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Water uptake dynamics for adult peach trees in a subtropical humid climate
Scientia Horticulturae 267 (2020) 109318
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Water uptake dynamics for adult peach trees in a subtropical humid climate a
a,
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Carlos Zambrano-Vaca , Lincoln Zotarelli *, Kelly T. Morgan , Kati W. Migliaccio , Richard C. Beeson Jr.d, José X. Chaparroa, Mercy A. Olmsteada a
Horticultural Sciences Department, University of Florida, Gainesville, FL, 32611, USA Southwest Florida Research and Education Center University of Florida, Immokalee, FL, 34142, USA Agricultural and Biological Engineering Department, University of Florida, Gainesville, FL, 32611, USA d Mid-Florida Research and Education Center, University of Florida, Apopka, FL, 32703, USA b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Crop coefficient Soil water depletion Stress coefficient Water demand
Low-chill peach [Prunus persica (L.) Batsch] cultivars allow Florida growers to become competitive by offering fruits earlier than northern states for premium prices. Due to the predominance of sandy soils in Florida, irrigation is required to ensure fruit production in peaches. Available irrigation recommendations and peach crop coefficients (Kc) were determined in Mediterranean and temperate climates, and they can overestimate water demand in humid subtropical climates. Thus, a two-year study was conducted in Citra, FL, aiming to determine the water requirements and Kc values for four-year-old peach trees in humid subtropical conditions. A daily soil water balance and the crop evapotranspiration (ETc) was determined using soil volumetric water content recorded every 10 min. Five soil probes, each equipped with soil moisture sensors at 10, 20, 40, and 80 cm of soil depth representing soil depth layers of 0–15, 15–30, 30–60, and 60–90 cm, respectively placed under the tree canopy. The ratios of daily ETc and Penman-Monteith FAO-56 reference evapotranspiration were used to estimate Kc for phenological stages of adult (≥4-year-old) peach trees. A soil water depletion coefficient (Ks) was estimated obtaining a threshold of 25.8 % available soil water depletion before trees undergo water stress. Daily ETc ranged from 0.22 mm d−1 during dormancy to 3.65 mm d−1 during shoot development. Values of Kc for peach trees ranged from 0.30 during dormancy to 0.69 during fruit maturity. The use of peach tree Kc adapted for subtropical humid conditions reduce the estimated crop water requirements by 21 % compared to the Kc values reported in FAO-56 guidelines.
1. Introduction
prolonged time. Rainfall comes primarily from conventional thunder and tropical storms with a higher incidence during the summer than winter months. Major differences in air temperature, relative humidity, and vapor pressure deficit (VPD) between humid and Mediterranean regions lead to different evapotranspirative demands, consequently different crop water requirements. Johnson et al. (2004) reported that VPD higher than 2 kPa can result in up to 24 % higher water requirement in peaches grown in a study conducted in California. Therefore, the use of Kc values or irrigation recommendations from other regions may overestimate the water requirements of peaches under humid conditions, where VPD rarely exceeds 2 kPa. In humid subtropical climates, winters are generally mild thus, ‘lowchill’ peach cultivars are a viable option because they require as low as 100 chilling hours (Sarkhosh et al., 2018). Peach trees cultivated in subtropical climates have a different length of phenological stages compared to peaches grown in temperate conditions. The timing and duration of crop phenological stages should be accounted for in the
Application of proper irrigation volume that meets the tree water demand based on weather conditions, soil characteristics, and crop phenological stages is key to the efficient use of agricultural water. Despite the expansion of peach [Prunus persica (L.) Batsch] cultivation to humid climates, available information about water requirements for peaches originates from Mediterranean (Abrisqueta et al., 2011, 2013; Ayars et al., 2003; Johnson et al., 2000, 2004) and temperate regions (Massai and Remorini, 2000). There is limited information on management practices for peach trees growing under humid subtropical conditions (Williamson and Crane, 2010). Florida has a humid subtropical climate (Cfa) according to Köppen classification, during hotter months of the year, daily maximum air temperatures are above 30 °C, and the daily average is around 22 °C or higher. Winters are generally mild and during the colder months, the average air temperature is below 18 °C, but higher than −3 °C. Frost can occur but not for a
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Corresponding author. E-mail address: lzota@ufl.edu (L. Zotarelli).
https://doi.org/10.1016/j.scienta.2020.109318 Received 4 September 2019; Received in revised form 23 February 2020; Accepted 26 February 2020 0304-4238/ © 2020 Elsevier B.V. All rights reserved.
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2016 and 21 December of 2017. The trees were fertilized with a blend of soluble fertilizer (70 %) and controlled release fertilizer (30 %) for each application, for a total annual application rate of 100 kg ha-1 of N, 33 kg ha-1 of P2O5, 50 kg ha-1 K2O split in three equal applications in late January, May, and August.
determination of peach water requirements throughout the year under humid subtropical conditions. One of the most common methods to estimate the crop evapotranspiration (ETc) is the product of the reference evapotranspiration (ETo) and crop coefficient (Kc). The ETc represents the evapotranspirative demand of the crop plus the water evaporation from the soil surface. The estimation of ETc is valid when soil water availability is non-limiting. However, when plant water uptake is limited by soil water availability, which is common in sandy soils, the use of a water depletion coefficient (Ks) is needed (Allen et al., 1998). The total soil available water (TAW) for plant uptake can be defined by a soil water content (θ) between field capacity (FC) and wilting point (WP). Within the TAW interval, there is a limited range of water depletion where plant water uptake is not affected (Allen et al., 1998). This range is known as the readily available water (RAW), and below this threshold point, the plant is under water stress before reaching the permanent wilting point (WP) (Allen et al., 1998). The proportion of TAW represented by RAW is used to estimate Ks, and its value changes depending on soil physical characteristics. Therefore, Ks works as a correction coefficient to estimate ETc from ETo. When the Kc is not affected by the θ, the value of Ks equals to 1. Due to the low water holding capacity of sandy soils, RAW can be less than 5% (Morgan et al., 2006), whereas it can be approximately 50 %, as suggested previously for loam and loamy clay soils (Allen et al., 1998). Under the assumption of a threshold point, it can be expected to have a maximum daily ETc when θ is near or at FC and have a reduced water consumption when values of Ks are lower than 1. Therefore, the soil water content should be maintained near or at FC, ensuring adequate soil water availability to the crop, avoiding crop water stress, and preventing soil water percolation below the root zone due to excess irrigation. A two-year field experiment was established to determine water requirements for peaches grown under humid subtropical conditions in the southeast USA. The objectives of this study were: i) measure depletion in daily soil volumetric water content (VWC, cm3 cm−3) in the root zone to estimate daily peach ETc, ii) to estimate Kc for adult peach trees in humid subtropical conditions based on their phenological stage and day of the year, and iii) estimate a Ks value for peach production in sandy soils based on reduced daily ETc for a given the phenological crop stage at selected daily available soil water content depletion.
2.2. Weather and soil water measurements The weather parameters such as rainfall, air temperature, solar radiation, wind speed, and relative humidity were measured every 15 min and aggregated to hourly data during using a weather station (UT30 Campbell Scientific, Logan, UT, USA) from the Florida Automated Weather Network (FAWN) station in Citra, FL, (latitude 29.41, longitude -82.17). The daily VPD and daily ETo were calculated using the FAO-56 equations (Allen et al., 1998). 2.3. Hydraulic soil characterization Soil hydraulic characterization under the peach tree canopy was determined before soil moisture sensors installation on 15 June 2016. Four random locations were selected to collect undisturbed soil cores (6 cm height by 5.4 cm diameter) at 15- and 30-cm soil depth for each location. An additional, four soil samples were collected on the same day and sent soil carbon analysis (Waters Agricultural Laboratory, Camilla, GA, USA). Soil water retention curves were determined by using pressure plates according to the methodology described by Gallage et al. (2013). Subsequently, data were averaged and fitted using the van Genuchten equation (Eq. 1) (van Genuchten, 1980) using the RETC software v. 6.02 for Windows (van Genuchten et al., 1991). θ = θr + (θs - θr) / [(1+(αh)n)]m
(1) −3
Where, θ is the soil water content (cm cm ), θr is the soil residual water content (cm3 cm−3), θs is the soil saturated water content (cm3 cm−3), h is soil water pressure head (cm), α is a scale parameter inversely proportional to mean pore diameter (cm-1), n and m are the shape parameters of soil water characteristic curve, m = 1 − 1/n, 0 < m < 1. 3
2.4. Irrigation management 2. Materials and methods Irrigation was supplied by two micro-sprinklers per tree (Maxijet Max-240 Peach 1.83 m radius, 39.75 L∙h−1, 240° *10 blue mediumangle streams, Dundee, FL, USA) to simulate commercial settings in Florida. The soil water content of the irrigated area (10.51 m2) under each tree was independently maintained within 25 % available soil water depletion (ASWD) until refill back to the soil FC point after the irrigation during fruit production and 50 % during shoot development. Irrigation was controlled using an automated irrigation control system with switching tensiometers installed at 15 and 30 cm depths in the irrigated zone (ASS-506 and AAS- 512, low tension w/AC switch closed past setting, Irrometer, CA, USA). The tensiometers were set at 15 kPa as the threshold to start irrigation. A modular controller unit (Rainbird ESP4ME modular, Rainbird, CA, USA) controlled irrigation application by bypassing time clock-initiated irrigation events if soil moisture values of both tensiometers was above the threshold. All irrigation events occurred in the morning between 1:00 and 5:00 h to minimize surface evaporation from solar radiation or wind.
2.1. Experimental orchard and management This study was conducted from 1 September 2016 to 31 August 2018 corresponding to the growing seasons 2016-17 and 2017-18 at the University of Florida, Plant Science Research and Education Unit, Citra, FL, USA (longitude 29.41, latitude -82.16). The soil at the site is classified as Arredondo sand, siliceous, semiactive, hyperthermic Grossarenic Paleudults sand with 0% slope (USDA-NRCS, 2017). Three four-year-old trees of variety ‘TropicBeauty’ grafted on ‘Flordaguard’ were randomly selected from a 0.8 ha orchard. The variety ‘TropicBeauty’ is classified as a low-chill requiring 150 ‘chill-hour’ to flower. The growth habit of a peach trees was divided into crop phenological stages according to the “Biologische Bundesanstalt, Bundessortenamt und Chemische Industrie” (BBCH): 0 - dormancy (occurring between 335-20 DOY); 1 - leaf development (21–105 DOY); 3 - shoot development (106–300); and 9 - senescence (301–354 DOY) (Meier et al., 1994). Trees were planted 4.6 m apart in-row, and 6.1 m apart in between rows, with an allocated tree space of 27.87 m2, resulting in 358 trees·ha−1 plant population. Herbicide and manual weeding were used to maintain a nearly weed-free strip 4.57 m wide. Pruning was conducted twice a year on 22 December 2016, 10 June 2017, 23 December 2017, and 12 June 2018. An annual spray application of a 4% solution zinc sulfate to defoliate the peach trees was conducted on 20 December
2.5. Soil moisture probes set up Soil VWC was measured every 10 min using a set of EnviroSCAN capacitance probes (Sentek Pty. Ltd., Stepney, Australia) equipped with soil moisture sensors at 10, 20, 40, and 80 cm of soil depth representing soil depth layers of 0–15, 15–30, 30–60, and 60–90 cm, respectively (Fig. 1A). A total of five probes were used per tree, three probes were 2
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Fig. 1. Field diagram of soil moisture probes arrangement in the peach tree irrigated area. A) Lateral view of the three probes installed from north to south and the soil moisture probes on each probe located at 10, 20, 40, and 80 cm soil depths; B) above view of the wetted area divided by five zones associated to each probe. The trapezium symbols represent the probe location in each zone.
installed in-row, oriented north to south; and two probes were between tree rows, oriented east to west. The distance of the first probe from the trunk and between probes was 60 cm apart. The criteria utilized for the probe distribution considered the total wetted area by the sprinklers, irrigated pattern resulted from the use of two sprinklers of 240° radius, and the expected peach root distribution. To calculate crop water uptake, the wetted area was divided into five zones, as proposed by Morgan et al. (2006). One capacitance probe was installed in representative points of each zone, and soil water measurements from each sensor represented the moisture beneath each zone and correspondent soil depth layer (Fig. 1B). Probes in Zone 1 and Zone 4 were installed 60 cm from the trunk in the center of a triangular region of 1.29 m height each. Another two probes were installed 1.2 m from the trunk and corresponded to Zone 2 (in-row) and Zone 5 (between rows). The probe installed 1.8 m from the trunk (in-row) represented soil moisture content in Zone 3. Zones 4 and 5 corresponded to the overlapped area of the sprinklers. The soil moisture measurement from each sensor was assumed to be uniform in each assigned zone and correspondent soil depth layer. Each sensor was calibrated for the soil type following the recommendations from the manufacturing company, with weekly checks to avoid sensor malfunctions.
Volume of water lost (L) = VWC depletion (cm3 cm−3) * assigned zone (m2) * soil layer width (m) *1000 (3)
2.7. Kc and ks calculations The ratios of daily ETc, calculated from the soil water depletion calculation to daily ETo, were used to estimate daily Kc values for peach tree using Eq. 4 (Allen et al., 1998). It was assumed that on those days Ks was equal to 1 because VWC was at least 95 % of FC. The calculated Kc values were used on two regression models plotted against day of the year (DOY) to obtain an estimated daily Kc throughout the year. First regression model comprised of a three-segment linear regression (1–100 DOY), (101–300 DOY), and (301–365 DOY), whereas the second model was a quadratic regression including data for the entire year. The estimated Kc values obtained from the regression models were then used to estimate daily Ks using the ratios of ETc to (ETo * Kc) (Eq. 4). The relationship between ASWD and the correspondent values of Ks was determined by linear regression analysis. The resulting equation was then used to calculate the threshold point of ASWD by replacing the Ks value by 1. The threshold point for ASWD indicates the lowest soil VWC in which tree water uptake is not limited, ensuring trees are not under water stress conditions.
2.6. Soil water depletion and ETc calculations
ETc = ETo* Kc* Ks
The soil water balance was calculated only for the days that the VWC of all five zones and soil depth layers averaged ± 5% of FC in the early hours of the day. For those specific days, the soil water depletion was calculated using the VWC readings every 10 min. The assumption was that since the VWC values were near to the FC, the soil moisture equilibrium was reached (Fares et al., 2000). Thus, soil water percolation was considered to be negligible and after soil VWC equilibrated at FC soil water depletion was assumed to be due to ETc. The average of all replications for each zone was used to calculate daily soil water depletion for each soil depth layer by using Eq. 3. Then, the volume of water depleted of each assigned zone and soil layer was calculated. Afterward, the four soil depth layers were added to account for the soil profile where roots were found down to a depth of 0.90 m. Subsequently, soil water in the five assigned zones was added and multiplied by 2 to obtain the total volume of water (L∙m−2) depleted from the wetted area per tree (Fig. 1B). The volume of water depleted from Eq. 3 was then divided to the allocated area of each tree (27.87 m2) to obtain ETc (L m−2) or (mm).
(4)
2.8. Statistical analysis All linear and quadratic regressions were analyzed at p < 0.05 of significance. Kc estimations were analyzed using a Pair t-test at p < 0.05 of significance using SigmaPlot 14.0 software (Systat Software, San Jose, CA, USA). 3. Results and discussion 3.1. Soil hydraulic characterization The soil profile at the experiment site had a similar soil texture up to 0.9 m of soil depth (USDA-NRCS, 2017), which was the depth where roots were observed after excavation. The soil organic matter was 10.8 g kg−1 at the 0−30 cm soil depth. The 0−15 cm and 15−30 cm soil depth layers were averaged, and the soil water characteristic curve was determined by fitting the van Genutchen equation (r2 = 0.96) (Fig. 2). The fitting parameters for the Arredondo sand were θr (0.01);
VWC depletion (cm3 cm−3) = VWC initial (cm3 cm−3) – VWC final (cm3 cm−3) (2) 3
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between February and April, average air temperature ranged between 17 and 22 °C in 2017 and 16–20 °C in 2018. During the shoot development stage (April to October), air temperature ranged from about 5–35 °C (Fig. 3A) and average daily temperatures between 18 and 28 °C in both seasons. The relatively high air temperature (Fig. 3A) and high relative humidity characteristic of north-Florida climate, resulted in VPD reaching up to 1.9 kPa in May of each year during the shoot development stage (Fig. 3B). The VPD was at the lowest during the dormancy stage in lateDecember to mid-January and ranged from 0.09 to 1.1 kPa in 2016-17 and 0.02–1.1 kPa in 2017-18 (Fig. 3B). Johnson et al. (2002) suggested that peach trees have a strong positive response to VPD, resulting in high water demand associated with high VPD. The values of VPD reported for north Florida did not exceed 2.0 kPa (Fig. 3B) in contrast to VPD of around 4 kPa common in Mediterranean regions. Johnson et al. (2004) reported that underestimation of peach water demand might occur when VPD is higher than 2.0 kPa and adjusted Kc are needed to avoid this issue. Thus, the difference in VPD between the two regions can result in overirrigation if recommendations from Mediterranean climates are adopted in a humid subtropical climate. Daily rainfall and ETo from September 2016 to August 2018 are presented in Fig. 3C. From February to April, rainfall was scattered and represented for about 17 % of the annual rainfall. About 70 % of the annual rainfall occurred from May to October (rainy season), which corresponded to the shoot development stage. Rainy and cloudy days resulted in low ETo days during the rainy season (Fig. 3C). Summer and fall seasons are characterized by the occurrence of tropical storms and hurricanes. The hurricane “Irma” accounted for about 221 mm of rainfall on 10 and 11 September 2017, which represented 15 % of the rainfall in 2017. Overall, the daily ETo had a bell-shaped curve ranging from 0.74 mm day−1 during the dormancy stage (late-December to lateJanuary) and peaking at 5.4 mm day−1 during the shoot development stage (May-October). After reaching the highest ETo rates during the shoot development stage, ETo progressively decreased from mid-July throughout December (Fig. 3C).
Fig. 2. Soil water characteristic curve for Arredondo sand (r2 = 0.962); FC (0.114 cm3 cm−3) and WP (0.017 cm3 cm−3). Fitting parameters θr (0.01); θs (0.399); α (0.023); n (1.625); m (0.3846); and Ks (3627.12 mm∙d-1).
θs (0.399); α (0.023); n (1.625); m (0.3846); and Ksat (3627.12 mm d−1). The FC was determined to be at 0.114 cm3 cm-3, WP at 0.017 cm3 cm-3, and TAW 0.097 cm3 cm-3. 3.2. Weather characterization and reference evapotranspiration Daily air temperatures during the period of the study are presented in Fig. 3A. Throughout the year, January was the coldest month and averaged 15.5 and 10.6 °C in 2017 and 2018, respectively, while the daily average air temperature in January ranged from about 3–22 °C (Fig. 3A). During the time of the study, peach trees accumulated 196 and 324 chilling hours in 2016-17 and 2017-18 seasons, respectively. During the dormancy stage, the minimum air temperature reached −2 °C on 8 January 2017 and −5 °C on 3 and 7 January 2018, (Fig. 3A). Air temperatures below 0 °C occurred when the trees were still dormant and lasted for 5 h or less. During the flowering, leaf development, and harvest periods which correspond to the months
3.3. Soil water content in the tree wetted area Soil VWC monitored at four different soil depths in three months of
Fig. 3. Daily maximum and minimum air temperature (A); vapor pressure deficit (B), and daily reference evapotranspiration (ETo) and rainfall; (C), from September 2016 to August 2018 in Citra, FL. 4
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Fig. 4. Volumetric water content (cm3 cm−3) from 0-15, 15-30, 30-60, and 60-90 cm of soil depth in the irrigated zone in Citra, FL. A) From 1-13 January 2018 corresponded to peach dormancy period, B) from 18-30 March 2017 period correspondent to peach fruit development, and C) 13-26 June 2018 period correspondent to peach tree shoot development. Arrows indicate irrigation events.
the year are presented in Fig. 4. The soil VWC in the upper soil depth layers was maintained near FC as much as possible throughout the year with supplemental irrigation to avoid plant water stress. Peach trees have variable water demand throughout the year not only because of the changes in ETo (Fig. 3C) but also due to the changes in phenological stages. January was characterized by the lowest ETo of the year, when peach trees were dormant for most parts of the month resulting in a very low crop water demand compared to the other phenological stages (Allen et al., 1998). From 1–13 January 2018 (dormancy stage), there were some rainfall events and the major event was 31 mm, which were enough to maintain the VWC was near FC (Fig. 4A) due to the very low ETo for the period (Fig. 3C). Between 18 and 30 of March 2017, the period corresponding to the fruit development stage, a 0.05 mm rainfall occurred on the 22 March, and irrigation was applied daily to replenish soil VWC at FC (Fig. 4B). In the summer, a higher frequency of rainfall events was reported compared to the spring months (Fig. 3C). The summer corresponded to peach shoot development stage. Due to the high frequency of rainfall during summer, much lower volumes of irrigation were needed. For example, from 13 to 25 June 2018, multiple rainfall events, ranging from 3 to 28 mm, maintained the VWC above FC in the upper soil depth layers. They also resulted in an increased VWC at the 60−90 cm soil depth layer, indicating soil water percolation (Fig. 4C). In this way, Fig. 4 exemplifies the different soil-waterplant-atmosphere dynamics throughout the year. The critical growth stages for peach trees when water stress can be detrimental to yield is followed by the dormancy period corresponding to March and April. In early March, the ETo increased progressively until reaching the highest portion of the bell-shaped curve in late June (Fig. 3C). March and April are months when peach fruit development occurs. Therefore, maintaining VWC near FC during that time prevents small fruit size and reduced yield due to water stress (Mahhou et al., 2006). Shoot development took place from May to October. This was the period characterized by the highest crop water demand. However, due to the high rainfall, irrigation was less needed to replenish the soil water storage back to the FC. Close monitoring of weather and soil VWC is needed during the summer months due to the erratic pattern of rainfall. The combination of high rainfall incidence and over-irrigation can cause excessive peach shoot growth and nutrient leaching. Conversely, water stress can lead to reduced stem and tree growth (Berman and Dejong, 1997).
Fig. 5. Daily crop evapotranspiration (ETc in mm, mean of three replicates) from September 2016 to August 2018 in Citra, FL. Numbers above the graphic represent the crop phenological stages: 0 (dormancy 335-20 DOY); 1 (leaf development 21-100 DOY); 3 (shoot development 101-300); 9 (senescence 301334 DOY).
from 0.3 to 0.8 mm in the 2017-18 season. With the transition from dormancy to the leaf development stage, ETc increased and ranged from 0.9 to 1.8 mm in 2016-17 and from 0.4 to 1.7 mm in 2017-18. The peach ETc peaked during the shoot development stage (DOY 101–300) at 3.5 mm in 2016-17 and 3.7 mm in 2017-18. Towards the end of each year, peach ETc decreased gradually from 1.5 mm around DOY 301 to 0.5 mm around DOY 334 in 2016-17, and from 1.4 to 0.7 mm in 201718 for the same period. The difference in ETc ranges between growing seasons was due to the variation in ETo factors such as minimum/ maximum air temperature, and relative humidity. Nevertheless, a paired t-test analysis between the daily ETo of the growing seasons resulted in no significant difference (P > 0.27). The estimated values of ETc in the present study did not exceed the measured ETo values (Fig. 3C), contrary to the findings reported by Abrisqueta et al. (2013) in Mediterranean conditions, where ETc was about 9% higher than ETo at the end of the shoot development stage (DOY 240–300). In another study, Ayars et al. (2003) reported peach ETc values ranging from 0.2–9.5 mm, while ETo ranged from 0.3–7.7 mm throughout the year. The 12 % higher ETc than ETo reported by Ayars et al. (2003) occurred between DOY 180–240 coinciding with the fruit development and harvest, which occurred between DOY 205–235 (late-season peach). Therefore, the highest ETc reported by Ayars et al. (2003) occurred when fruits were still on the tree, which explains the high-water demand by peach trees. In the present study,
3.4. Quantifying soil water depletion and ETc The peach tree ETc (mm) was estimated by the daily ASWD for the irrigated zone previously calculated using Eq. 2 and 3 for the period from September 2016 to August 2018 (Fig. 5A). During the dormancy stage (DOY 335-20), the ETc ranged from 0.7 to 1.4 mm in 2016-17, and 5
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Johnson et al., 2005) and for citrus in Florida (Morgan et al., 2006). The crop coefficient equations for the quadratic fit follow:
peach harvest occurred much earlier around DOY 85–120 (Fig. 5A), which was a period when ETc and ETo were still increasing before peaking around DOY 140–220 (Figs. 3C and 5 A). These observations support the different water requirements during phenological stages and due to presence of fruits on peach trees; thus, the importance of considering these factors in the proper determination of Kc values. The daily ASWD represents the soil evaporation and plant transpiration. The ASWD was calculated for the 0−60 cm soil depth layer for three-time period intervals correspondent to DOY 1–100, DOY 101–300, and DOY 301–365, and also the annual average of the ASWD (Fig. 5B). The first and the third periods had similar percentages of daily soil water depleted. On average 76 % of the total daily water depletion originated from the 0−60 cm soil depth layer. For the second period (DOY 101–300), 81 % of the daily water depleted originated from the top 60 cm of soil. The findings of this study are similar to the results from Garnier et al. (1986), where about 80 % of the peach tree water uptake occurs in the first 60 cm of the soil profile in well-watered soil. Therefore, irrigation frequency is a key component to maintain the proper soil moisture content in the first 60 cm of soil depth. Thus, avoiding prolonged time between irrigation events would prevent excessive soil water depletion on the upper soil layers.
Kc = 0.318 + (0.00379 * DOY) - (0.00000955* DOY2) for DOY 1–365 (r2 0.55) (8) Despite the different approaches for the determination of Kc, the overall values of Kc were quite similar. The estimated Kc for the dormancy period was 0.29 from the linear fit, while 0.32 from the quadratic fit. Allen et al. (1998) reported Kc after peach leaf drop of 0.20, which is lower than the estimated Kc from this study (0.30) (Fig. 6). This may be due to the mild-winters air temperatures in Florida, which requires more water for plant maintenance than in regions with cold winters. Additionally, the trees do not lose foliage naturally like in northern latitudes, but they had to be defoliated during late December. This might have affected the water consumption during this time compared to regions were the trees enter dormancy naturally. During the shoot development stage, Kc values were 0.66 and 0.69 for the linear and quadratic fits, respectively (Fig. 6). A paired t-test analysis between the three-segment linear fit and the quadratic fit resulted in no significant difference (P > 0.87) between Kc values obtained by either approach. The determination of Kc for other environments revealed major discrepancies with the present study which are mainly attributed to the higher evapotranspirative demand that the peach trees were subjected. For example, Ayars et al. (2003) reported Kc ranging from 0.25 to 1.40 for the DOY 60–310, respectively for California. Ferreira et al. (1996) reported Kc values between 0.4 and 0.6 using the heat balance sap flow method and modeling soil evaporation for the DOY 165–210 (June and July) in Mediterranean climate. Additionally, Ferreira et al. (1997) reported values between 0.4 and 0.6 using the eddy covariance method during the same period. Another critical factor to consider is the method for calculation of the area occupied by the tree. In both studies cited previously, authors mentioned that Kc values of 0.7–1.2 were observed if the projected area of the canopy was used as the base for the evapotranspirative area. Ferreira et al. (1996) and Ayars et al. (2003) determined Kc values from 0.7 to 1.3 using the area occupied by the tree for similar crop stages. While, the FAO-56 (Allen et al., 1998) reports Kc values for crop stage, Kc ini 0.55, Kc mid 0.9, Kc end 0.65 and Kc 0.20 after leaf drop; however, there is no specification how these Kc values were obtained. Overall, Kc values from the studies cited above were generally higher than the ones from the present study indicating that recommendations for other climates might overestimate peach water demand on subtropical humid conditions. The only exception is a study conducted by Paço et al. (2006), in which Kc values of 0.5 and 0.7 for June and July (only months reported) for temperate regions. The ETc values reported by Paço et al. (2006) peaked at about 3.5 mm day−1, which was similar to the ETc from the present study; however, ETo was not reported on Paço
3.5. Peach kc quantification The calculated Kc values for days when VWC was within ± 5% of the FC from September 2016 to August 2018 are presented in Fig. 6. The Kc values for the peach dormancy period, represented by DOY 33520 ranged from 0.25 to 0.35. There was an overall increase in the Kc value due to the shoot development during DOY 30 and 95. The Kc values for this period ranged from 0.38 to 0.68. Between DOY 100 and 330, corresponding to the summer and fall months, the Kc values ranged from 0.5 and 0.8 (Fig. 6). Regression analysis was used to determine the relationship between Kc values and DOY. The first approach was the three-segment linear regression (Eqs. 5,6, and 7) for a given period of the year determined by the peach crop stage. This first approach had a significant relationship only for the periods 1–100 DOY (Eq. 5) and 301–365 DOY (Eq. 7), and non-significant for the period of 101–300 DOY (Eq. 6). Kc = 0.294 + (0.00390 * DOY) for DOY 1–100 (r2 0.60) 2
Kc = 0.659 - (0.0000313 * DOY) for DOY 101–300 (r 0.001) 2
Kc = 2.498 - (0.00587 * DOY) for DOY 301–365 (r 0.57)
(5) (6) (7)
The second approach was fitting a quadratic function to the data regardless of the crop stage (Eq. 8). A significant relationship between DOY and Kc was observed. This approach had been reported for peaches in Mediterranean conditions (Abrisqueta et al., 2013; Ayars et al., 2003;
Fig. 6. Daily calculated, three-segment linear fit, quadratic fit, and FAO-56 crop coefficients in Citra, Florida from September 2016 to August 2018. Numbers above the graphic represent a phenological stage 0 (dormancy 335-20 DOY); 1 (leaf development 21-105 DOY); 3 (shoot development 106-300); 9 (senescence, beginning of dormancy 301-334 DOY). Values used for FAO-56 Kc initial = 0.55, Kc mid-season = 0.9 Kc end =0.65 and Kc dormancy = 0.2.
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temperate climates. A short dormancy period and an early harvest are two of the main characteristics of peach production in Florida. Therefore, the use of Kc values developed for humid conditions that account for water usage year-round would increase water saving and prevent water stress during critical periods, such as fruit development. Furthermore, soil VWC should be monitored regularly to prevent ASWD to be higher than 25.8 %; therefore, irrigation frequency is a key component to have proper irrigation management. Using the Kc values determined for humid conditions and avoiding excessive ASWD can result in improved peach production practices to increase water savings, reduce excessive irrigation, and prevent reduction on fruit yield due to water stress. 4. Conclusion The Kc values determined in the present study suggest that water requirements for peaches in humid subtropical conditions are much lower than in Mediterranean regions and previously reported in the FAO-56 publication. Daily volumetric soil moisture variation was successfully used to estimate daily ETc when soil moisture was near FC. Peach ETc ranged from 0.8 to 3.7 mm∙d−1 in season 1 and 0.3 to 3.5 mm∙d−1 in season 2. The estimated Kc values from the quadratic fit ranged from 0.3 for the dormancy and Kc 0.69 for the shoot development. The use of peach tree Kc adapted for subtropical humid conditions may potentially reduce crop water requirements by 21 % compared to the Kc values reported in FAO-56 guidelines. Ks values lower than 1 were observed when ASWD reached 25.8 %, which corresponded to the soil volumetric water content threshold in which ETc may become limited due to soil water stress. The finding of the present study will support future research on irrigation scheduling focus on minimizing excessive irrigation water application and nutrient leaching in sandy soils. The accurate determination of crop water allows growers to adopt more sustainable irrigation practices to improve crop yield and profitability.
Fig. 7. Estimated soil water depletion coefficient (Ks) as a function of available soil water depletion (ASWD%) for Arredondo sand soil in Citra, FL.
et al. (2006) study. The peach water demand is mainly driven by the climatic factor affecting evapotranspirative demands but also by the phenology of the plant. In humid subtropical conditions, the dormancy period can, in some years, last less than 30 days, and most of the Kc previously reported in other studies disregard tree water demand for at least four months during the year. Therefore, a Kc recommendation for the yearly cycle of peach trees, as proposed by the quadratic model can be very useful for irrigation scheduling in humid climates. If Kc values presented in FAO-56 recommendation were used in this study, the irrigation volume applied in 2017 would have been about 140 mm or 21 % higher than the requirement estimated by using the Kc values of the present study.
3.6. Ks and maximum ASWD % quantification CRediT authorship contribution statement Daily Ks values were obtained using Eqs. 3–4. For this estimation, the measured ETo, Kc values obtained from Eqs. 3–8, and the corresponding ETc were used. The estimated Ks values were plotted against the corresponding ASDW for a given day (Fig. 7). A significant relationship (P < 0.001) between Ks and ASDW% was determined, indicating that for the Arredondo sand, when VWC values were below 25.8 % of ASWD (when Ks < 1) plant water stress starts occurring, which can limit ETc. This ASWD threshold is lower than those previously reported by Ferreira et al. (1996). They reported a threshold of 40 % for the 0−80 cm sandy soil layer, and 80−100 cm loamy sand layer, in contrast to the homogenous soil texture along with the profile in the present study, which might explain the lower ASWD threshold. Koo (1963, 1978) suggested that a maximum of 33 % AWSD on a Lakeland fine sand should be allowed for citrus in subtropical conditions. Garrot et al. (1993) suggested ASWD values between 45 % and 50 % for citrus in Mediterranean climate on a Pima clay loam soil. These variations result from the interaction of crop-soil-environment and show that the application of a stress coefficient can help to improve irrigation management. Therefore, VWC should not be lower than 0.089 (cm3 cm−3) for peaches growing on this soil type to avoid water stress during critical stages (i.e., fruit growth). Nevertheless, the effect soil VWC below the ASWD of 25.8 % during shoot development (stage 3) is uncertain as a strategy to prevent excessive vegetative growth after harvest without compromising future fruit production. Implementing the Kc values derived in this study can improve irrigation practices for peach production in Florida. Because of the humid conditions, the VPD is lower than in the Mediterranean climate; thus, resulting in lower peach trees water demand in Florida. Additionally, due to the use of low-chill cultivars, the duration of the phenological stages is different in humid climates compared to Mediterranean and
Carlos Zambrano-Vaca: Conceptualization, Investigation, Writing original draft. Lincoln Zotarelli: Conceptualization, Investigation, Project administration, Supervision, Writing - review & editing. Kelly T. Morgan: Conceptualization, Validation. Kati W. Migliaccio: Conceptualization, Validation. Richard C. Beeson: Conceptualization, Validation. José X. Chaparro: Conceptualization, Validation. Mercy A. Olmstead: Conceptualization, Funding acquisition. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the Florida Department of Agriculture and Consumer Services, Florida Specialty Crop Block Grant Program, USDA-AMS-SCBGP-2016, Federal Award n. 10.170. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.scienta.2020.109318. References Abrisqueta, I., Vera, J., Conejero, W., Abrisqueta, J.M., Ruiz-Sánchez, M.C., Tapia, L.M., 2011. Water use by drip-irrigated early-season peach trees. Acta Hortic. 889,
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Johnson, S.R., Williams, L.E., Ayars, J.E., 2005. Weighing lysimeters aid study of water relations in tree and vine crops. Calif. Agric. 59, 133–136. Koo, R.C.J., 1963. Effect of frequency of irrigation on yield of orange and grapefruit. Proceedings of the Florida State Horticultural Society 1–5. Koo, R.G.J., 1978. Response of densely planted “Hamun” orange on two rootstocks to low volume irrigation. Proceedings of the Florida State Horticultural Society 8–10. Mahhou, A., Dejong, T.M., Shackel, K.S., Cao, T., 2006. Water stress and crop load effects on yield and fruit quality of Elegant Lady peach [Prunus persica (L.) Batch]. Fruits 61, 407–418. https://doi.org/10.1051/fruits. Massai, R., Remorini, D., 2000. Estimation of water requirements in a young peach orchard under irrigated and stressed conditions. Acta Hortic. 537, 77–86. https://doi. org/10.17660/ActaHortic.2000.537.6. Meier, U., Graf, M., Hess, W., Kennel, W., Klose, R., Mappes, D., Seipp, D., Stauss, R., Streif, J., van den Boom, T., 1994. Phänologische Entwicklungsstadien des Kernobstes (Malus domestica Borkh. Und Pyrus communis L.), des Steinobstes (Prunus-Arten), der Johannisbeere (Ribes-Arten) und der Erdbeere (Fragaria x ananassa Duch.). Nachrichtenblatt des Dtsch. Pflanzenschutzdienstes 46, 141–153. Morgan, K.T., Obreza, T.A., Scholberg, J.M.S., Parsons, L.R., Wheaton, T.A., 2006. Citrus water uptake dynamics on a sandy Florida entisol. Soil Sci. Soc. Am. J. 70, 90. https://doi.org/10.2136/sssaj2005.0016. Paço, T.A., Ferreira, M.I., Conceição, N., 2006. Peach orchard evapotranspiration in a sandy soil: comparison between eddy covariance measurements and estimates by the FAO 56 approach. Agric. Water Manag. 85, 305–313. https://doi.org/10.1016/j. agwat.2006.05.014. Sarkhosh, A., Olmstead, M., Chaparro, J., Andersen, P., Williamson, J., 2018. Florida Peach and Nectarine Varieties. [WWW Document]. Univ. Florida IFAS Dep. Hortic. Sci. URL https://edis.ifas.ufl.edu/mg374 (accessed 2.24.19). . USDA-NRCS, 2017. Web Soil Survey [WWW Document]. Soil Surv. Staff. Nat. Resour. Conserv. Serv. United States Dep. Agric. Web Soil Surv. URL https://websoilsurvey. sc.egov.usda.gov/ (Accessed 3.15.19).. . van Genuchten, M.T., 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Sci. Soc. Am. J. 44, 892. https://doi.org/10.2136/ sssaj1980.03615995004400050002x. van Genuchten, M.T., Leij, F.J., Yates, S.R., 1991. The RETC Code for Quantifying the Hydraulic Functions of Unsaturated Soils, Version 1.0. EPA Report 600/2-91/065. U.S. Salinity Laboratory, USDA, ARS, Riverside, California. Williamson, J.G., Crane, J.H., 2010. Best management practices for temperate and tropical/subtropical fruit crops in Florida : current practices and future challenges. Horttechnology 20, 111–119.
175–180. https://doi.org/10.17660/ActaHortic.2011.889.19. Abrisqueta, I., Abrisqueta, J.M., Tapia, L.M., Munguía, J.P., Conejero, W., Vera, J., RuizSánchez, M.C., 2013. Basal crop coefficients for early-season peach trees. Agric. Water Manag. 121, 158–163. Allen, R.G., Pereira, L.S., Raes, D., Smith, M., 1998. Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements. FAO Irrigation and Drainage Paper 56. Food Agric. Organ, United Nations. Ayars, J.E., Johnson, R.S., Phene, C.J., Trout, T.J., Clark, D.A., Mead, R.M., 2003. Water use by drip-irrigated late-season peaches. Irrig. Sci. 22, 187–194. https://doi.org/10. 1007/s00271-003-0084-4. Berman, M.E., Dejong, T.M., 1997. Crop load and water stress effects on daily stem growth in peach (Prunus persica). Tree Physiol. 17, 467–472. Fares, A., Alva, A.K., Nkedi-Kizza, P., Elrashidi, M.A., 2000. Estimation of soil hydraulic properties of a sandy soil using capacitance probes and Guelph Permeameter. Soil Sci. 165, 768–777. Ferreira, M.I., Valancogne, C., Daudet, Fa, Ameglio, T., Pacheco, C.a, Michaelsen, J., 1996. Evapotranspiration and crop-water relations in a peach orchard. In: Camp, C.R., Dadler, E.J., Yoder, R.E. (Eds.), Proceedings of the International Conference on Evapotranspiration and Irrigation Scheduling. ASAE/ IA/ICIC.. pp. 61–68. Ferreira, M.I., Valancogne, C., Michaelsen, J., Pacheco, C., Ameglio, T., Daudet, F.-A., 1997. Evapotranspiration, water stress indicators and soil water balance in a Prunus persica orchard, in central Portugal. Acta Hortic. 449, 379–384 doi:doi:10.17660/ actahortic.1997.449.53. Gallage, C., Kodikara, J., Uchimura, T., 2013. Laboratory measurement of hydraulic conductivity functions of two unsaturated sandy soils during drying and wetting processes. Soils Found. 53, 417–430. https://doi.org/10.1016/j.sandf.2013.04.004. Garnier, E., Berger, A., Rambal, S., 1986. Water balance and pattern of soil water uptake in a peach orchard. Agric. Water Manag. 11, 145–158. https://doi.org/10.1016/ 0378-3774(86)90027-2. Garrot, D.J., Kilby, M.W., Fangmeier, D.D., Husman, S.H., Ralowicz, A.E., 1993. Production, growth, and nut quality in pecans under water stress based on the crop water stress index. J. Am. Soc. Hortic. Sci. 118, 694–698. Johnson, R.S., Ayars, J.E., Trout, T.J., Mead, R.M., Phene, C.J., 2000. Crop coefficients for mature peach trees are well correlated with midday canopy interception. Acta Hortic. 537, 455–460. Johnson, R.S., Hsiao, T., Ayars, J., 2002. Modeling young peach tree evapotranspiration. Acta Hortic. 107–113. Johnson, R.S., Ayars, J., Hsiao, T., 2004. Improving a model for predicting peach tree evapotranspiration. Acta Hortic. 664, 341–346.
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Water uptake dynamics for adult peach trees in a subtropical humid climate a
a,
b
T
c
Carlos Zambrano-Vaca , Lincoln Zotarelli *, Kelly T. Morgan , Kati W. Migliaccio , Richard C. Beeson Jr.d, José X. Chaparroa, Mercy A. Olmsteada a
Horticultural Sciences Department, University of Florida, Gainesville, FL, 32611, USA Southwest Florida Research and Education Center University of Florida, Immokalee, FL, 34142, USA Agricultural and Biological Engineering Department, University of Florida, Gainesville, FL, 32611, USA d Mid-Florida Research and Education Center, University of Florida, Apopka, FL, 32703, USA b c
A R T I C LE I N FO
A B S T R A C T
Keywords: Crop coefficient Soil water depletion Stress coefficient Water demand
Low-chill peach [Prunus persica (L.) Batsch] cultivars allow Florida growers to become competitive by offering fruits earlier than northern states for premium prices. Due to the predominance of sandy soils in Florida, irrigation is required to ensure fruit production in peaches. Available irrigation recommendations and peach crop coefficients (Kc) were determined in Mediterranean and temperate climates, and they can overestimate water demand in humid subtropical climates. Thus, a two-year study was conducted in Citra, FL, aiming to determine the water requirements and Kc values for four-year-old peach trees in humid subtropical conditions. A daily soil water balance and the crop evapotranspiration (ETc) was determined using soil volumetric water content recorded every 10 min. Five soil probes, each equipped with soil moisture sensors at 10, 20, 40, and 80 cm of soil depth representing soil depth layers of 0–15, 15–30, 30–60, and 60–90 cm, respectively placed under the tree canopy. The ratios of daily ETc and Penman-Monteith FAO-56 reference evapotranspiration were used to estimate Kc for phenological stages of adult (≥4-year-old) peach trees. A soil water depletion coefficient (Ks) was estimated obtaining a threshold of 25.8 % available soil water depletion before trees undergo water stress. Daily ETc ranged from 0.22 mm d−1 during dormancy to 3.65 mm d−1 during shoot development. Values of Kc for peach trees ranged from 0.30 during dormancy to 0.69 during fruit maturity. The use of peach tree Kc adapted for subtropical humid conditions reduce the estimated crop water requirements by 21 % compared to the Kc values reported in FAO-56 guidelines.
1. Introduction
prolonged time. Rainfall comes primarily from conventional thunder and tropical storms with a higher incidence during the summer than winter months. Major differences in air temperature, relative humidity, and vapor pressure deficit (VPD) between humid and Mediterranean regions lead to different evapotranspirative demands, consequently different crop water requirements. Johnson et al. (2004) reported that VPD higher than 2 kPa can result in up to 24 % higher water requirement in peaches grown in a study conducted in California. Therefore, the use of Kc values or irrigation recommendations from other regions may overestimate the water requirements of peaches under humid conditions, where VPD rarely exceeds 2 kPa. In humid subtropical climates, winters are generally mild thus, ‘lowchill’ peach cultivars are a viable option because they require as low as 100 chilling hours (Sarkhosh et al., 2018). Peach trees cultivated in subtropical climates have a different length of phenological stages compared to peaches grown in temperate conditions. The timing and duration of crop phenological stages should be accounted for in the
Application of proper irrigation volume that meets the tree water demand based on weather conditions, soil characteristics, and crop phenological stages is key to the efficient use of agricultural water. Despite the expansion of peach [Prunus persica (L.) Batsch] cultivation to humid climates, available information about water requirements for peaches originates from Mediterranean (Abrisqueta et al., 2011, 2013; Ayars et al., 2003; Johnson et al., 2000, 2004) and temperate regions (Massai and Remorini, 2000). There is limited information on management practices for peach trees growing under humid subtropical conditions (Williamson and Crane, 2010). Florida has a humid subtropical climate (Cfa) according to Köppen classification, during hotter months of the year, daily maximum air temperatures are above 30 °C, and the daily average is around 22 °C or higher. Winters are generally mild and during the colder months, the average air temperature is below 18 °C, but higher than −3 °C. Frost can occur but not for a
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Corresponding author. E-mail address: lzota@ufl.edu (L. Zotarelli).
https://doi.org/10.1016/j.scienta.2020.109318 Received 4 September 2019; Received in revised form 23 February 2020; Accepted 26 February 2020 0304-4238/ © 2020 Elsevier B.V. All rights reserved.
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2016 and 21 December of 2017. The trees were fertilized with a blend of soluble fertilizer (70 %) and controlled release fertilizer (30 %) for each application, for a total annual application rate of 100 kg ha-1 of N, 33 kg ha-1 of P2O5, 50 kg ha-1 K2O split in three equal applications in late January, May, and August.
determination of peach water requirements throughout the year under humid subtropical conditions. One of the most common methods to estimate the crop evapotranspiration (ETc) is the product of the reference evapotranspiration (ETo) and crop coefficient (Kc). The ETc represents the evapotranspirative demand of the crop plus the water evaporation from the soil surface. The estimation of ETc is valid when soil water availability is non-limiting. However, when plant water uptake is limited by soil water availability, which is common in sandy soils, the use of a water depletion coefficient (Ks) is needed (Allen et al., 1998). The total soil available water (TAW) for plant uptake can be defined by a soil water content (θ) between field capacity (FC) and wilting point (WP). Within the TAW interval, there is a limited range of water depletion where plant water uptake is not affected (Allen et al., 1998). This range is known as the readily available water (RAW), and below this threshold point, the plant is under water stress before reaching the permanent wilting point (WP) (Allen et al., 1998). The proportion of TAW represented by RAW is used to estimate Ks, and its value changes depending on soil physical characteristics. Therefore, Ks works as a correction coefficient to estimate ETc from ETo. When the Kc is not affected by the θ, the value of Ks equals to 1. Due to the low water holding capacity of sandy soils, RAW can be less than 5% (Morgan et al., 2006), whereas it can be approximately 50 %, as suggested previously for loam and loamy clay soils (Allen et al., 1998). Under the assumption of a threshold point, it can be expected to have a maximum daily ETc when θ is near or at FC and have a reduced water consumption when values of Ks are lower than 1. Therefore, the soil water content should be maintained near or at FC, ensuring adequate soil water availability to the crop, avoiding crop water stress, and preventing soil water percolation below the root zone due to excess irrigation. A two-year field experiment was established to determine water requirements for peaches grown under humid subtropical conditions in the southeast USA. The objectives of this study were: i) measure depletion in daily soil volumetric water content (VWC, cm3 cm−3) in the root zone to estimate daily peach ETc, ii) to estimate Kc for adult peach trees in humid subtropical conditions based on their phenological stage and day of the year, and iii) estimate a Ks value for peach production in sandy soils based on reduced daily ETc for a given the phenological crop stage at selected daily available soil water content depletion.
2.2. Weather and soil water measurements The weather parameters such as rainfall, air temperature, solar radiation, wind speed, and relative humidity were measured every 15 min and aggregated to hourly data during using a weather station (UT30 Campbell Scientific, Logan, UT, USA) from the Florida Automated Weather Network (FAWN) station in Citra, FL, (latitude 29.41, longitude -82.17). The daily VPD and daily ETo were calculated using the FAO-56 equations (Allen et al., 1998). 2.3. Hydraulic soil characterization Soil hydraulic characterization under the peach tree canopy was determined before soil moisture sensors installation on 15 June 2016. Four random locations were selected to collect undisturbed soil cores (6 cm height by 5.4 cm diameter) at 15- and 30-cm soil depth for each location. An additional, four soil samples were collected on the same day and sent soil carbon analysis (Waters Agricultural Laboratory, Camilla, GA, USA). Soil water retention curves were determined by using pressure plates according to the methodology described by Gallage et al. (2013). Subsequently, data were averaged and fitted using the van Genuchten equation (Eq. 1) (van Genuchten, 1980) using the RETC software v. 6.02 for Windows (van Genuchten et al., 1991). θ = θr + (θs - θr) / [(1+(αh)n)]m
(1) −3
Where, θ is the soil water content (cm cm ), θr is the soil residual water content (cm3 cm−3), θs is the soil saturated water content (cm3 cm−3), h is soil water pressure head (cm), α is a scale parameter inversely proportional to mean pore diameter (cm-1), n and m are the shape parameters of soil water characteristic curve, m = 1 − 1/n, 0 < m < 1. 3
2.4. Irrigation management 2. Materials and methods Irrigation was supplied by two micro-sprinklers per tree (Maxijet Max-240 Peach 1.83 m radius, 39.75 L∙h−1, 240° *10 blue mediumangle streams, Dundee, FL, USA) to simulate commercial settings in Florida. The soil water content of the irrigated area (10.51 m2) under each tree was independently maintained within 25 % available soil water depletion (ASWD) until refill back to the soil FC point after the irrigation during fruit production and 50 % during shoot development. Irrigation was controlled using an automated irrigation control system with switching tensiometers installed at 15 and 30 cm depths in the irrigated zone (ASS-506 and AAS- 512, low tension w/AC switch closed past setting, Irrometer, CA, USA). The tensiometers were set at 15 kPa as the threshold to start irrigation. A modular controller unit (Rainbird ESP4ME modular, Rainbird, CA, USA) controlled irrigation application by bypassing time clock-initiated irrigation events if soil moisture values of both tensiometers was above the threshold. All irrigation events occurred in the morning between 1:00 and 5:00 h to minimize surface evaporation from solar radiation or wind.
2.1. Experimental orchard and management This study was conducted from 1 September 2016 to 31 August 2018 corresponding to the growing seasons 2016-17 and 2017-18 at the University of Florida, Plant Science Research and Education Unit, Citra, FL, USA (longitude 29.41, latitude -82.16). The soil at the site is classified as Arredondo sand, siliceous, semiactive, hyperthermic Grossarenic Paleudults sand with 0% slope (USDA-NRCS, 2017). Three four-year-old trees of variety ‘TropicBeauty’ grafted on ‘Flordaguard’ were randomly selected from a 0.8 ha orchard. The variety ‘TropicBeauty’ is classified as a low-chill requiring 150 ‘chill-hour’ to flower. The growth habit of a peach trees was divided into crop phenological stages according to the “Biologische Bundesanstalt, Bundessortenamt und Chemische Industrie” (BBCH): 0 - dormancy (occurring between 335-20 DOY); 1 - leaf development (21–105 DOY); 3 - shoot development (106–300); and 9 - senescence (301–354 DOY) (Meier et al., 1994). Trees were planted 4.6 m apart in-row, and 6.1 m apart in between rows, with an allocated tree space of 27.87 m2, resulting in 358 trees·ha−1 plant population. Herbicide and manual weeding were used to maintain a nearly weed-free strip 4.57 m wide. Pruning was conducted twice a year on 22 December 2016, 10 June 2017, 23 December 2017, and 12 June 2018. An annual spray application of a 4% solution zinc sulfate to defoliate the peach trees was conducted on 20 December
2.5. Soil moisture probes set up Soil VWC was measured every 10 min using a set of EnviroSCAN capacitance probes (Sentek Pty. Ltd., Stepney, Australia) equipped with soil moisture sensors at 10, 20, 40, and 80 cm of soil depth representing soil depth layers of 0–15, 15–30, 30–60, and 60–90 cm, respectively (Fig. 1A). A total of five probes were used per tree, three probes were 2
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Fig. 1. Field diagram of soil moisture probes arrangement in the peach tree irrigated area. A) Lateral view of the three probes installed from north to south and the soil moisture probes on each probe located at 10, 20, 40, and 80 cm soil depths; B) above view of the wetted area divided by five zones associated to each probe. The trapezium symbols represent the probe location in each zone.
installed in-row, oriented north to south; and two probes were between tree rows, oriented east to west. The distance of the first probe from the trunk and between probes was 60 cm apart. The criteria utilized for the probe distribution considered the total wetted area by the sprinklers, irrigated pattern resulted from the use of two sprinklers of 240° radius, and the expected peach root distribution. To calculate crop water uptake, the wetted area was divided into five zones, as proposed by Morgan et al. (2006). One capacitance probe was installed in representative points of each zone, and soil water measurements from each sensor represented the moisture beneath each zone and correspondent soil depth layer (Fig. 1B). Probes in Zone 1 and Zone 4 were installed 60 cm from the trunk in the center of a triangular region of 1.29 m height each. Another two probes were installed 1.2 m from the trunk and corresponded to Zone 2 (in-row) and Zone 5 (between rows). The probe installed 1.8 m from the trunk (in-row) represented soil moisture content in Zone 3. Zones 4 and 5 corresponded to the overlapped area of the sprinklers. The soil moisture measurement from each sensor was assumed to be uniform in each assigned zone and correspondent soil depth layer. Each sensor was calibrated for the soil type following the recommendations from the manufacturing company, with weekly checks to avoid sensor malfunctions.
Volume of water lost (L) = VWC depletion (cm3 cm−3) * assigned zone (m2) * soil layer width (m) *1000 (3)
2.7. Kc and ks calculations The ratios of daily ETc, calculated from the soil water depletion calculation to daily ETo, were used to estimate daily Kc values for peach tree using Eq. 4 (Allen et al., 1998). It was assumed that on those days Ks was equal to 1 because VWC was at least 95 % of FC. The calculated Kc values were used on two regression models plotted against day of the year (DOY) to obtain an estimated daily Kc throughout the year. First regression model comprised of a three-segment linear regression (1–100 DOY), (101–300 DOY), and (301–365 DOY), whereas the second model was a quadratic regression including data for the entire year. The estimated Kc values obtained from the regression models were then used to estimate daily Ks using the ratios of ETc to (ETo * Kc) (Eq. 4). The relationship between ASWD and the correspondent values of Ks was determined by linear regression analysis. The resulting equation was then used to calculate the threshold point of ASWD by replacing the Ks value by 1. The threshold point for ASWD indicates the lowest soil VWC in which tree water uptake is not limited, ensuring trees are not under water stress conditions.
2.6. Soil water depletion and ETc calculations
ETc = ETo* Kc* Ks
The soil water balance was calculated only for the days that the VWC of all five zones and soil depth layers averaged ± 5% of FC in the early hours of the day. For those specific days, the soil water depletion was calculated using the VWC readings every 10 min. The assumption was that since the VWC values were near to the FC, the soil moisture equilibrium was reached (Fares et al., 2000). Thus, soil water percolation was considered to be negligible and after soil VWC equilibrated at FC soil water depletion was assumed to be due to ETc. The average of all replications for each zone was used to calculate daily soil water depletion for each soil depth layer by using Eq. 3. Then, the volume of water depleted of each assigned zone and soil layer was calculated. Afterward, the four soil depth layers were added to account for the soil profile where roots were found down to a depth of 0.90 m. Subsequently, soil water in the five assigned zones was added and multiplied by 2 to obtain the total volume of water (L∙m−2) depleted from the wetted area per tree (Fig. 1B). The volume of water depleted from Eq. 3 was then divided to the allocated area of each tree (27.87 m2) to obtain ETc (L m−2) or (mm).
(4)
2.8. Statistical analysis All linear and quadratic regressions were analyzed at p < 0.05 of significance. Kc estimations were analyzed using a Pair t-test at p < 0.05 of significance using SigmaPlot 14.0 software (Systat Software, San Jose, CA, USA). 3. Results and discussion 3.1. Soil hydraulic characterization The soil profile at the experiment site had a similar soil texture up to 0.9 m of soil depth (USDA-NRCS, 2017), which was the depth where roots were observed after excavation. The soil organic matter was 10.8 g kg−1 at the 0−30 cm soil depth. The 0−15 cm and 15−30 cm soil depth layers were averaged, and the soil water characteristic curve was determined by fitting the van Genutchen equation (r2 = 0.96) (Fig. 2). The fitting parameters for the Arredondo sand were θr (0.01);
VWC depletion (cm3 cm−3) = VWC initial (cm3 cm−3) – VWC final (cm3 cm−3) (2) 3
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between February and April, average air temperature ranged between 17 and 22 °C in 2017 and 16–20 °C in 2018. During the shoot development stage (April to October), air temperature ranged from about 5–35 °C (Fig. 3A) and average daily temperatures between 18 and 28 °C in both seasons. The relatively high air temperature (Fig. 3A) and high relative humidity characteristic of north-Florida climate, resulted in VPD reaching up to 1.9 kPa in May of each year during the shoot development stage (Fig. 3B). The VPD was at the lowest during the dormancy stage in lateDecember to mid-January and ranged from 0.09 to 1.1 kPa in 2016-17 and 0.02–1.1 kPa in 2017-18 (Fig. 3B). Johnson et al. (2002) suggested that peach trees have a strong positive response to VPD, resulting in high water demand associated with high VPD. The values of VPD reported for north Florida did not exceed 2.0 kPa (Fig. 3B) in contrast to VPD of around 4 kPa common in Mediterranean regions. Johnson et al. (2004) reported that underestimation of peach water demand might occur when VPD is higher than 2.0 kPa and adjusted Kc are needed to avoid this issue. Thus, the difference in VPD between the two regions can result in overirrigation if recommendations from Mediterranean climates are adopted in a humid subtropical climate. Daily rainfall and ETo from September 2016 to August 2018 are presented in Fig. 3C. From February to April, rainfall was scattered and represented for about 17 % of the annual rainfall. About 70 % of the annual rainfall occurred from May to October (rainy season), which corresponded to the shoot development stage. Rainy and cloudy days resulted in low ETo days during the rainy season (Fig. 3C). Summer and fall seasons are characterized by the occurrence of tropical storms and hurricanes. The hurricane “Irma” accounted for about 221 mm of rainfall on 10 and 11 September 2017, which represented 15 % of the rainfall in 2017. Overall, the daily ETo had a bell-shaped curve ranging from 0.74 mm day−1 during the dormancy stage (late-December to lateJanuary) and peaking at 5.4 mm day−1 during the shoot development stage (May-October). After reaching the highest ETo rates during the shoot development stage, ETo progressively decreased from mid-July throughout December (Fig. 3C).
Fig. 2. Soil water characteristic curve for Arredondo sand (r2 = 0.962); FC (0.114 cm3 cm−3) and WP (0.017 cm3 cm−3). Fitting parameters θr (0.01); θs (0.399); α (0.023); n (1.625); m (0.3846); and Ks (3627.12 mm∙d-1).
θs (0.399); α (0.023); n (1.625); m (0.3846); and Ksat (3627.12 mm d−1). The FC was determined to be at 0.114 cm3 cm-3, WP at 0.017 cm3 cm-3, and TAW 0.097 cm3 cm-3. 3.2. Weather characterization and reference evapotranspiration Daily air temperatures during the period of the study are presented in Fig. 3A. Throughout the year, January was the coldest month and averaged 15.5 and 10.6 °C in 2017 and 2018, respectively, while the daily average air temperature in January ranged from about 3–22 °C (Fig. 3A). During the time of the study, peach trees accumulated 196 and 324 chilling hours in 2016-17 and 2017-18 seasons, respectively. During the dormancy stage, the minimum air temperature reached −2 °C on 8 January 2017 and −5 °C on 3 and 7 January 2018, (Fig. 3A). Air temperatures below 0 °C occurred when the trees were still dormant and lasted for 5 h or less. During the flowering, leaf development, and harvest periods which correspond to the months
3.3. Soil water content in the tree wetted area Soil VWC monitored at four different soil depths in three months of
Fig. 3. Daily maximum and minimum air temperature (A); vapor pressure deficit (B), and daily reference evapotranspiration (ETo) and rainfall; (C), from September 2016 to August 2018 in Citra, FL. 4
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Fig. 4. Volumetric water content (cm3 cm−3) from 0-15, 15-30, 30-60, and 60-90 cm of soil depth in the irrigated zone in Citra, FL. A) From 1-13 January 2018 corresponded to peach dormancy period, B) from 18-30 March 2017 period correspondent to peach fruit development, and C) 13-26 June 2018 period correspondent to peach tree shoot development. Arrows indicate irrigation events.
the year are presented in Fig. 4. The soil VWC in the upper soil depth layers was maintained near FC as much as possible throughout the year with supplemental irrigation to avoid plant water stress. Peach trees have variable water demand throughout the year not only because of the changes in ETo (Fig. 3C) but also due to the changes in phenological stages. January was characterized by the lowest ETo of the year, when peach trees were dormant for most parts of the month resulting in a very low crop water demand compared to the other phenological stages (Allen et al., 1998). From 1–13 January 2018 (dormancy stage), there were some rainfall events and the major event was 31 mm, which were enough to maintain the VWC was near FC (Fig. 4A) due to the very low ETo for the period (Fig. 3C). Between 18 and 30 of March 2017, the period corresponding to the fruit development stage, a 0.05 mm rainfall occurred on the 22 March, and irrigation was applied daily to replenish soil VWC at FC (Fig. 4B). In the summer, a higher frequency of rainfall events was reported compared to the spring months (Fig. 3C). The summer corresponded to peach shoot development stage. Due to the high frequency of rainfall during summer, much lower volumes of irrigation were needed. For example, from 13 to 25 June 2018, multiple rainfall events, ranging from 3 to 28 mm, maintained the VWC above FC in the upper soil depth layers. They also resulted in an increased VWC at the 60−90 cm soil depth layer, indicating soil water percolation (Fig. 4C). In this way, Fig. 4 exemplifies the different soil-waterplant-atmosphere dynamics throughout the year. The critical growth stages for peach trees when water stress can be detrimental to yield is followed by the dormancy period corresponding to March and April. In early March, the ETo increased progressively until reaching the highest portion of the bell-shaped curve in late June (Fig. 3C). March and April are months when peach fruit development occurs. Therefore, maintaining VWC near FC during that time prevents small fruit size and reduced yield due to water stress (Mahhou et al., 2006). Shoot development took place from May to October. This was the period characterized by the highest crop water demand. However, due to the high rainfall, irrigation was less needed to replenish the soil water storage back to the FC. Close monitoring of weather and soil VWC is needed during the summer months due to the erratic pattern of rainfall. The combination of high rainfall incidence and over-irrigation can cause excessive peach shoot growth and nutrient leaching. Conversely, water stress can lead to reduced stem and tree growth (Berman and Dejong, 1997).
Fig. 5. Daily crop evapotranspiration (ETc in mm, mean of three replicates) from September 2016 to August 2018 in Citra, FL. Numbers above the graphic represent the crop phenological stages: 0 (dormancy 335-20 DOY); 1 (leaf development 21-100 DOY); 3 (shoot development 101-300); 9 (senescence 301334 DOY).
from 0.3 to 0.8 mm in the 2017-18 season. With the transition from dormancy to the leaf development stage, ETc increased and ranged from 0.9 to 1.8 mm in 2016-17 and from 0.4 to 1.7 mm in 2017-18. The peach ETc peaked during the shoot development stage (DOY 101–300) at 3.5 mm in 2016-17 and 3.7 mm in 2017-18. Towards the end of each year, peach ETc decreased gradually from 1.5 mm around DOY 301 to 0.5 mm around DOY 334 in 2016-17, and from 1.4 to 0.7 mm in 201718 for the same period. The difference in ETc ranges between growing seasons was due to the variation in ETo factors such as minimum/ maximum air temperature, and relative humidity. Nevertheless, a paired t-test analysis between the daily ETo of the growing seasons resulted in no significant difference (P > 0.27). The estimated values of ETc in the present study did not exceed the measured ETo values (Fig. 3C), contrary to the findings reported by Abrisqueta et al. (2013) in Mediterranean conditions, where ETc was about 9% higher than ETo at the end of the shoot development stage (DOY 240–300). In another study, Ayars et al. (2003) reported peach ETc values ranging from 0.2–9.5 mm, while ETo ranged from 0.3–7.7 mm throughout the year. The 12 % higher ETc than ETo reported by Ayars et al. (2003) occurred between DOY 180–240 coinciding with the fruit development and harvest, which occurred between DOY 205–235 (late-season peach). Therefore, the highest ETc reported by Ayars et al. (2003) occurred when fruits were still on the tree, which explains the high-water demand by peach trees. In the present study,
3.4. Quantifying soil water depletion and ETc The peach tree ETc (mm) was estimated by the daily ASWD for the irrigated zone previously calculated using Eq. 2 and 3 for the period from September 2016 to August 2018 (Fig. 5A). During the dormancy stage (DOY 335-20), the ETc ranged from 0.7 to 1.4 mm in 2016-17, and 5
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Johnson et al., 2005) and for citrus in Florida (Morgan et al., 2006). The crop coefficient equations for the quadratic fit follow:
peach harvest occurred much earlier around DOY 85–120 (Fig. 5A), which was a period when ETc and ETo were still increasing before peaking around DOY 140–220 (Figs. 3C and 5 A). These observations support the different water requirements during phenological stages and due to presence of fruits on peach trees; thus, the importance of considering these factors in the proper determination of Kc values. The daily ASWD represents the soil evaporation and plant transpiration. The ASWD was calculated for the 0−60 cm soil depth layer for three-time period intervals correspondent to DOY 1–100, DOY 101–300, and DOY 301–365, and also the annual average of the ASWD (Fig. 5B). The first and the third periods had similar percentages of daily soil water depleted. On average 76 % of the total daily water depletion originated from the 0−60 cm soil depth layer. For the second period (DOY 101–300), 81 % of the daily water depleted originated from the top 60 cm of soil. The findings of this study are similar to the results from Garnier et al. (1986), where about 80 % of the peach tree water uptake occurs in the first 60 cm of the soil profile in well-watered soil. Therefore, irrigation frequency is a key component to maintain the proper soil moisture content in the first 60 cm of soil depth. Thus, avoiding prolonged time between irrigation events would prevent excessive soil water depletion on the upper soil layers.
Kc = 0.318 + (0.00379 * DOY) - (0.00000955* DOY2) for DOY 1–365 (r2 0.55) (8) Despite the different approaches for the determination of Kc, the overall values of Kc were quite similar. The estimated Kc for the dormancy period was 0.29 from the linear fit, while 0.32 from the quadratic fit. Allen et al. (1998) reported Kc after peach leaf drop of 0.20, which is lower than the estimated Kc from this study (0.30) (Fig. 6). This may be due to the mild-winters air temperatures in Florida, which requires more water for plant maintenance than in regions with cold winters. Additionally, the trees do not lose foliage naturally like in northern latitudes, but they had to be defoliated during late December. This might have affected the water consumption during this time compared to regions were the trees enter dormancy naturally. During the shoot development stage, Kc values were 0.66 and 0.69 for the linear and quadratic fits, respectively (Fig. 6). A paired t-test analysis between the three-segment linear fit and the quadratic fit resulted in no significant difference (P > 0.87) between Kc values obtained by either approach. The determination of Kc for other environments revealed major discrepancies with the present study which are mainly attributed to the higher evapotranspirative demand that the peach trees were subjected. For example, Ayars et al. (2003) reported Kc ranging from 0.25 to 1.40 for the DOY 60–310, respectively for California. Ferreira et al. (1996) reported Kc values between 0.4 and 0.6 using the heat balance sap flow method and modeling soil evaporation for the DOY 165–210 (June and July) in Mediterranean climate. Additionally, Ferreira et al. (1997) reported values between 0.4 and 0.6 using the eddy covariance method during the same period. Another critical factor to consider is the method for calculation of the area occupied by the tree. In both studies cited previously, authors mentioned that Kc values of 0.7–1.2 were observed if the projected area of the canopy was used as the base for the evapotranspirative area. Ferreira et al. (1996) and Ayars et al. (2003) determined Kc values from 0.7 to 1.3 using the area occupied by the tree for similar crop stages. While, the FAO-56 (Allen et al., 1998) reports Kc values for crop stage, Kc ini 0.55, Kc mid 0.9, Kc end 0.65 and Kc 0.20 after leaf drop; however, there is no specification how these Kc values were obtained. Overall, Kc values from the studies cited above were generally higher than the ones from the present study indicating that recommendations for other climates might overestimate peach water demand on subtropical humid conditions. The only exception is a study conducted by Paço et al. (2006), in which Kc values of 0.5 and 0.7 for June and July (only months reported) for temperate regions. The ETc values reported by Paço et al. (2006) peaked at about 3.5 mm day−1, which was similar to the ETc from the present study; however, ETo was not reported on Paço
3.5. Peach kc quantification The calculated Kc values for days when VWC was within ± 5% of the FC from September 2016 to August 2018 are presented in Fig. 6. The Kc values for the peach dormancy period, represented by DOY 33520 ranged from 0.25 to 0.35. There was an overall increase in the Kc value due to the shoot development during DOY 30 and 95. The Kc values for this period ranged from 0.38 to 0.68. Between DOY 100 and 330, corresponding to the summer and fall months, the Kc values ranged from 0.5 and 0.8 (Fig. 6). Regression analysis was used to determine the relationship between Kc values and DOY. The first approach was the three-segment linear regression (Eqs. 5,6, and 7) for a given period of the year determined by the peach crop stage. This first approach had a significant relationship only for the periods 1–100 DOY (Eq. 5) and 301–365 DOY (Eq. 7), and non-significant for the period of 101–300 DOY (Eq. 6). Kc = 0.294 + (0.00390 * DOY) for DOY 1–100 (r2 0.60) 2
Kc = 0.659 - (0.0000313 * DOY) for DOY 101–300 (r 0.001) 2
Kc = 2.498 - (0.00587 * DOY) for DOY 301–365 (r 0.57)
(5) (6) (7)
The second approach was fitting a quadratic function to the data regardless of the crop stage (Eq. 8). A significant relationship between DOY and Kc was observed. This approach had been reported for peaches in Mediterranean conditions (Abrisqueta et al., 2013; Ayars et al., 2003;
Fig. 6. Daily calculated, three-segment linear fit, quadratic fit, and FAO-56 crop coefficients in Citra, Florida from September 2016 to August 2018. Numbers above the graphic represent a phenological stage 0 (dormancy 335-20 DOY); 1 (leaf development 21-105 DOY); 3 (shoot development 106-300); 9 (senescence, beginning of dormancy 301-334 DOY). Values used for FAO-56 Kc initial = 0.55, Kc mid-season = 0.9 Kc end =0.65 and Kc dormancy = 0.2.
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temperate climates. A short dormancy period and an early harvest are two of the main characteristics of peach production in Florida. Therefore, the use of Kc values developed for humid conditions that account for water usage year-round would increase water saving and prevent water stress during critical periods, such as fruit development. Furthermore, soil VWC should be monitored regularly to prevent ASWD to be higher than 25.8 %; therefore, irrigation frequency is a key component to have proper irrigation management. Using the Kc values determined for humid conditions and avoiding excessive ASWD can result in improved peach production practices to increase water savings, reduce excessive irrigation, and prevent reduction on fruit yield due to water stress. 4. Conclusion The Kc values determined in the present study suggest that water requirements for peaches in humid subtropical conditions are much lower than in Mediterranean regions and previously reported in the FAO-56 publication. Daily volumetric soil moisture variation was successfully used to estimate daily ETc when soil moisture was near FC. Peach ETc ranged from 0.8 to 3.7 mm∙d−1 in season 1 and 0.3 to 3.5 mm∙d−1 in season 2. The estimated Kc values from the quadratic fit ranged from 0.3 for the dormancy and Kc 0.69 for the shoot development. The use of peach tree Kc adapted for subtropical humid conditions may potentially reduce crop water requirements by 21 % compared to the Kc values reported in FAO-56 guidelines. Ks values lower than 1 were observed when ASWD reached 25.8 %, which corresponded to the soil volumetric water content threshold in which ETc may become limited due to soil water stress. The finding of the present study will support future research on irrigation scheduling focus on minimizing excessive irrigation water application and nutrient leaching in sandy soils. The accurate determination of crop water allows growers to adopt more sustainable irrigation practices to improve crop yield and profitability.
Fig. 7. Estimated soil water depletion coefficient (Ks) as a function of available soil water depletion (ASWD%) for Arredondo sand soil in Citra, FL.
et al. (2006) study. The peach water demand is mainly driven by the climatic factor affecting evapotranspirative demands but also by the phenology of the plant. In humid subtropical conditions, the dormancy period can, in some years, last less than 30 days, and most of the Kc previously reported in other studies disregard tree water demand for at least four months during the year. Therefore, a Kc recommendation for the yearly cycle of peach trees, as proposed by the quadratic model can be very useful for irrigation scheduling in humid climates. If Kc values presented in FAO-56 recommendation were used in this study, the irrigation volume applied in 2017 would have been about 140 mm or 21 % higher than the requirement estimated by using the Kc values of the present study.
3.6. Ks and maximum ASWD % quantification CRediT authorship contribution statement Daily Ks values were obtained using Eqs. 3–4. For this estimation, the measured ETo, Kc values obtained from Eqs. 3–8, and the corresponding ETc were used. The estimated Ks values were plotted against the corresponding ASDW for a given day (Fig. 7). A significant relationship (P < 0.001) between Ks and ASDW% was determined, indicating that for the Arredondo sand, when VWC values were below 25.8 % of ASWD (when Ks < 1) plant water stress starts occurring, which can limit ETc. This ASWD threshold is lower than those previously reported by Ferreira et al. (1996). They reported a threshold of 40 % for the 0−80 cm sandy soil layer, and 80−100 cm loamy sand layer, in contrast to the homogenous soil texture along with the profile in the present study, which might explain the lower ASWD threshold. Koo (1963, 1978) suggested that a maximum of 33 % AWSD on a Lakeland fine sand should be allowed for citrus in subtropical conditions. Garrot et al. (1993) suggested ASWD values between 45 % and 50 % for citrus in Mediterranean climate on a Pima clay loam soil. These variations result from the interaction of crop-soil-environment and show that the application of a stress coefficient can help to improve irrigation management. Therefore, VWC should not be lower than 0.089 (cm3 cm−3) for peaches growing on this soil type to avoid water stress during critical stages (i.e., fruit growth). Nevertheless, the effect soil VWC below the ASWD of 25.8 % during shoot development (stage 3) is uncertain as a strategy to prevent excessive vegetative growth after harvest without compromising future fruit production. Implementing the Kc values derived in this study can improve irrigation practices for peach production in Florida. Because of the humid conditions, the VPD is lower than in the Mediterranean climate; thus, resulting in lower peach trees water demand in Florida. Additionally, due to the use of low-chill cultivars, the duration of the phenological stages is different in humid climates compared to Mediterranean and
Carlos Zambrano-Vaca: Conceptualization, Investigation, Writing original draft. Lincoln Zotarelli: Conceptualization, Investigation, Project administration, Supervision, Writing - review & editing. Kelly T. Morgan: Conceptualization, Validation. Kati W. Migliaccio: Conceptualization, Validation. Richard C. Beeson: Conceptualization, Validation. José X. Chaparro: Conceptualization, Validation. Mercy A. Olmstead: Conceptualization, Funding acquisition. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the Florida Department of Agriculture and Consumer Services, Florida Specialty Crop Block Grant Program, USDA-AMS-SCBGP-2016, Federal Award n. 10.170. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.scienta.2020.109318. References Abrisqueta, I., Vera, J., Conejero, W., Abrisqueta, J.M., Ruiz-Sánchez, M.C., Tapia, L.M., 2011. Water use by drip-irrigated early-season peach trees. Acta Hortic. 889,
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