THE VARIABILITY OF EVAPORATION DURING THE HAPEX-SAHEL INTENSIVE OBSERVATION PERIOD

The variation in evaporative fraction and actual evaporation is examin ed for three sample days in the HAPEX-Sahel Intensive Observation Peri od (IOP), including data from all the vegetation types and sites. The trends in evaporative fraction over the IOP are also presented for eig ht sites. The high rate of evaporation from bare soil in the days foll owing rainfall produces a variability in evaporation which makes diffe rences between sites difficult to interpret on a day-to-day basis, but over the whole IOP it is shown that the millet uses a smaller proport ion of the available energy for evaporation than the tiger bush or fal low savannah. The combined effect of differences in the total energy u sed and its partitioning into evaporation and sensible heat flux is de monstrated from the trends in cumulative total energy use and evaporat ion at the three southern sites, where it is shown that there is syste matically less evaporation from the millet than from the savannah or t iger bush sites.