Patterns of low flow and intermittency in the rivers of water surplus zone

Abstract:

Every year, the number of intermittent rivers is increasing worldwide, and human activities and climate change have a huge impact on it. Nowadays, little is known about intermittent rivers, as until recently this component of low flow was ignored as an object of researches. This is especially true for cold areas as in warm regions like the Mediterranean region, extreme changes of low flow are more investigated. However, evaluation of changes of low flow and river intermittency are also important in water surplus zone where precipitation is higher that evaporation. In this work, research objects are located in Lithuania, where there are 22,000 rivers per 65,300 km² and the ratio between annual precipitation and evaporation reaches 1.47. Task of this study is evaluation of changes of patterns of low flow and intermittency in the rivers of water surplus zone in temporal and spatial aspects. In addition, the impact of physico-geographical and climate factors is considered on variability of low flow.

Main data sources were daily hydrological data of 183 water gauging stations for the different time periods, soil maps in shp-format and satellite images. Low flow parameters and indices of river intermittency as well as available catchment-scale data were analysed using statistical methods (correlation and multiple linear regression analysis) and GIS-instruments. Characteristics of low flow (Q30, 7Q10, timing of the dry 30-day period), river intermittency parameters (the mean annual frequency of zero-flow periods and the mean timing of the first zero-flow event), physico-geographical and climate factors were used for analysis. These characteristics and factors were analysed for periods 1961-1990 and 1991-2018. Correlation coefficients showed the influence of the catchment area on the intermittency indices: as the catchment area increases, the likelihood of drying out decreases. Comparison of low flow parameters in two periods (1961-1990 and 1991-2018) showed that low flow is affected by climate changes in last decades and risk of river intermittency will have increasing tendency.