The long-term role of mountains as water providers to lowlands is threatened by shrinking glaciers due to anthropogenic climate change. Modelling this dependence and uncovering past indigenous responses can inform adaptive responses.
Controlling water has been a cornerstone of flourishing civilizations1. However, humanity’s historical success at accessing mountain water is being challenged by soaring populations and development coupled with climate-driven decreases in montane glaciers and meltwater and increases in rainfall variability2. Understanding how much, when and where water will be available is vital. In this issue of Nature Sustainability, two studies address these questions for areas downstream of mountains; one showcasing the power of twenty-first century analytical tools and another the potential of indigenous engineering acumen.
In so doing, these papers inadvertently dialogue about the mountain–water–agriculture nexus through cases thousands of kilometres and hundreds of years apart. Biemans et al.3 report that decreasing meltwater will diminish the water supply to the Indo-Gangetic Plain (see Fig. 1), compromising the livelihoods of 129 million farmers and the system’s capacity to modulate monsoonal and riverine fluctuations. However, this dire prospect is brightened as Ochoa-Tocachi et al.4 analyse an ancient Peruvian indigenous system in which upstream water was canalled to hillslopes during the wet season, where it infiltrated and subsequently recharged natural springs downstream. These flows were regulated by a network of water bodies (that is, ponds), with the entire system using the water’s trip through the mountain to delay its movement downstream. This system extended crop-growing periods by increasing dry-season water availability.
Understanding how montane water resources affect crops and people requires considering multiple factors together, from hydrogeological processes to ecological and social dynamics. This challenge is heightened by our limited understanding of montane ecosystems, human systems and the interactions between them5. To analyse when and where Himalayan meltwater impacts irrigation, Biemans et al. coupled simulations of cryospheric–hydrological dynamics; downstream water availability, crop production and water demand; and crop yields. Through simulations of the resulting crop carbon assimilation and associated growth, the study quantitatively differentiates in which growing stage key crops will be using meltwater, realistically coupling crop timing to decreasing meltwater dynamics. In the Indus Basin, for example, the authors found that 9% of wheat, 15% of rice, 28% of cotton and 17% of sugarcane relies on meltwater3.
On the other side of the world, Ochoa-Tocachi et al. offer an assessment of a pre-Inca water-infiltration system, contextualizing findings for the Himalayas today with those of the Andes yesteryear. The authors combined participatory mapping — focus groups, interviews and surveys — with hydrological monitoring and used dye tracers to track water flows from mountain canals through the earth to the downstream springs. They discovered that this ancient system worked as designed, increasing spring flows and thus water availability during the dry season. Like the Himalayan meltwater, the infiltration system helps water users cope with seasonal, and now climatic, water availability and lengthens the crop-growing season. It also highlights the human potential for innovation to adapt to changing water circumstances.
How these papers consider human factors is important. Biemans et al.’s model varies water availability while assuming constant technology and human behaviour6. This limitation, while perhaps reasonable, needs to be considered to ensure that resulting policy considers the stressors that threaten water security and the opportunities for change7. Crop systems and human communities co-evolve with their social and ecological conditions, such as available water and land. Possible responses may include, for instance, introducing water-stress-tolerant crops and limiting agribusiness expansion. Further, powerful non-climatic pressures in the highlands, such as mining and hydropower plants, and in the lowlands, such as urbanization and industry, compete for water and must be considered in overarching models and policies regulating water use8.
Ochoa-Tocachi et al.’s study focuses on a past human adaptive response and its present potential, but does not query the social arrangements that allowed it then and those that would be required now. The authors estimate that the technology could recover 99 × 106 m3 yr–1 in the Rimac basin, which houses Lima, Peru’s capital (with about 10 million inhabitants). Whether current social conditions are able to support this technology remains to be assessed. The authors do not analyse the social aspects that enabled the development and maintenance of such technology, a shortcoming and perhaps a focus for future work. Indeed, a social–hydrological perspective could present other crucial questions about the efficacy of innovations more generally and about this technology specifically. For example, understanding why 30 ponds and 19 ancient infiltration canals were abandoned or clogged in the first place is crucial before we can recommend scaling up indigenous technologies. The sustainability of technological replicas rests largely on having the social capacity and structure to build, maintain, and manage such infrastructure.
The dialogue between these papers suggests potential for cross-fertilization, though the new research questions would be challenging. For example, the appeal of replicating and scaling out the Himalayan analysis to other meltwater-dependent basins, such as the Andes, begs questions about scale and resolution. Which basin size(s) are most suitable? What are the trade-offs of using other grid resolutions? Likewise, the potential of the Andean technique to be scaled-up there or applied elsewhere hinges on digging more deeply into the requisite sustainable sociotechnical arrangements and regional differences in hydrology. They also pique interest into how ancient Himalayan societies responded to water shortages and whether those approaches could be adopted, adapted and scaled now. It is worth taking these issues and questions seriously.
While water shortages are not a new problem for human society, climate change, population expansion and growing affluence are compounding this historic challenge7,9. Taken together, Biemans et al. and Ochoa-Tocachi et al. illustrate the importance of projecting future cryosphere impacts in order to be prepared while also not losing sight of past responses. Ancient innovations may help address future shortages. These studies also illustrate the potential of combining insights from indigenous and scientific-knowledge systems and from integrating social scientists, natural scientists and practitioners to generate policies for adapting to water scarcity.
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Postigo, J.C. Bridging past and future to address water stress. Nat Sustain 2, 543–544 (2019). https://doi.org/10.1038/s41893-019-0333-z
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