1. Introduction
Evapotranspiration (ET) is one of the key parameters in the simultaneous
processes of heat and water transfer to the atmosphere via transpiration
and evaporation in the soil–plant– atmosphere system (Sentelhas,
Gillespie, & Santos, 2010), thereby playing an important role in water
balance calculations, water allocation and water irrigation management.
Thus, accurate estimates of evapotranspiration could improve water
management strategies and promote the efficient use of water resources,
especially in regions suffering water shortages (Ljungqvist et al.,
2016; Sun et al., 2011).
To-date, direct measurements of ET have been achieved by a variety of
methods such as the Bowen Ratio Energy Balance System (A. Irmak &
Irmak, 2008; S Irmak, Allen, & Whitty, 2003; S. Irmak, Howell, Allen,
Payero, & Martin, 2005; Si et al., 2005), lysimeters (Jia, Dukes,
Jacobs, & Irmak, 2006; Liu et al., 2017) and the eddy covariance
technique (Gu et al., 2008; Novick et al., 2009). Alternatively, ET can
be indirectly assessed by applying various reference evapotranspiration
equations. Several models of reference evapotranspiration have become
widely used for the calculation of ET, and can be classified into three
types: radiation-based models (Doorenbos, 1975; Hargreaves and Samani,
1985), temperature-based models (Trajkovic, 2005; Valipour & Eslamian,
2014), and combination models (Monteith, 1965; Penman, 1963).While the
development of these models has undoubtedly benefited the calculation of
ET, it still difficult to choose the optimal one due to the availability
of the observed data the together with large number of models also adds
confusion and arbitrariness to model selection, because most models have
not been evaluated against lysimeter measurements across a range of
regions and climates. To select the best performing models, many studies
have been conducted to assess model performance under various climates.
For instance, the Food and Agriculture Organization of the United
Nations (FAO) recommend the Penman–MonteithFAO-56 combination equation
(PM-56) as the standard equation for estimating reference
evapotranspiration (Allen, Pereira, Raes, & Smith, 1998), and this has
been widely used worldwide when compared with other equations (Cai, Liu,
Lei, Pereira, & Luis, 2007). The advantages of the Penman–Monteith
equationare that it is does not require any local calibration because it
incorporates both physiological and aerodynamic parameters, and it has
been well tested by a variety of lysimeters (Trajkovic, 2009). Although
many models have been widely used to estimate ET, it should be noted
that most previous models have only been evaluated with reference to
FAO-56 PM (Cao, Yu-Zhong, Liu, Zhong, & Zhao, 2015; Martı́Nez-Cob &
Tejero-Juste, 2004), with few being tested against lysimeter
measurements (Jensen, Burman, & Allen, 1990; Ventura, Spano, Duce, &
Snyder, 1999; Yoder, Odhiambo, & Wright, 2005). Furthermore, the
application of the PM-56 equation needs many meteorological inputs, such
as wind speed, temperature, humidity and solar radiation, that are often
not available in regions with harsh environments (Hossein Tabari, Kisi,
Ezani, & Talaee, 2012), especially in developing countries where have
limited number of meteorological stations. Thus, it is essential to
develop an relatively accurate reference evapotranspiration equation
that requires fewer meteorological parameters, to allow more simplified
estimates of ET than those of PM-56, applicable across a range of
climatic conditions (Hossein Tabari, 2010; H. Tabari & Talaee, 2011).
So far, many models have been developed: for example, Tabari (2010)
assessed four reference evapotranspiration models in an arid climate,
and found that the Turc model performed the best. Meanwhile, the
Hargreaves equation performed best in semiarid regions (Sabziparvar,
Tabari, Aeini, & Ghafouri, 2010; Hossein Tabari, 2010). Liu et al.
(2017) compared 16 models for reference evapotranspiration against
weighing lysimeter measurements, and found that the combination models
performed best for estimating ET in semiarid regions. Overall, most
previous studies have been conducted in low-humidity conditions at low
altitude (i.e. arid and semiarid regions) (Liu et al., 2017; Sentelhas
et al., 2010), with few studies in humid climates, particularly in
alpine ecosystems.
The Qinghai-Tibetan Plateau (QTP),with an average altitude of 4000 m, is
the world’s highest alpine ecosystem and is also known as the ”Asian
tower”, playing an important role in ensuring the safety of water
resources in China and southeast Asia (Dai, Guo, Zhang, et al., 2019;
Zou et al., 2017). The alpine meadow and alpine grassland across the QTP
account for almost 60% of the plateau area (Dai, Ke, et al., 2019));
therefore, accurate assessment of ET in an alpine ecosystem are not only
provides new insights into the water cycle, but also benefit the
formulation of water resource management strategies. Furthermore, given
the uncertainty and confusion in the selection of ET equations across
different regions and climates, it is critical to thoroughly understand
the performance of the various models in a humid alpine meadow (Liu et
al., 2017). In this study, we compared 13 reference evapotranspiration
models against weighing lysimeter measurements on the northern Tibetan
Plateau, with the aims of selecting the best fit model in applications
in this region to estimate the ET.
2 Materials and methods