**1. Introduction**

Latent heat flux (*LE*) measurement is very critical in the field of micrometeorology for the efficient management of available water resources. The precise estimation of *LE*, also termed as evapotranspiration (ET), is vital due to its great influence on precipitation, plant growth, and amount of irrigation water runoff. There is constant pressure on the available water resources for different crops in arable agricultural areas. Therefore, there is always a need for accurate estimation of crop evaporation to maximize the available water resources and crop water use efficiency. Traditionally, crop water evaporation was estimated from climatic data and reference evapotranspiration methods and by using the crop coefficient approach [1]. In recent decades, the instrumentation, techniques, and different approaches for the estimation of ET have been improved a lot. In order to spread scientifically approved techniques into commercial practice, simpler techniques are preferred. Various micrometeorological methods have been used for estimating *LE* (i.e., scintillometer, eddy covariance (EC), Bowen ratio (BR) [2], lysimeter, surface renewal (SR), and flux variance (FV)) [3]. Use of a lysimeter is a reliable method for ET measurements, but it is seldom used outside of experimental stations [4,5]. The Bowen ratio (BR) method requires extensive fetch and responsiveness to the biases of the instrument used for estimating the air temperature and water vapor pressure at two levels [2]. The scintillometer method is a high-cost method based on the Monin–Obukhov similarity theory (MOST) and high skills are required for

**Citation:** Wang, J.; Buttar, N.A.; Hu, Y.; Lakhiar, I.A.; Javed, Q.; Shabbir, A. Estimation of Sensible and Latent Heat Fluxes Using Surface Renewal Method: Case Study of a Tea Plantation. *Agronomy* **2021**, *11*, 179. https://doi.org/10.3390/agronomy 11010179

Received: 26 November 2020 Accepted: 14 January 2021 Published: 18 January 2021

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correct operation. In addition, its estimations are disrupted by optical interception of rainfall, insects, frost, and vertical air temperature to differentiate between the ascending and descending directions of sensible heat flux (*H*) [6]. The EC system allows the direct measurement of surface fluxes (sensible and latent heat fluxes), and a lysimeter can also provide estimations of these fluxes with better accuracy. In spite of their many advantages, these techniques are not easily available for daily usage by farmers due to high cost and operational complexity; hence, simple and low-cost methods for ET measurement are required [7]. There are few studies of ET estimation over different homogeneous surfaces and crops using these methods. Most of these methods are costly and their instrumentation is sensitive to damage, requiring site homogeneity and relatively wide fetch. The SR method has been proposed as a reliable alternative to methods such as the EC system, first proposed by Van Atta in 1977 [8]. The theoretical basics of the SR method have been reviewed and applied over different crop canopies and surfaces with better results [9–16]. The SR method uses high-frequency (~10 Hz) air temperature measurements obtained from fine-wire thermocouples for the estimation of the sensible heat flux (*H)* above the plant canopy. The SR method has been deployed over natural surfaces with mixed results and is considered as an attractive replacement for other available meteorological methods due to its low-cost and relatively simple instrumentation, which makes it easier to apply close to the canopy surface and with limited fetch [17–19].

This study tests the performance of the surface renewal method for estimating the surface fluxes, including sensible and latent heat fluxes, over a tea plantation in Danyang. Tea plants grow well all year round in temperate and humid climates. During the last decade, sprinkling irrigation has been gradually applied to tea plantations and drip irrigation has been recently adopted with the extension of fertigation technology. Usually, growers start irrigation based on only soil moisture content, without knowing the exact crop water requirement [20]. This study was performed with the aim to provide a relatively low-cost technique for estimation of the crop water requirement of tea plants. The performance of the SR method was evaluated for the estimation of surface fluxes including sensible and latent heat fluxes and the results were compared against the measurements of the eddy covariance system.
