Reviewing the evidence: How do nature-based solutions affect water flows in agriculture and rangelands

Water security presents a crucial development challenge, with water shortages and floods leading to economic losses, loss of life and livelihoods, and impacts to resources. Nature-based solutions (NBSs) may complement or in some cases even substitute for business-as-usual approaches to addressing water insecurity without some of the failure points of "grey" infrastructure. However, large-scale application of NBSs are limited by a lack of knowledge regarding their effects on water quantity. We present a novel review of evidence on the water quantity effects of NBSs conducted through the Science for Nature and People Partnership (SNAPP). We systematically review the literature for empirical evidence of the effects of rangeland and agriculture restoration and best-management practices on streamflow, soil moisture, infiltration, and groundwater recharge at multiple timescales. This is the first such review to our knowledge, and the database will be made publicly available. We also summarize previous reviews of water quantity changes from forest and wetland protection and restoration for comparison. Finally, we develop a conceptual model to describe the synthesized results and clarify them for practitioners.

We find that NBSs can have the desired effects of increasing infiltration and reducing surface runoff locally, but that there is little published evidence for significant water quantity effects from NBSs at the regional scale. We attribute this to the current limited implementation scale of most NBSs, which modify ecosystem properties less widely than would be needed to measurably change regional water flows given the variability of climatic drivers. As expected due to ET effects of increased vegetation, we find that most NBSs reduce total discharge but can shift the location and timing of water fluxes. Further, we find somewhat more variability than expected in the response to many agricultural and rangeland BMPs, including some small-magnitude reductions in infiltration or local water availability. This synthesis of knowledge and tools will be a valuable resource for future application of NBSs, and reveals knowledge gaps to guide future research.