4.2 The performance comparison of radiation and temperature
models against lysimeter measurements
For the performance comparison of radiation models against lysimeter
measurements, we found that the PT and DK models yielded the best
performances of the radiation-based models, and were respectively
44.11% and 42.71% more accurate than the poorest (IRMAK2) model, which
was in line with the previous study conducted in humid areas that the PT
method exert the good accuracy estimate for ET (Trajkovic & Kolakovic,
2009). There is increasing evidence indicated that the input parameters
was the dominated factors affecting their performance (Lang et al.,
2017), we thus conclude that the better performances of the PT and DK
models which might associated with the use of only the most important
meteorological factors affecting ET such as net radiation
(Rn), was supported by our results (Fig.6). Compare with
PT and DK model, the other radiation models just use the
Rs as the mainly driving variable, the ET thus may
overestimate because some Rs was reduced through
reflecting into the atmosphere due to the high albedo in this region
(Zhang et al., 2018). Furthermore, each model was developed from its
specific underlying surface and climate conditions. For instance, the PT
was established in a humid climate condition, which was suitable for our
humid alpine meadow. Most importantly, the PT and DK models required
fewer meteorological inputs when compared with combination models.
Combine those factors, we can prefer to recommend the PT and DK model
for use in a humid alpine meadow on the northeastern Qinghai-Tibetan
Plateau, especially when considering the difficulty in obtaining ET in
this harsh climate.
For the performance comparison of temperature models against lysimeter
measurements, previous study reported that the Hargreaves versions
equation as one of the most simple empirical methods were widely used
for the ET estimation due to its less meteorological data input,
especially considering not all the meteorological data required in the
standard PM-56 model (Jensen et al., 1990). To further select the best
Hargreaves versions equation, we compare the performance of original
(HAR) and two modified versions (HAR1 and HAR2) of the Hargreaves
equations, and found that the original HAR model had the lowest error
(RMSE =1.28 mm d−1, MBE=0.17 mm d−1
and PE=5.92%), which was consistent with previous studies conducted on
humid region (Hossein Tabari, 2010) but contrast to these study
conducted in arid region that the modified Hargreaves equation display a
more accurate estimation of evapotranspiration compared with the
original Hargreaves equation (Ravazzani, Corbari, Morella, Gianoli, &
Mancini, 2011). Overall, the temperature models display a poor
performance compared with radiation models owing to the Hargreaves
method was established in semiarid areas (Hossein Tabari, 2010), thus a
local calibrations was required to improve the accuracy of Hargreaves
method in other region.