WATER DEMAND AND WATER USE EFFICIENCY IN ‘PALMER’ MANGO CULTIVATION IN THE LOW-MIDDLE SÃO FRANCISCO VALLEY

ABSTRACT The study aimed to determine the evapotranspiration (ETc) of the mango cv. ‘Palmer’ over two productive cycles, analyzing water-use efficiency (WUE) and crop water productivity (CWP) to propose average crop coefficient (Kc) values for improving irrigation management under cultivation conditions in the Low-Middle São Francisco Valley. The study was conducted from July 2019 to May 2021 in a commercial ‘Palmer’ mango orchard in Petrolina, State of Pernambuco, Brazil. Micrometeorological data was collected throughout the experimental period. The crop evapotranspiration (ETc) was determined using Bowen ratio energy balance (BREB), and then the Kc, WUE, and CWP were determined. The highest ETc values occurred during the floral induction phase (5.14 ± 0.85 mm day-1), with a Kc of 0.85; however, the lowest values were observed during the fruit maturation phase (3.60 ± 0.73 mm day-1), with a Kc of 0.91. Average water consumption per cycle was 1445 mm, with a daily average of 4.39 mm day-1. WUE and CWP were 16.9 and 24.5 kg ha-1 mm-1, respectively. Average Kc values of 0.81, 0.76, 0.85, 0.90, 0.95, and 0.91 are recommended for the vegetative growth, rest period and shoot maturation, floral induction, flowering, fruit development, and fruit maturation phases, respectively.


INTRODUCTION
Mango (Mangifera indica L.) is among the main fruits produced (around 1.5 million tons in 2020) and exported (over 243,000 tons and revenue of nearly US$ 247 million) by Brazil (IBGE, 2020).About 90% of the exported volume comes from the Northeastern Semi-Arid Region, especially the São Francisco River Valley, particularly the Low-Middle region where the fruit farming hubs of Petrolina, State of Pernambuco, and Juazeiro, State of Bahia, are located.This area covers about 30,000 hectares cultivated with mango, accounting for approximately 40% of the national production (Lima et al., 2018;IBGE, 2020).
Annual rainfall in the Low-Middle of the São Francisco Valley is below 500 mm year -1 , which is associated with a negative water balance for most of the year, making irrigation essential for mango cultivation (Wei et al., 2017;Medeiros et al., 2018).With the current high and increasing demand for water in agriculture, often applied inefficiently, determining water demand parameters, such as crop evapotranspiration (ETc), is essential to improve irrigation management, aiming for more precise and efficient water use, while maintaining high yields and ensuring sustainable development under conditions of low water availability (Alves et al., 2018;Simões et al., 2021).
Engenharia Agrícola, Jaboticabal, v.43, n.3, e20220189, 2023 Water consumption of mango trees in the Low-Middle region has been studied by several authors (Lopes et al., 2001;Azevedo et al., 2003;Teixeira et al., 2008;Silva, 2019).In these studies, micrometeorological methods were especially applied, focusing on the Bowen Ratio method.Mainly the Tommy Atkins and Kent varieties were studied, which showed an average crop evapotranspiration ranging between 2.3 and 5.9 mm day -1 , with the most intense consumption during the fruiting phase, for which the highest crop coefficient (Kc) values, close to 1.0, were obtained (Lopes et al., 2001;Azevedo et al., 2003;Teixeira et al., 2008;Silva et al., 2015;Silva, 2019).However, some aspects remain unclear in these studies, such as potential effects of morphophysiological differences between cultivars and detailed variations throughout a complete cultivation cycle, as most studies focus on post-flowering phases, neglecting the vegetative phase, which spans a significant period of the productive cycle.
In this context, considering the commercial importance of mango varieties like 'Tommy Atkins', 'Kent', and 'Palmer', it is crucial to explore research more extensively to determine water use by these varieties, especially for the 'Palmer', which lacks dedicated studies even though being one of the predominant planted variety in the Low-Middle of the São Francisco Valley (Lima et al., 2018).
Therefore, detailing the water consumption and the water use throughout the entire phenological cycle of the 'Palmer' mango grown in the Low-Middle of the São Francisco Valley can provide new reference parameters to improve irrigation management of the crop in the region.Hence, the objective of this study was to determine the evapotranspiration of the mango cv.'Palmer' over two productive cycles, analyzing water use efficiency and crop water productivity to propose average crop coefficient values aiming to improve irrigation management under the cultivation conditions of the Low-Middle of the São Francisco Valley.

Description of the area and experimental period
The study was conducted in a commercial orchard at Andorinhas Farm (9°27'24" S; 40°36'86" W at an altitude of 380 m above mean sea level), located in the municipality of Petrolina, Pernambuco State (Figure 1a), in the Low-Middle São Francisco River Valley, a semi-arid region in Northeast Brazil.The climate is classified as Bswh', according to the Köppen's classification, with a rainy season concentrated between January and April (Álvares et al., 2013).Climatic normals for the site are shown in Figures 1b and 1c.
Engenharia Agrícola, Jaboticabal, v.43, n.3, e20220189, 2023 Engenharia Agrícola, Jaboticabal, v.43, n.3, e20220189, 2023 The experimental area of the orchard covers 5.08 ha, cultivated with 'Palmer' mangoe, grafted onto cv.'Espada', planted in November 2002, oriented East-West, spaced at 4 m x 7 m (357 plants ha -1 ).The soil at the studied area is of the Quartzipsamment type (Mouco, 2008), at least 2 m deep, sandy textured, and well-drained.In soil management, with the orchard well established, soil coverage was accomplished through the deposition of vegetative residues from pruning, which were shredded and kept in the planting line.Additionally, each cycle had 30 liters of goat manure added per plant.These practices are crucial for improving soil water retention and availability under such texture conditions.
A micro-sprinkler irrigation system was employed, with field tests showing a flow rate of 21 L h -1 (input pressure of 4.5 kgf cm -2 and output of 3.9 kgf cm -2 ), achieving a water application efficiency of 85%.The crop was irrigated daily, split into two sessions, one in the morning (around 09h00) and other at dawn (around 01h00).
Adjustments were made based on farm water needs and the crop's phenological stages.Non-irrigation only occurred when rainfall exceeded 5 mm per day or due to irrigation system issues.The total water applied in the mango system is shown in Figure 2d.
The first evaluation cycle (cycle I) started on 03/07/2019; no production pruning occurred in this cycle as the farm adjusted its phytotechnical management to establish a new harvest season.The beginning of the cycle was marked by cleaning pruning after the previous harvest, where only the remains of inflorescence structures, fruiting, and some branches were removed; the end of this productive cycle was on 29/06/2020, with the harvest.This was followed by a resting period and the start of the second cycle (cycle II) happened with production pruning on 14/07/2020, ending with the harvest on 04/05/2021.The phenological stages are highlighted in Table 1, with their respective durations.Fertilization was performed at the beginning of each cycle, applying nitrogen (30 kg), phosphorus (20 kg), and potassium (30%).Nutritional needs were determined by farm technicians based on soil analyses (Supplementary Material).At 100 days, counted from the beginning of the cycle, potassium, sulfur, boron, and zinc were applied weekly for about 40 days.At 140 days, dormancy was broken using calcium and potassium-based products.After full flowering, potassium and nitrogen were applied, repeating potassium application 20 days before harvest.After two leaf flushes, Paclobutrazol (PBZ) was applied to the soil as growth regulator at dosages of 35.0 ml plant -1 and 4.0 L ha -1 .
Figure 2 describes the environmental conditions observed in the orchard throughout the experimental period.

Micrometeorological monitoring
A seven meter tall micrometeorological tower was installed inside the orchard.It was equipped with sensors to monitor daily micrometeorological variables (Table 2).All sensors were connected to a CR5000 data acquisition system (Campbell Scientific Inc., Logan, Utah, USA), which was programmed to take measurements every 30 seconds and store averages every 10 minutes.

Bowen Ratio Method
Net radiation (Rn) and soil heat flux (G) were measured respectively by the net radiometer and the soil heat flux sensor.The latent heat flux (LE) was estimated using Bowen ratio energy balance (BREB) as already used for mango trees (Silva, 2019;Souza et al., 2018), using the following expression: in which: The values of β were calculated from the equation: in which: ∆ -slope of the saturated vapour pressure curve, kPa °C-1 ; γ -psychrometric constant, kPa °C-1 ; tu1 e tu2 -wet bulb temperatures at two levels (levels 1 and 2) of measurement above the crop canopy, °C, and ts1 e ts2 -dry bulb temperature at two levels (levels 1 and 2) of measurement above the crop canopy, °C.
Engenharia Agrícola, Jaboticabal, v.43, n.3, e20220189, 2023 Potential inconsistencies in the BREB method were assessed by examining the errors that resulted in invalid β and LE values, as defined by Perez et al. (1999).This assessment was based on measurements of vapor pressure profiles at various times, the precision of temperature and vapor pressure sensors, and a consistent signal conversion in accordance with the flow-gradient Besides these criteria, values of β < -0.75 were deemed to cause inconsistencies in determining energy flows (Ortega-Farias et al., 1996).
To fill in invalid values, correlations were proposed using valid LE data obtained by traditional BERB from both cycles and LE data calculated using the modification proposed by Lin et al. ( 2016) (Equation 3), or with Rn measured at the micrometeorological tower when conditions did not allow using the method of Lin et al. (2016).
in which: T -absolute temperature, K, and RH -relative humidity, decimal of %.

Evapotranspiration and Crop Coefficient
Crop evapotranspiration (ETc) was determined for 10-minute intervals, based on consistent LE data, according to Expression 4. These ETc10min values were later integrated into daily average ETc data (mm day -1 ), considering the period when Rn -G > 0. Daily crop coefficients (Kc) were obtained during the two evaluated productive cycles, according to the following relationship: in which: ETc -crop evapotranspiration, mm day -1 , and ETo -reference evapotranspiration, mm day -1 .
Daily ETo was estimated by the Penman-Monteith-FAO 56 method (Allen et al., 1998).To do so, we used data collected at the agrometeorological station located 500 m from the orchard on the study farm.
For results presentation, all daily data were converted to averages per phenological phase of each cycle (Table 1), considering the mango management under irrigated tropical conditions.

Water-Use Efficiency and Crop Water Productivity
Water-use efficiency (WUE, kg ha -1 mm -1 ) was obtained for each production cycle through the expression: in which: CP -crop productivity, kg ha -1 , and I -irrigation, mm.
For productivity determination, all fruits from 10 selected plants were harvested, counted, and weighed.Productivity computation was performed considering the average fruit weight multiplied by the average fruit quantity per plant and the number of plants in one hectare (357 plants ha -1 ).
Crop water productivity (CWP, kg ha -1 mm -1 ) was determined for each evaluated cycle using the relationship: in which: CP -crop productivity, kg ha -1 , and ETc -crop evapotranspiration, mm.

Crop Evapotranspiration
Figure 3 shows the daily variation of crop evapotranspiration (ETc) for the 'Palmer' mango orchard across two evaluated productive cycles, along with reference evapotranspiration (ETo).ETc ranged between 0.94 and 7.48 mm day -1 and it was lower at the beginning and end of each cycle, which corresponded to the months of May and July.These periods encompass the vegetative growth and fruit maturation phases.On the other hand, ETc was higher during floral induction and flowering phases, which coincided mostly with the last quarter of each year.
ETc is proportional to ETo, which is higher for most of the period, indicating that the crop water demand is below the atmospheric demand.However, in the final phase of cycle I, ETc exceeded ETo on certain days during fruit development.This occurred when ETo dropped because of reduced incoming radiation in the orchard, which continued into the fruit maturation phase.On average, the incoming radiation during this time was 16.8 MJ m -2 day -1 (Figure 2b).During the same period, soil moisture levels were higher than in other phases (Figure 2d).This increase in soil moisture resulted from an increased rainfall (Figure 2d), allowing ETc to approach ETo values.In contrast, during the same phase in the following cycle, ETo was higher due to increased incoming radiation, while ETc remained like the previous cycle.
After rainfall events, ETc rises and approaches ETo values.The micro-sprinkler irrigation system used allows a wetted area concentrated beneath the canopy (Simões et al., 2018).As a result, on days without rain, values calculated using the Bowen ratio method typically reflect low transpiration and evaporation levels beneath the canopy.In contrast, after rain, the entire surface is moistened, including the inter-row spaces, resulting in increased LE, likely due to increased soil evaporation.As a micrometeorological method, the Bowen ratio energy balance (BREB) accounts for all surface fluxes, including evapotranspiration, without distinguishing between transpiration and evaporation.

Crop Coefficient
The crop coefficient (Kc) in Figure 4 highlights the impact of crop development on water demand.Average Kc rises from the vegetative growth phase to the vegetative rest and branch maturation phases in both cycles.The Kc varies between the two cycles during vegetative rest and branch maturation to fruit development, with lower values during floral induction and flowering in cycle I than those in cycle II, wherein Kc values remain close to 0.9.Finally, during fruit maturation, there was a decrease in Kc from the previous phase in both cycles.
According to Wei et al. (2017), fruiting requires more water, especially during the first six weeks of fruit formation.During this time, water scarcity can lead to fruit falling off the trees prematurely, as well as a decrease in fruit size and weight in the final phase.On the other hand, adequate water supply ensures optimal physiological processes and productive success.Engenharia Agrícola, Jaboticabal, v.43, n.3, e20220189, 2023 This study presented Kc curve similar to previous studies in the region with other mango varieties.For example, Teixeira et al. (2008) studied the Tommy Atkins cultivar and found Kc values between 0.6 and 1.2 over two production cycles.
Table 3 presents the mean of ETc per phenological phase for each cycle, along with ETo and Kc values.ETc increased during Cycle I and Kc decreased in average from vegetative growth (Kc = 0.82) to vegetative rest and branch maturation (Kc = 0.72).This suggests that during this phase, the plants consume less water than atmospheric conditions allow and requires more water during new leaf growth compared to the vegetative rest and branch maturation stages.Higher water consumption during rest and branch maturation, when ETc was 4.69 mm day -1 , was conditioned by atmospheric conditions that enhanced ETc, confirmed by higher ETo at this stage.Kc increased in the followed phases when it reaches 1.0 in average during the fruit development and maturation, reflecting as well the crop water needs that approaches the atmospheric demand.In cycle II, Kc values followed a similar pattern to the previous period, but the value of 0.80 was maintained during vegetative growth and rest, followed by an increase during floral induction, higher than the previous cycle (Kc = 0.91).One reason was that the soil water availability was higher because of the rainfall events, despite the reduced irrigation, being higher in this period than the previous one mainly in the inter row parts of the orchard.Another difference was the decrease in Kc during maturation (Kc = 0.82) compared to the previous cycle, which may be related to changes in crop management, reducing the duration of this phase and distinct environmental conditions like radiation and soil water availability.

Water Use Efficiency and Crop Water Productivity
Figure 5 shows that irrigation applied 2,393.1 mm in cycle I and 1,918.3mm in cycle II.Accumulated rainfall was 448.5 mm in cycle I and 377.7 mm in cycle II.In both cycles, water inputs were excessive compared to accumulated crop evapotranspiration of 1,554.4mm in cycle I and 1,439.4mm in cycle II.
In Cycle I, the total orchard productivity was 27.53 t ha -1 , equivalent to 134 fruits per plant.In contrast, Cycle II saw an increase to 42.77 t ha -1 , or 218 fruits per plant.The average productivity in the first cycle was close to the regional average for mangoes in the Low-Middle of the São Francisco Valley, which is 28.7 t ha -1 , as reported by IBGE (2020); this value was exceeded in the second cycle.This difference emphasizes the impact of varying management practices between cycles.Cycle I started with a cleaning pruning conducted after the previous harvest to align the production cycle with buyer demands, whereas Cycle II involved a production pruning with an already standardized production system.These conditions led to a higher production of inflorescences in cycle II, which may be linked to higher temperatures during floral induction in this cycle (Figure 2a), resulting in more fruits per plant.Additionally, these factors likely contributed to enhanced leaf development, thereby increasing the resources available for photosynthetic processes, biomass accumulation, and fruit production.
Irrigation-based WUE was equivalent to 11.5 kg ha -1 mm -1 in cycle I and 22.3 kg ha -1 mm -1 in cycle II.The difference in WUE between cycles is due to the management practices adopted, as the crop has better productivity in cycle II compared to cycle I.Simões et al. (2021) obtained values of 22.7 and 36.6 kg ha -1 mm -1 for two cycles of mango cv.'Kent' grown in Petrolina-PE, using an irrigation depth of 100% of the recommended ETc.In a study conducted in Spain under a subtropical Mediterranean climate, Zuazo et al. (2019) obtained a WUE of 21.2 kg ha -1 mm -1 for cv.'Tommy Atkins'.Therefore, irrigation can be managed more efficiently by developing strategies to reduce water application under similar conditions to the evaluated orchard, while maintaining or even increasing productivity, as highlighted by Simões et al. (2021).This would reduce costs associated with irrigation and increase WUE (Hahn et al., 2022).CWP was 17.7 kg ha -1 mm -1 in cycle I and 32.1 kg ha -1 mm -1 in cycle II.The difference between cycles again arises due to the distinct management practices employed in each cycle.The higher value for cycle II is closer to the value obtained by Teixeira & Bastiaanssen (2009), for the same region, on the cv.'Tommy Atkins', with a CWP of 40.0 kg ha -1 mm -1 .

FIGURE 1 .
FIGURE 1. Location and climatological conditions of the 'Palmer' mango orchard in Petrolina, State of Pernambuco, Brazil.

TABLE 1 .
Duration of the phenological stages of the 'Palmer' mango variety grown in the Low-Middle São Francisco River Valley.

TABLE 2 .
Sensors used for micrometeorological monitoring of 'Palmer' mango orchard in Petrolina, state of Pernambuco, Brazil.