Abstract A reliable estimate of crop water requirements is one of the key design criteria required for the planning and operation of irrigation schemes. This in turn requires accurate estimation of evaporation for which the Penman form of the combination equation is widely used. This paper examines the sensitivity of evaporation estimates to errors in input data, and to uncertainties in the values of the parameters used in the equation. Results from a wide range of stations over the globe show that estimates are most sensitive to temperature, and that this sensitivity can have a marked seasonal fluctuation. The effects of uncertainty in the equation's parameter values are small and comparable to the sensitivities of the other input variables; wet bulb depression, wind speed and sunshine hours.