Coefficients for Penman-based estimation of daily potential evapo-transpiration in Botswana

Reference potential evapo-transpiration (PET) for Botswana was estimated using a Penman-based equation in order to determine the appropriateness of five different sets of values for the 10 input coefficients. Calculated estimates were compared to the only existing set of observed PET values from two free drainage lysimeters at the headquarters of the Department of Meteorological Services (DMS) in Gaborone, Botswana. These values consisted of 36 five-day mean values of PET for grass cover during the period March 21, 1989–20 March, 1990 distributed as follows: fall 9, winter 8, spring 10, summer 9. Estimates were most sensitive to the wind coefficient in the aerodynamic term of the Penman-based equations. Analysis of the residual error between observed and estimated PET showed that the coefficients currently used by DMS best matched the lysimeter observations over the 4 seasons. However, only further comparisons between estimates and long-term, accurate, and multi-locational lysimeter measurements of reference crop PET would establish the set of coefficients appropriate for Penman-based PET estimation across Botswana.