Crop Water Stress Index Values Research Articles

Evaluation of on-farm irrigation scheduling methods for potatoes

The neutron probe, infrared thermometry and crop water stress index (CWSI), and a computer-assisted irrigation scheduling method were evaluated in terms of their effect on tuber yield, tuber quality, and water use. The experiment was conducted during 1990 and 1991 near Othello, central Washington, using Russet Burbank potatoes grown in a silt loam soil. Irrigation treatments did not commence until after tuber initiation. In general, no differences in total number of tubers and total tuber yield resulted from the different scheduling methods. However, the canopy temperature method showed reduction in the yield of number one tubers in 1990. The least total irrigation water was applied during the growing season with the neutron probe method. Using CWSI values above 0.5 to 1.0 (scale 0 to 10) for two consecutive days as a threshold to schedule irrigations appeared to be adequate for potatoes grown in silt loam soils. However, shortcomings of infrared thermometry suggested that this method may not be practical for scheduling irrigation of potatoes.

Red Clover Seed Production: I. Crop Water Requirements and Irrigation Timing

Information about water management and crop stress response for red clover (Trifolium pratense L.) seed production is limited. This study determined red clover seed crop water requirements and evaluated crop water stress index (CWSI) and fraction of available soil‐water used for timing supplemental irrigation. Experiments were conducted at Corvallis, OR, on a Woodburn silt loam (fine‐silty, mixed, mesic Aquultic Argixeroll) in 1990 and 1991. Six irrigation treatments were used to assess within‐ and between‐crop season water requirements. As a consequence of root disease in 1991, the fraction of availables oil water used by plants in the different treatments was generally less in 1991 than 1990. Also, seed production water‐use efficiency was lower in 1991 than 1990. Two distinct non‐stressed baselines for canopy‐air temperature differences versus vapor pressure deficit were identified that were related to increasing leaf senescence and crop aging. The CWSI values across all treatments were negatively associated with plant water potentials (r = −0.87). The CWSI values were similar within similar treatments in both years indicating that root rot damage rather than available soil‐water was limiting plant water uptake in 1991. The CWSI generally increased faster than fraction of available soil‐water used. Unlike fraction of available soil‐water used, CWSI integrated the total plant environment, detecting changes due to root disease in 1991. Under the conditions of this experiment, a single irrigation filling the actives oil profile at CWSI ≈0.28 substantially increased seed yield compared with the nonirrigated control. The CWSI was a useful indicator of red clover water stress status and can be used to schedule clover seed irrigations under typical western Oregon climatic conditions.

Quantifying Wheat Water Stress with the Crop Water Stress Index to Schedule Irrigations

AbstractThe relationship between the timing and amount of applied irrigation water and grain yield has not been well defined for durum wheat (Triticum turgidum L. var. durum) grown under irrigation in arid regions. With rising water costs and decreasing water supplies, grain growers will have to optimize yield for given levels of water inputs if they are to remain profitable. Small‐plot field studies were conducted in 1986 and 1987 at Marana, AZ, to (i) test the feasibility of using the crop water stress index (CWSI) to schedule wheat irrigations and (ii) determine relationships among CWSI, grain production, and water applied. Highest grain production was attained when irrigations were scheduled at CWSI values averaging 0.37 units in 1986 and 0.30 units in 1987. Scheduling irrigations with lower CWSI values did not increase grain production, but required more water. Delaying irrigation by scheduling irrigations at CWSI values exceeding the optimum values reduced grain production.

Crop Water Stress Index Models for Bermudagrass Turf: A Comparison

AbstractA field study assessed the accuracy and consistency of crop water stress index (CWSI) estimates determined by two empirical models and one theoretical model for evaluation of bermudagrass [Cynodon dactylon (L.) Pers. cv. Midiron] water status. The simplest empirical model, describing canopy temperature minus air temperature (Tc — Ta) of well‐watered turf as a linear function of air vapor pressure deficit, was inadequate for estimating CWSI, since seasonal averages of CWSI for well‐watered, moderately stressed, and stressed turf had corresponding coefficients of variation of 1170, 160, and 86, respectively. The CWSI values were also highly influenced by net radiation, as indicated by coefficients of determination for regressions of calculated CWSI on net radiation of 0.68, 0.76, and 0.33 for the three levels of irrigation treatments, respectively. The second empirical model included net radiation in the regression analysis of of Tc — Ta. The inclusion of net radiation in the equation for the unstressed baseline increased the coefficient of determination from 0.76 to 0.90 as compared with Model 1. The coefficient of determination for the upper baseline (which included net radiation) was 0.872, indicating a strong dependency of CWSI on net radiation. The theoretical CWSI model yielded the most accurate estimates of turfgrass water stress, as indicated by coefficients of variation for seasonal averages of CWSI for well‐watered, moderately stressed, and stressed turf of 76, 3, and 86, respectively. Midday estimates of CWSI were related to percent available extractable water, with coefficients of determination for a regression of midday CWSI on percent available extractable water of 0.988, 0.989, and 0.991 for the empirical, modified empirical, and theoretical models, respectively. The theoretical CWSI appears to be the most promising approach for a turfgrass crop water stress index for irrigation scheduling.

Evaluation and interpretation of the crop water stress index for well-watered potatoes

During the 1988, 1989 and 1990 growing seasons, canopy and air temperatures, solar and net radiation, relative humidity and wind speed were measured at 10 minute intervals during daytime hours in a field of wellwatered Russet Burbank potatoes in southeast Idaho. A linear relationship between canopy temperature minus air temperature and vapor pressure deficit (TC-TA = 0.2 -2.0 VPD) was developed for use in the Crop Water Stress Index (CWSI). Values of CWSI for well-watered potatoes were generally in the -0.4 to 0.4 range, and exhibited a diurnal trend of decreasing values towards midday and increasing values during the afternoon. Wind speed appeared to increase the CWSI and may account for some of the observed variability.

Alfalfa Yield and Plant Water Relations with Variable Irrigation

In the arid and semiarid western USA, alfalfa (Medicago sativa L.) grown for forage requires large amounts of water for high productivity. Managing alfalfa to achieve the best possible water‐use efficiency (WUE) is essential for the crop to remain competitive for water supplies both within and outside agriculture. This study was conducted in the San Joaquin Valley of California, to define alfalfa forage yield and plant water relation responses of three alfalfa cultivars (CUF 101, Moapa 69, and WL 318) to contrasting irrigation intensities and establish critical plant water‐status values for irrigation scheduling. A single line‐source sprinkler system provided a variable water supply. The soil was a Hanford sandy loam, silty substratum (coarse‐loamy, mixed, nonacid, thermic typic xerorthent). Maximum total season crop evapotranspiration (ETc) of 1000 mm gave 26.3 Mg ha−1 of hay yield that was similar for the three cultivars. A linear hay yield (Yh)−ETc relationship was defined as Yh = −0.212 + 0.0265ETc (r2 = 0.82). Water‐use efficiency, 23.1 Mg ha−1 dry matter per meter of water used as ETc, was comparable with other C3 species. As midday plant water potential (ψw) declined below −1 MPa, yield reductions were observed for all cultivars. Increased crop water stress index (CWSI) was correlated (r = 0.84) with declining ψw below the −1 MPa yield‐limiting ψw threshold. Absolute values of ψw or CWSI associated with a given yield reduction were cultivar‐dependent; Moapa 69 and WL 318 responded alike, but each differed from CUF 101. Plant‐based water‐status measurements provided a practical and reliable index for assessing the adequacy of supplied irrigation water.

Water Stress and Storage Environment Affect Pillowy Fruit Disorder in Cucumber

Mean daily stomatal resistance was higher (58%; to 384%), photosynthesis values lower (11% to 49%), and crop water stress index values higher (92% to 95%) in stressed cucumber (Cucumis sativus L.) plants than in irrigated control plants in three experiments performed in 1987 and four in 1988. Pillowy fruit disorder (PFD) was more frequent (110% to 150%) and more severe (59% to 81%) in freshly harvested fruits from stress plots when compared to controls. No after-storage differences in PFD were detectable between water stress treatments. Mean PFD ratings of processed fruit following postharvest storage at 26.5C and either 60% or 75% relative humidity were significantly higher than ratings of fruit stored at either 10.5 or 15.5C and 85% relative humidity. The progressive development of pillowy and the observed enhancement of PFD symptoms following storage at higher storage temperatures indicated that postharvest changes can occur in fruit mesocarp tissue and that the development of PFD can be altered, to some extent, during storage. Negative correlations (- 0.18 to - 0.78) between fruit quality and PFD ratings suggested that these changes can affect processed product quality. Pillowed tissue of processed fruit was significantly softer (33% to 39%) than nonpillowed tissue.

Remotely measured canopy temperature of greenhouse strawberries as indicator of water status and yield under mild and very mild water stress conditions

Strawberry plants were submitted to mild and very mild water stress regimes in a tunnel (simple semicircular greenhouse) by planting them in two plots fertirrigated when the soil matric potential reached −0.07 MPa and −0.01 MPa, respectively. The plants were monitored for water stress by measuring the foliage temperature with a hand-held infrared thermometer. Parallel to this, weather variables, the difference between leaf and air temperature, the derived crop water stress index (CWSI), the soil matric potential, the leaf water potential, the photosynthetic gas exchange rates, the transpiration rates, photosynthetic pigments, sugars, starch, canopy structure and accumulated yield were measured. The wet treatment (WT) presented a higher yield and higher leaf area index (LAI). In WT, leaves were disposed mostly in a monolayer oriented to the south, whereas in dry treatment (DT) leaves were distributed in a multilayer pattern and oriented to the north. During the hotter part of cloudless days and before irrigation took place, the canopy temperature of WT was about 3°C less than that of DT. Accumulated stress degree days (SDD) were then higher in DT. WT presented lower average CWSI values, between 0.045 and 0.54, while those of DT were between 0.32 and 0.70. It was concluded that leaf temperature, its difference with air temperature (δ T) and the derived indices, such as SDD and CWSI, are useful for the assessment of even these mild and very mild water stresses in strawberries under protected conditions. Regression analysis showed that under the very mild water stress conditions tested in WT, the contribution of air vapor pressure deficit to variation in leaf water potential was significant. This did not happen under the mild water stress of DT.

Relationships between normalized leaf water potential and crop water stress index values for acala cotton

A technique using an infrared thermometer to measure plant water stress in cotton from stand establishment to final irrigation is defined. This study used the crop water stress index (CWSI) to measure plant water stress and relates these measurements to leaf water potential values. Two non water stress baselines are generated and used for calculating CWSI values for SJ-2 Acala cotton during two segments of the growing season. Two regression lines were generated of CWSI values with leaf water potential values for these segments of the growing season. Applications of the growth regulator mepiquat chloride (PIX) were found to have no effect on the ability of the CWSI method to measure plant water stress. The technique presented makes use of the CWSI method through the entire growing season in a manner that is preferred to the leaf water potential method when measured with the pressure chamber.

Nitrogen Fertility Influence on Water Stress and Yield of Winter Wheat

AbstractAvailable soil water and N fertility are the primary factors limiting dryland winter wheat (Triticum aestivum L.) yields in the Central Great Plains. The objective of this field study was to determine how the level of N fertilization influences water use, water stress, and yield of dryland winter wheat grown in this area. The study was conducted during the 1988 and 1990 growing seasons on a Platner loam (fine, montmorillonitic, mesic Aridic Paleustoll) near Akron, CO. Nitrogen fertilizer was broadcast as NH4NO3 at 0, 28, 56, 84, and 112 kg N ha−1. Canopy temperatures were measured with an infrared thermometer and used to compute the crop water stress index (CWSI). Evapotranspiration was computed and rooting depth inferred from weekly neutron probe readings of soil water content. In general, plant height, above‐ground biomass, leaf area index, rooting depth, water use, and grain yield increased with increasing N. Values of CWSI declined after rain and increased as soil water again became limiting. Nitrogen treatment effects on CWSI varied with the severity of water stress. When CWSI <0.38, increasing N rate decreased water stress because a slight increase in rooting volume resulted. When CWSI >0.38, increasing N rate increased water stress because the excessive transpirational demand of the resulting larger leaf area and vegetative mass was not fully compensated by the increased rooting volume. Grain yield averaged over the 2 yr increased with increasing N up through the 56 kg ha−1 rate. The effect of N fertility on water use efficiency (WUE) was significantly different between years, with WUE increasing with increasing N up through the 56 and 84 kg ha−1 rates in 1988 and 1990, respectively. Grain yield was linearly correlated with cumulative evapotranspiration. Increasing levels of N fertility can be detrimental to winter wheat yields when water‐limiting conditions reduce evapotranspiration rates to less than 62% of potential evapotranspiration.

Crop water stress index for seed alfalfa: influences of within-season changes in plant morphology

A field study was conducted to determine the utility of the crop water stress index ( cwsi) for assessing plant water status of seed alfalfa ( Medicago sativa L.). Six irrigation treatments consisting of combinations of three rates of irrigation and two irrigation frequencies were used in assessing the influence of vegetative, flowering, and pod-filling periods on non-stressed canopy-air temperature differences versus vapour pressure deficit baselines and calculated cwsi values. Three distinct non-stressed baselines were determined and found to be related to the following crop morphological stages: (1) fully vegetative (few or no flowers); (2) flowering (few drying pods); and (3) pod development and maturation. Crop water stress index values across all treatments were highly and linearly correlated with stem water potentials. A less distinct curvilinear relationship existed between cwsi and leaf conductance. Use of the vegetative period non-stressed baseline to calculate cwsi values during flowering and pod-filling resulted in cwsi values 0.1 to 0.2 higher than those determined using growth-stage-dependent baselines. For irrigation scheduling, use of two baselines is suggested, one for the vegetative period and another baseline determined using pooled data from the flowering and pod-filling periods.

RELATIONSHIPS BETWEEN CROP WATER STRESS INDEXES AND PHOTOSYNTHESIS, CONDUCTANCE, OR COLD HARDINESS OF PEACHES

Water stress in mature `Redhaven' / Lovell peach [Prunus persica (L.) Batsch] trees was imposed, during the 1988 growing season. Trickle irrigation was reduced from 100% to 25% of a calculated weekly evaporation amount on 22 June, 11 July, and 8 and 27 Aug. Trees were isolated from rainfall by tents under the canopy and from horizontal water movement between root systems on 4 sides to a depth of 0.5 m by a water-proof barrier. Canopy to air temperature differentials monitored throughout the growing season were developed into 3 stress indexes: crop water stress index (CWSI); cumulative crop water stress index (CCWSI); and postharvest cumulative crop water stress index (PCCWSI). CWSI values varied from 0 to 0.6, while both CCWSI and PCCWSI increased through late Sept. Mean PCCWSI of the 22 June 25% treatment increased at a greater rate than the other treatments. Significant linear regressions were found with some of the indexes and net photosynthesis or stomatal conductance; however, the r-square values were low. In general, no linear relationships were found between either CCWSI of PCCWSI and the Index of Injury for cold hardiness.

Scheduling irrigations for soybeans with the Crop Water Stress Index ( CWSI)

A Crop Water Stress Index ( CWSI) has been related to water use and plant water-stress parameters, but irrigation scheduling based on CWSI values has only been reported for corn and wheat. The objectives of this study were to evaluate irrigation scheduling of soybean ( Glycine max L. Merrill) with CWSI as computed from measurements of infrared canopy temperature, air temperature, and vapor-pressure deficit, and to determine if amount of water applied per irrigation influenced water use and yield obtained from irrigation scheduling by CWSI. Soybeans were grown in field plots with drip irrigation (25 mm water per irrigation) and under a rainout shelter with flood irrigation (25 or 51 mm water per irrigation). Irrigations were initiated when CWSI reached threshold values of 0.1, 0.2, 0.4, or 0.6. Analysis of the data showed the CWSI baseline equation to be incorrect. A new baseline was computed and reanalysis of the CWSI data showed true irrigation thresholds had been 0.2, 0.3, 0.4, and 0.5. These four threshold values resulted in total irrigation amounts of 181, 180, 174, and 145 mm applied to the drip-irrigated plots, respectively. Under the rainout shelter, total irrigation amounts of 347, 271, 195, and 195 mm when 25 mm was applied per irrigation, and 406, 356, 356, and 305 mm when 51 mm was applied per irrigation, resulted from the 0.2, 0.3, 0.4, and 0.5 CWSI thresholds, respectively. Respective yields for these plots were 2656, 2566, 2430, and 2189 kg ha −1 for the drip-irrigated plots, 3375, 2826, 2435, and 2365 kg ha −1 for the rainout-shelter plots with 25 mm per irrigation, and 3575, 3551, 3110, and 2108 kg ha −1 for the rainout-shelter plots with 51 mm per irrigation. Under deficit-irrigation conditions the relationship between CWSI, soil water content, and leaf water-potential appears to change. Knowing how much water to apply per irrigation is important information for effectively using CWSI to schedule irrigations in soybeans. Cloudy sky conditions do not occur with sufficient frequency in the central Great Plains to inhibit the timely use of the infrared thermometer for irrigation scheduling.

Crop water stress index relationships with crop productivity

Field experiments between 1983 and 1987 were used to study the effect of crop development on crop water stress index (CWSI) parameters and the relationship of CWSI with the yield of cotton and grain sorghum. The absolute slopes of nonstressed baselines (NSBL) generally increased until canopy cover reached 70% (Table 1). NSBL derived from data collected when canopy temperature exceeded 27.4 °C had greater absolute slopes and higher R 2-values than NSBL that included all diurnal measurements (Table 1). Average CWSI values of cotton and grain sorghum grown under varying soil water regimes were negatively correlated with yield. Grain sorghum yield was more sensitive to CWSI values than was cotton lint yield (Figs. 1 and 2). Multiyear data analysis indicated that yields from cotton that experienced a completely stressed condition during part of each day during the boll setting period would be 40% of those from completely nonstressed cotton (Fig. 3). Negative values of CWSI computed for cotton growing under non-water stressed conditions were associated with uncertainties in calculations of aerodynamic resistance (r aand in estimating canopy resistance at potential evapotranspiration (r cp).

Use of Crop Water Stress Index to Schedule Irrigation of Freeway Landscape Plants

A study was conducted to determinethe feasibility of using crop water stress index (CWSI) to schedule irrigation of eight species of freeway landscape plants, Acacia redolens B.R. Maslin, Acacia salicina Lindl., Caesalpinia pulcherrima Sw., Cassia nemophila A. Cunn. ex Vogel, Cercidium floridum Benth., Eucalyptus microtheca F.J. Muell., Nerium oleander L., and Prosopis chilensis Mol. Nerium oleander and C. pulcherrima were suited to the use of the CWSI, tolerated repeated exposures to CWSI values of 0.6, and remained aesthetically acceptable. Irrigation of N. oleander via the CWSI resulted in a 19% reduction in water use, compared to the conventional method. CWSI data of other species were too variable, and, thus, irrigation could not be scheduled by CWSI values. Variability was attributed, in part, to lack of a dense canopy, which is necessary to fill the view of the infrared thermometer.

The Possible Use of the Crop Water Stress Index as an Indicator of Evapotranspiration Deficits and Yield Reductions in Sweet Corn

Abstract The crop water stress index (CWSI) may be useful for optimal irrigation timing. This preliminary study evaluates the relationships between the CWSI and evapotranspiration (ET) and yield. The CWSI was also characterized on an hourly basis. Once-daily CWSI measurements after full ground cover was established and hourly CWSI measurements on 4 days were made in sweet corn (Zea mays L. ‘Jubilee’) irrigation experiments in 1984 and 1985. The gradient of water applied included five irrigation levels established from 0% to 100%, with the 100% level intended to refill the root zone to field capacity, after 50% depletion of available water, at each irrigation. CWSI values, obtained hourly throughout the day, were highest between 1000 and 1700 hr. CWSI values tended to be higher in the less-irrigated plots (40% and less) than in those that received greater amounts of water (57% to 100% treatment levels). Seasonal average CWSI values (midday measurements) were closely related to the seasonal ET deficit (r2 ranged from 0.45 to 0.96), but there was not the expected 1:1 relationship of CWSI and ET deficit. The yield deficit of good, husked ears was also closely related to CWSI (r2 ranged from 0.82 to 0.93), but differences in these relationships between years and experiments indicate that CWSI measurements must be improved.

Cotton Water Stress Under Trickle Irrigation

ABSTRACT Cotton was trickle irrigated two to three times weekly in 1984 and 1985 using buried perforated tubing under each row. Water application rates were about 0.6, 1.0, and 1.3 times estimated consumptive use (CU). The cotton was monitored for water stress using the crop water stress index (CWSI), soil water content, and yield. Significant differences in seasonal average crop water stress values, average soil water contents, and yields were obtained for the three water treatments. The wettest treatment with average CWSI values near 0.1 gave the highest yield and had the highest soil water contents before irrigation. The yield increased nearly linearly with decreasing CWSI while the water use efficiency (yield per unit of water) was highest for the 1.0 CU treatment.

Relationship between crop water stress index and other physiological plant water status indicators in guayule

Guayule ( Parthenium argentatum Gray, cv. N565 II) was grown at Phoenix, Arizona, in well-watered and water-stressed conditions. The relationships between the crop water stress index ( CWSI) and other physiological indicators of plant water status were examined. The drought treatment lasted for 70 days (29 May through 5 August 1986) during which time the available soil moisture content declined from 100 to 0%, while the well-watered plants were maintained at greater than 70% available soil moisture. The CWSI results were normalized to reduce environmental effects by subtracting the values obtained for the well-watered plants from those of the water-stressed plants. Significant ( P<0.01) linear relationships were obtained between the normalized CWSI and the similarly normalized values of leaf water potential ( r 2=0.75), leaf osmotic potential ( r 2=0.70), and stomatal conductance ( r 2=0.62). Turgor potentials of the plants in both treatments were similar until their difference in CWSI reached approximately 0.6, above which the water-stressed plants were no longer able to maintain turgor. No difference in net photosynthesis was observed between the wet and dry plots even though their difference in CWSI was as great as 0.80. During an 11-day period following irrigation of the water-stressed plants, their CWSI values remained significantly higher than the well-watered plants even though the soil moisture content of the two treatments was similar.

Canopy temperature and water stress of cotton crops with complete and partial ground cover

The use of canopy and air temperature differences to compute a crop water stress index (CWSI) for assessing plant water status was investigated using cotton crop canopies that either fully or partially covered the ground. The complete ground cover canopy condition was studied in a well watered moisture regime in a rainout shelter with measurements made on six Texas cotton race stocks. The partial ground cover canopy situation was investigated in a well watered moisture regime of a commercial cotton variety ‘Paymaster 266’ grown in the field. The slope of the nonstressed baseline of the CWSI for a cotton canopy with about 50% ground cover was approximately one-half that reported for full canopies. Values of CWSI calculated with “theoretical” and “empirical” procedures agreed more closely under a complete canopy condition than under a partial canopy situation. Values of aerodynamic resistance (r a ) and canopy resistance for well watered soil moisture conditions (r ep )were estimated in order to use the theoretical procedure of computing CWSI. Values of r a ranged from 10 to 15 sm−1 and r cp from 50 to 60 sm−1. Both the theoretical and empirical procedures showed much promise, but more information is needed to develop techniques for evaluating r a and r cp under differing canopy and environmental conditions.

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.