Hypothesis
The most physically complete equation, Penman-Monteith, gives the best global daily reference potential evaporation for hydrological models.
What failed
Evaluated against Penman-Monteith values derived from CRU data, Penman-Monteith applied to CFSR did not outperform the simpler equations; it returned relatively low values in arid regions such as the Sahara, central Australia and the US deserts, which pushed discharge too high for dry basins including the Orange, Murray and Zambezi. The cell-specific recalibrated Blaney-Criddle fitted best but proved unstable under changing climate and less reliable for daily series, and in many basins the choice of method barely changed simulated river flow at all.
Broken assumption
The most physically complete evaporation equation is the most accurate when driven by reanalysis data.
Why
The authors blame the equation’s heavy data demand and its sensitivity to inaccuracy in the input data, and recommend a recalibrated Hargreaves form instead.
From the published paper: Weiland et al. Hydrology and earth system sciences, 2012. ↗CC BY