Drainage Systems and their Water Management Function with regard to Probable Climatic and Hydrological Changes

In case that the climatic or farming conditions have changed in a region, it is possible to anticipate that the core parameters of drainage constructions will not be adequate for the current needs. Some of these constructions might be over-dimensioned, which would be inconsistent with current terms of nature and water resources protection. On the other hand, the valuable contribution the regulation of the water regime of originally waterlogged lands and swamps indisputably brought along and thus enabled the agricultural use of drained lands would be depreciated to a certain extent. In this paper, some scenarios of probable climatic changes in the Czech Republic's territory and the anticipated effect of these changes on the components of the hydrologic water discharge from drained agricultural and forest catchments are studied. The function of drainage systems on selected experimental lands is examined. However, not only probable changes in precipitation, temperature and water runoff should be taken into account, but also changes in the way of farming, i.e. changes in the agricultural conditions and data that played a decisive role in the calculation of the basic parameters of these construction projects, for instance, the spacing of parallel drains or trenches or the depth of their laying. In the Czech Republic about 1.1 million ha of total agricultural land was drained by the end of the twentieth century. In some localities of Eastern and Southern Bohemia up to 80% of agricultural land was drained. To what extent the above-mentioned climatic changes and the changes in the way of farming influence the drainage system and whether adjustments of these systems are required are the questions we tried to answer at least partly in this contribution. The effect of climatic changes on the hydrologic balance and/or on the runoff from the catchments significantly varies up to ± 150% provided that we compare average runoff. The influence of changes in the way of farming on the hydrologic balance manifests itself in a more concrete, but negative way, and that demands a concrete reaction on the part of the water management control. It is therefore vital to take such measures of regulation of the water regime of soil for the existing drainage systems that will ensure both the drainage phase and the phase of runoff retardation. Single-function and obsolete drainage systems should be converted into systems with controlled drainage and irrigation functions - the double-function control systems.