This book is an update of a 1964 publication that discusses the results of research work performed by the author to measure the potential evapotranspiration (ETP) in a tropical location: ETP is defined as the depth of water consumed by a vegetated cover that is green, short and uniform, that completely shades the soil, that has no limitations of water or nutrients and that is actively growing. It also correlated ETP with the evaporation Ev from a US Weather Bureau Type ‘A’ pan: both quantities are related by a coefficient of proportionality. The definition of ETP implies that all other crops in the zone have a rate of real evapotranspiration equal or less than that of ETP. The devices utilised in this research were the assembly of a floating lysimeter situated in a 1.4 hectare field planted with bermuda grass (Cynodon dactylon) and meteorological instruments in situ.
The term potential evapotranspiration was coined by professor H. L. Penman in 1946 in the United Kingdom. In his work, and for optimal conditions, he eliminated the plant factor in his calculations; by detaching the plant factor from his formulae Penman expanded the concept of ETP beyond its utilisation for agricultural purposes and made it applicable, with certain provisos, to other areas: for this reason it is often considered a geographical constant.
The review of the literature at the time of the research revealed that most work on this subject emanated from the northern temperate zones; no entries were found from tropical latitudes and it can be surmised that this study was the first one done in the tropics. Since then numerous works on this field have been produced in tropical latitudes many of which have important refinements regarding methodology and also more precise results.
The author performed this work as a young professor at the Central University of Venezuela (UCV) after graduating from California Polytechnic State University (Cal Poly). The research was performed in coordination with professor William O. Pruitt at the University of California at Davis and it was part of parallel research done by him and others in Australia. Valuable contributions were made by professors Wynne Thorne and Sterling A. Taylor of Utah State University during visits to the project site: their conformity with the instruments, the methodology utilised and the managing of the effort was a welcome support to this endeavour.
The text of the book has been carefully edited mostly to improve the syntax. The original calculations, the figures inserted in the text and the maps and tables in the appendices were left untouched, the latter captured as images in JPEG format. Four tables interspersed with the text and two drawings of the lysimeter assembly in the appendices were redone. A glossary of common terms used throughout the text was added to the appendices.
The term potential evapotranspiration was coined by professor H. L. Penman in 1946 in the United Kingdom. In his work, and for optimal conditions, he eliminated the plant factor in his calculations; by detaching the plant factor from his formulae Penman expanded the concept of ETP beyond its utilisation for agricultural purposes and made it applicable, with certain provisos, to other areas: for this reason it is often considered a geographical constant.
The review of the literature at the time of the research revealed that most work on this subject emanated from the northern temperate zones; no entries were found from tropical latitudes and it can be surmised that this study was the first one done in the tropics. Since then numerous works on this field have been produced in tropical latitudes many of which have important refinements regarding methodology and also more precise results.
The author performed this work as a young professor at the Central University of Venezuela (UCV) after graduating from California Polytechnic State University (Cal Poly). The research was performed in coordination with professor William O. Pruitt at the University of California at Davis and it was part of parallel research done by him and others in Australia. Valuable contributions were made by professors Wynne Thorne and Sterling A. Taylor of Utah State University during visits to the project site: their conformity with the instruments, the methodology utilised and the managing of the effort was a welcome support to this endeavour.
The text of the book has been carefully edited mostly to improve the syntax. The original calculations, the figures inserted in the text and the maps and tables in the appendices were left untouched, the latter captured as images in JPEG format. Four tables interspersed with the text and two drawings of the lysimeter assembly in the appendices were redone. A glossary of common terms used throughout the text was added to the appendices.
