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Hydrological conseąuences of human action..

infiltration into soil and restrict overland flow. Runoff in periods with excess water, particularly in spring, is delayed or reduced, which attenuates floods. In dry periods, however, supply from underground reserves restricts the formation of Iow water. This regulating role of the forest also depends on local conditions.

It is morę dificult to determine the influence of farming on water relationships. A big role is played by the character of farming:

—    the method of soil cultivation conditioning evaporation, infiltration, overland flow and soil retention,

—    the kind of cultivated plants differing strongly with regard to water reąuirements, i.e. affecting the magnitude of evapotranspiration and very differently protecting soil from overland flow,

—    the intensity of soil cultivation on which the amount of used fertilizers depends, which effects the magnitude of soil moisture storage.

In the conditions of the Łęczna-Wlodawa Lakę Region changes in landuse must have affected above all the magnitude of evapotranspiration. With shallow groundwater occurrence evapotranspiration in forests could be maximum in the given thermal conditions. Replacing forests with cultivated fields or pastures decreased ground evaporation, which resulted in a rise in the groundwater-table, and thus in the extended rangę of wetlands.

Runoff conditions also changed. In spring, waters retained in snów cover are released faster on cultivated areas. On the fiat land of the Lakę Region large areas were covered with water, the runoff of which was impeded due to a poorly-developed river network. Thus we can assume that a forest landscape changed to forest-agricultural caused an increase in wetlands, particularly in the spring season. There was also a bigger contrast between the amount of water running off in spring and that in the period of smali autumn supply when river flow is sustained only by poor, easily-exhaustible underground reserves.

These changes cannot be expressed quantitatively because hydrotechnical work was conducted in parallel to changes in land use, and these also modify retention and runoff conditions.

HYDROTECHNICAL WORKS

The digging of drainage ditches to get rid of excess spring water was practised in the Łęczna—Włodawa Lakę Region a very long time ago. No documentation of these primitive hydrotechnical works is available, but it can be supposed that they consisted in facilitating runoff by deepening and straightening the natural, poorly-developed drainage system and connecting depressions without outflow with ditches. The natural character has been preserved only by the lower and middle courses of the rivers: Wlodawka with tributaries, Tarasinka and Krzemianka in the Bug river basin, and on the west side, those of the Tysmienica river with the two Piwonia rivers