4.0. Discussion
4.1 Product accuracy
The ETIa-WPR results are comparable the improved MODIS global
terrestrial evapotranspiration algorithm, MAPE of 24.6% as compared to
EC measurement, when driven by driven by the tower meteorological data
(Mu, Zhao & Running 2011). The ETIa-WPR error estimates, on average,
are also close the average errors in EC measurements (20-30%) (Allen,
Pereira, Howell, & Jensen, 2011; Blatchford, Mannaerts, Zeng, Nouri, &
Karimi, 2019), however, it appears that the ETIa-WPR is regularly
overestimating ETIa, which is evident at local to basin level. Figure 12
shows the bias and number of observations between ETIa-WPR and ETa-EC
for all EC observations disaggregated based on 0.5mm/day ETa-EC
increments. The results are further defined based on non-irrigated
sites, irrigated agriculture and all stations. For non-irrigated sites,
there is a positive bias (ETIa-WPR>ETa-EC) when the ETa-EC
is less than 2.5mm/day and becomes negative when the ETa-EC is greater
than 2.5mm/day. This bias increases, both positive and negative, as the
ETa-EC deviates from 2.5mm/day. The underestimation is further
exacerbated by the fact that ETa-EC estimations can lead to
underestimation of the latent energy or ETa-EC by 20% (Wilson et
al. , 2002; Glenn et al. , 2007). Underestimation bias is larger
than overestimation bias and increases with increasing ETIa-WPR.
However, Africa as a continent is dry with long term (2010-2015) average
daily ETIa-WPR for the continent being 1.5mm/day. Therefore, the
ETIa-WPR frequently overestimates at the annual, basin scale. The
irrigated sites (EG-SAA, EG-SAB and EG-ZAN) are overestimated for nearly
all ETa-EC. The irrigated sites strongly influenced the overall bias, as
these sites have many observation points. When irrigated and
non-irrigated results are combined, the changing point where ETIa-WPR is
greater than ETa-EC occurs when ETa-EC exceeds 3.5mm/day.