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Abstract
The vegetative productivity of forests, grassy plains or agricultural fields depends on many factors, including geological features, soil quality, precipitation, solar radiation, vegetation cover and climate. In arid and semi-arid regions, precipitation is of central concern, with biomass production largely sustained by rainfall and its seasonal distribution. Probing the movement of water into and out of the atmosphere and soil, through the vegetation – as well as its passage amongst them - is central to understanding ecosystem productivity and sustainability indryland areas. Such tallying of water movement, known for short as the “water balance,” allows planners to evaluate whether human activities such as afforestation or farming would be sustainable (in terms of available water) or whether a net water deficit would occur. In the latter case, expensive water from without the region would be needed to support these ventures.
In Israel's northern Negev, researchers at the Blaustein Institutes for Desert Research and the Forestry Department of the Keren Kayemeth LeIsrael - Jewish National Fund are investigating the water-vegetation interrelationship in Yatir Forest, 23-km northeast of Beer Sheva. Detailed moisture measurements in the forest, planted in the late sixties and early seventies to reduce soil erosion, were the first comprehensive study of a forest in a dryland ecosystem. It provides an instructive basis for future forest planning in arid regions
In these investigations, researchers measure water in its various depositories, including the atmosphere, soil, as well as its movement among these stores and growing vegetation. The offset between water inputs and outputs in an ecosystem is known as the ecosystem water balance, which – over a multiyear time scale – can be positive, negative, or zero. Because water-balance components fluctuate throughout the year with changing environmental factors (temperature, rainfall, etc.), the intensity of plant growth, among other ecological features, varies as well. Despite these short-term cyclic changes, ratios of the annual averages of water-balance components to rainfall do not change greatly from year to year provided that precipitation remains fairly constant. When undisturbed, this tranquil situation can continue for many decades. However, when indigenous populations decide to reorganize land use (by initiating intensive agriculture in uncultivated regions, abandoning previously cultivated areas, afforestation, deforestation, etc.), the water balance and the relationship between its various components can be affected.
Environmentalists therefore seek to measure and predict the outcomes of natural and man-produced changes on water balance and bioproductivity. Measurements in the Yatir Forest have shown, for example, that this particularly sparse tree canopyhardly intercepts rainfall and almost all precipitation reaches the ground. (In closed forest canopies, as much as 30 percent of the rainfall can be trapped by the canopy and lost to evaporation.) Moreover, runoff through the forest is negligible. Therefore, practically all the rainwater is stored in the soil and later used by trees and/or annual herbs and/or it evaporates via the soil surface. Blaustein Institutes researchers are also examining agroforestry systems, in which crops are raised in areas between trees. Several approaches have developed to achieve maximum availability of water for agricultural production, and the novel use of runoff water for irrigating arid-region agroforestry systems has been advanced.
Water Balance Components
The Water-Balance Equation
Forest Ecosystem
Yatir Forest: A Model Study
Findings of the Yatir Study
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