Masters Theses

Date of Award

12-1963

Degree Type

Thesis

Degree Name

Master of Science

Major

Agronomy

Major Professor

W.L. Parks

Committee Members

H.C. Smith, R.S. Dotson

Abstract

Evapotranspiration consists of two processes, namely, evaporation and transpiration. It has been defined as the sum of water removed by the vegetation and that lost by evaporation from the soil for a particular area during a specified time (42). Evaporation is dependent upon the moisture content of the soil and an integrated effect of the meteorological elements which determine evaporating power of the atmosphere, whereas transpiration is dependent upon certain plant, soil and meterological factors. Thornthwaite (44) originated the concept of potential evapotranspiration (P.E.) in 1944 and defined it as "the amount of water which will be lost from a surface completely covered with vegetation if there is sufficient water in the soil at all times for the use of the vegetation." The actual evapotranspiration (ET.) may or may not approach P.E., depending upon how the conditions for P.E. have been satisfied. Explaining further, Thornthwaite stated that unless a large area with uniform environment is provided while estimating the P.E., the estimates of P.E. may differ from the ET. The proportion of irrigated area to the surrounding dry area is frequently small in arid regions and advection of dry air occurs and this influences ET. Several workers have modified the original concepts and formulae based on the meteorological factors in an effort to predict the ET., for controling the irrigation practices for maintaining the soil moisture at an optimum level for crop growth. Apart from arid and semiarid regions, a large amount of interest has also been created in humid regions where supplemental irrigations may be used to avoid yield reductions due to drouth. In order to maintain the soil moisture balance, it is essential to know the amount of available moisture in the rooting zone of the crop, the moisture additions due to rains or irrigation, and the ET. for the growth periods of the crop. There are differences of opinion regarding the relative availability of soil moisture to crops from field capacity to the permanent wilting point. Although it may be assumed that water is available from 0.3 to 15.0 atmospheres tension for meeting the transpiration requirements the crops, the growth of most of the crops is usually found to be affected when the moisture tension exceeds 4 atmospheres (41). The extent of yield reduction depends upon the nature of the crop and the stage of crop growth at the time the moisture stress occurs. The optimum for each crop can be ascertained by conducting experiments with different levels of moisture tension and fertility. Under field conditions the ET. may be determined by soil sampling at the desired depths and ascertaining the moisture content by gravimetric methods or by indirect measurement of soil moisture through the use of tensiometers, electrical resistance blocks. or by neutron scattering technique. The use of these methods require time, equipment, skill and training and they may not be practicable for the farmer. ET. has been recognized as being largely a function of climate. The climatic elements such as temperature, solar radiation, humidity, air movement, precipitation and evaporation may be measured at some representative weather stations in the locality. If the ET. could be predicted by making use of these records, it would be of great use to the farmers in scheduling irrigation practices. Many workers have shown a close relationship between ET. and loss of water from evaporimeters and atmometer bulbs. Their use is simple and, if found to be suitable for the purpose, could be advantageously used by the farmers. This pioneering study was to analyze the effect of some climatic factors on ET. in corn under Tennessee conditions. The object of the study was to ascertain the relationship of ET. with the meteorological elements such as temperature, solar radiation, wind, sunshine, humidity, and evaporation from open pan and atmometers. The ultimate aim is to explore the possibility of guiding the farmers in scheduling their irrigation practices based on the climatic data and also to relate crop yield to certain climatic factors so as to use the weather data to establish probabilities of occurrence of these conditions.

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