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 on the basis of 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 1961–2000. Within the period 1981–2000 alone, the trend of decreasing runoff is barely noticeable. 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 water courses 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.

After two years with abundant quantities of water in Slovenia, the total annual water runoff in 2015 amounted to only 12,045 million m³ (382 m³/s). In the period 1961–2015, only four years were drier than 2015 (2003, 2011, 1983 and 2007). Although temperatures and precipitation exceeded the long-term average in 2015 (for more details see: T. Cegnar, 2015, Podnebne značilnosti leta 2015, Naše okolje, letnik XXII, št. 12; Agrometeorološki pregled leta 2015, Naše okolje, letnik XXII, št. 12), no catastrophic hydrological drought (in which river discharges were near or even below historic recorded minimums and the majority of Slovenia's territory would be affected), was recorded. The most affected by summer drought was south-western Slovenia, while in most of the country, the drought was periodically interrupted by summer thunderstorms. December was hydrologically dry as well. Droughts in October did not exceed annual return periods.

(further reading: I. Strojan, Hidrološko suho in toplo leto 2015, http://www.sos112.si/slo/tdocs/ujma/2016/030-038.pdf).

The periodical trend decreasing river discharge persists.