Modification of direct runoff in a small forest catchment of the Krajenskie Lakeland as a result of the watercourse development

The field investigations were carried out in a small forest catchment situated in the area of the Krajeńskie Lakeland, in the Lipka Forest District, the Biskupice Forest Range. The catchment covers the area of 182 ha; 95% is covered by forests and 5% by arable land and meadows. Field measurements comprised continuous recording of water level at the Thompson’s weir and weekly measurements of groundwater levels in ten wells. Construction development was introduced in the area of the watercourse during the conducted research: six damming devices (installations) constant weirs - were constructed there. The annual outflow coefficient from the catchment equals to 0.330. It confirms the necessity of developing the discussed watercourse in order to create the so-called small retention. However, no significant influence was found of the development on the water balance components of the catchment. The influence of the watercourse bank development can be clearly described by conducting an analysis of direct runoff. 14 recorded large floods were subject to analysis; 6 prior to the development and 8 following it. Each of the waves was described by applying Nash’s conceptual model. A constant number of 2 reservoirs in a cascade was assumed. Means of time-constants for high water waves after the development were higher by approx. 50% than for the waves prior to the construction. Resulting from it a hypothesis can be constructed here stating that the time of runoff water deposition in the catchment as a result of the weir development was significantly prolonged. It can be thus assumed that systems of small and basic weirs should be applied in forest small retention programmes.