Pamela A. Green

Global threats to human water security and river biodiversity

Protecting the world’s freshwater resources requires diagnosing threats over a broad range of scales, from global to local. Here we present the first worldwide synthesis to jointly consider human and biodiversity perspectives on water security using a spatial framework that quantifies multiple stressors and accounts for downstream impacts. We find that nearly 80% of the world’s population is exposed to high levels of threat to water security. Massive investment in water technology enables rich nations to offset high stressor levels without remedying their underlying causes, whereas less wealthy nations remain vulnerable. A similar lack of precautionary investment jeopardizes biodiversity, with habitats associated with 65% of continental discharge classified as moderately to highly threatened. The cumulative threat framework offers a tool for prioritizing policy and management responses to this crisis, and underscores the necessity of limiting threats at their source instead of through costly remediation of symptoms in order to assure global water security for both humans and freshwater biodiversity.

Anthropogenic sediment retention: major global impact from registered river impoundments

In this paper, we develop and apply a framework for estimating the potential global-scale impact of reservoir construction on riverine sediment transport to the ocean. Using this framework, we discern a large, global-scale, and growing impact from anthropogenic impoundment. Our study links information on 633 of the worlds largest reservoirs (LRs) (≥0.5 km3 maximum storage capacity) to the geography of continental discharge and uses statistical inferences to assess the potential impact of the remaining >44,000 smaller reservoirs (SRs). Information on the LRs was linked to a digitized river network at 30′ (latitude×longitude) spatial resolution. A residence time change (ΔτR) for otherwise free-flowing river water is determined locally for each reservoir and used with a sediment retention function to predict the proportion of incident sediment flux trapped within each impoundment. The discharge-weighted mean ΔτR for individual impoundments distributed across the globe is 0.21 years for LRs and 0.011 years for SRs. More than 40% of global river discharge is intercepted locally by the LRs analyzed here, and a significant proportion (≈70%) of this discharge maintains a theoretical sediment trapping efficiency in excess of 50%. Half of all discharge entering LRs shows a local sediment trapping efficiency of 80% or more. Analysis of the recent history of river impoundment reveals that between 1950 and 1968, there was tripling from 5% to 15% in global LR sediment trapping, another doubling to 30% by 1985, and stabilization thereafter. Several large basins such as the Colorado and Nile show nearly complete trapping due to large reservoir construction and flow diversion. From the standpoint of sediment retention rates, the most heavily regulated drainage basins reside in Europe. North America, Africa, and Australia/Oceania are also strongly affected by LRs. Globally, greater than 50% of basin-scale sediment flux in regulated basins is potentially trapped in artificial impoundments, with a discharge-weighted sediment trapping due to LRs of 30%, and an additional contribution of 23% from SRs. If we consider both regulated and unregulated basins, the interception of global sediment flux by all registered reservoirs (n≈45,000) is conservatively placed at 4–5 Gt year−1 or 25–30% of the total. There is an additional but unknown impact due to still smaller unregistered impoundments (n≈800,000). Our results demonstrate that river impoundment should now be considered explicitly in global elemental flux studies, such as for water, sediment, carbon, and nutrients. From a global change perspective, the long-term impact of such hydraulic engineering works on the worlds coastal zone appears to be significant but has yet to be fully elucidated.

Pre-industrial and contemporary fluxes of nitrogen through rivers: a global assessment based on typology

This paper provides a global synthesis of reactive nitrogen (Nr) loading to the continental landmass and subsequent riverine nitrogen fluxes under a gradient of anthropogenic disturbance, from pre-industrial to contemporary. A mass balance model of nitrogen loading to the landmass is employed to account for transfers of Nr between atmospheric input sources (as food and feed products) and subsequent consumer output loads. This calculation produces a gridded surface of nitrogen loading ultimately mobilizable to aquatic systems (Nmob). Compared to the pre-industrial condition, nitrogen loading to the landmass has doubled from 111 to 223 Tg/year due to anthropogenic activities. This is particularly evident in the industrialized areas of the globe where contemporary levels of nitrogen loading have increased up to 6-fold in many areas. The quantity of nitrogen loaded to the landscape has shifted from a chiefly fixation-based system (89% of total loads) in the pre-industrial state to a heterogeneous mix in contemporary times where fertilizer (15%), livestock (24%) and atmospheric deposition (15%) dominate in many parts of the industrialized and developing world. A nitrogen transport model is developed from a global database of drainage basin characteristics and a comprehensive compendium of river chemistry observations. The model utilizes constituent delivery coefficients based on basin temperature and hydraulic residence times in soils, rivers, lakes and reservoirs to transport nitrogen loads to river mouths. Fluxes are estimated for total nitrogen, dissolved inorganic nitrogen, and total organic nitrogen. Model results show that total nitrogen fluxes from river basins have doubled from 21 Tg/year in the pre-industrial to 40 Tg/year in the contemporary period, with many industrialized areas of the globe showing an increase up to 5-fold. DIN fluxes from river basins have increased 6-fold from 2.4 Tg/year in the pre-industrial to 14.5 Tg/year in the contemporary period. The amount of nitrogen loading delivered to river mouth as flux is greatly influenced by both basin temperatures and hydraulic residence times suggesting a regional sensitivity to loading. The global, aggregate nitrogen retention on the continental land mass is 82%, with a range of 0–100% for individual basins. We also present the first seasonal estimates of riverine nitrogen fluxes at the global scale based on monthly discharge as the primary driver.

Geospatial Indicators of Emerging Water Stress: An Application to Africa

Abstract This study demonstrates the use of globally available Earth system science data sets for water assessment in otherwise information-poor regions of the world. Geospatial analysis at 8 km resolution shows that 64% of Africans rely on water resources that are limited and highly variable. Where available, river corridor flow is critical in augmenting local runoff, reducing impacts of climate variability, and improving access to freshwater. A significant fraction of cropland resides in Africas driest regions, with 39% of the irrigation nonsustainable. Chronic overuse and water stress is high for 25% of the population with an additional 13% experiencing drought-related stress once each generation. Paradoxically, water stress for the vast majority of Africans typically remains low, reflecting poor water infrastructure and service, and low levels of use. Modest increases in water use could reduce constraints on economic development, pollution, and challenges to human health. Developing explicit geospatial indicators that link biogeophysical, socioeconomic, and engineering perspectives constitutes an important next step in global water assessment.

Global N removal by freshwater aquatic systems using a spatially distributed, within-basin approach

2.6-1000 km 2 ), large rivers, lakes, and reservoirs, using a 30 0 latitudelongitude river network to route and process material from continental source areas to the coastal zone. Mean annual aquatic TN removal (for the mid-1990s time period) is determined by the distributions of aquatic TN inputs, mean annual hydrological characteristics, and biological activity. Model-predicted TN concentrations at basin mouths corresponded wellwithobservations(medianrelativeerror= � 12%,interquartile rangeofrelativeerror= 85%), an improvement over assumptions of uniform aquatic removal across basins. Removal by aquatic systems globally accounted for 14% of total N inputs to continental surfaces, but represented 53% of inputs to aquatic systems. Integrated aquatic removal was similar in small rivers (16.5% of inputs), large rivers (13.6%), and lakes (15.2%), while large reservoirs were less important (5.2%). Bias related to runoff suggests improvements are needed in nonpoint N input estimates and/or aquatic biological activity. The within-basin approach represented by FrAMES-N will improve understanding of the freshwater nutrient flux response to anthropogenic change at global scales.

Extreme rainfall, vulnerability and risk: a continental-scale assessment for South America

Extreme weather continues to preoccupy society as a formidable public safety concern bearing huge economic costs. While attention has focused on global climate change and how it could intensify key elements of the water cycle such as precipitation and river discharge, it is the conjunction of geophysical and socioeconomic forces that shapes human sensitivity and risks to weather extremes. We demonstrate here the use of high-resolution geophysical and population datasets together with documentary reports of rainfall-induced damage across South America over a multi-decadal, retrospective time domain (1960–2000). We define and map extreme precipitation hazard, exposure, affectedpopulations, vulnerability and risk, and use these variables to analyse the impact of floods as a water security issue. Geospatial experiments uncover major sources of risk from natural climate variability and population growth, with change in climate extremes bearing a minor role. While rural populations display greatest relative sensitivity to extreme rainfall, urban settings show the highest rates of increasing risk. In the coming decades, rapid urbanization will make South American cities the focal point of future climate threats but also an opportunity for reducing vulnerability, protecting lives and sustaining economic development through both traditional and ecosystem-based disaster risk management systems.

Assessment of regional threats to human water security adopting the global framework: A case study in South Korea

Water resources have been threatened by climate change, increasing population, land cover changes in watersheds, urban expansion, and intensive use of freshwater resources. Thus, it is critical to understand the sustainability and security of water resources. This study aims to understand how we can adequately and efficiently quantify water use sustainability at both regional and global scales with an indicator-based approach. A case study of South Korea was examined with the framework widely used to quantify global human water threats. We estimated the human water threat with both global and local datasets, showing that the water security index using global data was adequately correlated with the index for regional data. However, particularly poor associations were found in the investment benefit factors. Furthermore, we examined several different aspects of the index with the local datasets as they have relatively high spatial and temporal resolution. For example, we used cropland percentage, population and moderate water use as surrogate indicators instead of employing the approximately 20 original indicators, and we presented a regression model that was able to capture the spatial variations from the original threat index to some extent. This finding implies that it would be possible to predict water security or sustainability using existing indicator datasets for future periods, although it would require regionally developed relationships between water security and such indicators.