# Panta Rhei

## Young Water Fractions of Global Rivers

##### BY CHUNLIN SONG

Recently, a paper entitled Substantial proportion of global streamflow less than three months old was published by Nature Geoscience. Jasechko et al. shown that about 1/3 of global streamflow are young water, as defined in the paper, 2.3 ± 0.8 months.

The foundation of this study is based on two newly published companion papers in the journal Hydrology and Earth System Sciences by James W. Kirchner. James criticized the traditional approach of using seasonal cycles in chemical or isotopic tracers to estimate mean transit time (MTT) of stream water. He proved in his long papers with mathematics deduction that these calculations are mostly wrong by several hundred percent because of aggregation bias. So where can we go next? While, James proposed an alternative storage metric, the young water fraction (Fyw). Fyw is a better metric than MTT because Fyw can be accurately estimated from the amplitude ratio of seasonal tracer cycles in precipitation and runoff within a precision of a few percent. Fyw has much less uncertainty than the MTT thus more reliably for the heterogeneous real world catchments.

Jasechko et al. compiled a global precipitation and streamflow isotope database which includes 254 rivers. With solid methodology by James W. Kirchner, they conducted periodic regression analyses with the isotope data to get $$\delta$$18O cycle amplitudes of precipitation and streamwater. After that, they calculated the (Fyw) of each catchment by dividing the river $$\delta$$18O cycle amplitude by the precipitation $$\delta$$18O cycle amplitude. Also, according to the paper, there is a negetive relationship of young water fraction and catchment topographic gradient.

The time spent by river water is crucial for predicting the retention, mobility and fate of solutes, nutrients and contaminants. This study tells us that young streamflow represents roughly one-third of global river runoff. It means even watersheds with long MTT can transmit substantial fractions of soluble contaminant inputs to aquatic systems in very short time spans. Besides, the rivers of plain areas may have more young water than the rivers in mountainous areas. Since young water are mainly derived from a thin layer at the top of the aquifer, this paper conclude that the small fraction of continental aquifer will have disproportionate influence on stream water quality.

More and more deep groundwater is likely to be extracted for expanded population, I’m wondering will this change the young water fraction? Since we already know the older water fraction (one minus Fyw, older than three month) of global river, what are the age composition of the old water fraction? What is the fraction of older water over one year or ten years old? Where is the older water come from? To what extent the older water impact water quality?

More information: Kirchner, J. W. Aggregation in environmental systems – Part 1: Seasonal tracer cycles quantify young water fractions, but not mean transit times, in spatially heterogeneous catchments, Hydrol. Earth Syst. Sci., 20, 279-297, doi:10.5194/hess-20-279-2016, 2016.

Kirchner, J. W. Aggregation in environmental systems – Part 2: Catchment mean transit times and young water fractions under hydrologic nonstationarity, Hydrol. Earth Syst. Sci., 20, 299-328, doi:10.5194/hess-20-299-2016, 2016.

Scott Jasechko et al. Substantial proportion of global streamflow less than three months old. Nature Geoscience 9, 126–129 (2016), doi:10.1038/ngeo2636.

categories: HYDROLOGY