A review of the empirical evidence
Our research confirmed that context matters. Surface and groundwater resources (in locations & quantities where people can take advantage of them) are generated by a complex interaction of weather, vegetation, soils, geology, and terrain, and nature-based solutions can only influence some of these factors. Restoring forests and wetlands can often help to reduce peak flows, and implementing best practices in production lands can improve water retention in soils, making them more drought-resistant.
Information gaps
We found that most field studies are not measuring and tracking the right metrics at a scale to match the ambition of proponents of nature-based solutions. Existing evidence helps with predictions of how small-scale projects can affect local water cycling in some geographies, but we still have a lot to learn about how a growing number of small-scale tree-planting efforts or other commonly promoted nature-based solutions in agriculture or rangelands will scale up to influence regional downstream water flows.
Some basic guidance
The science is clear that protection of intact natural ecosystems is much more effective at delivering water security benefits than restoring an ecosystem once it’s been lost. We also know that the scale of implementation needs to match the scale of ambition — we cannot expect to improve water security for downstream populations or industries through small investments in nature upstream. If we invest as much in the science-based design of specific nature-based projects as we would in conventional engineered water infrastructure, we can make informed decisions with the best combination of approaches.