3.2 Comparison of daily ET between reference evapotranspiration
models and lysimeter measurements during the study period
The comparison of 13 reference evapotranspiration equations against the
lysimeter measurements is presented in Fig.3 and Table 2, showing that
the relationship between daily ET0 calculated by the 13
reference evapotranspiration equations and lysimeter measurements are
each significantly (P <0.01), with higher coefficients of
determination (R 2) ranging from 0.61 to 0.81.
For the combination models, FAO 24 Penman yielded the highest
correlation, followed by Pen-63 and FAO-56 PM. For radiation-based
models, PT and DK obtained the highest correlation, followed by Makkink,
Makkink(1967) and Makkink (1957), IRMAK1 and IRMAK2. For
temperature-based models, HAR, HAR1 and HAR2 obtained the same
correlation. The daily estimates of combination models and
radiation-based models (except Makkin and Makkin (1967)) generally
underestimated the ET values measured by lysimeter: the mean difference
in ET between lysimeter measurements and model estimates (i.e. MBE)
ranged from -0.47 mm d−1 to -0.04 mm
d−1 for combination models, and ranged from -0.28 mm
d−1 to -0.02 mm d−1 for
radiation-based models. However, the temperature-based models generally
overestimated the ET values measured by lysimeter, with MBEs ranging
from 0.10 mm d−1to 0.39 mm d−1.
Overall, the combination models and radiation-based models, on average,
underestimated ET by 12.79% and 5.93%, respectively, but the
temperature-based models overestimated ET by an average of 7.99%.
During the whole study period, the RMSE of combination models ranged
from 1.19 to 1.36 mm d−1 and averaged 1.26 mm
d−1, the RMSE for radiation-based models ranged from
1.03 to 1.47mm d−1and averaged 1.09 mm
d−1, and the RMSE for temperature-based models ranged
from 1.28 to 1.32 mm d−1 and averaged 1.29 mm
d−1. Based on the RMSE, the performances of the 13
reference evapotranspiration models decreased in the order:
DK>PT>Pen-63>FAO-24
Pen>HAR>HAR2>HAR1>IRMAK1>FAO-56PM
>Makkink>Makkink(1967)>Makkink(1957)>IRMAK2.
The best model (DK) was, respectively, 49% and 33% more accurate than
the poorest (BR) and the commonly used FAO-56 PM equation. Furthermore,
the Pen-63 and FAO-24 Pen demonstrated better performance than the
commonly used FAO-56 PM equation. Overall, for the whole study period,
the radiation-based models yielded the best performance, followed by the
combination models and temperature-based models.