Groundwater is the most extracted natural resource in the planet and it provides around 45% of human freshwater needs (Eckstein 2017). Yet, the understanding and comprehension of our dependence on groundwater and its interactions with natural and human systems, is as limited and superficial as the existent — or nonexistent — regulatory and management systems in place.

More than a decade ago, the World Bank estimated that all easily available surface water and exclusively national sources of water (groundwater) have already been developed or are in the process of development (Varis et al. 2008). Consequently, approximately 40% of the world’s population rely on transboundary river/aquifer basins to supply their water needs (Eckstein 2017).

As competition for fresh water increases, border communities around the world are facing growth in the dependence, reliance, risk, conflict, and cooperation with their neighbors. Water security is not a new challenge for these regions; however, resilience and sustainability risks coupled with water scarcity, climate change, environmental degradation, and population growth have placed additional pressure on the level of threats to water security in border regions around the world. The strategic and geopolitical value of groundwater resources located across political boundaries, as well as the physical complexity intrinsically attached to transboundary groundwater systems, encompasses critical considerations at the technical and institutional level. Border communities that depend on shared water resources around the world face the highest threat in terms of water security given deficiencies on the policy development for the management of shared water resources, especially groundwater (Varis et al. 2008). Currently, groundwater governance regimes seem to be limited or at the very least, not prepared to address the transboundary nature or the transboundariness , of a growing dependable resource traditionally assumed to have a never‐ending supply.

Water resources in the border region between Mexico and the United States (U.S.) have historically been a matter of national security. In a region with around 15 million inhabitants, the risks associated with shared surface and groundwater resources, have escalated over the last decades as water scarcity, extreme drought and flood events, and population growth have put serious pressure on the availability of water resources and therefore the resilience of border communities. Public health threats associated with degrading water quality, lack of research and data on current groundwater conditions, limited binational efforts on holistic basin water management, and unsustainable growing population centers highly dependent on groundwater resources, are some of the most pressing water security threats in the region (Sanchez et al. 2016). Additionally, the vulnerability of border communities, particularly toward the easternmost part of the border, is exacerbated by the less than state average standards in education, public health, poverty, security, and income per capita (US Census Bureau 2018). Water security in the border region as well as water policy development becomes then a multidimensional, transboundary challenge as environmental, socioeconomic, and hydrogeological conditions tend to be more related and responsive to a transboundary regional logic than to a national jurisdiction or national policy.

In the border region between the U.S. and Texas, particularly in the Middle Rio Grande (from El Paso to Del Rio), groundwater is the major source of water for domestic and agriculture use. Contrastingly, in the Lower Rio Grande (from Del Rio to Brownsville), surface water becomes the main source of water for agriculture, and groundwater remains the primary source for domestic and potable use for urban centers. Because surface sources of water (Rio Grande River) are fully allocated and drought conditions are becoming the norm throughout the region, groundwater has become the alternative new source of water, though extraction and treatment costs are highly variable across the basin. In the border region between New Mexico, Arizona, and Mexico, groundwater dependency for border communities and economic development is even more critical. For example, groundwater supply for all types of uses for border communities that overlay the aquifer basins of the Mimbres in New Mexico, and Santa Cruz and San Pedro in Arizona, can be close to 90%. In the case of California, groundwater dependency for the Mexicali Valley is a well‐known study case that has proven the sensitive connectivity of the Colorado River to the regional transboundary aquifers, and therefore water intensive agriculture and protected areas that require minimum base flow for its sustainability, have been historically important drivers of binational negotiations over shared water resources.

Currently, there is limited and in some cases, nonexistent reliable information regarding the location, boundaries, and condition of transboundary aquifer systems between Mexico and the U.S. Current research suggests there could be up to 36 transboundary aquifers located at the border between Mexico and the U.S. (Sanchez et al. 2016). However, studies that are more recent have proven that between Mexico and Texas only, there may be at least 25 transboundary aquifers (Sanchez et al. 2018). These findings demonstrate that the number of purely physical unknowns supersede by far what the current research has been able to accomplish. Aside from the undisputable need for more research and funding to address the data lacking on the physical conditions of the groundwater systems that traverse political borders, the key challenge stems from the reliance on antiquated and poorly developed institutional and regulatory frameworks that do not acknowledge the shared nature of groundwater resources in their policies and management practices.

The power of the unseen, uncertainty of the models, limited regulation, disagreement on criteria and methods of boundary delineation, as well as limited data, and sharing mechanisms (mostly at international level), irremediably generates a negligent environment toward the sustainability of transboundary aquifers.

This feature collection attempts to paint a portrait of some challenges that particular regions within the U.S. and across its southern border with Mexico are currently facing as transboundary aquifer systems transform jurisdictions into virtual barriers that are urging institutions and legal regimes to respond to pressing conditions and potential vulnerabilities of shared groundwater systems. Additionally, this edition attempts to offer a variety of options, recommendations, and perspectives to understand, and address transboundary water issues under the current legal constraints and looking toward potential adaptation to worsening water conditions in those regions highly dependent on groundwater for economic and population growth.

Contributing authors provide interesting legal and institutional assessments both at domestic and international level, to highlight innovative modeling and technical capabilities, as well as new approaches to address the challenges that this research area has just begun to call the attention of researchers and authorities at domestic and international level. The analyses provided do not pretend to propose an exhaustive number of study cases nor its complete implications. The reality of challenges that remain to be addressed, understand, and even discover in the groundwater flow systems across political boundaries, extend beyond our scientific endeavors, but papers in this collection do provide important guidelines, technical simulations, and experiences worth sharing that might be applicable to other regions of the world. In addition, our objective is to contribute to the growing empirical literature of the topic in much need of development.

We believe that the studies presented here provide a good starting point to understand the multiple dimensions and complex nature of transboundary aquifers along the U.S.–Mexico border that will help fuel future research and inform policy dialog across management institutions in both countries that share this hidden yet valuable resource.

地下水是地球上提取最多的自然资源，可满足人类淡水需求的45％（Eckstein，2017年）。但是，对我们对地下水及其与自然系统和人类系统的相互作用的依赖的理解和理解，与现有（或不存在的）监管和管理系统一样有限和肤浅。

十多年前，世界银行估计，所有容易获得的地表水和国家专用水（地下水）都已经开发或正在开发中（Varis等，2008）。因此，世界上约40％的人口依靠跨界河流/含水盆地来满足其用水需求（Eckstein，2017年）。

随着对淡水的竞争加剧，世界各地的边境社区正面临着依赖，依赖，风险，冲突以及与邻国合作的增长。对于这些地区来说，水安全并不是一个新的挑战。但是，抵御力和可持续性风险，再加上水资源短缺，气候变化，环境恶化和人口增长，给世界各地边境地区的水安全威胁带来了更大的压力。跨越政治边界的地下水资源的战略和地缘政治价值，以及跨界地下水系统固有的物理复杂性，在技术和机构层面都包含重要的考虑因素。2008）。目前，地下水治理制度似乎被限制或至少是不准备解决跨界性质或transboundariness，传统上认为有一个永无止境的供应越来越可靠的资源。

墨西哥和美国（美国）之间的边界地区的水资源历来是国家安全问题。在过去的几十年中，由于人口稀少，极端干旱和洪水事件以及人口增长，对拥有约1500万居民的地区而言，与地表和地下水共享的风险日益加剧，给水资源的可获得性带来了巨大压力，因此边境社区的复原力。与水质下降有关的公共卫生威胁，缺乏有关当前地下水状况的研究和数据，两国在整体流域水管理方面的努力有限以及高度依赖地下水资源的不可持续的人口增长中心，是该区域最紧迫的水安全威胁。地区（Sanchez等2016）。此外，由于教育，公共卫生，贫困，安全和人均收入低于州平均水平，边境社区尤其是边境最东部地区的脆弱性进一步加剧（美国人口普查局2018年）。由于与国家管辖权或国家政策相比，环境，社会经济和水文地质条件与跨界区域逻辑之间的相关性和响应性往往更高，因此边境地区的水安全以及水政策的制定成为一个多维，跨界的挑战。

在美国和德克萨斯州之间的边界地区，特别是在中里奥格兰德州（从埃尔帕索到德尔里约），地下水是家庭和农业用水的主要来源。相反，在下里奥格兰德州（从德尔里奥到布朗斯维尔），地表水成为农业用水的主要来源，而地下水仍然是城市中心家庭和饮用水的主要来源。由于地表水源（里奥格兰德河）已得到充分分配，干旱条件正在成为整个地区的常态，因此地下水已成为替代性新的水源，尽管整个流域的开采和处理成本变化很大。在新墨西哥州，亚利桑那州和墨西哥之间的边界地区，对边界社区和经济发展的地下水依赖性更加重要。例如，覆盖新墨西哥州Mimbres含水层盆地以及亚利桑那州圣克鲁斯和圣佩德罗的边界社区所有类型用途的地下水供应量将接近90％。在加利福尼亚州，墨西卡利河谷对地下水的依赖性是一个著名的研究案例，已经证明了科罗拉多河与区域跨界含水层之间的敏感连通性，因此，水力密集型农业和保护区要求其最低基础流量可持续性一直是两国就共享水资源进行谈判的重要推动力。

当前，关于墨西哥和美国之间跨界含水层系统的位置，边界和状况的可靠信息有限，在某些情况下还不存在，目前的研究表明，在墨西哥和美国之间的边界可能有多达36个跨界含水层。 （Sanchez等人2016）。但是，最近的研究证明仅在墨西哥和德克萨斯州之间，可能至少有25个跨界含水层（Sanchez等人，2018年）。）。这些发现表明，纯物理未知数的数量远远超过了当前研究已能够完成的数量。无可争议的是，除了需要更多的研究和资金来解决缺乏穿越政治边界的地下水系统物理条件的数据外，主要的挑战还来自对过时且发展欠佳的机构和监管框架的依赖，而这些框架和框架并没有承认共同点。政策和管理实践中地下水资源的性质。

看不见的力量，模型的不确定性，有限的法规，对边界划定的标准和方法的分歧以及有限的数据以及共享机制（主要是国际一级的），不可避免地为跨界含水层的可持续性产生了疏忽的环境。 。

此功能集试图描绘一些特定挑战的画像，因为跨界含水层系统将辖区转变为虚拟壁垒，促使美国机构及其法律制度对紧迫的条件和潜力做出回应，从而在美国及其与墨西哥南部接壤的整个南部地区面临某些挑战。共享地下水系统的脆弱性。此外，该版本试图提供各种选择，建议和观点，以了解和解决当前法律约束下的跨界水问题，并在可能高度依赖地下水以促进经济和人口增长的地区，寻求潜在的适应日益恶化的水状况的方法。

投稿作者在国内外提供了有趣的法律和机构评估，以强调创新的建模和技术能力，以及应对该研究领域刚刚开始引起国内外研究者和当局注意的挑战的新方法。国际水平。提供的分析并不假装提出大量的研究案例，也不暗示其完整的含义。在跨政治边界的地下水流系统中，仍然有待解决，理解甚至发现的挑战的现实，超出了我们的科学努力，但本系列中的论文确实提供了重要的指导方针，技术模拟和值得分享的经验适用于世界其他地区。此外，

我们认为，此处介绍的研究为理解美墨边境跨界含水层的多个维度和复杂性质提供了一个很好的起点，这将有助于推动未来的研究，并为两国共享这一隐蔽而又尚未解决的管理机构之间的政策对话提供信息宝贵的资源。