The anticipated impacts of climate-change induced glacier shrinkage on the water security of mountains and downstream lowlands is a major global concern. However, the connections between climate change, glacier shrinkage, water security and local adaptive capacity are multi-dimensional and non-linear. In many regions of the world including Peru, the physical and human processes that govern them are poorly understood. Therefore, understanding these process, their impacts and implementing adequate science-based adaptation strategies requires an interdisciplinary approach. This approach should combine advancing the state-of-the-art of glaciological and hydrological process understanding, with new insights in current and future levels of water security, human vulnerability, and adaptive capacity. We propose to address this challenge by developing an integrated glacier - water security assessment model to transform our understanding of the impact of glacier shrinkage on water security and to inform policy practices in Peru. We identify the lack of glaciological, hydro-climatological, and water resources data as a major bottleneck to achieve this. Therefore, we propose participatory water resources monitoring as a radically new approach to transform our knowledge of physical processes, constraining water resources models, and supporting evidence-based policy-making. We have assembled a world-leading consortium that combines high-level expertise in field monitoring and computer simulation of glaciers and water resources in Peru, with pioneers of participatory data collection for sustainable development and policy-support. This consortium is ideally placed to generate a breakthrough in data availability on the link between glacier reduction and current and future water security. This is needed to build the next generation of glaciological and hydrological models that can support the design and implementation of adequate climate adaptation strategies. We will use the Vilcanota-Urubamba Basin in southern Peru as our case study. This basin hosts the largest tropical ice cap (Quelccaya) and it is characterised by a very complex water management context and high data scarcity. Our project will follow a "source to tap" paradigm, in which we will deliver the first fully integrated water resources vulnerability assessment framework for glacier-fed basins, comprising state-of-the-art glaciology, hydrology, water demand characterisation, and water security assessment. We will design targeted glacio-hydrological and water resources monitoring campaigns, to complemented existing monitoring efforts of our project partners and collaborators, and new remotely sensed data sets. This campaign will be implemented using the principles and tools of participatory monitoring and knowledge co-creation that our team has pioneered in the tropical Andes. The datasets produced by this approach, combined with existing monitoring implemented by our team and collaborators, will allow us to build an integrated water supply-demand-vulnerability assessment model for glacierized basins, and to use this to evaluate adaptation strategies at the local scale. For the latter, we have engaged with a set of policy stakeholders in Peru that play a key role in the implementation of recent transformative legislation on Peruvian water resources management, and in particular in the new law on the implementation of water funds to invest in catchment interventions (Law 30215). Working directly with these stakeholders will ensure that our approach focuses on locally relevant adaptation strategies, including novel approaches such as the use of nature-based solutions and the restoration of ancestral water "seeding and harvesting practices", thus providing both the scientific basis and the operational tools that support the implementation of this legal framework.

The Peruvian Andes is home to 71% of the world's tropical glaciers, and the meltwater they supply is an essential resource for people downstream who depend on it for irrigation and sanitation. Further, hydropower plants driven by glacial meltwater provide more than 40% of Peru's electricity. However, Peru's glaciers are receding rapidly, threatening this supply, as well as releasing sediment to valley areas and revealing topographic depressions that may become natural reservoirs for glacier runoff. These thawing landscapes are also very active and can pose risks to downstream people and infrastructure. PEGASUS will assess the opportunities and threats that rapidly evolving landscapes, and natural resources, will bring to the people and businesses of three glacierised Cordilleras of the Peruvian Andes - Urubamba, Vilcabamba and Vilcanota - and make recommendations that will maximise the potential prosperity that can be gained in the face of continued environmental change. Modelling the climate of mountain catchments such as those in Peru is complex because of the interaction of large-scale weather systems with local-scale winds and extreme relief. Uncertainties in modelling the climate feed into projections of glacier change, which themselves are limited by a lack of data on previous glacier behaviour for calibration, and downstream river flows for validation. Robust climate modelling is also required for predictions of permafrost (freezing) heights, which are a key control on ice and bedrock stability, and thus avalanche risk. PEGASUS will produce new and refined projections of climate that will drive cutting edge glacier and permafrost models, to yield firm predictions of how the glaciers and freezing levels will change on a 5-yearly interval from now until the end of the century. As the glaciers recede and hillslopes become more active, sediment will be released into the valleys, and lakes will develop where ice existed. Some of the sediment will be trapped within these glacial lakes, and some will be transferred downstream by river flows. The rate of sediment release by glaciers in advanced states of recession is poorly known, and the role of lakes in capturing the sediment is also poorly quantified. PEGASUS will perform field measurements and modelling to improve understanding of the role of glacial lakes in removing, conveying and storing sediment being released from the glaciers, and characterise the impact this will have on downstream water quality and critical hydropower infrastructure. The locations of future glacial lakes can be predicted by modelling the thickness of the current glaciers and identifying subglacial depressions that will be revealed as the ice recedes. Using a Digital Elevation Model (DEM) of this ice-free terrain, it is possible to make a quantitative assessment of the hazard that these new lakes, as well as existing glacial lakes, pose to downstream areas if they were to burst catastrophically. PEGASUS will carry out this assessment for the largest lakes in the Urubamba-Vicabamba-Vilcanota study area and then undertake additional fine-resolution and physically-based numerical modelling to robustly quantify the effects of flooding and debris flows on people, land, the downstream river dynamics, and hydropower infrastructure. PEGASUS will then identify the barriers and opportunities that exist to the use of these lakes for water storage and hydropower development. This assessment will integrate consultations with government (CORECC), a large hydropower company (EGEMSA) and, crucially, communities living in the catchments of the lakes we have analysed. The recommendations that follow will provide information on the sustainability of existing and future hydropower schemes, how to manage water use in future decades and formulate policies that reflect the needs of all stakeholders, and the potential hazards that unstable mountain environments may pose to lives and livelihoods in future years.

Hundreds of millions of people live close to, and depend upon, the world's large rivers for water, food, transport and the maintenance of a thriving ecosystem. However, these rivers are increasingly vulnerable to the effects of a wide range of natural and human-induced disturbances, including climate change, construction of large dams, river engineering works, deforestation, agricultural intensification, and mining activity. Over the past 20 years, climate change and deforestation have impacted on the hydrology and sediment fluxes within the Amazon River Basin. However, the Amazon has remained one of the few large river systems that has been largely unaffected by dams. This situation is changing rapidly, because widespread hydropower dam construction in Brazil, Bolivia, Peru and Ecuador now threatens the basin, with >300 dams planned or under construction. These dams are expected to trigger severe hydro-physical and ecological disturbances throughout the basin, including massive reductions in sediment and nutrient delivery to the lowland Amazon and its floodplains, substantial degradation of river beds and banks, significant changes in river water levels and flooding, and adverse impacts on river and floodplain ecosystems, on which the human population depends. Recent high profile studies highlight the need for international action to assess and mitigate these impacts, both in the Amazon and elsewhere. However, our capacity to do this is severely restricted by an absence of quantitative models that can predict how environmental disturbances propagate through large rivers and floodplains, over continental distances, and decadal to centennial time periods. Critically, environmental disturbances driven by dams, climate and land cover change promote dynamic river responses (e.g., changes in river width, depth, slope, sediment size, degree of branching and rate of floodplain reworking), which in turn control changes in flood conveyance and downstream sediment delivery. Despite advances in modelling of river dynamics over short distances (<100 km), hydrological models that are applied to continental-scale drainage basins treat rivers and floodplains as static conduits. Consequently, such models are unable to represent or predict the future impacts of environmental change on flooding, sediment fluxes or river and floodplain functioning. This project will deliver a step-change in our ability to model, predict and understand how the world's large rivers are impacted by, and respond to, environmental change. We will achieve this by implementing a research strategy that involves six elements: First, we will develop a new multi-scale numerical modelling approach that enables the effects of river dynamics on environmental disturbance propagation through continental-scale drainage basins to be simulated. Second, we will develop a suite of environmental scenarios representing climate and land cover changes and dam construction throughout the Amazon Basin for the recent past (1985-2015) and future (up to 2200). Third, we will collect new field datasets at sites on the Amazon River that are required to test key components of the model. Fourth, we will work with an international team of project partners to assemble high-resolution field, satellite and model datasets that quantify channel and floodplain processes, and river morphology and dynamics throughout the Amazon Basin. Fifth, we will use these data to carry out rigorous testing of our new model. Sixth, we will apply the model to predict the future evolution of the Amazon River and its tributaries for a wide range of environmental change scenarios, and quantify the controls on hydro-geomorphic disturbance propagation within large drainage basins. We will work with our project partners to disseminate our model code, datasets and project outcomes to non-academic stakeholders, both nationally and internationally.