Determination of Crop Water Stress Index and

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Determination of Crop Water Stress Index and Irrigation Timing on Olive Trees Using a Handheld Infrared Thermometer

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Erhan Akkuzu, Ph.D. 1; Ünal Kaya, Ph.D. 2; Gökhan Çamoğlu, Ph.D. 3; Gülay Pamuk Mengü, Ph.D. 4; and Şerafettin Aşik, Ph.D. 5 Abstract: Leaf temperature has long been recognized as an indicator of water availability. The stress level in a plant can be quantified from leaf temperature by using the crop water stress index (CWSI). In this study, it was investigated whether infrared thermometer measurements and accordingly CWSI could be used to create irrigation schedules for olive trees (cultivated variety Memecik). The research was conducted at the olive tree plantation of the Olive Research Station between 2009 and 2010. In the study, the effects of different irrigation treatments on the yield, canopy temperature, and CWSI of olive trees were investigated, and the optimum irrigation schedule was determined according to the findings. Seven different water application treatments were created using the drip irrigation method. Five treatments consisted of irrigating at a rate equivalent to 25% (S-0.25), 50% (S-0.50), 75% (S-0.75), 100% (S-1.00), and 125% (S-1.25) of the cumulative evaporation in 5 days from a Class A evaporation pan. The other two treatments consisted of a treatment in which the humidity lost at a soil depth of 0–90 cm was replenished each time to the field capacity (Control, S-C) and a treatment in which no irrigation was performed and cultivation was carried out under completely rain-based conditions (Stress, S-0). In the study, the amounts of irrigation water applied to the treatments ranged from 0 to 809 mm, and the crop water consumption values varied from 127 to 853 mm according to the average of both years. The highest water-use efficiency was obtained in the S-0 treatment, whereas the highest irrigation water–use efficiency was obtained from the S-0.50 treatment. One of the important findings of this study was that handheld infrared thermometer can be used for stress detection and irrigation scheduling of olive trees. When the mean CWSI values in the experimental years were examined in terms of the irrigation treatments, the CWSI values ranged from 0 to 0.68 in 2009 and from 0.02 to 0.71 in 2010. In both years, the highest values were recorded in the S-0 treatment, and the lowest values in the S-1.25 treatment. When water-use efficiency is evaluated along with CWSI values, irrigation can be recommended at half of the evaporation from a Class A evaporation pan (S-0.50, when CWSI values reach 0.39). In conditions in which water sources are insufficient, it can be recommended that irrigation be started when evaporation is a quarter (S-0.25), that is, when CWSI values reach 0.49. DOI: 10.1061/(ASCE)IR.1943-4774.0000623. © 2013 American Society of Civil Engineers. CE Database subject headings: Remote sensing; Evapotranspiration; Trickle irrigation; Thermal stress; Turkey; Crops. Author keywords: Olive; Water stress; Irrigation scheduling; Canopy temperature; Mediterranean; Irrigation management; Plant monitoring.

Introduction Olive (Olea europaea L.) is an emblematic tree of the Mediterranean region. The countries in the world with the biggest olive production are, in order, Spain, Italy, Greece, and Turkey [Food and Agriculture Organization Corporate Statistical Database (FAOSTAT) 2008]. Olive and olive oil consumption has increased in Associate Professor, Faculty of Agriculture, Ege Univ., İzmir 35100, Turkey (corresponding author). E-mail: [email protected] 2 Researcher, Bornova Olive Research Station, İzmir 35100, Turkey. E-mail: [email protected] 3 Research Assistant, Faculty of Agriculture, Çanakkale Onsekiz Mart Univ., Canakkale 17020, Turkey. E-mail: [email protected] 4 Associate Professor, Faculty of Agriculture, Ege Univ., İzmir 35100, Turkey. E-mail: [email protected] 5 Professor, Faculty of Agriculture, Ege Univ., İzmir 35100, Turkey. E-mail: [email protected] Note. This manuscript was submitted on February 21, 2012; approved on April 8, 2013; published online on April 10, 2013. Discussion period open until February 1, 2014; separate discussions must be submitted for individual papers. This paper is part of the Journal of Irrigation and Drainage Engineering, Vol. 139, No. 9, September 1, 2013. © ASCE, ISSN 0733-9437/2013/9-728-737/$25.00. 1

recent years with the increase in concern for long and healthy living. Hence, it is expected that the increase in olive production and consumption will continue in the future (Fabbri 2004). It is an inescapable fact that the plant ecology of the Mediterranean Basin will be faced with increasingly severe droughts because of the likely increase in potential crop water consumption in connection with global climate change and that scarce water resources will decrease further (Houghton et al. 1995; Feller 2006). For this reason, olive production, as all agricultural production, must be carried on in such a way as to derive the maximum possible product per unit of water. For stable production, high yield, and good quality of product, a good irrigation program must be established. Good irrigation management ensures that an adequate supply of soil moisture is maintained throughout the season (Goldhamer 2011). The plant response to irrigation is a function of the plant water status, which is influenced by a range of factors including the soil water potential (i.e., the energy required to remove water from the soil), the interface (i.e., resistance and area) between the soil and plant roots, hydraulic conductivity within the plant, and evaporative demand (i.e., atmospheric conditions) to which the plant is exposed. Hence, methods used to schedule irrigations

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commonly involve measuring either atmospheric conditions or soil moisture or plant stress (White and Raine 2008). Soil water content and atmospheric demand can never determine when a plant needs irrigation as effectively as the plant’s own internal water condition. Therefore, methods to determine the internal water status of the crop are commonly used in preparing irrigation programs (Reginato and Howe 1985; Yazar 1993). Results of studies carried out have shown that methods reliant on the crop have great potential in irrigation programming. It has been stated that measurements of changes in crop water potential, canopy temperature, sap flow, and trunk diameter can be used for this purpose (Fernandez and Cuevas 2010). Leaf temperature has been recognized as an indicator of water availability for a long time (Gates 1964; Wiegand and Namken 1966; Begg 1980; Idso et al. 1981; Jackson et al. 1981; Jackson 1982; Jensen et al. 1990; Colaizzi et al. 2003a, b). Leaf temperatures of water-stressed plants are typically 2–4°C higher than those of nonstressed plants, but differences as great as 15°C have been recorded (Drake 1976; Mattson and Haack 1987; Kent et al. 2004). As the plant transpires, the temperature of the leaf falls to less than the air temperature because energy coming to the surface of the plant is used in evaporating water. When water in the soil is reduced, making water uptake by the plant less, transpiration slows, and the stomata in the leaves slowly begin to close, so the exchange of latent heat between the leaf surface and the atmosphere slows down. In this situation, incoming solar energy does not cause transpiration but an increase in the surface temperature of the leaf. When leaf temperature exceeds the air temperature, transpiration has become very slow (Taiz and Zeiger 2008). Thus, the difference between the canopy temperature of the crop and the air temperature can be used as a stress indicator in assessing the amount of water available for use in the root zone. The stress level in crops can be quantified by using the crop water stress index (CWSI) defined by Idso et al. (1981) and Jackson et al. (1981). The approach of Idso et al. (1981) depends on the measurement of the difference between canopy and air temperatures (Tc − Ta, °C) as a function of atmospheric vapor pressure deficit (VPD, kPa). Irrigation is needed when the crop reaches a certain stress index value. This threshold value varies according to climate and production techniques (Massai et al. 2000; Yazar 2009). Researchers have established the water stress index for a large number of field crops; studies have also been carried out for trees, but these are relatively few compared with those of fields crops (Tormann 1986; Glenn et al. 1989; Andrews et al. 1992; Garrot et al. 1993; Sepaskhah and Kashefipour 1994; Garcia et al. 2000; Testi et al. 2008; Ben-Gal et al. 2009; Berni et al. 2009; Wang and Gartung 2010; Zia et al. 2011; Paltineanu et al. 2013). In recent years, studies on CWSI have benefited from satellite imaging (Sepulcre-Canto et al. 2006; Ben-Gal et al. 2009; Berni et al. 2009), but the widespread use of this technology has been limited by the high cost of high-resolution satellite imaging and the time and highly sophisticated programs that are necessary for processing. However, the handheld infrared thermometer is more practicable for widespread use at the field level. The cost of this instrument is falling, it is easy for anyone to use, and data can be obtained quickly and accurately. The aim of this study was to determine the usability of infrared thermometers in determining stress and irrigation time in olives, a typical crop of the Mediterranean, observed when in recent years it has undergone a rapid conversion from rain-fed cultivation to irrigation.

Materials and Methods Study Site and Experimental Design This study was performed in 2009 and 2010 in Bornova (west of Turkey; 38°43′ N; 27°25′ E; 20 m above sea level), Izmir Province, Turkey. It has a typical Mediterranean climate, hot and dry in the summer and cool and wet in the winter. The soil of the experimental area has a loamy structure, an infiltration rate 8 mm=h, electrical conductivity of 0.8 dS=m, a pH of 7.4. The available waterholding capacity at a soil depth of 0–90 mm is 94.41 mm. The olive trees used were of the Memecik variety and were of a productive age and had been irrigated by a drip irrigation system since planting. The trees were 22 years old in 2009 and planted at distances 7 × 5 m. The Memecik variety is used both for oil and as table olives. This variety is widely cultivated in Turkey and is preferred by producers because although it shows periodicity it is adapted to difficult conditions and has high oil content. Water used for irrigation in the experiment had a pH of 7.9 and electrical conductivity of 0.5 dS=m. Trees were irrigated by using drip irrigation system. Polyethylene pipe (16 mm) was used as lateral. For each row of trees in the experiment, two lateral lines were laid 90 cm apart, and online pressure-regulated drippers set to a flow of 8 L=h were placed at intervals of 90 cm on each line. The experiment consisted of seven irrigation treatments, with three replicated in a random block design. On each plot, four trees were assessed, apart from the edge effect. The study was carried out in parallel with another project (TUBİTAK Project No. 108O135), which was performed with the same treatments and on the same plot between 2008 and 2010, and yield data values were obtained from this project (Aşık et al. 2011). Seven different irrigation treatments were applied in the experiment. In treatments other than S-0 and S-C, a percentage of the 5-day cumulative evaporation from a Class A evaporation pan was taken into account. 1. S-0: treatment without irrigation; 2. S-0.25: treatment with irrigation water applied every 5 days at 25% of the evaporation of a Class A evaporation pan (kpc ¼ 0.25); 3. S-0.50: treatment with irrigation water applied every 5 days at 50% of the evaporation of a Class A evaporation pan (kpc ¼ 0.50); 4. S-0.75: treatment with irrigation water applied every 5 days at 75% of the evaporation of a Class A evaporation pan (kpc ¼ 0.75); 5. S-1.0: treatment with irrigation water applied every 5 days at 100% of the evaporation of a Class A evaporation pan (kpc ¼ 0.1.00); 6. S-1.25: treatment with Irrigation water applied every 5 days at 125% of the evaporation of a Class A evaporation pan (kpc ¼ 0.1.25); and 7. S-C: treatment in which the reduced moisture at a depth of 0–90 cm was refilled to field capacity every 5 days. Irrigation was started when the capacity of available soil moisture in the top 90 cm of the soil fell to half and was ended with the autumn rains. Harvesting was carried out by hand in November. The computer program SPSS 13.0 was used to carry out statistical analysis of yield. The data were subjected to analysis of variance (PROC ANOVA), means were compared by using Duncan’s test, and significance was set at p < 0.05. The analysis was performed by taking into account the average of the 3 years because of the effect of periodicity.

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The crop water consumption for each experimental treatment was calculated by the water-balance method (James 1988). For this purpose, moisture measurements were carried out in the 0–30 cm soil layer by the gravimetric method and in the 30–60, 60–90, and 90–120 cm soil layers by neutron probe. The 90–120 cm soil layer was examined for downward seepage and groundwater movement. Water-use efficiency (WUE) and irrigation water–use efficiency (IWUE) values were calculated from crop water consumption and irrigation water values and yields (Zhang et al. 1999).

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Field Data Collection Canopy Temperature Measurement and Calculation of the Crop Water Stress Index Plant canopy temperatures, which are affected by the environment, the phenological stage of the plant, and shortages of moisture in the soil, were measured in the experiment with a handheld infrared thermometer (Fluke 574, Everett, Washington) to determine crop water stress. The response range of the infrared thermometer was between 8 and 14 μm, and its emissivity was adjusted to 0.98. Canopy temperature (Tc) measurements were carried out when the sky was completely clear or when clouds did not obscure the sun, between 1100 and 1400 hrs on the days before irrigation. Measurements were made, with three replicated on the east, west, north, south, southeast, and southwest of each of the two trees in the center of the plot. During measurement, the thermometer was held approximately 2 m from the tree canopy and at a height of approximately 2 m from the ground, parallel to it (zenith angle ≈ 90°). Care was taken while measuring that only the tree canopy came into the field of view. The average canopy temperature of that plot was calculated by averaging the measurements (Glenn et al. 1989; Andrews et al. 1992). The method of Idso et al. (1981) was used in determining the CWSI. For this, a baseline was created by linear regression of the values of Tc − Ta and VPD determined from measurements taken once an hour between 1000 and 1600 hrs on four different days in the season, 1 day after irrigation, on the well-irrigated treatment (S-C). The Tc − Ta value at the upper limit, accepted as when plants were completely under water stress and not transpiring, was found according to Idso et al. (1981). A basic graph was formed when lower and upper baselines were obtained. This graph was used to calculate CWSI values with the help of Eq. (1) (Gençoğlan and Yazar 1999; Colaizzi et al. 2003a; Erdem et al. 2005). CWSI ¼

½ðTc − TaÞ − LL UL − LL

ð1Þ

where Tc = canopy temperature (°C); Ta = air temperature (°C); LL = lower limit, when plant is not under water stress (the limit value at which plants transpire at their potential rate); and UL = upper limit, when the plant is totally under water stress (the limit value when the plant is assumed not to be transpiring). At the beginning and end of the infrared thermometer measurements in the study, readings were taken from a wet-dry bulb thermometer in the experimental area in the shade at a height of 2 m. The VPD at the time of measurement was calculated with basic psychrometer equations (Allen et al. 1998). Comparison of Vertical and Horizontal Measurement Techniques In studies to determine canopy temperature in trees, very different techniques have been used. Therefore, a study was made on 4 days of the first year of the experiment to compare techniques in which

tree canopy temperature is measured horizontally and vertically. This was done from August 12 to 15, 2009, when the amount of moisture in the soil of the nonirrigated plot in the study was very low and the air temperature was very high. Vertical and horizontal measurements were made at 2-h intervals during the day between 0700 and 1900 hrs in the stress treatment (S-0) and the fully irrigated treatment (control, S-C). For horizontal measurements, the technique previously described was used. Vertical measurements were carried out from above the tree canopy by using a platform attached to a tractor. In this method, canopy temperature measurements were performed on the south of the row of trees, from approximately 1 m above the tree canopy, and with the infrared thermometer held in a vertical position (nadir position; zenith angle = 130°–135°). Measurements were carried out above the tree canopy in five different directions and with three replications.

Results and Discussion Irrigation Water and Crop Evapotranspiration Irrigation applications were started when approximately 50% of available moisture capacity in the soil was used up and terminated when rains began. The irrigation season began in the third week of June 2009 and ended in the third week of September. In 2010, spring rains ended late, so irrigation began in the first week of July and ended in the third week of September, as in 2009. Table 1 shows the seasonal and average total amounts of irrigation water (mm) applied to the treatments and crop water consumption values (mm). There was no precipitation during the irrigation seasons. The total amounts of irrigation water applied to the treatments varied from 0 to 883 mm in 2009 and from 0 to 734 mm in 2010. The average of the 2 years was between 0 and 809 mm (Table 1). The seasonal evapotranspiration values of olives were 121–895 mm in 2009 and 132–811 mm in 2010 according to the treatment. The average seasonal evapotranspiration value for the 2 years varied between 127 and 853 mm. Table 1 shows that the crop water consumption values obtained from treatment Table 1. Total Amounts of Irrigation Water Applied and Seasonal Evapotranspiration in the Experiment Treatments S-0 I ETa S-0.25 I ETa S-0.50 I ETa S-0.75 I ETa S-1.00 I ETa S-1.25 I ETa S-C I ETa

2009

2010

Average

0 121

0 132

0 127

177 226

147 268

162 247

354 394

294 401

324 398

530 560

441 539

486 550

706 699

587 655

647 677

883 895

734 811

809 853

632 663

559 594

596 629

Note: I = amount of irrigation water (mm); ETa evapotranspiration (mm).

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= actual

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S-C were between those of S-0.75 and S-1.00. This shows that the decrease in moisture in the soil was between 75 and 100% of the evaporation from a Class A evaporation pan. In a study performed in the same region (Izmir) and on the same olive cultivar (cultivated variety Memecik), seasonal water consumption was 616 mm, and the total irrigation water requirement was 235 mm (Özkara and Özyılmaz 1989). In a study in Greece, irrigation water requirements for drip-irrigated olive trees were found to be between 325 and 596 mm (Metheney et al. 1994). A study in Spain found that the crop evapotranspiration of mature olive trees varied from 393 to 899 mm according to treatment (Moriana et al. 2003). The results of this study are similar to those of these studies, which were carried out in similar climatic conditions. Also, a study in Argentina found that the total amounts of irrigation water applied to drip-irrigated olives varied from 334 to 914 mm according to the irrigation treatment (Correa-Tedesco et al. 2010). Yield Table 2 shows yield and water-use efficiency values per tree and per hectare by treatments. As explained in the “Materials and Methods” section, this project was run in parallel with TUBİTAK Project No. 108O135, carried out between 2008 and 2010 in the same place, and yield data values for 2008 were taken from this project (Aşık et al. 2011). In evaluating data, the 3 years from 2008 to 2010 were taken into account. The yield per tree varied from 7.05 to 57.71 kg according to treatment, with averages between 16.49 and 37.35 kg (Table 2). The lowest yield was obtained from treatment S-0 (with no irrigation), and the highest came from treatment S-1.25. The difference between treatments S-1.00 and S-1.25 was not significant at p < 0.05. S-0.50, S-075, and S-C are included in the same group, and S-0 and S-0.25 are included in different groups. Aurora et al. (2007) obtained a yield of 38.2 kg=tree from unirrigated Manzanilla olives and a maximum yield of 52.7 kg=tree from irrigated trees. They found no difference between irrigation applications and therefore stated that a small amount of water given to olive trees would be enough to provide an increase in yield. In Spain, Gomez-Rico et al. (2007) carried out a study over 4 years in which one treatment was unirrigated and three others were irrigated at different levels. They found no difference in yield per tree between the irrigated treatments. Ben-Gal et al. (2011) carried out a study in Israel with five different irrigation treatments on two local olive cultivars. They found that yield generally increased with an increase in the amount of irrigation water applied. In Tunisia, Ben-Ahmed et al. (2007) applied water by drip irrigation at three different levels (0, 33, and 66% of crop water consumption) to the Chemlali olive cultivar. They found no difference between the

33 and 66% treatments but did find a significant difference between these two treatments and the unirrigated treatment. Water-use efficiency values varied between 1.25 and 3.72 kg=mm, and irrigation water–use efficiency values varied between 0 and 1.50 kg=mm. The highest water-use efficiency was in treatment S-0, whereas the highest irrigation water–use efficiency was obtained from treatment S-0.50. When average yield was evaluated according to WUE and IWUE, treatment S-0.50 came out ahead. Thus, it may be recommended that irrigation be carried out at half the rate of evaporation from a Class A evaporation pan in conditions when the water source is inadequate. Variation of Canopy-Air Temperature Difference (T c − T a) Canopy temperature (Tc) measurements began in June 26, 2009 [day of year (DOY) 177], and ended on September 3 (DOY 247). In 2010, they started on July 7 (DOY 187) and ended on September 14 (DOY 257). Figs. 1 and 2 show the variation over time of differences in canopy and air temperatures used to calculate the CWSI. The air temperatures measured during the course of the study varied in 2009 from 28.5 to 39°C and from 31 to 41°C in 2010. In 2009, the VPD varied between 1.7 and 5.1 kPa on days when measurements were taken, whereas in 2010 this value varied from 2.0 to 5.4 kPa. Measurements for 2010 were carried out in conditions in which the air temperature and VPD were higher than those in 2009. Fig. 1 shows temporal variations in the differences between canopy and air temperatures (Tc − Ta) for 2009 used in the calculation of the CWSI. Fig. 1 shows that although the Tc − Ta values for treatment S-0 took a positive value throughout the year, or the canopy temperature was higher than the air temperature, values in treatments S-1.00, S-1.25, and S-C were negative until the end of August and were positive after that. From the end of August, canopy temperatures in all treatments were measured as higher than air temperature because on these days the VPD fell to 2.1 kPa or less. On days when the VPD was very high (DOY 207 and 232), there was a reduction in Tc − Ta values for all treatments, and this reduction was much greater on irrigated treatments than on nonirrigated treatments on a day (DOY 207) when the VPD was 5.1 kPa. This can be explained by an increase in the rate of transpiration in the irrigated treatments and the cooling effect of transpiration. Fig. 2 shows temporal variations in Tc − Ta values for 2010 used in the calculation of CWSI. Values of Tc − Ta remained negative in treatment S-0 until the end of July (DOY 207) and then became positive. In other treatments, Tc − Ta values remained negative until the end of the season. On DOY 187 and 227, when

Table 2. Yield and Water-Use Efficiency Values for Olives in the Experiment Water-use efficiency (kg=mm)

Yield Treatments S-0 S-0.25 S-0.50 S-0.75 S-1.00 S-1.25 S-C a

2008 (kg=tree)

2009 (kg=tree)

2010 (kg=tree)

7.05 16.76 25.48 25.48 38.27 40.14 22.70

25.64 26.36 28.00 26.27 15.54 14.19 31.14

16.79 31.52 46.91 43.04 57.55 57.71 29.52

Averagea (kg=tree) 16.49 24.88 33.47 31.60 37.12 37.35 27.79

C BC AB AB A A AB

Average (kg=ha)

WUE

IWUE

4,710 7,110 9,560 9,030 10,610 10,670 7,940

3.72 2.88 2.41 1.64 1.57 1.25 1.26

0.00 1.48 1.50 0.89 0.91 0.74 0.54

Treatments with the same letter are not significantly different by Duncan’s test at p < 0.05. JOURNAL OF IRRIGATION AND DRAINAGE ENGINEERING © ASCE / SEPTEMBER 2013 / 731

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Fig. 1. Variation in Tc − Ta values for olives by treatments in 2009

the VPD was 5.3 and 5.4 kPa, respectively, Tc − Ta shows a steep fall on the graph for all treatments. When the Tc − Ta graphs for 2009 and 2010 are compared, some important differences stand out because of higher soil humidity (it was rainy in the spring and June of 2010) and VPD in 2010 than in 2009. In 2009, the maximum temperature difference between treatments S-0 and S-1.25 was 2.7°C, and the vapor pressure deficit was 4.1 kPa on DOY 212. In 2010, the maximum temperature difference between these two treatments was 2.6°C, and the vapor pressure deficit was 5.4 kPa on DOY 227. These findings are similar to those of other studies on this topic. Akkuzu et al. (2010) studied the diurnal behavior of leaf water potential and canopy temperature in irrigated and nonirrigated mature olive (cultivated variety Memecik) trees on four consecutive days in 2009. The researchers found that the maximum difference between the canopy temperatures of the two treatments was 2.7°C. Sepulcre-Canto et al. (2006) found that the difference in canopy temperature between irrigated and nonirrigated olive trees reached

4°C. In a study by Jiménez-Berni et al. (2007) of olives, the differences found between the canopy temperatures in an olive orchard between well-irrigated trees and stressed trees were approximately 2°C. In a study using images from the ASTER satellite by SepulcreCanto et al. (2009) of canopy temperatures of irrigated and nonirrigated olive trees, the difference in canopy temperatures between irrigated and nonirrigated olive orchards could rise in the summer to 2°C, but this difference was reduced in the winter. Upper and Lower Baselines for Olives In the approach of Idso et al. (1981), the direct relationship between the difference between plant canopy and air temperatures (Tc − Ta) in a plant that is transpiring at its potential rate and the VPD is called the lower baseline. It has been established that this relationship can be accepted over a wide geographical area, depending on the plant species. The upper baseline of the plant canopy–air temperature difference, independent of VPD but dependent on air temperature, is determined from plants in a state

Fig. 2. Variation in Tc − Ta values for olives by treatments in 2010 732 / JOURNAL OF IRRIGATION AND DRAINAGE ENGINEERING © ASCE / SEPTEMBER 2013

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Fig. 3. Upper and lower baselines for olives in 2009

of maximum water stress. Figs. 3 and 4 show the variations in the graphs of the upper and lower baselines of the olive trees in the study for 2009 and 2010, respectively. To establish the lower base curve, canopy temperature and wet-dry bulb thermometer readings were measured on treatment S-C between 1000 and 1600 hrs on four different days after irrigation in July and August 2009 and 2010. The line of regression giving the relationship between the vapor pressure deficit and Tc − Ta was then accepted as the lower baseline. The Tc − Ta value at the upper limit, accepted as when the plant is completely under water stress and is not transpiring, was found according to the suggestions of Idso et al. (1981). Figs. 3 and 4 show the variations in the graphs of the lower and upper baselines used to calculate the crop water stress index for the olive trees in the study for 2009 and 2010. Compared with the

2009 graph, that for 2010 is lower and shows a wider scatter because of the differences in the weather between the summers of the 2 years. Horst et al. (1989) stated that the lower limit without water stress depended on the plant species and variety and on environmental conditions; wind speed, net radiation, and the plant canopy also had an effect. Berni et al. (2009), in a study carried out in Spain, calculated the nonwater stressed baseline as Tc − Ta ¼ −0.35 VPD þ 2.08, (R2 ¼ 0.67). Variation of CWSI The values of CWSI vary according to the level of stress between 0 (minimum stress) and 1 (maximum stress). When plants are transpiring at their potential rate, leaf temperature is lower than air temperature, and CWSI ¼ 0. When plants are under water

Fig. 4. Upper and lower baselines for olives in 2010 JOURNAL OF IRRIGATION AND DRAINAGE ENGINEERING © ASCE / SEPTEMBER 2013 / 733

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Fig. 5. Variation in CWSI values of olive trees by treatments in 2009

Fig. 6. Variation in CWSI values of olive trees by treatments in 2010

stress, transpiration decreases, leaf surface temperature rises, and CWSI increases. When plants cannot transpire, CWSI ¼ 1 (Reginato 1983). Figs. 5 and 6 show variations in 2009 and 2010 of CWSI values calculated for the treatments of different irrigation levels carried out on olives in this study by using the graphs in Figs. 3 and 4. Fig. 5 shows variations in CWSI values in 2009. The CWSI values for treatment S-0 began at 0.4 and approached 1 toward the end of the season because in the nonirrigated S-0 treatment, soil moisture in the plant root zone declined progressively from the beginning to the end of the study period, so the rate of transpiration in the crop also declined. In treatments S-0.75, S-C, S-1.00, and S-1.25, however, CWSI values varied between 0 and 0.20 throughout the season. Toward the end of the season, they fell further because of low VPD values and became negative. The values of treatment S-1.25 also became negative within the season, which can be explained by the lower base curve having been set up according to treatment S-C. More irrigation water was applied to S-1.25 than to S-C, so canopy temperatures were lower, causing CWSI values to be negative. In treatments S-0.25 and S-0.50, CWSI values varied between 0.2 and 0.6. Because irrigation was applied as a proportion of the evaporation from a Class A

evaporation pan, soil moisture content decreased over time, thereby increasing the stress level in the plants simultaneously. Fig. 6 shows variations in CWSI values for 2010. The CWSI values for treatment S-0 in 2010 also began slightly above 0.4 and approached 1 by the end of the season because, as in 2009, the transpiration rate in the crop in treatment S-0, which was not irrigated, fell as soil moisture in the root zone declined progressively from the beginning to the end of the season. In treatments S-0.25 and S-0.50, CWSI values were at first very close to 0 but later rose and varied between 0.4 and 0.8 because, as in 2009, irrigation was applied as a proportion of the evaporation from Table 3. Average CWSI Values by Irrigation Treatment Treatments

2009

2010

Average

S-0 S-0.25 S-0.50 S-0.75 S-1.00 S-1.25 S-C

0.68 0.43 0.33 0.11 0.07 0.00 0.07

0.75 0.54 0.44 0.14 0.14 0.02 0.18

0.71 0.49 0.39 0.12 0.10 0.01 0.12

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Table 4. T-Test Results Relating to Values of Tc − Ta Obtained by Horizontal and Vertical Measurement Techniques (N ¼ 28) Treatments S-C

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S-0

Measurement technique Horizontal Vertical Horizontal Vertical

Average

Standard error average

Difference from average

Difference of standard error

Significance

0.202 0.194 0.203 0.195

0.129

0.280

0.648

−0.068

0.282

0.810

−1.254 −1.382 −0.029 0.039

the evaporation pan, so soil moisture content fell progressively, and stress levels in the crop rose over time. In treatments S-0.75, S-C, S-1.00, and S-1.25, CWSI values varied over the season from 0 to 0.40. In treatments S-1.00 and S-1.25, which received more water than treatment S-C, negative values were occasionally observed. The graphs for the 2 years of the study show that the curves for the treatments conflict with each other. In particular, this conflict is greater in treatments S-0.75, S-C, S-1.00, and S-1.25; therefore, stress is not experienced after treatment S-0.75. This conflict is caused by the high variability between measurement values of Tc for the same treatment. This variability can be explained by the trees’ morphology, the shading of one leaf by another, and the intrusion of nontranspiring organs such as branches and fruit into the measurement area. Therefore, very different measurement values were obtained from the same irrigation treatment. Taking into account measurement averages, higher canopy temperatures can be calculated for treatments to which more water was applied. An attempt was made to overcome this problem by increasing the number of measurements. The average CWSI values by irrigation treatment were between 0 and 0.68 in 2009 and between 0.02 and 0.71 in 2010 (Table 3). In both years, the highest values were obtained from treatment S-0, and the lowest came from treatment S-1.25. Testi et al. (2008) examined crop water stress variations in two different treatments of full and restricted irrigation in mature pistachio trees. In that study, CWSI values only occasionally exceeded 0.2 in the treatment in which water was fully replenished; however, in the restricted treatment in which water was replenished at a rate of 40% in the shell-hardening period, CWSI values reached 0.8–0.9 at the end of the season. They stated that when VPD was less than 2 kPa, the use of measured canopy temperature values in the calculation of CWSI would cause significant errors. Also, Paltineanu et al. (2013) examined the relationship between soil moisture content and CWSI in peach trees in Romania. They calculated CWSI from both leaves in the sun and leaves in the shade and reported that calculation of CWSI from sunny leaves gave a more accurate result. Again, when determining irrigation timing in medium-textured and relatively homogeneous soils, they reported that 0.18–0.20 could be used as a CWSI value calculated by taking account of sunny leaves. Comparison of Horizontal and Vertical Measurement Techniques To compare the horizontal and vertical techniques of measuring canopy temperature, a 4-day experiment was conducted in the first year of the study from August 12 to 15, 2009. Horizontal and vertical measurements were made in treatments S-0 (stressed) and S-C (control) of the study in the day between 0700 and 1900 hrs at 2-h intervals. Diurnal variation of canopy temperatures in these days and canopy temperature differences between the treatments can be examined in detail in Akkuzu et al. (2010). Table 4 shows the results of the t-test for Tc − Ta values calculated by the horizontal and vertical techniques.

The lowest air temperature (Ta) recorded on days when measurements were taken was 18.9°C, and the highest was 34.0°C. Tree canopy temperatures (Tc) for the fully irrigated treatment (S-C) varied between 16.7 and 32.5°C, whereas those of the nonirrigated treatment (S-0) varied between 17.6 and 35.0°C. Early in the morning, the canopy temperature was low; it rose to its maximum at approximately noon and later began to decline. Analysis of the Tc − Ta values calculated according to horizontal and vertical measurement techniques found no difference between horizontal and vertical measurements on either the stressed treatment or the control at p < 0.05 (Table 4). In other words, the measurement results obtained by the two methods were similar. The findings of this study are parallel with those of a study on peaches by Glenn et al. (1989).

Conclusion The 2-year average amounts of water applied to the various treatments in the study varied between 0 and 809 mm, and evapotranspiration varied between 127 and 853 mm. Three-year average yields per tree varied from 16.49 to 37.35 kg according to treatment. The highest water-use efficiency was observed in treatment S-0, and the highest irrigation water–use efficiency was in treatment S-0.50. When average yield in the study was assessed according to water-use efficiency and irrigation water–use efficiency, treatment S-0.50 came out in front. An irrigation rate equivalent to half of the amount of evaporation of a Class A evaporation pan is adequate. Moreover, the irrigation water–use efficiency of treatment S-0.25 was high. According to this result, it may be suggested that, in conditions of inadequate water resources, irrigation be carried out equivalent to a quarter of the amount of evaporation of a Class A evaporation pan. One of the findings of this study is that handheld infrared thermometer can be used for irrigation scheduling of olive trees. Analysis of the average CWSI values in the years of the study for the irrigation treatments has shown that the CWSI values of the treatments were between 0 and 0.68 in 2009 and between 0 and 0.71 in 2010. In both years, the highest values were obtained from S-0, and the lowest values came from S-1.25. It was observed in the study that the lower and upper baselines for 2009 and 2010 used in calculating the water stress indices of the olive trees were considerably different probably because of the differences of the 2 years in weather during spring and summer. Hence, longer-term work should be carried out for upper and lower baselines to be recommended for use in determining the time of irrigation for olives by region. When water-use efficiency and CWSI values are assessed together, it may be recommended that irrigation be carried out at a rate equivalent to half (S-0.50, when CWSI values reach 0.39) or, in cases where the water resource is inadequate, a quarter (S-0.25, when CWSI values reach 0.49) of the evaporation of a Class A evaporation pan.

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Another finding of this study was that no significant difference was found between Tc − Ta values calculated according to horizontal or vertical measurement techniques for either the control (S-C) or the stressed treatment (S-0). The results of measurements obtained using both techniques were similar. Therefore, when using CWSI in irrigation programming, the horizontal measurement technique is recommended, as it is more practical.

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