Soil Water Continuum Essay

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Soil-Plant-Atmosphere continuum
This chapter explains the concept of soil-plant-atmosphere continuum (SPA) and its fundamental principles. It also examines the role of water in the SPA continuum. A model for soil-water balance, its components and the way in which it can be determined will be also presented, taking into account the different uncertainties that could affect the measurements.
1.1 Terminology
The SPA continuum is defined by the movement of water from the soil to the root through the stem and from the leaf to the atmosphere (Lambers et al 2008). The system which is composed components involving soil, plants leaves’ and roots, xylem and the atmosphere is named the Soil-Plant-Atmosphere system, which represent a pathway for the
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Water potential is defined as energy amount (joules) obtained from a moving mass of water (in kilograms) from the soil to a certain elevation (plant height), and it usually negative which means that energy must be expended to extract the water. With the drying processes, water in the soil becomes less and less available and its potential decrease, making it more difficult for the plant to absorb its needs. There is therefore a direct relationship between water potential and water content. The relationship between soil water content and how tightly the water is held by the soil (soil matric potential) is typically described using a "soil water retention" curve. Once water goes into the plant through the root, it moves according to a decreasing gradient to the leaves throw the vascular system. Hence, the resistance of flow transit in the stem xylem is less than the one of the root. Moreover, according to the conservation principles, the flow going out from the root is equal to the one entering to stem xylem and the soil-root water potential difference is usually higher than the root-leaf potential difference. The evaporation process draws water from the stomata leaf cells. This water is hydraulically connected by cohesion forces to a continuous column of water up the root. When water draws from the stomata, a flux goes through the xylem to …show more content…
It is the combination of two separate processes of direct evaporation from an evaporating surface and transpiration from vegetation. Crops absorb water by the roots, parts of it is lost through the stomata of the leaves, while the rest is used for photosynthesis. Transpiration and evaporation depend essentially from the heat supply and the available water in the soil. Energy is the driving force determining evapotranspiration (ET) rates for unstressed crops and; therefore, understanding energy balance is critical to understanding ET. Energy is needed to break the hydrogen bonds and separate H2O molecules from the liquid water. The energy for ET comes mainly from radiation although heat transfer from the air and soil can also contribute to ET. The key to understanding ET is to understand how much energy is available within the crop canopy to evaporate water (Courault et al., 2005). The energy balance of a crop canopy is expressed using the following equation:

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