Field sugarcane transpiration based on sap flow measurements and root water uptake simulations: Case study on Tanegashima Island, Japan
Introduction
Food, energy, and water (FEW) are indispensable and fundamental resources for the sustainability of human beings. In modern society, FEW resources are in a complicated nexus, each having a trade-off relationship with the others (Cosgrove and Loucks, 2015, Scanlon et al., 2017). Therefore, we have to efficiently utilize and manage FEW resources. Recently, sugarcane has been used not only as an agricultural product but also as a biofuel crop (Grecco et al., 2019). Total global biofuel production by sugarcane, corn, and sorghum is 50 billion L a year; sugarcane solely accounts for approximately 40% and is now a major contributor to biofuel production (Lam et al., 2009). Therefore, many integrated studies have been conducted on the nexus among water resources, agricultural production, and energy. Additionally, the expansion of sugarcane production for biofuel may function against the conservation of water resources. In our study, sugarcane, the main agricultural crop in the southern islands of Japan, was selected as the target crop.
Only a few studies have been conducted on the water use and evapotranspiration (ET) of sugarcane in the southern islands of Japan. ET studies on field sugarcane are intensively carried out on the Okinawa Islands (Fig. 1). For example, the daily mean ET on Okinawa Island ranges from 5.9 mm d−1 to 6.4 mm d−1 in the summer season, reaching a maximum in August, and the crop coefficient Kc is 1.24 in August based on long-term field water balance lysimeter experiments (Hossain et al., 2005). ET observed by the Bowen ratio energy balance method on the Okinawa Island reaches to maximum 6.01 mm d−1 in August, and its annual average is 2.91 mm d−1 (Hiyane, 2008). High ET occurs during the summer season under high air-temperature and solar radiation conditions. To the best of our knowledge, field research on sugarcane transpiration in these regions has not been reported. Despite many studies on ET in sugarcane fields (Cabral et al., 2012, Hiyane et al., 2004), field sugarcane transpiration has rarely been reported (Chabot et al., 2005, Nassif et al., 2014) because of the difficulties in accurate direct measurements of transpiration and in separate evaluation of ET as evaporation and transpiration under actual field conditions (Bastidas-Obando et al., 2017, Eksteen et al., 2014, Kool et al., 2014, Qiu et al., 1999, Rafi et al., 2019). Previous studies on the partitioning of ET into soil surface evaporation (E) and crop transpiration (T) suggest that E accounts for 20–40% of ET in crops such as corn, cotton, wheat, and sorghum; E/ET can be large for the initial crop development stage (Kool et al., 2014). In the sorghum experiments conducted by Qiu et al. (1999), E ranged from 3% to 20% of ET, and the measured values of E in August were mostly less than 1.5 mm d−1 under a well-developed sorghum canopy. Moreover, in a grain sorghum with a leaf area index greater than 1 in Texas, USA, E accounted for 13% of ET (Ritchie, 1972). Thus, E often constitutes a large fraction of ET and may not be negligible under the mid-season growth stage condition. Soil evaporation, however, is a loss of water from the point of view of agricultural water use, and transpiration, not ET, has the major effect on the crop growth (James et al., 1983). Thus, water use by field sugarcane should be examined from field sugarcane transpiration.
Sap flow measurements are an effective way to directly evaluate the transpiration of herbaceous crops in small plots where it is difficult to apply micrometeorological approaches. Approaches to measure sap flow such as heat pulse, stem heat balance, and constant heater methods (Cohen et al., 1993, Gerdes et al., 1994, Miner et al., 2017, Sakuratani, 1981) are thermal-based techniques used to capture water flow through the plant stem. For example, according to the heat balance method, four sap flow sensors were applied to estimate the field sugarcane sap flow in southern Brazil for ten days in summer (Nassif et al., 2014), and T by sap flow ranged from 3.15 mm d−1 to 5.98 mm d−1. Chabot et al. (2005) showed that during the mid-season stage, over ten days in August in Morocco, the sap flow measurement for 14 sugarcane stems resulted in an overestimation by 35% compared to the ET results in which the soil surface evaporation was neglected. The heat balance method for long-period usage usually requires sensor maintenance, in which the sensor is regularly removed from the stem to prevent stem strangulation (Kool et al., 2014). In this study, we used the heat-pulse (HP) method (Cohen et al., 1988, Cohen et al., 1993) to measure the sap velocity in sugarcane stem and estimate sugarcane transpiration for about two months in Tanegashima Island (about 31° N), Japan. The first objective of this study is to evaluate the relationship between field sugarcane transpiration determined by the HP method and the reference crop ET determined by the FAO Penman–Monteith (FAO–PM) method (Allen et al., 1998). We discuss the daily and hourly sap flow responses to solar radiation and vapor pressure deficit (VPD; the difference between the saturation vapor pressure of air and the actual vapor pressure). The second objective is to investigate the relationship between the root water uptake and transpiration in the sugarcane field. We discuss the response of root water uptake to atmospheric transpiration demand on the basis of numerical simulation of soil moisture dynamics in the root zone.
Section snippets
Study sites
Field experiments were performed on the Tanegashima Islands in southern Japan, where sugarcane (Saccharum spp.) is the dominant crop. The study site was a sugarcane field (48 m × 29 m) at the Kagoshima Prefectural Institute for Agricultural Development (30.7315° N, 131.0261° E; altitude 51 m) (Fig. 1). The sugarcane cultivar was NiF8, which is a Japanese cultivar and most popular in the study site. Single-row planting with 1.2-m spacing between rows is normally used. According to the local
Calibration of heat-pulse method
The sap flow rate was evaluated using the HP method, with a calibration constant of c = 1.3 (Fig. 4). This constant was determined from the slope of the regression line (1.32) between the sap flow rate determined by the HP method and the transpiration rate of potted sugarcane measured using an electronic balance. At c = 1.3, the sap flow rate corresponded well with transpiration. The accumulated sap flow rates on July 3 and 4 by the HP method were 1.61 L d-1 and 1.64 L d-1, respectively (Fig. 4
Conclusions
The aim of this study is to help optimize irrigation water management for sugarcane fields at the study site. We investigated the relationship between transpiration and reference crop evapotranspiration in sugarcane fields on Tanegashima Island in Japan based on the sap flow measurements obtained using the HP method and the numerical analysis of soil moisture movement considering the root water uptake. At present, 3 mm d−1 of irrigation water is commonly used for the irrigation of sugarcane
Funding
This research was supported by the Grant-in-Aid for Scientific Research (JSPS KAKENHI, Grant Number: JP18K05887) from the Japan Society for the Promotion of Science.
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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