Evaporation of grass under non-restricted soil moisture conditions

The behaviour of various formulas for evapotranspiration of grass in nonrestricted soil water conditions is considered. These are the expressions based on the Penman formula, i.e. "old" Penman, Penman-Monteith, Thom-Oliver and the Version recommended more recently by the FAO. Moreover, the Priestley-Taylor and the Makkink formulas are considered, which are radiation-based. Comparisons are made between daily mean values estimated with these formulas and direct measurements. The latter were collected over grass in the period 1979-1982 in the catchment area of the Hupselse Beek (The Netherlands). It was found that if all required input data were measured, the Priestley-Taylor and the "old" Penman formula yielded the best results. The assumption that soil heat flux can be neglected introduces a systematic and a random error of roughly 5%. The empirical estimates for net radiation from sunshine duration, temperature and humidity appear to perform rather poorly. These estimates improved significantly if solar radiation was measured directly. The empirical expression proposed by Slob (unpublished) that requires incoming solar radiation only as input, provided better results than the other more complicated expressions. Moreover, this study reveals that evaporation of unstressed grass is primarily determined by the available energy, i.e. good evaporation estimates can be obtained by using simply lambda E = 0.86(R-n - G). The Makkink method appears to be attractive for practical applications. These findings support the use of Makkink's formula for routine calculations of crop-reference evapotranspiration as has been done by the Royal Netherlands Meteorological Institute since 1987.