Water Vapor Flux in Agroecosystems Methods and Models Review

Abstract

The water vapor flux in agroecosystems is the second largest component in the hydrologycal cycle. Water vapor flux or evapotranspiration (ET) from the vegetation to the atmosphere is a widely studied variable throughout the world. ET is important for determining the water requirements for the crops, climatic characterization, and for water management. The estimation of ET from vegetated areas is a basic tool to compute water balances and to estimate water availability and requirements. During the last sixty year several methods and models to measure the water flux in agroecosystems have been developed. The aim of this chapter is to provide a literature review on the subject, and provide an overview of methods and models developed which are widely used to estimate and/or measure ET in agroecosystems. Evapotranspiration constitutes an important component of the water fluxes of our hydrosphere and atmosphere (Conroy et al., 2003), and is a widely studied variable throughout the world, due to it applicability in various disciplines, such as hydrology, climatology, and agricultural science. Pereira et al., (1996) has reported that the estimation of ET from vegetated areas is a basic tool for computing water balances and to estimate water availability and requirements for plants. Measurement of ET is needed for many applications in agriculture, hydrology and meteorology (Suleiman & Crago, 2004). ET is a major component of the hydrologic water budget, but one of the least understood (Wilson et al., 1992). ET permits the return of water to the atmosphere and induces the formation of clouds, as part of a never-ending cycle. ET also permits the movement of water and nutrients within the plant; water moving from the soil into the root hairs, and then to the plant leaves. ET is a complicated process because it is the product of the different processes, such as evaporation of water from the soil, and water intercepted by the canopy, and transpiration from plant leaves. Physiological, soil and climatic variables are involved in these processes. Symons in 1867 described evaporation as “...the most desperate art of the desperate science of meteorology” (Monteith, 1997). The first vapor flux measurements were initiated by Thornthwaite and Holzman in 1930s, but that works was interrupted by World War II (Monteith, 1997). In the late 1940s Penman (1948) published the paper “Natural Evaporation from open Water, Bare Soil and Grass” in which he combined a thermodynamic equation for