The influence of the land drainage on the river runoff in Lielupe river basin district

The evolution and influence of drainage of too wet lands in the Musa, Levuo, Tatula and Nemunelis basins located in the Lielupe river basin district (RBD) on the river runoff in the period of 1960-2011 was analyzed. It was determined the wetland areas in the Musa, Levuo and Nemunelis basins exceed 70% and those of the Tatula basin - even 90% of the total basin area. Before 1961, the drainage areas covered only 4-7% of the territory of the basin and in 2000 these areas covered already 58-80%. Lately, any new drainage systems almost are not developed, so the drainage areas have changed little. Water surplus is most intensively removed during spring floods and summer high waters; therefore, due to draining influence the runoff in rivers varies only during the seasons of the same year.