Ocena efektów kształtowania retencji na zalesionych gruntach porolnych we Wrocławiu

The paper presents the results of a study on water relations conducted in the hydrological years 2000−2010 in the catchment of Ługowina and Ługowinka in the western part of Wrocław (SW Poland). The studied object was created in the mid−1990s by afforestation of nearly 100 ha of meliorated agricultural land. The research objective was to determine the changes in the resources of ground water retention under the effect of damming structures in diverse hydrometeorological conditions. The analyses concerned the changes in the mean level of the ground water table in an area with outflow regulated by three dams and, for comparison, in an area with natural water management. The study showed that in the period from March to June, i.e. the time of the highest water demand from trees, the level of the ground water table in the area with regulated outflow was higher by about 30 cm compared to the control area. Correct operation of the damming structures, consisting in annual regulation of outflow, allows effective restoration of springtime resources of ground water retention. At the end of the vegetation period, the level of the ground water table was usually relatively stable over the entire object, which indicates cyclic depletion of retention resources of the local micro−catchment basin. During the vegetation period those resources could be restored only in the case of occurrence of high precipitation. The effectiveness of the regulated outflow in the first half of the vegetation period was noted also in dry years 2002−2005, with simultaneous overall decrease of water resources in the scale of the country. The observations demonstrated also that sporadically (in very wet periods) the water−melioration system was unable to drain the excess water. That caused the occurrence of local floodings that threatened the health status of the trees. One can conclude that the use of water−melioration infrastructure typical for agricultural areas for the regulation of outflow from a local micro−catch−ment basin in the conditions of a lowland river valley is an effective method of improvement of ground water resources. The highest effectiveness in terms of increased retention was noted in the period of the most intensive growth of trees. In case of afforestation of post−agricultural lands this may have a highly positive effect on the quality of cultivations and their subsequent development and health status.