Лекция: Human Influences

Human influences can alter soil water conditions in a large number of ways, ranging from irrigation schemes, which considerably increase the amount of water entering the soil, to the construction of large impermeable surfaces in urban areas, which prevent water from infiltrating into the soil. These effects describes in engineering and agricultural texts. Accordingly, only a brief account of a few examples given below.

Agricultural practices have the most widespread effect on soil water conditions. Irrigation and artificial drainage are used throughout the world as a means to increase crop production. Agricultural drainage schemes comprise open ditches or surface pipes. These are deeper and closer together than the natural stream channels, so increasing the hydraulic gradient in the soil and lowering the water table more rapidly between storms than would otherwise occur. A detailed account of the distribution and purpose of field drainage in England and Wales, the most intensively drained part of Europe, was given by Robinson and Armstrong.

Tillage and cultivation operations may also alter the movement and distribution of soil water. Ploughing increases the pore spaces in the upper soil and may encourage lateral flow in the topsoil, with less downflow into the subsoil. It is showed by tracer studies that ploughing disrupts the vertical continuity with pores in the soil below. Infiltrating water is found to penetrate to greater depths on land that had not been ploughed.

A change in agricultural land use from grassland to arable cropping may also affect interception and evaporation losses, especially if the arable farming leaves the soil bare at times of the year. Heavy rainfall on land with little vegetation cover may lead to crusting and sealing of the soil surface, reducing infiltration. Forestry may have a large effect on interception and evaporation losses, causing soils under trees to be much drier than under other types of vegetation. In areas where the natural water table is close to the ground surface, groundwater abstraction may lower the water table, causing significant drying of the soil and a reduction in plant growth. The most extreme case of human influence on soil water conditions, however, perhaps is found in areas of steep topography, where deforestation and bad farming practices lead to accelerated erosion and may, in severe cases, ultimately result in the complete destruction of the soil.

Упражнение 11.

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