Field Measurement of Evapo-Transpiration in the Sahele Area of Republic of Mali, West Africa

The precise evaluation of evapo-transpiration is indispensable to develop the water resources in the arid and/or the semi-arid areas such as the Sahele in Africa. The evapo-transpiration were successfully measured in the Sahele by using a new equipment that was developed by the present authors. This measurement was carried out during 17/Aug. /1994-28/Aug. /1994 at several locations in the vicinity of the Nara city of Republic of Mali.The equipment used is mainly composed of two parts; a transparent box and a electric wind fan. A part of the ground surface was covered with the box and air was injected into the box through a pipe by the wind fan. Then, air in the box was exhaused through another pipe installed on the other side of the box. The evapo-transpiration from the ground surface and the plants covered with the box can be calculated from the air flow discharge and the absolute humidity difference between the injected and the exhaused air.The obtained results are as follows:(1) The evapo-transpiration was mainly affected by the solar radiation.(2) More than 60% of water poured on the ground surface in a day time was rapidly evaporated during 3 hours after the water supply.(3) The transpiration from plant was about 3-4 times larger than the evaporation from the bare soil surface.(4) The evapo-transpiration rate obtained by the equipment was about 1/3-1/5 of the evaporation rate from water surface measured by the pan-type and the PICH-type evaporation meter.