Estimation of evapotranspiration and crop coefficient of drip-irrigated orange trees under a semi-arid climate

https://doi.org/10.1016/j.agwat.2021.106769Get rights and content

Highlights

  • The average evapotranspiration for orange trees (southern Iran) was 5.2 mm day-1 with crop coefficient of 0.65 to 0.95.

  • The average irrigation rate performed traditionally in the region was 19% higher than the actual requirement.

  • gs ranged from 0.11 to 0.12 molm−2 s−1 in fully irrigated trees, to 0.04–0.08 molm−2 s−1 in highly stressed trees.

Abstract

A sustainable agricultural system requires increasing water use efficiency and enhancing knowledge of crop water use. This prerequisite is more pronounced in the regions with inadequate water resources and limited observational data such as southern Iran. Therefore, this study aimed at finding the water requirement of mature orange trees (Citrus sinensis (L.) Osbeck, cv. Tarocco Ippolito) by identifying standard evapotranspiration rate and crop coefficients (single and dual). Seventy-two orange trees in a drip-irrigated orchard with loam soil were classified into six treatments and irrigated at 100%, 90%, 80%, 70%, 60%, and 50% of reference evapotranspiration rate during 2017 and 2018. Soil moisture variability and crop physiological responses, including stem water potential (Ψstem), net photosynthesis (An), and stomatal conductance (gs) were measured. Our results showed that irrigating at 90% ETo provided the full water requirements of the trees. The mean crop evapotranspiration rate was calculated as 5.2 mm day−1, with the crop coefficient ranging from 0.65 to 0.95. The average irrigation rate performed traditionally in the region was 19% higher than the actual requirement. Analysis of physiological response highlighted the controlling role of stomata in regulating transpiration and maintaining leaf turgor. During the peak water-stress, gs ranged from 0.11 to 0.12 molm−2 s−1 in fully irrigated trees, to 0.04–0.08 molm−2 s−1 in highly stressed trees. Our findings will provide a useful guideline for the local growers and agencies to achieve better irrigation scheduling and higher water productivity for the region.

Introduction

World citrus production and consumption have increased rapidly since the mid-1980s. FAO estimates a nearly 40% increase in the world citrus production during the last 30 years (FAOSTAT, 2017). More than 40 million tons (MT) of the productions come from Asian countries, and Iran contributes to 4.3 MT of the total productions with Fars province in Southern Iran as the largest contributors (Ahmadi et al., 2015). Southern Iran is characterized by dry summers and mild winters (semi-arid to arid regions) and has the agriculture as the prime driver of the economy (Jamshidi et al., 2019b). Nevertheless, the limited water sources availability, along with the recent prolonged droughts, have posed severe challenges to the sustainability of the agricultural productivity of the regions. The current situation necessitates the provision of water saving practices for enhancing agriculture production and alleviating the current orchard vulnerability. The applicability of water-saving approaches at core requires accurate information on crop water use (i.e., crop coefficient and evapotranspiration) (Fernández et al., 2019). Considering the limited academic research in southern Iran on orchard water use, it is crucial to carry out in-situ and local experiments to identify crop-specific information and water-consumption for field-grown orchards.

Improving crop productivity requires a wide range of contributions, including developing measurement devices (Henderson et al., 2018, Vagulabranan et al., 2016), improving crop and soil-based model simulations (Noshadi and Jamshidi, 2014), and optimizing irrigation scheduling (Nguyen et al., 2017, Perea et al., 2016). In this regards, many measurement platforms and models, like multi-resolution data sources for rainfall (Skofronick-Jackson et al., 2017) and multiple evapotranspiration schemes and datasets (Abatzoglou, 2013, Niyogi et al., 2020) as the two main drivers that affect crop yield and productivity have been developed. Nevertheless, the major challenge in improving crop-water productivity is still associated with collecting reliable in-situ measurements in data-sparse regions, particularly for crop coefficient (Kc) and the crop evapotranspiration rate (ETc) (Jamshidi et al., 2020). The importance of accurate ETc estimates has been highlighted in several studies (De Medeiros et al., 2005, Er-Raki et al., 2008, Jamshidi et al., 2019a) with particular emphasis on crop coefficient (Maestre-Valero et al., 2017, Niziński et al., 2017, Peddinti and Kambhammettu, 2019). Nevertheless, different studies have reported a range of values for Kc, ETc, even for a specific crop type.

The Kc values recommended by FAO for mature citrus trees for a Mediterranean climate varies from 0.65 to 0.75. Other studies on citrus, however, reported different values, including 0.63–1.17 for 14-year-old Sweet Lime by Sepaskhah and Kashefipour (1995) in a semi-arid region, 0.96–1.9 for 33-year-old Navel orange in an Arid climate by Snyder and O’Connell (2007), 0.3–0.9 for 8-year-old citrus orchard in tropical savanna climate by Peddinti and Kambhammettu (2019), and 0.67–0.96 for 14-year-old Navel oranges by Jamshidi et al. (2020). The discrepancies in the Kc, and ETc values can stem from the physiological specification of the crop (Consoli et al., 2006), management practices (Zitouna-Chebbi et al., 2015), the climate of the study region (Yang et al., 2003) and soil characterization (Maestre-Valero et al., 2017). Applying water-saving practices based on the information measured and calibrated for other regions could lead to exacerbating the water use efficiency. Therefore, the information on crop coefficient and evapotranspiration needs to be locally measured, particularly considering the importance of every drop in water-scarce regions (Villalobos et al., 2009).

A sustainable implication of water-saving practices requires monitoring crop water status (Rodríguez-Gamir et al., 2010). The stem water potential (Ψstem) and stomatal conductance (gs) are two water-stress indices that properly reflect plant water status (de Lima et al., 2015, Taylor et al., 2015). For a mature citrus tree in an optimum growing environment (i.e., no water or heat stress) the average values for gs and Ψstem have been reported in a range of 0.12–0.15 molm−2 s−1 and − 0.9–1.1. MPa, respectively (Ballester et al., 2013a, Romero Trigueros et al., 2014). When the soil moisture availability starts to deplete below a certain threshold, the plant regulates its leaf gas exchange by partially closing the stomata. Gomes et al. (2004) reported the midday Ψstem below − 1 MPa in young orange trees could trigger releasing the abscisic acid (ABA) and stomatal closure. Villalobos et al. (2009) used the daily gs of Clementine mandarin (⁓average 1.85 mm s−1) and developed a model to calculate the transpiration of the orchards. Romero Trigueros et al. (2019) have also reported a high level of agreement between the crop water stress index (reflecting water stress condition of the plant) and the stem water potential in grapefruit trees.

While Ψstem has been highlighted to have the most substantial role in governing the opening and closing of stomata, the mechanism between Ψstem and gs varies among different crop types and environmental conditions (Liu et al., 2006). Establishing the feedback mechanism between these two indicators can significantly help understand how the plants are functioning and responding to the water availability and irrigation strategy. For example, Jamshidi et al. (2020) reported an isohydric behavior of Navel orange trees due to maintaining a relatively constant leaf turgor in water stress conditions by increasing stomatal resistance.

In this study, we attempted to identify the crop coefficient and standard evapotranspiration rate of matured citrus trees (Citrus sinensis (L.) Osbeck, cv. Tarocco Ippolito) under the semi-arid climate conditions of southern Iran. Additionally, the study aimed to identify the plant feedback in terms of Ψstem, An, and gs responses to the different irrigation levels. The finding of this study helps the local growers to identify the exact crop water use and develop more accurate irrigation scheduling to preserve the limited water sources.

Section snippets

Study site

The experiments were conducted for two consecutive years during 2017 and 2018, in a clean cultivated citrus orchard located 35 km west of Kazeroon city (Latitude: 28° 32′ N, Longitude: 51° 43′ E, Elevation: 920 m a.s.l.), Fars province, Southern Iran. The climate of the study is classified as semi-arid with an annual mean rainfall of 352 mm (Soufi, 2004). The mean air temperature rises to over 40 °C during the summer months and drops down to 3 °C during the colder months. The orchard soil type

Reference evapotranspiration

The time series of daily reference evapotranspiration along with the primary meteorological variables, including relative humidity, wind speed, rainfall and air temperature during the study years (2017 and 2018) are presented in Fig. 1. ETo rates showed the lowest variability during winter (January in particular) with the mean value of 2.55 mm day−1, while it reached to just above 9 mm day−1 during summertime (June in particular). Most of the rainfall occurred from January to April, with a

Conclusion

In this study, we attempted to identify the full irrigation requirement of Orange trees (Citrus sinensis (L.) Osbeck, cv. Tarocco Ippolito) on a field-grown condition in southern Iran. Our results indicated irrigating at 90% of ETo as the full water requirement for which the crop coefficient ranged from 0.65 (in January) to 0.95 (in June). The conventional irrigation application was found to be significantly higher than the actual requirement, particularly during the fruit set and fruit

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.

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      The issue of water scarcity in the area has been made worse by climate change and agricultural development (Marieme et al., 2017), which is confirmed by the low values of the aridity index. The different patterns of ETo under semi-arid climates reported in the literature (Er-Raki et al., 2009, 2008; Jafari et al., 2021; Tabari et al., 2012) do not differ from the pattern of the experiment. However, ETa values fall in the conventional range of 700–1300 (mm/year), with an average of 1000 (mm/year) (El-Otmani et al., 2020).

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