Beschreibung:
L'estimation de la vitesse d'érosion d'un système karstique est une donnée qui se heurte à la variabilité du débit et du chimisme des eaux, caractéristique majeure des systèmes aquifères karstiques. Les méthodes empiriques sont généralement entachées d'une forte incertitude liée à la méconnaissance de la superficie des bassins-versants hydrogéologiques, voire même du nombre total d'exutoires d'un massif karstique. Les formules théoriques donnant la vitesse de dissolution en fonction d'un seul paramètre climatique (la pluviosité) sont criticables. On montre que l'emploi d'un modèle mathématique de la dissolution, basé sur le calcul des précipitations efficaces et sur la connaissance de l'équilibre calco-carbonique, fournit des résultats comparables avec une marge d'erreur moindre.
The specific behaviour of karsts makes the estimation of karstic denudation very difficult : discharge and water chemical variabilities are in fact major characteristics of aquifer karstic systems and cannot be properly estimated by the way of random sampling. The classical empirical methods provide generally high relative errors due to the bad knowledge of the hydrogeological catchment basin and even of the total number of springs. In the case of CORBEL's or WILLIAM'S empirical formulas, average relative error can be estimated to about 100 % for a normally known aquifer karstic system : therefore it is impossible to compare different karsts that have not been studied with the same accuracy. The theoretical statistical relationships between karstic denu¬ dation and a single climatic parameter (rainfall) are open to criticism : main authors tell that effective rainfall and pedological CO2 are essential parameters of karstic denudation, which are never taken into account. For example, there are different PULINA 's formulas for different climatic types indicating that it is necessary to use another climatic parameter different from rainfall ! Moreover this way of modelling the data restrains the statistical repre-sentativity of each formula and introduces a difficult choice for karsts which are at the border of two climatic types (or with mountainous parts). Another problem is the case of polar countries karsts where most precipitation is snowy and does not participate in karstic denudation. Therefore a mathematical modelling of carbonate dissolution is shown, based on infiltration rate calculation and knowledge of calco-carbonic equilibrium. Temperature and rainfall are taken into account to determine the efficient part of precipitation, the productivity of pedogenetic CO2 and the carbonate solubility constants. This theoretical approach gives the same results but with relative errors under 50 %. Consequently it is easy to compare different karstic countries in the world : hot and wet climates are confirmed to have the main karstogenetic activity but the role of cold countries is rehabilitated. Then paleokarstic denudation can be estimated.