Twente Water Centre, University of Twente, 7500 AE Enschede, The Netherlands;
Institute of Water Policy, Lee Kuan Yew School of Public Policy, National University of Singapore, 259770, Singapore.
Proc Natl Acad Sci U S A. 2019 Mar 12;116(11):4893-4898. doi: 10.1073/pnas.1817380116. Epub 2019 Feb 25.
Green water--rainfall over land that eventually flows back to the atmosphere as evapotranspiration--is the main source of water to produce food, feed, fiber, timber, and bioenergy. To understand how freshwater scarcity constrains production of these goods, we need to consider limits to the green water footprint (WF), the green water flow allocated to human society. However, research traditionally focuses on scarcity of blue water--groundwater and surface water. Here we expand the debate on water scarcity by considering green water scarcity (WS). At 5 × 5 arc-minute spatial resolution, we quantify WF and the maximum sustainable level to this footprint (WF), while accounting for green water requirements to support biodiversity. We then estimate WS per country as the ratio of the national aggregate WF to the national aggregate WF We find that globally WF amounts to 56% of WF, and overshoots it in several places, for example in countries in Europe, Central America, the Middle East, and South Asia. The sustainably available green water flows in these countries are mostly or fully allocated to human activities (predominately agriculture and forestry), occasionally at the cost of green water flows earmarked for nature. By ignoring limits to the growing human WF, we risk further loss of ecosystem values that depend on the remaining untouched green water flows. We emphasize that green water is a critical and limited resource that should explicitly be part of any assessment of water scarcity, food security, or bioenergy potential.
绿水——降落在陆地上最终以蒸散形式返回大气的降水——是生产食物、饲料、纤维、木材和生物能源的主要水资源。为了了解淡水短缺如何限制这些商品的生产，我们需要考虑绿水足迹（WF）的限制，即分配给人类社会的绿水流量。然而，传统的研究侧重于蓝水——地下水和地表水的稀缺性。在这里，我们通过考虑绿水稀缺性（WS）来扩展关于水资源稀缺性的辩论。我们以 5×5 弧分的空间分辨率，量化了 WF 和这个足迹的最大可持续水平（WF），同时考虑了支持生物多样性的绿水需求。然后，我们根据国家总 WF 与国家总 WF 的比例来估计每个国家的 WS。我们发现，全球 WF 占 WF 的 56%，在一些地方超过了这个比例，例如欧洲、中美洲、中东和南亚的一些国家。这些国家可持续利用的绿水流量大部分或全部分配给了人类活动（主要是农业和林业），偶尔会以牺牲用于自然的绿水流量为代价。如果忽视不断增长的人类 WF 的限制，我们有可能进一步丧失依赖于未受干扰的绿水流量的生态系统价值。我们强调，绿水是一种关键且有限的资源，应该明确成为任何水资源短缺、粮食安全或生物能源潜力评估的一部分。
