Modelling the Effects of Changes in Forest Cover and Climate on Hydrology of Headwater Catchments in South-Central Chile

This study analyses the changes in the runoff of forested experimental catchments in south-central Chile, to determine to what extent observed trends can be attributed to effects of intensive forestry and/or climate change. For this, we applied the distributed TETIS® model to eight catchments (7.1−413.6 ha) representative of the land uses and forestry activities in this geographical area. Rainfall and runoff data collected between 2008 and 2015 were used for modelling calibration and validation. Simulation of three land uses (current cover, partial harvest and native forest) and 25 combinations of climatic scenarios (percentage increases or decreases of up to 20% of rainfall and evapotranspiration relative to the no-change scenario applied to input series) were used in each calibration. We found that changes in land use and climate had contrasting effects on runoff. Smaller catchments affected by the driest climatic scenarios experienced higher runoff when the forest cover was lower than under full forest cover (plantations or native forests). In contrast, larger catchments under all climatic scenarios yielded higher runoff below the full forest cover than under partial harvest and native forest. This suggests that runoff can be influenced, to a great extent, by rainfall decrease and evapotranspiration increase, with the model predicting up to a 60% decrease in runoff yield for the dry’s climatic scenario. This study proves to be relevant to inform ongoing discussions related to forest management in Chile, and is intended to minimize the impact of forest cover on runoff yield under uncertain climatic scenarios.The authors acknowledge the support from the Economy and Knowledge Department of the Catalan Government through the Consolidated Research Group ‘Fluvial Dynamics Research Group’—RIUS (2017 SGR 459)

Bayesian hydrograph separation in a minimally gauged alpine volcanic watershed in central Chile

This study examines the question of how much information one can extract from a tracer-based hydrograph separation in a remote and minimally gaged alpine catchment in Chile. We combine PCA-based endmember mixing analysis to identify the sources of flow contribution to the Diguillín River with a hierarchical Bayesian mixing model to integrate spatial and temporal variability in endmember concentration and quantify the source contributions to streamflow over time. The PCA-analysis shows that precipitation isotopes do not vary by elevation (e.g. snow and rainfall had identical signatures) but vary significantly by season, and that a third endmember is necessary to bound streamflow variability at the basin outlet, which was not captured by our field sampling. One of the main advantages of Bayesian methods is the quasi-machine learning capabilities, where we treated the third endmember as a parameter from which the mixing model could both estimate proportional contributions as well as posterior estimates for the tracer concentrations. The two tracer, three endmember hydrograph separation revealed groundwater to be the largest and precipitation (rain and snow) to be the smallest contributor, on average, to streamflow with the third unknown endmember contributing around 40% of streamflow during the Winter wet season. We hypothesize that interflow is occurring as the third endmember in the Alto Diguillín subwatershed, based on inferred tracer values and the presence of alluvium atop impermeable bedrock along certain reaches. More work is necessary to observe and sample these flowpaths, which was not possible during this study. The results of this work have implications for water resource management, since groundwater sustains the majority of streamflow in the Diguillín, and climate change will impact the timing and quantity of baseflow and interflow. Overall, we demonstrate the utility of combining PCA with Bayesian statistical modeling and inference to extract maximum information from a limited field dataset in a remote alpine catchment. The findings of this work can guide future water management in the Diguillín, but also provide clear questions for future research

Investigation of an integrated regional environmental watershed methodology

Recent public awareness of the environment has placed increased emphasis on the health and current state of the regional watershed. The watershed has been defined as that area in which water flowing across and beneath a given land surface drains into a specific stream or river, ultimately flowing through a single point or outlet on that stream or river. Since the processes involved are many and are analyzed in the literature on an individual basis, the current investigation attempts a more holistic approach by suggesting a methodology that integrates all elements of the hydrologic cycle. The investigation utilizes the area topography in the form of a digital elevation model (DEM) as the base for analysis. Basic to any watershed model is a characterization of the water flow in streams by a mathematical function expressed through the hydrograph. The investigation explores the hydrograph and proposes that it can be constructed from hydrological components in a feedback concept with precipitation as input and the volume of flow as output. Feedback, for example, is represented as ground water and infiltration. An approach is presented to develop the watershed hydrograph from a Taylor series expansion using the derivatives of measured flow as parameters. The expansion result is transformed through LaPlace techniques into a representation of the hydrograph. Once done, the resulting time function can be transformed by the Fourier operator and a unique spectral signature of the stream obtained. It is further asserted that the national network of stream gages can be a useful source of data for this construct. Included in the research is an investigation of the framework needed to package the information describing the watershed model. The Geographic Information System (GIS) is suggested as the ideal method to organize and provide clarity to the watershed model. Particularly important is the structured relational database required in this approach. Added to this are spatial geographic capabilities, which did not exist in the past. Lastly, an investigation into the project management tasks necessary for the successful pursuit of a watershed-monitoring program is outlined. Emphasis here is placed on the inclusion of all the interested parties in the care taking of the watershed. The analysis and modeling of watersheds are gaining increasing attention as managers and custodians become more acutely aware of the interactions of human activity and the environmental health of the watershed. Government investment in the streamgaging networks will contribute to this process by providing improved physical data to be used as input into the modeling efforts. The future holds greater promise to manage our natural resources through more comprehensive models of the environment

Swat Model Simulation of Bioenergy Crop Impacts in a Tile-Drained Watershed

Tile drains are an important component of agricultural production in the Midwest, and their inclusion in modeling studies is important in watersheds where they are a principal hydrologic pathway. The new tile drainage simulation method in the Soil Water Assessment Tool (SWAT) was parameterized and tile flow results were compared with reviewed literature. Streamflow, sediment, and nutrient outputs were compared to measured values and simulated crop yields were examined with respect to average county yields. Plant growth stressors were examined to account for differences between simulated and published yields. The bioenergy crop switchgrass (Panicum virgatum) was applied over the watershed in land use scenarios developed from a review of published modeling studies and scenario planning literature. Differences in water quality and quantity arising from these land use changes, simulated by SWAT, were quantified

Assessing the impacts of land-use change on the hydrology of the tropical Andes

Land-use and land-cover change (LUCC) has been identified as a major driver of change to the hydrological cycle. However, it is still a scientific challenge to quantify these effects. Land surface models are increasingly being used for such hydrological assessment because of their state-of-the-art representation of physical processes and versatility. A physically-based model has the advantage to map the modeller’s knowledge about the hydrological impacts of land-use and land-cover change into physically meaningful parameters. This PhD thesis explores the use of a land surface model (Joint UK Land-Environment Simulator, JULES) in combination with high temporal resolution in-situ data on streamflow, precipitation, and several weather variables, collected by a grassroots hydrological monitoring initiative (called iMHEA) in the tropical Andes. I find that the in-situ data can improve the hydrological simulation substantially, mainly by reducing uncertainty inherent in using large-scale precipitation data. The commonly used soil parameters based on pedotransfer functions lead to an underestimation of the flow. Therefore, I modified the soil parameterisation with experimental data for a more accurate representation of subsurface flow generation. Subsequently, I assessed the potential impacts of watershed interventions (grazing, afforestation, cultivation) using the calibrated soil parameters. A reduction in water yield and water regulation ability under these land use scenarios was identified, which is in line with observed impacts and relevant for water resources managers. In a next step, I implemented an open source land use change model, the lulcc R package, to analyse the regional land cover changes in the Andean region, and to generate predictive land use maps that can be used to drive the JULES model. For this purpose, the JULES model has been implemented at a regional scale using multiple sources of global data. The use of the JULES model allows the effects of LUCC to be assessed using knowledge about physical processes. My results show a further 3.7% of deforestation occurring in the region, which changes the flow by ±17% consequently.Open Acces

Rainwater harvesting techniques as an adaptation strategy for flood mitigation

The development of adaptation and mitigation strategies to tackle anthropic and climate changes impacts is becoming a priority in drought-prone areas. This study examines the capabilities of indigenous rainwater harvesting techniques (RWHT) to be used as a viable solution for flood mitigation. The study analyses the hydraulic performance of the most used micro-catchment RWHT in sub-Saharan regions, in terms of flow peak reduction (FPR) and volume reduction (VR) at the field and basin scale. Parametrized hyetographs were built to replicate the extreme precipitations that strike Sahelian countries during rainy seasons. 2D hydrodynamic simulations showed that half-moons placed with a staggered configuration (S-HM) have the best performances in reducing runoff. At the field scale, S-HM showed a remarkable FPR of 77% and a VR of 70% in case of extreme rainfall. Instead at the basin scale, in which only 5% of the surface was treated, 13% and 8% respectively for FPR and VR were obtained. In addition, the reduction of the runoff coefficient (Rc) between the different configuration was analyzed. The study critically evaluates hydraulic performances of the different techniques and shows how pitting practices cannot guarantee high performance in case of extreme precipitations. These results will enrich the knowledge of the hydraulic behavior of RWHT; aspect marginally investigated in the scientific literature. Moreover, this study presents the first scientific application of HEC-RAS as a rainfall-runoff model. Despite some limitations, this model has the effective feature of using very high-resolution topography as input for hydraulic simulations. The results presented in this study should encourage stakeholders to upscale the use of RWHT in order to lessen the flood hazard and land degradation that oppresses arid and semi-arid areas

Seasonal water storage and release dynamics of bofedal wetlands in the Central Andes

Tropical high-Andean wetlands, locally known as ‘bofedales’, are key ecosystems sustaining biodiversity, carbon sequestration, water provision and livestock farming. Bofedales' contribution to dry season baseflows and sustaining water quality is crucial for downstream water security. The sensitivity of bofedales to climatic and anthropogenic disturbances is therefore of growing concern for watershed management. This study aims to understand seasonal water storage and release characteristics of bofedales by combining remote sensing analysis and ground-based monitoring for the wet and dry seasons of late 2019 to early 2021, using the glacierised Vilcanota-Urubamba basin (Southern Peru) as a case study. A network of five ultrasound loggers was installed to obtain discharge and water table data from bofedal sites across two headwater catchments. The seasonal extent of bofedales was mapped by applying a supervised machine learning model using Random Forest on imagery from Sentinel-2 and NASADEM. We identified high seasonal variability in bofedal area with a total of 3.5% and 10.6% of each catchment area, respectively, at the end of the dry season (2020), which increased to 15.1% and 16.9%, respectively, at the end of the following wet season (2021). The hydrological observations and bofedal maps were combined into a hydrological conceptual model to estimate the storage and release characteristics of the bofedales, and their contribution to runoff at the catchment scale. Estimated lag times between 1 and 32 days indicate a prolonged bofedal flow contribution throughout the dry season (about 74% of total flow). Thus, our results suggest that bofedales provide substantial contribution to dry season baseflow, water flow regulation and storage. These findings highlight the importance of including bofedales in local water management strategies and adaptation interventions including nature-based solutions that seek to support long-term water security in seasonally dry and rapidly changing Andean catchments

The Impacts of Climate and Landscape Change on Catchment Scale Material Transport in High Mountain and Seasonally Cold Regions

Streamflow in high mountain and seasonally cold regions follows annual patterns that can make water quality and availability in these areas uniquely susceptible to changes in climate and land use. Precipitation in the form of snow accumulates during the winter months and gradually melts in the spring, leading to elevated streamflow that serves important ecological and human needs. This simple winter storage and spring release dynamic is especially sensitive to warming temperatures, which can result in a greater fraction of winter precipitation falling as rain and an earlier arrival of the hydrograph center of timing, with cascading effects on surrounding ecosystems. Changes in land use can similarly disrupt the natural storage and release balance by altering the pathways in which water is routed through a system. These changes necessarily impact the ways in which sediment and other solutes are delivered through stream systems. This dissertation investigates material fluxes at the watershed scale in high mountain and seasonally cold regions in an effort to better understand the processes impacting their magnitude, timing, and release and the broader implications for the management of water resources. Chapter 1 investigates these dynamics in the present day through the lens of a small urban headwater stream in the Northeastern United States characterized by frequent winter snowfall. In the surrounding watershed, sodium chloride is used seasonally as a deicing agent on roads and sidewalks, as is common in colder regions. Excess chloride loading poses a problem, however, because it can compromise sources of drinking water and threaten the health of surrounding ecosystems. The timing of chloride storage and release can also be altered in urban environments through the installation of armored stream channels and by increases in impervious surface cover. To investigate these landscape changes on chloride transport and storage, two years of streamflow and chloride concentration data were used to create continuous chloride load estimates for two contrasting reaches of an urban stream. The upstream reach is characterized by channelization and armored banks and is largely disconnected from groundwater while the downstream reach flows through a riparian floodplain and has a strong groundwater connection. Results from this study show that chloride loads in the channelized reach were similar to chloride application rates in the surrounding watershed. In contrast, chloride loads in the downstream reach were 50% lower than those delivered from upstream due to stream-groundwater interactions and water losses through subsurface flow paths. These findings show that longitudinal load estimates can be helpful in identifying areas of chloride storage and release, the magnitude of which may not always be apparent in urban settings. Chapters 2 and 3 consider both future and past influences on catchment scale sediment flux in the high Andes Mountains of Argentina and Chile. In this region, high mountain glaciers buffer streamflow during drier times of the year and water sourced from the uplands is critical to serving the needs of millions of people living in downstream communities such as Mendoza, San Juan, and Santiago. However, the high Andes are tectonically active, and sediment loads in regional streams and rivers can be high, posing a threat to the surrounding environment and human infrastructure alike. The area is also expected to face increases in temperature and decreases in precipitation in the coming decades. As a response, reservoirs have been built throughout the Andes, although the length of their usable lifespans is impacted by rates of sediment accumulation. In Chapter 2, future changes in streamflow and catchment scale sediment flux in the high Andes are modeled based a suite of end-member climate projections for temperature and precipitation in the coming decades. Results from this study show that reductions in precipitation propagate into even larger decreases in streamflow and sediment flux, although results from scenarios modeling warming without a change in precipitation were much more variable. In Chile, where annual precipitation is concentrated in the winter months, warming lead to an increase in high magnitude streamflow events as more storms that would have delivered snow in the high Andes delivered rain instead. These events were also associated with high sediment loads and could be connected to an increased risk in geohazards such as landslides. In Argentina, however, where the Andes act as an orographic barrier to winter storm events originating from the Pacific, the streamflow and sediment flux response to warming was more akin to simulations with reduced precipitation. These results show that different water management strategies may be needed on either side of the Andes in the coming decades. Chapter 3 takes a different approach, and investigates how sediment moves through the landscape over much longer timescales by utilizing cosmogenic radionuclides in several watersheds in the Argentine high Andes. First, 10Be in river sand was used to estimate catchment-wide erosion rates at the millennial scale and compared to decadal scale erosion rates from steam gauge data, with comparisons generally showing favorable agreement. Erosion rate estimates were also used to predict how long water reservoirs sited at the foot of the Andes can be expected to last before becoming filled with sediment, with predictions from 10Be for five different reservoirs occupying a fairly narrow range from 129 to 127 years. Finally, 10Be samples were paired with an analysis of in situ 14C in an effort to investigate sediment transport dynamics from the hillslope to catchment outlet. Low 14C/10Be ratios in this study suggest that despite agreement between decadal and millennial scale erosion rates, sediment transport times through these complex watersheds may range from a minimum of 10,000 to 19,000 years. This finding further indicates that sediments carried by streams out of the mountain front in the present may have been eroded from high elevation hillslopes in the early Holocene or even the late Pleistocene, connecting events in the geologic past to processes impacting humans today

Regional water balance analysis of glacierised river basins in the north-eastern Himalaya applying the J2000 hydrological model

The glacierised basins of the Northeast Himalayan region are highly vulnerable to climate-change impacts. The spatio-temporal hydroclimatic and physiographic variability impact the water balance of these glacierised basins across the region. This study assesses the glaciohydrological processes and dynamics in the data scarce region for the present as well future climate change scenarios by regional water balance analysis. The J2000 hydrological model was adapted to incorporate the frozen ground as well as glacier dynamics in a stepwise, nested basin calibration approach. The modelled ERA-Interim precipitation data cannot capture the high amplitude orographic and convective events. Therefore, Orographic correction factors were used to inversely correct the ERA-Interim precipitation data to account for the orographic as well as cyclonic precipitation in the region from reported glacier mass balance and evapotranspiration estimates. Monthly temperature lapse rate was adopted for correcting the ERA-Interim temperature dataset. The Beki basin was selected as the donor basin for model development and evaluation. The parameters from the Beki basin were regionalised to the receptor Lohit and the Noadihing basins by the Proxy-basin method. Multi-objective optimization criteria such as the Kling-Gupta efficiency (KGE) for temporal dynamics and flow distribution and Bias for overall water balance showed high to moderate conformity between measured and simulated discharge at the corresponding basin outlets. The variability in the water balance and runoff components among the three basins was primarily related to the spatio-temporal variation in the mean annual precipitation, runoff and evapotranspiration estimates. The impact of climate-change scenarios on the study basins indicated that water availability would sustain until the end of the century due to higher projected precipitation even though after the depletion of glaciers in the region

A Distributed Hydrological Modelling System to Support Hydrological Production in Northern Environments under Current and Changing Climate Conditions

The overarching goal of this project was to implement a distributed hydrological modelling system to support hydroelectric production in Yukon under current and changing climate conditions. Building from previous collaboration between YU and YEC, the project has increased the capacity for short and mid-term inflow forecasts for the Whitehorse (including Marsh Lake), Aishihik and Mayo Facilities and assess potential change in flow volume and extreme events due to climate change in terms of severity, timing and frequency.ReportThis report, including any associated maps, tables and figures (the “Information”) conveys general comments and observation only. The Information is provided by the Institut national de la recherche scientifique Eau Terre Environnement (INRS-ETE) on an “AS IS” basis without any warranty or representation, express or implied, as to its accuracy or completeness. Any reliance you place upon the information contained here is your sole responsibility and strictly at your own risk. In no event will the INRS-ETE be liable for any loss or damage whatsoever, including without limitation, indirect or consequential loss or damage, arising from reliance upon the Information.Final Report presented to: Yukon Energ