A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering of the Asian Institute of Technology, Bangkok, Thailand.

Thesis (M.Eng.) - Asian Institute of Technology, 1975

Different scientists at different times tried to find the evapotranspiration due to water hyacinth. They also tried to find the relation between it and the free water surface evaporation by calculating ratio between them. But they got ambiguous results. Our present study was aimed at to find the actual evapotranspiration capacity of water hyacinth and the ratio of evapotranspiration with simultaneously observed class A pan evaporation. It is established that evaporation is a good index of evapotranspiration and relation between U.S. Weather Bureau class A pan evaporation and evapotranspiration by maximum of crops are linear. Evapotranspiration due to water hyacinth obey thesis established fact. But one important thing is that though the relation between the class A pan evaporation and evapotranspiration by water hyacinth is linear but ratio of evapotranspiration to evaporation decreases with the increase of rate of evaporation. The slope of the regression line for the evapotranspiration on evaporation varies exponentially with the increase of densily of leaves coverage. The experiment was conducted for the coverage condition of 100%, 200%, 300% and 400%. With these four observed values it has been seen that exponential relationship between the slope of the regression lines and the increasing leaves coverage has a good fit. But it is not wise to say with this limited number of observations that the relationship between the slope of regression line for evapotranspiration on evaporation with the increment of coverage condition will remain exponential for more higher or lower coverage conditions. The intercepts of regression lines £0r the evapotranspiration on evaporation varies linearly with the increment of coverage condition.