3.5 Comparison of monthly averaged daily ET0 between reference evapotranspiration models and lysimeter measurements
The estimations of monthly mean daily ET0 for combination models were consistent with the pattern shown by lysimeter measurements (Fig. 6a), with the peak in July being 4.08±0.16 mm d-1, 3.76±0.16 mm and 3.50±0.14 mm d-1 for Pen-63, FAO-24Pen and FAO-56, respectively. As already noted, the combination models underestimated the measurements from May to October, and overestimated the measurements in the other months. Furthermore, Pen-63 yielded the best estimate of daily ET in the peak month, during which it underestimated the measurements by 1.50 mm d−1. The mean daily ET0 for temperature-based models also followed the pattern of lysimeter measurements (Fig. 6b), with the maximum in July reaching 4.60±0.10 mm d-1, 4.87±0.09 mm and 4.84±.11 mm d-1 for HAR, HAR1 and HAR2, respectively. Moreover, the temperature-based models generally overestimated the measured ET during most months (Fig. 6c). HAR1 yielded the most accurate estimate of ET in the peak month, when it underestimated the measurements by only 0.13 mm d−1 on average. The monthly mean daily ET0 for the radiation-based models also peaked in July, at 4.26±0.19 mm d-1, 4.31±0.19 mm d-1, 4.04±0.14 mm d-1, 3.79±0.13 mm d-1, 4.49±0.15 mm d-1, 3.75±0.11 mm and 3.74±0.13 mm d-1 for PT, DK, Makkink, Makkink (1959), Makkink (1967), IRMAK1 and IRMAK1, respectively. Again, the radiation-based models tended to underestimate the measured ET during the growing season, and overestimate the measured ET during the non-growing season.