Canopy temperature (Tc), besides serving as a parameter to estimate crop evapotranspiration (ETc), can also be used to define water stress indexes that have the advantage to serve as a reference for the rational scheduling for irrigation for a crop. One of these indexes, which has been well disseminated, is the crop water stress index (CWSI), based on the ratio between crop resistance and aerodynamic resistance (rc/ra), whose value can be estimated by the difference between the cropy canopy and the air (Tc - Ta). A study was carried out with the objective of obtaining CWSI values for beans (Phaseolus vulgaris L.), under field conditions, in which the crop was under different irrigation scheduling; 2, 4, 8, 12 and 16 day intervals. The experiment was conducted at Sete Lagoas, Minas Gerais State, Brazil, in the period from July through October, 1995. Tc values were measured by means of infrared thermometry at two times of the day; between 10:00 and 11:00 am, and between 01:00 and 02:00 p.m. The obtained results pointed to a limit of 0,15 as a reference for the beginning of water stress to occur, which is consistent with other work and other index obtained under similar conditions

ALBUQUERQUE, P. E. P.

GOMIDE, R. L.

KLAR, A. E.

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CROP WATER STRESS INDEX FOR BEANS OBTAINED FROMTEMPERA TURE DIFFERENCE BETWEEN CANOPY AND AIRALBUQUEfP.E.P.1998pE.P. ALBUQUERQUE and R.L. GOMIDEEMBRAPA, Sete Lagoas - MG, BrazilAE. KLARDept. Rural Engineering, State University of S. Paulo, Botucatu, BrazilAbstractCanopy temperature (Te), besides serving as a parameter to estimate cropevapotranspiration (ET c), can also be used to define water stress indexes that have theadvantage to serve as a reference for the rational scheduling for irrigation for a crop.One of these indexes, which has been well disseminated, is the crop water stress index(CWSI), based on the ratio between crop resistance and aerodynamic resistance (rc/ral,whose value can be estimated by the difference between the cropy canopy and the air(Te - Ta). A study was carried out with the objective of obtaining CWSI values forbeans (Phaseolus vulgaris L.), under field conditions, in which the crop was underdifferent irrigation scheduling: 2, 4, 8, 12 and 16 day intervals. The experiment wasconducted at Sete Lagoas, Minas Gerais State, Brazil, in the period from July throughOctober, 1995. Te values were measured by means of infrared thermometry at twotimes ofthe day: between 10:00 and 11:00 a.m., and between 01:00 and 02:00 p.m. Theobtained results pointed to a limit of 0.15 as a reference for the beginning of waterstress to occur, which is consistent with other work and other index obtained undersimilar conditions.Keywords: Crop water stress index, evapotranspiration, irrigation scheduling.1 Introductionlrrigation science advances will necessarily imply in the gradual acquismon andutilization of knowledge which will conduct toward an accurate and precise controlabout the quality and quantity of applied water. This control has the purpose ofoptimizing the irrigation and other crop production practices.Water and the Environment: Innovative Issues in Irrigation and Drainage. Edited by Luis S.Pereira and John W. Gowing. Published in 1998 by E & FN Spon. ISBN O419 23710 O190 ALBUQUERQUE et aiA way of rationalizing the use of irrigation water is to define the irrigation schedulingby utilizing the crop water stress index (CWSI) which has its bases on actual or realcrop evapotranspiration (ET c). The CWSI may serve as a parameter to scheduleirrigation and to reduce the amount of applied water and improve the crop water useefficiency, within welI established levels, without compromising desirable commercialand economical yield. The CWSI can be defined asETcCWSI=I---ETm(1)1where ETm is the maximum ar potential crop evapotranspiration. Both, ET c and ETmare expressed in mm/h, mm/day or mm/period. They can be express also as the latemheat flux per unit area (ÀEp in W/m2). The CWSI is an adimensional index. ETm canbe defined as the maximum water flow rate which is directly evaporated from the soiland plants (foliage intercepted water) and evaporated from the leaves stomatas 1(transpiration), with the plants presenting no water or health limitation. On the otherhand, water can be a limited parameter in the ETc situation. Thus, the CWSI valueswill change from O to I: The CWSI wilI be O when ETc reaches ETm (no water stress).The canopy temperature (Te) is used today as a parameter to predict ET c as weIl asto define CWSI ([1], [2], [3], [4], [5]). The relationship between plant water status andTe parameters are quite complexo Several other variables, which are not constamduring the plant growth period, should be taken into account in the relation between Te 1and ET c. However, Te has been easily measured and used as a key parameter for agreat number of stress indexes. According to Clawson et al. [3], ldso et al. [7] were 'the first to use the temperature difference between the crop canopy and the air (Te - Ta)as a way of establishing a crop water stress index.The Te values can be obtained from remote sensing equipments which are able todetect the surface temperature by means of infrared radiation. These instruments areknown as infrared thermometer (IRT). lnoue [6] pointed out advantages of IRT andalso verified IRT performance to detect (Te - Ta) and plant physiology depression inparameters such as photosynthesis rate, transpiration and stomata conductance in comand wheat.One of their first parameters utilized as a way to establish the crop water stress indexwas the welI known stress degree day (SDD) which presented good results in aridregions. Later, the SDD was normalized for humid regions conditions [3].ldso et aI. [7] also presented an empirical method for dealing with the CWSIproblem. However, the theoretical basis ofthe CWSI was studied by Jackson [1] whoused Tc measurements obtained from IRT to predict ETc' from a vegetated surfaceenergy balance equations.Several authors ([2], [4], [5], [8], [9], [10]) studied the CWSI developed by Jackson[1]. Grimes et al. [9], Silva [10] and Kobayashi [5] obtained CWSI values of about0.15, 0.3 and 0.20 for alfalfa, cotton and beans, respectively, as limits above which cropwater stress wilI begin. These limits could be used as reference for irrigationscheduling. Pitts et al. [4] and Folegatti [2] pointed out the occurrence of clouds as animportam limitation for the use ofthe CWSI.Crop water stress index 191The objective of the present work was to obtain CWSI values for beans (Phaseofusvu/garis L.) submitted to tive different irrigation frequency intervals (2,4, 8, 12, and 16days) under tield condition, utilizing the IRT technique approach (Jackson (1]).2 Material and methodsThe field experiment was conducted at EMBRAP A, in Sete Lagoas, Minas GeraisState, Brazil, during the months of July through October ofl 995.The crop was planted in 26/07/95 with a crop density equivalent to a stand of about220,000 plant per hectare. Completely randomized plots were used as statistical design,with 5 treatments and 4 replications. The treatments were formed with differentirrigation intervals (2, 4, 8, 12, and 16 days) which started being applied 29 days aftercrop seedling (DAS), when the plants presented the third leaf emission.The water volume to be added in each plot during irrigation was obtained by meansof the soil water content determination (tensiometer and gravimetric) in the day ofirrigation, in such a way as to bring the soil water levei up to the "field capacity". Thesoil water holding capacity curve was used to carry out this computation.Samples of the plants (3 for each plot) were collected weekly for determination ofplant height (hc) and leaf area index (LAI). Whenever e1ear sky conditions occurred, atselected crop growth stages, readings of abaxial stomata resistance (rs) were obtainedfrom some well developed and lightened leaves (from 4 plants in each plot) between10:00 and 12:00 hours, with a diffusion porometer. In the last sample, crop yield andyield components were evaluated.A portable electronic weather station, installed 2 m above soil surface, provided thefollowing local daily e1imatic data: average, maximum and minimum air temperature,rainfall, and global solar radiation (Rs). Also, a totalizing anemometer was installed 2m above soil surface, close to the field trial. Continuous data registration were obtainedfrom an analogical electranic diagram of net radiation (Rn), soil heat flux (G), andalbedo (oc). In order to carried out the Rn and o: measurements, a net radiometer andalbedometer sensors were installed 1.5 m above soil surface in the treatment irrigatedeach 2 days (T2). Soil heat flux plates sensors were placed 2 em below soil surface inthe treatments irrigated each 2, 8, and 16 days (T2, T8, and TI6 respectively) to obtainG readings. Other e1imatic parameters, such as Class A pan water evaporation,sunshine hours, atmospheric pressure, and air relative humidity were obtained frornEMBRAPA's main standard Climatological Weather Station, located about 1500 maway of the field trial.The canopy temperature (Te) was taken daily, within two periods: frorn 1000 to11:00 hours and from 13:00 to 14:00 hours, always with under e1ear sky conditions.The following IRT was used: 1 o C tield of view, temperature range -30 to 300 ()C,precision of ± 1.5 o C, sensibility of 0.1 o C, response time of about 1 s, spectral bandof 8 to 10 IJ.m,and emissivity of 0.98. Readings were taken by keeping the IRT fieldof view at about 45 o with the canopy surface and 2 m of distance from the target(leaves). Ten to twelve Te readings were used in the average Te calculation, and thevariation coefficients were maintained within 2 to 3 % limits.192 ALBUQUERQUE et aiThe aerodynamic resistance (ra> was estimated through the equation presented byAllen et ai. [11) and Jensen et ai. [12]. Daily CWSI were obtained for morning andafternoon periods with the following equation ( [13] and [1]):(2)in which the symbol .1 represents the slope of the saturated vapor pressure-temperaturerelation (Pa / "C), and was estimated according to Jackson et aI. [14] mathematicalmode!. The term y (in units of Pa / "C) is the psychrometric constant and was obtainedas described in Brunt's procedure, according to Smith et al. (15]. The ratio rc/r a wasdetermined according to Jackson [1], in terms of Rn, (Te- Ta), vapor pressure deficit(VPD), and ra. Afterwards, the rc values were obtained from (Te-Ta) and ra dailyvalues.The crop situation in the maximum evapotranspiration condition (ET m) wasprovided by Treatment 4, as well as the canopy resistance at ET m (rcp) and themodified psychrometric constant (y' in units of Pa / "C), according to the formula(3)3 Results and discussionAlI treatments received a total of 118.8 mm of applied water from seedling to 28 daysafter seedling (OAS), with 4-day average irrigation interva!. Just after the 28 DAS,treatment differentiation started, with the application of pre-deterrnined depths ofwater, according to the irrigation intervals. From seedling to harvest, the total amountsof applied water, incIuding 66 mm of rainfall, were 482.0, 439.6, 385.6, 326.5, and290.4 mm for T2, T4, T8, TI2 and TI6 treatments, respectively.The crop water stress index resuIts considered for analysis were from the growthperiod between lhe 29 and 80 DAS. Table 1 lists the crop yield and other yieldcomponents parameters. It was not observed statistical significance among thetreatments for the number of pods per plant. The same behavior was observed forharvest index (HI) and grain yield, with no response for irrigation frequencies of 2, 4,and 8 days. Also no responses were verified when comparing 8 and 12 days, and 12and 16 days irrigation intervals. Satisfactory results were obtained for HI, grain yield,number of seeds per pod, and the weight of 1000 seeds in the treatment T8. Resultsshow that irrigation frequency beyond 8-day intervals decreased the magnitude of thestudied parameters, causing injury to the crop.Table 1 also shows strong water stress levels for TI2 and TI6, resulting in reductionof grain yield and yield components parameters. Grain yield was the most affected one.Crop water stress index 193Table I. Number of pods per plant, number of seeds per pod, 1000-seed weigh. harvest index,:JI1dgrain yield responses to different irrigation frequencies on beans (Pha.l'eolu.\· vulgaris L.)IrrigatIon Pod Number Seed number 1000 seed Harvest Grain yield2frequency per plant per pod weight index 1 (Kg.ha-I)(days) (g)2 IIAa 4.8a 239.6ab 0.55 a-+ 11Ia 4.7ab 245.5 a 0.57 aS 9.5a 40b 241.6ab 0.53ab12 7A a 3.3 c 259A a OA6 b c16 6.8 a 3.2 c 225.0 b OA2 cAverage 9.2 4.0 242.2 0.51VC 25.16 7.33 3.73 5.652755 a2750 a2294 a b1583 b c1057 c208815A4I Dry marter ofthe grain / Total dry rnatter (at the harvest)2 Values expressed for 13 % humidityMeans followed by the same letters do not present statistical signifícance by the Tukey test at 5'X,probability level.VC = Variation CoefficientAssuming the grain yield of treatment T2 as the potential yield (no water stresscondition), it was verified a reduction of 0.181, 16.7,42.5, and 61.6 % in the grainweight caused water shortage in treatments T4, T8, Tl2, and T16, respectively.Figures I and 2 show the crop water stress index variation with the crop growth period(expressed as days after seedling) at four irrigation frequencies (T4, T8, T12, and T16)and two measuring periods (morning and afternoon), respectively. Treatment T2 wasassumed to have a crop with adequate water supply and maximum or potentialevapotranspiration rate in the CWSI determination. The results show that T4 presentedless oscillation in the CWSI values which were maintained elose to the zero leveI.Therefore, four-day irrigation frequency did not cause water stress damage to the crop.This result can also be verified in the Table I where the crop yield and yieldcomponents behavior of T2 and T4 treatments are quite similar. By analyzing the twomeasuring periods (morning and afternoon), it can be noticed a similar tendency in theCWSI variation. But the highest CWSI values were obtained during afternoonreadings. At first, this suggests that the best time for canopy temperature (Te) readingis between 1300 and 14:00 hours for CWSI studies.The treatment that caused most water stress was Tl6, where the CWSI reachedvalues of 0.26 and OAO for morning and afternoon periods, respectively. Table Iresults emphasize the damage caused to the crop.Overall, the results show that a CWSI value of about 0.15 may be used as a limit forirrigation water managemeot strategies to diferentiate the irrigated crop from a non-stressed to a stressed condition, in order to avoid signifficant yield loss.This limit of 0.15 derives from the observation that T4 and T8 treatments presentedmaximum CWSI values ofroughly 0.10 and 0.20, respectively. At T4, results indicatethat the crop was also adequately supplied with water (no-stress, similar to T2). On theother hand, at T8 the crop may have experienced a certain degree of water stress.These results agree with the results from Kobayashi [5] for the same crop.194 ALBUQUERQUE et ai-·-T4" /"\" --~-0-TI2::r ': \.",::'1 \ o o ~ !j / o~ i 1\; jN" ;-0"/'" V \" ,0\ \ \ ! "\ ,"" .,"0..00 -t----+' ---"-T-I'r"wt-'o--=-c" I-hl-"-+I----lf--+i-~ __ o ~ _0..300..250.150..10.o. OS.{l.OS-oioIrearrerts:0030 80day5 afier seedli rgFig. I. Crop water stress index versus crop growth period (expressed as days afterseedling) at four irrigation frequencies (T4, T8, Tl2, and T16) for measurements in themorning period (Erv1BRAP A, Sete Lagoas, MG, Brazil, 1995).Treatments:0.4z:~ 0.3~xIV-o0.2cIII~111 0.1IV 1iií~a. 0.0eo-0.130 40 50 60 70 80 90day s afler seedlingFig. 2. Crop water stress index versus crop growth period (expressed as days afterseedling) at four irrigation frequencies (T4, T8, Tl2, and Tl6) for measurements in theafternoon period (Erv1BRAP A, Sete Lagoas, MG, Brazil, 1995).Crop water stress index 195~ Conc1usionsThe potential grain yield (2,755 Kg.ha-1) was obtained with the irrigation frequency ofrwo days, under no water stress. A reduction of 018, 16.7,42.5, and 61.6 %was\erified in the grain weight due to treatments T4, T8, T12, and T16, respectively(tretments with water shortage). There was no statistical significance for grain yieldamong the 2-, 4-, and 8-day irrigation intervals.The total depth of applied water, including 66 mm of rainfall, were of 482.0, 4396,385.6, 326.5, and 290A mm for 2-, 4-, 8-, 12- and l6-day of irrigation frequency,respectivelylt was verified a similar tendency of the CWSI variation in the periods ofmeasurement (morning and afternoon). The highest CWSI values were obtained duringalternoon readings, suggesting that as the best period for canopy temperature (Te)measurements. The highest water stress was obtained with 16-day irrigation frequency,when the CWSI reached values of 0.26 and OAO, for morning and afternoon periods,respectively.A crop water stress index value of about 0.15 may be used as a limit for irrigationwater management to differentiate the irrigated crop from a non-stressed to a stressedcondition, as a criteria to avoid signifficant yield loss.5 Referencesl. l.Jackson, R.D. (1982) Canopy temperature and crop water stress Advances inirrigation. New York, Vol.l, ppA3-85.2. Folegatti, M.V. (1988) Avaliação do desempenho de um "scheduler" na detecçãodo estresse hídrico em cultura do feijoeiro (Phaseolus vulgaris L.) irrigada comdiferentes lâminas. Piracicaba: ESALQ, 1988. 188p. Thesis (Doctor inAgronomy) - Escola Superior de Agricultura "Luiz de Queiroz", Universidade deSão Paulo, Brazil.3. Clawson, K.L., Jackson, R.D. and Pinter Jr., PJ. (1989) Evaluating plant waterstress with canopy temperature differences. Agronomy Journal. Vo1.81, pp.858-63.4. Pitts, D.J., Wright, RE, Kimbrough, J.A. and Johnson, D.R. (1990) Furrowirrigated cotton on clayed soil in the Lower Mississipi River Valley. AppliedEngineering in Agriculture, Vo1.6, ppA46-52.5. Kobayashi, M.K. (1996) Determinação do índice de estresse hídrico da cultura dofeijoeiro (Phaseolus vulgaris L.) por meio da termometria a infravermelho, e dofator de disponibilidade de água no solo, em minilisímetro de pesagem. Viçosa:UFV, 1996. 90p. Thesis (M. Se, in Agricultural Engineering) - UniversidadeFederal de Viçosa, Brazil.6. lnoue, Y (1991) Remote and visualized sensing of physiological depression incrop plants with infrared thermal imagery. Japan Agricultural ResearchQuarterly, Vol.25, pp.l-S.7 ldso, S.B., Jackson, RD, Pinter Jr., PJ., Reginato, R.G. and Hatfield, J.L.(1981) Normalizing the stress-degree-day parameter for environmental variability.Agricultural Meteorology, Vo1.24, ppAS-55196 ALBUQUERQUE et ai8. Andrews, P.K., Chalmers, D.J. and Moremong, M. (1992) CanoPY-airtemperature differences and soil water as predictors of water stress of apple treesgrown in a humid, temperate climate. Journal American Society HorticulturalScience, Vol.117, pp.453-8.9. Grirnes, D.W., Wiley, P.L. and Sheesley, W.R. (1992) Alfalfa yield and plamwater relations with variable irrigation. Crop Science., Vo1.32, pp.1381-7.10. Silva, B.B. (1994) Estresse hídrico em algodoeiro herbáceo irrigado evidenciadopela termometria infravermelha. Campina Grande: UFPB, 1994. 139p. Thesis(Doctor in Civil Engineering) - Centro de Ciências e Tecnologia, UniversidadeFederal da Paraíba, Brazil.lI. Allen, R.G., Jensen, M.E., Wright, lL. and Burman, R.D. (1989) Operationalestimates of reference evapotranspiration. Agronomy Journal, VoI.81, pp.650-62.12. Jensen, M.E., Burrnan, R.D. and Allen, R.G. (1990) Evapotranspiration andirrigation water requirements. New York: ASCE, 332p.13. Jackson, R.D, Idso, S.B., Reginato, R.J. and Pinter Jr., PI. (1981) Canopytemperature as a crop water stress indicator. Water Resource Research, VoI.I7,pp.I133-8.14. Jackson, R.D., Kustas, W.P. and Choudhury, B.J. (1988) A reexamination of thecrop water stress index. Irrigation Science, Vol.9, pp.309-17.15. Smith, M., Segeren, A., Pereira, L.S., Perrier, A. and Allen, R. (1991) Report onthe expert consultation on procedures for revision of FAO guidelines forprediction of crop water requirements. Rome: FAO, 45p.

Crop water stress index for beans obtained from temperature difference between canopy and air.

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Crop water stress index for beans obtained from temperature difference between canopy and air.

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