Unsaturated hydraulic conductivity

The unsaturated zone plays an important role in the hydrological cycle. It forms the link between surface water and ground water and has a dominant influence on the partitioning of water between them. The hydraulic properties of the unsaturated zone determine how much of the water that arrives at the soil surface will infiltrate into the soil, and how much will run off and may cause floods and erosion. In many areas of the world, most of the water that infiltrates into the ground is transpired by plants or evaporated directly into the atmosphere, leaving only a small proportion to percolate deeper and join the ground water. Surface runoff and deep percolation may carry pollutants with them. Then it is important to know how long it will take for this water to reach surface or ground water resources. Besides providing water for plants to transpire, the unsaturated zone also provides oxygen and nutrients to plant roots, thus having a dominant influence on food and fiber production. Water content also determines soil strength, which affects anchoring of plants, root penetration, compaction by cattle and machinery, and tillage operations. To mention just one other role of the unsaturated zone, its water content has a great influence on the heat balance at the soil surface. This is well illustrated by the large diurnal temperature variations in deserts. To understand and describe these and other processes, the hydraulic properties that govern water transport in the soil must be quantified. Of these, the unsaturated hydraulic conductivity is, if not the most important, certainly the most difficult to measure accurately. It varies over many orders of magnitude not only between different soils but also for the same soil as a function of water content. Much has been published on the determination and/or measurement of the

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