Long-term changes in hydrological regime of the lakes Usma, Burtnieks and Rāzna

Changes in the hydrological regime of the lakes of Latvia have been caused by several natural and human factors. This publication summarises the results of research on the long-term and seasonal changes in the water level, and thermal and ice regimes of the three biggest lakes of Latvia (Usma, Burtnieks, and Rāzna) and their regional features in the period from 1926 to 2002. The levels of the lakes Usma and Rāzna have been controlled, but it can be considered that changes of the water level in Lake Burtnieks have been due to the impact of natural factors during the period from 1947 to 2002. Global climate warming has caused considerable changes in the hydrological regime of the lakes during the last decades, as the water level and temperature have increased and the number of days with ice cover and the thickness of ice have decreased. A positive trend in the freezing data and statistically reliable negative trend for the ice break-up date were observed for all the lakes. Lake Usma is located in the western part of Latvia, therefore, its hydrological regime, in particular, the thermal and ice regime, differs from those of lakes Burtnieks and Rāzna which are located in the northern and eastern part of Latvia, respectively.