation). The CWSI values were computed from measurements of canopy temperature, air temperature and vapor pressure defi cit. The total irrigations amount of 342, 280, 248 and 193 mm were applied to the 0.2, 0.4, 0.6 and 0.8 CWSI treatments, respectively. The maximum seasonal evapotranspiration (ET) as 412 mm was measured from 0.2 CWSI treatment. Irrigation levels signifi cantly affected fruit yield. Although the highest fruit yield (76.3 t ha-1) was obtained from the 0.2 CWSI treatment, the 0.4 and 0.6 of CWSI treatments were statistically in the same letter group with this treatment. Also, maximum water use effi ciency (WUE) and irrigation water use efficiency (IWUE) were obtained from 0.6 of CWSI treatment as 22.1 and 13.3 kg m-3, respectively. Therefore, based on these results, 0.6 of CWSI value should be used for irrigation time of watermelon under Tekirdag, Turkey conditions.

Application of a new method to evaluate crop water stress index

Optimum water management and irrigation require timely detection of crop water condition. Usually crop water condition can be indicated by crop water stress index (CWSI), which can be estimated based on the measurements of either soil water or plant status. Estimation of CWSI by canopy temperature is one of them and has the potential to be widely applied because of its quick response and remotely measurable features. To calculate CWSI, the conventional canopy-temperature-based model (Jackson’s model) requires the measurement or estimation of the canopy temperature, the maximum canopy temperature (T cu), and the minimum canopy temperature (T cl). Because extensive measurements are necessary to estimate T cu and T cl, its application is limited. In this study, by introducing the temperature of an imitation leaf (a leaf without transpiration, T p) and based on the principles of energy balance, we studied the possibility to replace T cu by T p and reduce the included parameters for CWSI calculation. Field experiments were carried out in a winter wheat (Triticum aestivum L.) field in Luancheng area, Hebei Province, the main production area of winter wheat in China. Six irrigation treatments were established and soil water content, leaf water potential, soil evaporation rate, plant transpiration rate, biomass, yield, and regular meteorological variables of each treatment were measured. Results indicate that the values of T cu agree with the values of T p with a regression coefficient r=0.988. While the values of CWSI estimated by the use of T p are in agreement with CWSI by Jackson’s method, with a regression coefficient r=0.999. Furthermore, CWSI estimated by the use of T p has good relations with soil water content and leaf water potential, showing that the estimated CWSI by T p is a good indicator of soil water and plant status. Therefore, it is concluded that T cu can be replaced by T p and the included parameters for CWSI calculation can be significantly reduced by this replacement.

Crop water stress index for potato under furrow and drip irrigation systems

This study was conducted to determine the crop water stress index (CWSI) for potato (Solanum tuberosum L.) grown under furrow and drip irrigation methods and subjected to three different irrigation levels (100, 50 and 0% replenishment of soil water depleted). The lower (non-stressed) and upper (stressed) baselines were determined empirically from measurements of canopy temperatures, ambient air temperatures and vapor pressure deficit values. Tuber yield decreased when mean CWSI prior to irrigation exceeded 0.68 in furrow and 0.81 in drip irrigation. The tuber yield was directly correlated with the seasonal CWSI values and the linear equations for furrow and drip irrigation methods, Y = −45.82 CWSI + 50.69 and Y = −52.65 CWSI + 58.44, respectively, can be used for yield prediction.

The effects of different irrigation regimes on cucumber ( Cucumbis sativus L.) yield and yield characteristics under open field conditions

A study was conducted to determine the effects of different drip irrigation regimes on yield and yield components of cucumber ( Cucumbis sativus L.) and to determine a threshold value for crop water stress index (CWSI) based on irrigation programming. Four different irrigation treatments as 50 (T-50), 75 (T-75), 100 (T-100) and 125% (T-125) of irrigation water applied/cumulative pan evaporation (IW/CPE) ratio with 3-day-period were studied. Seasonal crop evapotranspiration (ET c) values were 633, 740, 815 and 903 mm in the 1st year and were 679, 777, 875 and 990 mm in the 2nd year for T-50, T-75, T-100 and T-125, respectively. Seasonal irrigation water amounts were 542, 677, 813 and 949 mm in 2002 and 576, 725, 875 and 1025 mm in 2003, respectively. Maximum marketable fruit yield was from T-100 treatment with 76.65 t ha −1 in 2002 and 68.13 t ha −1 in 2003. Fruit yield was reduced significantly, as irrigation rate was decreased. The water use efficiency (WUE) ranged from 7.37 to 9.40 kg m −3 and 6.32 to 7.79 kg m −3 in 2002 and 2003, respectively, while irrigation water use efficiencies (IWUE) were between 7.02 and 9.93 kg m −3 in 2002 and between 6.11 and 8.82 kg m −3 in 2003. When the irrigation rate was decreased, crop transpiration rate decreased as well resulting in increased crop canopy temperatures and CWSI values and resulted in reduced yield. The results indicated that a seasonal mean CWSI value of 0.20 would result in decreased yield. Therefore, a CWSI = 0.20 could be taken as a threshold value to start irrigation for cucumber grown in open field under semi-arid conditions. Results of this study demonstrate that 1.00 IW/CPE water applications by a drip system in a 3-day irrigation frequency would be optimal for growth in semiarid regions.

Measurement and Analyses of Growth and Stress Parameters of Viburnum odoratissimum (Ker-gawl) Grown in a Multi-pot Box System

Two colors (white and black) of a recently introduced irrigation-plant production system [multi-pot box system (MPBS)] for container-grown nurseries were researched and results were compared with those obtained from the sprinkler-irrigated conventional (control) system (CS). Experiments were carried out in summer and fall of 2001 in Gainesville, Fla. Plant growth [growth index (GI), growth rate (GR), and dry matter] and stress parameters [stomatal resistance (rs), crop water stress index (CWSI), plant water potential (PWP), and substrate temperature (ST)] were measured and analyzed for Viburnum odoratissimum (Ker-gawl). In both seasons, plants grown in the white MPBS had significantly higher GI and GR as compared to the plants in the black MPBS and CS. In summer, plants in the white MPBS reached marketable size about 17 days and 86 days earlier than those in the black MPBS and CS, respectively. In fall, they reached marketable size about 25 and 115 days earlier than those plants in the black MPBS and CS, respectively. Plants in the white and black MPBSs showed exponential growth rate in summer with plants in the white MPBS having significantly higher growth rate (greater slope) than the other two treatments. In both seasons, plants in the white MPBS produced the highest amount of dry matter. In general, plants in the white MPBS had lower rs values to vapor transport compared to the other two treatments, and the black MPBS treatment had lower rs values than the CS in both seasons. The CWSI values of the plants in both white and black MPBSs were significantly lower than the CS. In both seasons, ST in the black MPBS and CS exceeded the critical value of 40 °C several times. The ST of &gt;40 °C is often reported to significantly reduce the plant growth and cause root death and/or injury for container-grown plants. Overall, the white MPBS provided a better environment for root development and plant growth under these experimental conditions. Results strongly suggest that there is a potential opportunity of using MPBS for irrigation and production of nursery plants. These important findings suggest that, in practice, producing nursery plants in a shorter period of time by using white MPBS will result in significant savings of energy, water, chemicals, and other inputs and thereby reducing the costs and increasing profits.

Use of Infrared Thermometry for Developing Baseline Equations and Scheduling Irrigation in Wheat

This study was conducted to develop baseline equations, which can be used to quantify crop water stress index (CWSI) for evaluating crop water stress in three winter wheat genotypes (Triticum aestivum L.) and to schedule irrigation and predict yield. Plants were grown under basin irrigation and subjected to five water treatments ranging from 100 to 0 % (100, 75, 50, 25, 0 %) replacement of evapotranspirational losses within 0.90 m soil profile. The highest yield and water use was obtained under fully irrigated conditions (100 % replenishment of soil water depleted). The lower (non-stressed) and upper (stressed) baselines were determined empirically from measurements of canopy and ambient air temperatures and vapour pressure deficit (VPD) on fully watered plants (100%) and under maximum water stress (0 %), respectively. The CWSI was determined by using the empirical approach for the five irrigation levels. The yield was directly correlated with the mean CWSI values and the linear equation for three genotypes (Saraybosna, Kate-A-1 and F-85), “Y = 1463.3 - 1062.3 CWSI”, “Y = 1483.8 - 1052.8 CWSI” and “Y = 1701.8 - 1367.7 CWSI” can be used for the yield prediction. CWSI values may also provide a valuable tool for monitoring water status and planning irrigation scheduling for wheat.

Crop water stress index for watermelon

This study was conducted to determine the suitability of a crop water stress index (CWSI) to schedule irrigation for watermelon ( Citrullus vulgaris) grown with trickle irrigation. The effects of five irrigation levels (100, 75, 50, 25 and 0% replenishment of soil water depleted from 0.90 m soil profile depth) on watermelon yields and resulting CWSI were investigated. The highest yield and water use was obtained under fully irrigated conditions (100% replenishment of soil water depleted) in 2 years. The CWSI was calculated from measurements of infrared canopy temperatures, ambient air temperatures and vapor pressure deficit values for five irrigation levels. The trends in CWSI values were consistent with the soil water contents induced by the deficit irrigations. Unlike the yield, CWSI increased with increased soil water deficit. An average CWSI of about 0.41 before irrigation produced the maximum yield. The yield was directly correlated with mean CWSI values and the linear equation ‘ Y=91.143−66.077 CWSI’ can be used for yield prediction. The CWSI value was useful for evaluating crop water stress in watermelon and should be useful for timing irrigation and predicting yield.

Determination of water stress index in sunflower

This study was conducted to quantify crop water stress index (CWSI) based on single leaf temperatures of sunflower and to determine if CWSI values were correlated with other measures of plant water stress. Plant were grown under furrow irrigation and subjected to five water treatments ranging from 100 to 0% (100, 75, 50, 25, 0%) replacement of evapo transpirational losses within 0.90 in soil profile. The yield and water use of fully irrigated sunflower were highest in both years. Trends in CWSI values were consistent with the soil water contents induced by deficit irrigations. An average CWSI of 0.59 before irrigation times produced maximum yield. An equation for calculating yield potential of sunflower was developed using the relationship between yield and seasonal mean CWSI. Moreover, statistically significant correlations were found between CWSI calculated from single leaf temperatures and stomatal resistance, leaf area index (LAI) and available water in the root zone.

Use of crop water stress index for monitoring water status and scheduling irrigation in wheat

The crop water stress index (CWSI) is a valuable tool for monitoring and quantifying water stress as well as for irrigation scheduling. This study was conducted during the 1990 and 1991 growing seasons at the Colorado State University Horticulture Farm near Fort Collins, CO, USA (40°35′N latitude, 105°05′W longitude and 1524 m elevation). The main objective was to develop a baseline equation, which can be used to calculate CWSI for monitoring water status and irrigation scheduling of wheat. The difference in crop canopy to air temperature ( T c− T a), measured above a crop was negatively related to the atmospheric vapor pressure deficiency (AVPD) [ R 2=0.88 and p=0.0001]. However, this relationship between ( T c− T a) and AVPD can be used to develop a non-stressed baseline equation and consequently the crop water stress index (CWSI). By using non-water-stressed baseline on data collected frequently through the growing season, CWSI values may provide a valuable tool for monitoring water status and planning irrigation scheduling for wheat and which is extendable to other similar agricultural crops.

Determination of Crop Water Stress Index for Irrigation Timing and Yield Estimation of Corn

Corn (Zea mays L.) grown under a Mediterranean semiarid climate requires supplemental irrigation to maximize the grain yield. Since the cost of irrigation application has been increasing, elimination of unnecessary irrigation applications would improve economics of corn production. There has been much interest in the crop water stress index (CWSI) as a potential tool for irrigation scheduling and yield estimation. An experiment was conducted to monitor and quantify water stress, and to develop parameters for irrigation scheduling and grain yield of summer‐grown corn as a function of CWSI under Mediterranean semiarid cropping conditions. Three irrigation treatments were based on replenishing the 0.9‐m deep root zone to field capacity when the soil water level dropped to 25, 50, and 75% of available water holding capacity (AWHC). A dryland treatment was also included. The lower (nonstressed) and upper (stressed) baselines were measured to calculate CWSI. An equation that can be used to calculate the yield potential of summer‐grown corn under a Mediterranean climate was developed using the relationship between the corn grain yield and the seasonal mean CWSI. Permitting the seasonal average CWSI value to exceed more than 0.22 resulted in decreased corn grain yield. The CWSI behaved as expected, dropping to near zero following an irrigation and increasing gradually as corn plants depleted soil water reserves. We concluded that CWSI is a useful tool to monitor and quantify the water stress of corn under a Mediterranean climate.

Evaluation of crop water stress index for LEPA irrigated corn

This study was designed to evaluate the crop water stress index (CWSI) for low-energy precision application (LEPA) irrigated corn (Zea mays L.) grown on slowly-permeable Pullman clay loam soil (fine, mixed, Torrertic Paleustoll) during the 1992 growing season at Bushland, Tex. The effects of six different irrigation levels (100%, 80%, 60%, 40%, 20%, and 0% replenishment of soil water depleted from the 1.5-m soil profile depth) on corn yields and the resulting CWSI were investigated. Irrigations were applied in 25 mm increments to maintain the soil water in the 100% treatment within 60–80% of the “plant extractable soil water” using LEPA technology, which wets alternate furrows only. The 1992 growing season was slightly wetter than normal. Thus, irrigation water use was less than normal, but the corn dry matter and grain yield were still significantly increased by irrigation. The yield, water use, and water use efficiency of fully irrigated corn were 1.246 kg/m2, 786 mm, and 1.34 kg/m3, respectively. CWSI was calculated from measurements of infrared canopy temperatures, ambient air temperatures, and vapor pressure deficit values for the six irrigation levels. A “non-water-stressed baseline” equation for corn was developed using the diurnal infrared canopy temperature measurements as Tc–Ta = 1.06–2.56 VPD, where Tc was the canopy temperature (°C), Ta was the air temperature (°C) and VPD was the vapor pressure deficit (kPa). Trends in CWSI values were consistent with the soil water contents induced by the deficit irrigations. Both the dry matter and grain yields decreased with increased soil water deficit. Minimal yield reductions were observed at a threshold CWSI value of 0.33 or less for corn. The CWSI was useful for evaluating crop water stress in corn and should be a valuable tool to assist irrigation decision making together with soil water measurements and/or evapotranspiration models.

Evaluation of Resistances for Bermudagrass Turf Crop Water Stress Index Models

AbstractEvaluation of the Penman‐Monteith‐based crop water stress index (CWSI), as a potential indicator of turf irrigation timing, requires reliable estimates of potential (minimum) canopy resistance, rcp, and aerodynamic resistance, ra. This paper compares various methods of estimating rcp and ra for evaluation of water stress of bermudagrass turfgrass (Cynodon dactylon cv. Midiron) in the desert Southwest of the USA. First, two empirical CWSI models were developed using field data collected from well‐watered and severely water‐stressed turf plots during the 1986 growing season in Tucson, AZ. Second, the regression constants in the empirical lower limits of CWSI (canopy temperature minus air temperature, Tc − Ta, of nonwater‐stressed turf) and the parameters associated with similar terms (net radiation and vapor pressure deficit) in the Penman‐Monteith equation were equated, thus solving for rcp and ra. This yielded mean rcp and ra values of 79 and 13 s m−1, respectively. Using these estimates, the predicted lower limits of Tc − Ta from the Penman‐Monteith equation were in good agreement with values measured over well‐watered turf. However, the predicted Tc − Ta for severely water‐stressed turf agreed very poorly with the measured values. The use of the log‐law equation yielded independent estimates of ra generally greater than 45 s m−1 and resulted in erroneous estimates of Tc − Ta by the Penman‐Monteith equation. Using a crop‐based method and data from the turf plots, estimates of the resistances were made in which constant rcp and ra values of 62.5 and 20 s m−1, respectively, were found. The crop‐based method was found to be not only simple to use, but also performed satisfactorily in predicting Tc − Ta. A well‐defined curvilinear relationship between estimates of midday canopy resistance (rc) and CWSI was determined for a wide range of turf water stress, implying that these two estimates nearly equally reflect the state of turf water stress. Our findings also suggest threshold midday CWSI and rc values for short bermudagrass turf of 0.16 and 125 s m−1, respectively, to indicate the need for irrigation.