Civil Engineering Papers

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Evapotranspiration in the tropics
(Colorado State University. Libraries, 1970) Schulz, E. F., author; Hossain, Aolad, author; Colorado State University, publisher
A study was made of monthly rainfall and runoff over a 105,000 square kilometer watershed in northern Thailand. The function of the watershed in producing the runoff hydrograph was simulated using a type of Stanford Watershed Model adapted for operation with an IBM 1130 digital computer. The evapotranspirational losses from the watershed were computed by using the Thornthwaite, Penman, Blaney-Criddle and Blaney-Morin methods of computing the evapotranspiration. By comparing the computed runoff with the observed runoff from the watershed, it is shown that the model most nearly predicted the observed runoff when the Penman or Blaney-Morin methods were used to estimate the evapotranspirational losses. It was concluded that for a tropical environment, evapotranspirational estimates must include a term measuring the relative humidity of the atmosphere. The evaporation of water is often limited by the ability of the atmosphere to carry away the water vapor produced. Those methods of estimating evapotranspiration based only on temperature and radiation or sunshine data consistently over-estimate the evapotranspiration from a tropical watershed.