The increase in temperatures across the Zambezi Basin will result in higher rates of evaporation and transpiration. Much of the Zambezi River Basin is semi-arid, and substantial water loss occurs due to evapotranspiration. At present, mean annual potential evapotranspiration across the basin is 1,560 mm, and potential evaporation exceeds rainfall during every month of the calendar year, in each of the 13 sub-basins. Over the next century, the Zambezi Rver Basin is expected to experience a significant increase in the rate of potential evapotranspiration, based on projected increases in temperature coupled with decreased humidity associated with reduced rainfall (below). Arnell (1999, as cited in IPCC 2001) projected an increased rate of evapotranspiration in the basin of 10-25% over the next 100 years.