Latest study confirms that more rain does not necessarily mean more river water

More rainfall does not necessarily translate into greater river flows, with a global study estabilishing that river discharge and precipitation trends have diverged across more than a quarter of the rivers analysed.

Researchers from Pennsylvania State University examined long-term precipitation and river-flow records from 10,179 sites worldwide between 1951 and 2020. In a study titled, Widespread Divergence Between Precipitation and Discharge Trends Across Global Rivers, researchers found that 27 per cent of the sites showed divergent trends, meaning changes in river discharge did not correspond with changes in precipitation.

Of the rivers with diverging trends, 40 per cent experienced declining discharge despite receiving more precipitation. The researchers found that the magnitude of this mismatch increased with aridity.

The study, published in Geophysical Research Letters on 26 August, suggests that precipitation alone is not enough to determine how much water ultimately reaches a river. The researchers identified the frequency of low- or high-precipitation days and the proportion of precipitation that becomes runoff as important factors behind the divergence.

The findings challenge the assumption that an increase in rainfall or snowfall will automatically result in greater river flows.

In some regions, more water may be lost through evaporation or may not translate into runoff reaching rivers.

The study also found that river discharge declined at 48 per cent of the sites examined, while precipitation declined at 52 per cent, highlighting that the two variables do not always move in tandem.

The researchers said understanding factors beyond precipitation will be important for predicting future freshwater availability as the climate changes.

Rivers and freshwater lakes are particularly important for human water supplies. The Guardian reported that large cities obtain nearly 80 per cent of their water from surface-water sources.

The study therefore points to the importance of monitoring river discharge alongside precipitation rather than using rainfall trends alone to assess future water availability.

Major findings

More than a quarter of the sites studied — 27.4 per cent — showed diverging trends in precipitation and river discharge, meaning the two changed in opposite directions. North America recorded the highest share at 28.5 per cent, followed by Europe at 27.9 per cent and South America at 22.4 per cent, while Australia had the lowest at 12.3 per cent.

Globally, 16.4 per cent of sites saw river discharge increase even as precipitation declined, while 11 per cent recorded declining river discharge despite increasing precipitation. The latter pattern was most common in Europe, particularly in Great Britain, Switzerland and Austria.

The study also found that the divergence between rainfall and river flow was generally stronger in more arid locations, suggesting that precipitation trends alone cannot reliably indicate how river discharge will change.