Evaporation from a pine forest - using the aerodynamic method and bowen ratio method

Abstract

Hourly values of the water balance of a pine forest in the southern part of the Upper Rhine valley in southwest Germany were calculated. The evaluations covered a period of 1.5 years. We estimated the wind profile parameters from 1981 to 1986 and calculated the sensible heat flux during the period from August 1985 to December 1986 using the aerodynamic method. Using the energy balance equation, we derived evaporation values. We found a clear dependence of the zero plane displacement, d, and the roughness length, z 0, on wind speed; d decreased with increasing wind speed and z 0 increased with wind speed. Evaporation was 736 mm using the aerodynamic method; with the water balance we obtained a value of 826 mm during the monitoring period. The latter value is probably 5–10% too high because of a possible loss through percolation. The aerodynamic method is very sensitive to the value of d. A 10% difference in d leads to a corresponding difference in the evaporation estimate. Additional calculations to compare the results with the Haude formula and the Priestley-Taylor equation confirmed that they should be applied only for “potential evapotranspiration” estimates. The water balance calculated by means of the profile method is suitable for examinations of empirical evapotranspiration formulae.