Tuesday, 11 July 2006

Drainage Index (DI) as a Climate Factor that Controls the Distribution of Zonal Soils. The Case of Spain.

G. Gasco1, A. Saa1, Ana M. Tarquis2, and M.C. Diaz1. (1) Dpto. Edafología - E.T.S. Ing. Agrónomos - Polytechnic University of Madrid, Ciudad Universitaria sn, Madrid, 28040, Spain, (2) Dpto. Matemática Aplicada - E.T.S. Ing. Agrónomos - Polytechnic University of Madrid, Ciudad Universitaria sn, Madrid, 28040, Spain

Zonal soils are typical soils of great morphogenetic regions having distinctive climates. The influence of living organisms, while very profound, is usually dependent on the other factors of soil formation, i.e. parent material and topography or local relief exerting a modifying influence, and time required for other four factors to act. In general, under similar environments, defined by the five factors of soil formation, soils are similar, and this regularity permits to predict the location of different types of soil. In particular, the kinds of zonal soils in large areas are related to distinctive weathering and leaching patterns depending on regional patterns of climate. These patterns of climate can be used to predict the kinds of soil in geomorphic-climatic regions. Climate defines the sum total of the weather of a region as expressed by temperature, moisture and wind. The higher the temperature and the wind, the greater is the evapotranspiration and the less is the precipitation water available for leaching of the soil. Great morphogenetic regions have distinctive sets of geomorphic processes associated with distinctive climates (glacial, periglacial, arid, semiarid-subhumid, humid-temperate and humid-tropical climatic regions). Other major regions characterized by seasonal climatic variation are also recognized (Mediterranean and Monzonic regions). Pedoclimate is defined by soil temperature regime (pergelic, cryic, frigid, mesic, thermic, hyperthermic) and soil moisture regime (aquic, aridic or torric, udic, ustic, xeric). Hypothetic soil moisture and temperature regimes can be estimated from climate records in order to classify soils. But these estimations are difficult to achieve, and some weathering and leaching indexes has been defined in relation to soil genesis and classification, for example: a) the soil temperature regime, defined by the annual mean temperature at 50 cm depth, and b) the Henin-Aubert drainage index (HADI) which can be calculated from mean annual precipitation and mean annual soil temperature, besides soil texture. In this study, we have used soil temperature regime (ST) and a drainage index (DI = P2/ETP) to predict the kinds of soil in large areas. This drainage index (DI) showed a strong relation to HADI and can be used to predict actual soil genesis in large areas of Aridisols, Inceptisols, Alfisols, Ultisols, Spodosols, Histosols and, out of Spain Oxisols. Other soil features related to paleoclimate, parent material and local relief, which can exert a modifying influence for soil forming processes, were not considered.

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