Info

Date

Figure 8: Average soil-water contents at the 0-60 cm depth in the Cambisol and Fluvisol plots during the 2005 Sugarbeet irrigation season. The dashed lines show the corresponding Field Capacities (after Utset et al., 2007b).

However, water contents are higher than the field capacity at the earlier stages of sugarbeet development, when the sugarbeet roots are not long enough and crop water use is lower. The results suggest that water application might be excessive at this stage. The difference between the measured soil water contents and the field capacity is higher in the Fluvisol, where clay contents are higher and water moves slowly across the soil profile.

Figure 9 shows the actual SWAP-simulated sugarbeet evapotranspiration, the maximum sugarbeet evapotranspiration, calculated using the Penman-Monteith reference evapotranspiration and the KC coefficients shown in Table 5, as well as the actual crop evapotranspiration estimated from water balance and the water content measurements shown in Figure 8. The evapotranspiration values given in Figure 9 comprise both the Cambisol and the Fluvisol data and were calculated on a weekly basis.

As shown in the figure, simulations can follow the temporal changes of both measured ETC values and those computed from reference evapotranspirations. The measured ETC values are higher than the simulated and maximum rates computed from weather data at the beginning of the irrigation season. The water balance was calculated neglecting the percolation and capillary rising components of the balance. However, at these earlier sugarbeet stages, irrigation excess might yield to significant water loss by percolation, which is in accordance with the water contents being higher than the field capacity for the same period, as shown in Figure 8. The simulated evapotranspirations are lower than the maximum, as expected.

All the computed evapotranspirations indicate that sugarbeet water use at the end of the cropping season is lower than at the beginning of the root growing period, as pointed out by Velicia (1998).

The correlation coefficient between the actual SWAP-simulated evapotranspiration and the maximum weather-dependent sugarbeet evapotranspiration was 0.81; whereas the correlation coefficient between the simulated and actual estimated water-balance evapotranspirations was 0.75.

Figure 9 also shows that simulated ETC values correlate better with the maximum evapotranspiration than with the measured ETC. In practice, irrigation management at both plots was able to keep soil water contents above field capacity throughout the sugarbeet growing season. Therefore, actual crop evapotranspiration must be close to the highest possible maximum, since water requirements were mainly satisfied. This can explain the good ratios between the SWAP simulations and the weather-based maximum ETC.

Figure 9: SWAP-simulated sugarbeet actual evapotranspiration (Simulated ETc), sugarbeet maximum evapotranspiration calculated from the Penman-Monteith reference evapotranspiration (ETc Penman-Monteith) and actual evapotranspiration estimated by water balance from the measured soil water contents (after Utset et al., 2007b).

Figure 9: SWAP-simulated sugarbeet actual evapotranspiration (Simulated ETc), sugarbeet maximum evapotranspiration calculated from the Penman-Monteith reference evapotranspiration (ETc Penman-Monteith) and actual evapotranspiration estimated by water balance from the measured soil water contents (after Utset et al., 2007b).

However, ETC calculations by water balance were made ignoring any eventual percolation. However, water loss by percolation seems to be significant at the beginning of the irrigation season, since the water contents measured during the said period were higher than the field capacity, as shown in Figure 8. SWAP estimates that all the components of the water balance and the simulated evapotranspirations do not comprise percolation. This could explain the relatively lower correlation between SWAP-estimated and measured ETC.

Figure 10A shows the ratios between sugarbeet evapotranspirations estimated by water balance and the SWAP-simulated evapotranspirations.

As the figure shows, simulated and field-measured evapotranspirations are close to the 1:1 line, particularly for the lower rates of the measured ETc. As indicated below, sugarbeet ETC could be overestimated by water-balance estimations. Figure 10B shows the absolute differences between ETC values simulated by SWAP and the ETC estimated by water balance. Absolute differences depend on the measured ETC values. Differences are much higher for the water-balance measured ETC that is higher than 30 mm. This value is higher than the Readily Available Water usually computed for sugarbeet (Morillo, 1993). Therefore, actual sugarbeet evapotranspiration could hardly reach such high values (Allen et al., 1998).

Water-balance measured ETc (mm)

Was this article helpful?

0 0

Post a comment