Waters flowing from surface rivers and streams efficiently transport sediments, organic matter, and nutrients, inter alia, from hills and land areas to downstream lakes, reservoirs, and the ocean. Along the way, rivers and streams (hereinafter collectively referred to as streams) provide important resources for our communities and support rich and complex ecosystems. Non-perennial flows, which do not flow throughout the year, are essential in this context. However, because non-perennial streams are less reliable sources of surface water than perennial ones, they are less studied than their perennial counterparts. Writing in Nature, The Messenger ETC..1 provide a much-needed estimate of the total percentage of the world stream network, by length, that is non-perennial, and find that most fall into this category.
Messager and their colleagues combined flow data from countries around the world with information describing hydrology, climate, physical geography, and land cover at those sites to model the probability that water would not flow for at least one day a year. They then extended their forecasts to all power segments recorded in a global flow network database (RiverATLAS)2.
The authors report that 5160% of world streams do not flow for at least one day a year, and that 4453% of global current length is dry for at least a month (about 30 days) each year. Their modeling shows that non-perennial flows occur in all climates and biomes on each continent (see Fig. 1 of the paper1) The model also shows that 95% of the power grid in hot and dry regions representing 10% of the global land dries up every year (Fig. 1). Surprisingly, even segments of major rivers, such as the Niger River in West Africa, are projected to dry up in these arid regions. The high prevalence of non-perennial streams in such places highlights how even non-continuously flowing streams significantly affect water availability and water quality. The results highlight the need for more detailed mapping of perennial and non-perennial flows at the regional and local scales, and for further studies on how non-perennial flows affect overall water availability and quality.
Small streams of water sources (those without branches) make up 7080% of the length of streams worldwide3, similar to the way in which the collective length of those fingers is much greater than the length of the palm of the hand. The Messager model and co-workers predict that, even in the wettest regions, such as the Amazon River Basin and parts of Central Africa and Southeast Asia, up to 35% of these runoff stops flowing at some point in the year. However, it should be noted that watercourses are monitored by relatively few stream meters, which tend to be located in larger, perennial downstream rivers. Therefore, the model can provide very uncertain estimates for upstream regions of flow networks.
Lack of flow data is a common problem for modeling watercourses, and so data collection efforts are being implemented to fill this knowledge gap. For example, France has developed the Observatoire National des tiages (ONDE) network, which complements the national electricity metering network but focuses on watercourses. However, these programs are costly and require significant resource investments.
Current meters are also scarce for non-perennial flows in general. In the analysis of Messager and colleagues, for example, there was no meter in non-perennial currents in Argentina; only one in New Zealand; and 10 in the United States North Pacific, from a network of 250 meters. Improving models designing perennial and non-perennial flows will require low-cost field observations, coupled with the development of high-resolution remote sensing technology that often detects or at least predicts surface flow in streams.
Message and contributor analysis provides a robust, quantitative confirmation of the ubiquity of non-perennial rivers. Their results show the need for a fundamental change in the fields of science and management of rivers and streams, in which non-perennial streams are largely bypassed.4. In arid regions, the predominance of non-perennial streams can be a major driver of water availability and quality. And in areas where man-made services are not available, ecosystem services such as streams are used to meet basic needs and, in part, will determine the well-being and prosperity of the people in that area.5. The new findings, therefore, illuminate the need for global calculation of perennial and non-perennial flows.
Furthermore, changes in stream distribution can have far-reaching effects on carbon and biogenic cycles on a global and continental scale6, and on the survival of organisms living in streams, including many endangered species7. A global benchmark for the prevalence of perennial and non-perennial flows is essential for assessing the effects of future changes on their distribution associated with climate change and land use. Finally, regional and local flow models are needed, as well as better data on water resources and non-perennial parts of the flow network, to further enhance the value of global models.
The author does not declare competing interests.
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