This indicator shows the annual river balance of the Republic of Slovenia as a whole. The balance is composed of the river water inflow and runoff expressed in million m3/year. Both elements are calculated based on mean annual discharge (flow, Qs) at the water measuring stations catching the majority of inflow and runoff of river water in and from Slovenia. The river balance of Slovenia represents one of the fundamental and more dynamic elements of the Slovenian water balance, which is also composed of precipitation, evaporation, changes in groundwater reserves and water use. Discharge measurements are reliable and well-established, meaning a longer time series of comparable data is available. Based on the accurate assessment of direct anthropogenic effects on the river regime, the river balance may serve as a good indicator of potential effects of climate change on the quantities of available water.

Due to a great annual variability of river discharges, a longer time series of data is required for trend assessment. According to the operation of relevant water gauging stations, the most appropriate time series would be 1961–2020. Throughout the period, a declining trend in total river runoff was observed. In the thirty-year period 1971–2000, the decline in river runoff is obvious. In the period 2000 - 2021, there is a characteristic deviation from the average in both directions, and the general tendency of decreasing runoff is less pronounced. The year 2022 was again markedly below-average, as the net river-runoff from Slovenia represented just under 73 percent of the average in the comparison period 1991–2020 and just over 70 percent of the average net river runoff in the comparison period 1961–2020. River runoff trends indirectly indicate an increased or decreased probability of low water occurrence (drought) and flood risk. However, annual river runoff trends do not always correspond to trends of high or low water level.

Most Slovenian rivers have torrential characteristics. This means that discharges tend to increase and decrease very quickly and that most water runs off at high water levels or during flooding. In most rivers, high water occurs in spring and autumn, usually as a result of thaws, snow melting and abundant precipitation. Intensive local precipitation causes local flooding of a torrential character that only affect smaller catchment areas. Torrential watercourses are particularly vulnerable during periods of low discharge and hydrological drought, which are not distinctly seasonal and usually occur in late summer or early autumn. Hydrological droughts coincide with long periods of below-average precipitation and high air temperatures, which also results in agricultural droughts. As ecologically acceptable river discharges are maintained during such periods, shortages of irrigation water as well as deterioration of water quality may occur.

The above-average years 2013 and 2014 were followed by below-average year 2015 and average period 2016 – 2021 of annual runoff. In the year 2022 annual runoff was below-average. In the period 2016 – 2021, the driest years were 2003, 2011, 1983, 2022, 2007 and 2015. The downward trend of river runoff from the territory of Slovenian currently persists.