Effect of Atmospheric Pressure on Evaporation in Central Ethiopia

The authors have implemented a meteorological observation in the Oromia State, central Ethiopia. The region is located at the Great Rift Valley and classified as semi-arid area. Generally, the area is exposed to intense solar radiation, moderate wind, and low relative humidity resulting in high driving force of evaporation. Further, the authors suggest that low atmospheric pressure associated with high elevation would accelerate the evaporation ratio since water is vaporizable when the atmospheric pressure is reduced. The current study aims to monitor annual change in the atmospheric pressure and analyze the effect of it on the evaporation ratio in central Ethiopia. The evaporation ratio was computed by the Bowen ratio energy balance method. The atmospheric pressure in central Ethiopia was almost constant throughout a year ranging from 83.9 to 84.5 kPa independent of air temperature and rainfall. Taking into account the actual atmospheric pressure measured at the experimental field, the evaporation ratio computed by the Bowen ratio energy balance method was 4.5% larger than that calculated under the standard atmosphere. Although this difference (4.5%) would seem small, this would cause considerable error (i.e. underestimation) when the amount of evaporation is assessed cumulatively for longer period (ex. months or years). The results of the current study also suggest that highlands in arid lands have a high potential to dry due to its low atmospheric pressure. This implies fragility of eco-systems in other similar environments and accounts in part for difficulty to reestablish forests in highlands of the arid lands once the forests are cleared.