1,720,986 research outputs found
A study on numerical problems of a coupled modelling of regional water balance and anthropogenic landcover change
No full textThis thesis describes the approaches applied to attempt to solve the numerical problems of the regional atmospheric model incorporated in the coupled modelling of regional water balance and anthropogenic land cover change in Amazon basin. For computational efficiency, the previous atmospheric model is evaluated at monthly scale. In order to cope with the numerical instability, nearest neighbouring averaging interpolation is iteratively performed to smooth the solutions as a transitional approach. Therefore a subsequent study is conducted to investigate the origin of the numerical instability and whether there are feasible measures to fix the numerical problem of the modelling. Chapter 1 serves as an introduction, which briefly introduces the background and research question — Whether there is any possible remedy that can solve the numerical instability of the monthly-timestep regional water balance model and obtain convergent solutions? Chapter 2 contains 7 sections, each of which gives statement of the specific problem, the experimental method applied, the corresponding results and related discussions. It has been concluded from the series of experiments that — a. adding diffusion terms makes no sense; b. applying smaller fractions of wind helps alleviate instability but the applied monthly timestep length seems to make the model paradoxic and inherently not convergent; c. instability is not really relevant with the iterative method; d. after correcting a dormant error in previous research, the results gets no better; e. the model may be so oversimplified that cannot reflect the reality; f . wind and monstrous timestep length are the keys to the problem, especially the latter is more problematic. Chapter 3 summarizes the discussions and conclusions from chapter 2 and proposes several recommendations for future research such as trade-offs between model complexity and efficiency, a heuristic way of making wind endogenous and reconsideration of the model architecture
The hydrological change under extreme drought in the United States: Separating climate and landscape impacts
No full textCurrent climate change characterized by increasing temperature has led to an increase in the intensity and frequency of extreme droughts that have more prolonged and profound ecohydrological and social impacts. By paying attention to the hydrological change before and after extreme drought and the patterns of drought recovery of ecohydrological system, it is possible to better understand the consequences of extreme drought on ecohydrological system. Both climate change and landscape change have an influence on catchment hydrological condition. The drought-related hydrological change is, therefore, the combination of changes induced by these two drivers. To further explore extreme drought impacts and the root causes of hydrological change under extreme drought events, it is necessary to separate the impact of drought-related climate change from the impact of landscape change. This study aims to characterize the variations in hydroclimatic conditions before and after extreme drought by studying the hydroclimatic movements in Budyko space, explore post-drought ecohydrological system recovery, and further separate and investigate the effects of climate and landscape change on catchment hydrological conditions. Monthly SPEI at a 12-month timescale was used to characterize and define the extreme drought events. The Budyko framework was applied to study the hydroclimatic changes of 63 basins in the United States induced by extreme drought events from 1990 to 2013 by quantifying the hydroclimatic movements in Budyko space. The climate effect on precipitation partitioning was distinguished from the landscape effect that is mainly related to vegetation response to extreme drought events. The contributions of precipitation and potential evaporation were quantified to further understand the effect of climate change which is caused by alterations of these climatic variables. To understand the effect of drought-related vegetation change on catchment precipitation partitioning, NDVI was applied to examine the response of vegetation to drought in terms of alteration in vegetation greenness and patterns of vegetation recovery. There were significant hydroclimatic changes in the basins before and after extreme drought. In post-drought period, more precipitation tended to be partitioned into evaporation in most basins. Change in streamflow was larger than the change in evaporation. 63.5% of all the basins experienced wetter conditions and more precipitation after drought. All basins gradually recovered in post-drought period, but not fully restored to their pre-drought states. The hydrological change under extreme drought was not explained by climate change alone in these basins, suggesting the existence of landscape drivers. The climate and landscape effects on precipitation partitioning could either enhance or counteract each other. The landscape drivers contributed more to change in catchment precipitation partitioning. In terms of the climatic effect that associated with the change in aridity index, climate change affects catchment precipitation partitioning by changing the precipitation and potential evaporation, among which precipitation is a more crucial climatic driver. From a vegetation-related landscape perspective, vegetation greenness reverted to pre-drought level within three years in most basins. The rapid or slow recovery, regrowth and even degradation of vegetation in post-drought period cause landscape-driven changes in catchment precipitation partitioning through directly changing vegetation transpiration and streamflow.Water Managemen
The water cycle with climate change: A study on atmospheric moisture transport using GFDL climate forecasts
No full textClimate change causes temperatures to rise worldwide. Up until now it is unclear what effect this has on the global water cycle. In this study the output of two GFDL model experiments were thoroughly investigated and used in the moisture tracking model WAM-2layers, accounting for model runs in a past case in the end-20th century and future case in the end-21st century, based on RCP8.5. This is done in order to acquire knowledge on what will happen to the global as well as regional water cycle in the future and to find out what processes provide climate change to have an effect on the water cycle. Changes in precipitation and evaporation rates are spatially highly dissimilar, therefore regional differences can be distinguished. Past studies have suggested that a DDWW paradigm - where dry regions dry out further and wet regions get wetter - will take place with climate change. Even though in some regions this might happen, results show that on land this is not necessarily the case - at least, if precipitation rate is the benchmark for a region to get drier or wetter. Processed GFDL data concludes that dry region Western Sahara gets wetter while dry region Middle East gets drier and wet region Indonesia gets wetter while wet region Amazonia gets drier. The DDWW paradigm covers another aspect as well, that there will be a larger spatial variability for variables characterising a region to be wet or dry. However, while everywhere on the globe the temperatures will rise according to the GFDL data, the spread of the yearly mean temperatures over the globe will actually decrease, meaning that mean yearly temperature in the coldest place will lie closer to the mean yearly temperature in the warmest place. This is also the case for mean yearly precipitation and evaporation rates. The mean yearly precipitation and evaporation rate in the driest place will lie closer to the mean yearly precipitation and evaporation rate in the wettest place in the future case. Continents show divergent effects in water cycle due to climate change. The continents with relatively larger sources of terrestrial moisture - North America, Europe and Asia - have an increased water cycle with higher precipitation and evaporation rates in the future. The increases in precipitation rates on these continents will originate from terrestrial evaporation and a higher percentage of evaporation will return as precipitation on land. South America shows a distinct effect, different than all other continents. This continent shows a decreased water cycle with lower precipitation and evaporation rates in the future. Of this precipitation a lower percentage comes from land and the evaporated moisture will return less on land. Africa and Oceania show another pattern. Both these continents will experience more precipitation in the future case, but this moisture will come from oceanic evaporation. Two case study areas are examined in more depth by looking at seasonality effects. A case study in the Amazon forest shows a distinct dry season in the future case, the effect of possible land use change in the Amazon forest. Less evaporation that is an effect of deforestation of the rain forest gives less moisture for clouds to form and precipitate. Moisture evaporates from the oceans on the East of Amazon region but moves over the region before it can precipitate. A case study in Western Africa shows a magnified rain season. The analyses show that during the wet season in the future case there in an increased amount of moisture coming from the oceans West and South of Western Africa and meanwhile there was still moisture coming in from the tropical rain forest East of the area
Descriptor variables of the root zone storage capacity in Canada
No full textThe root zone storage capacity is a critical determinant in hydrology, playing a major role in the partitioning of precipitation into evaporation and runoff. Besides, it is an important parameter in climatological and hydrological models. Understanding of the root zone storage capacity and its major determining processes is therefore fundamental in environmental sciences. Several studies have investigated root zone storage capacity magnitude and its descriptor variables, but mainly in snow absent regions. Computation and analysis of root zone storage capacities in snow dominant regions is therefore underexposed. As such, additional understanding of the major descriptor variables of the root zone storage capacity in boreal regions and in particular the influence of snow on the root zone storage capacity is desired. The aim of this study is therefore to quantify catchment average root zone storage capacities, identify its main descriptor variables and their regional variability and determine the influence of snow on root zone storage capacities in Canada. Root zone storage capacities were computed for 230 Canadian catchments using a simple water balance approach with additional snow module and were found to be normally distributed with mean magnitude of 183 mm and a standard deviation of 70 mm. Individual correlation of climate, discharge and landscape variables showed most relevant relationship between root zone storage capacities and yearly potential evaporation, runoff coefficient and seasonality index, although with considerable variance. Subsequent investigation on the mutual effect of several variables showed that the aridity index, runoff coefficient and seasonality timing index are major descriptor variables of the root zone storage capacity, by how they indicate the allocation of water for transpiration in a catchment and describe the degree of synchronisation between liquid input and atmospheric water demand. Application of a multiple linear regression model using the aridity index, runoff coefficient and seasonality timing index showed these variables can be used to predict root zone storage capacities in Canada with an R2 of 0.72. Subsequent tests of the predictive capability of this model Finland resulted in an R2 of 0.62. The influence of snow on the root zone storage capacity in Canada was identified by comparing its magnitude computed with and without a snow module. Whenever significantly present, snow effects showed a decrease in root zone storage capacity magnitude, caused by increased overlap between liquid input and transpiration output in a catchment. These effects are encapsulated by the seasonality timing index. To determine the regional variability of root zone storage capacity descriptors in Canada, catchments were clustered based on similar functioning. The results indicated that different variables have an effect on the root zone storage capacity in different functionally comparable regions and that a large part of the functional behaviour of the clusters can be explained by the geographical location of their catchments. The influence of these regionally dependent variables on the root zone storage capacity is encapsulated in the earlier defined main descriptor variables aridity index, runoff coefficient and seasonality timing index.Water Management | Hydrolog
Extreme precipitation return levels for multiple durations on a global scale
Get PDFQuantifying the magnitude and frequency of extreme precipitation events is key in translating climate observations to planning and engineering design. Past efforts have mostly focused on the estimation of daily extremes using gauge observations. Recent development of high-resolution global precipitation products, now allow estimation of global extremes. This research aims to quantitatively characterize the spatiotemporal behavior of precipitation extremes, by calculating extreme precipitation return levels for multiple durations on the global domain using the Multi-Source Weighted-Ensemble Precipitation (MSWEP) dataset. Both classical and novel extreme value distributions are used to provide insight into the spatial patterns of precipitation extremes. Our results show that the traditional Generalized Extreme Value (GEV) distribution and Peak-Over-Threshold (POT) methods, which only use the largest events to estimate precipitation extremes, are not spatially coherent. The recently developed Metastatistical Extreme Value (MEV) distribution, that includes all precipitation events, leads to smoother spatial patterns of local extremes. For durations of 5 and 10 days, however, there are less events per year to fit the distribution (37 and 22 on average, respectively), leading to larger inter-annual variability and possible overestimation of the extremes. While the GEV and POT methods predict a consistent shift from heavy to thin tails with increasing duration, the MEV method predicts a relatively constant heaviness of the tail for any precipitation duration, opening up an important research question on what is the ‘correct’ tail behavior of extreme precipitation for different durations. The generated extreme precipitation return levels and corresponding parameters are provided as the Global Precipitation EXtremes (GPEX) dataset. These data can be useful for studying the underlying physical processes causing the spatiotemporal variations of the heaviness of extreme precipitation distributions.</p
Multi-fold increase in rainforest tipping risk beyond 1.5-2 °C warming [Elektronisk resurs]
No full textTropical rainforests rely on their root systems to access moisture stored in soil during wet periods for use during dry periods. When this root zone soil moisture is inadequate to sustain a forest ecosystem, they transition to a savanna-like state, losing their native structure and functions. Yet the influence of climate change on ecosystem's root zone soil moisture storage and the impact on rainforest ecosystems remain uncertain. This study assesses the future state of rainforests and the risk of forest-to-savanna transitions in South America and Africa under four Shared Socioeconomic Pathways (SSP1-2.6, SSP2-4.5, SSP3-7.0, and SSP5-8.5). Using a mass-balance-based empirical understanding of root zone storage capacity (Sr), defined as the maximum volume of root zone soil moisture per unit area accessible to vegetation's roots for transpiration, we project how rainforest ecosystems will respond to future climate changes. We find that under the end-of-the-21st-century climate, nearly one-third of the total forest area will be influenced by climate change. As the climate warms, forests will require a larger Sr than they do under the current climate to sustain their ecosystem structure and functions, making them more susceptible to water limitations. Furthermore, warming beyond 1.5-2 degrees C will significantly elevate the risk of a forest-savanna transition. In the Amazon, the forest area at risk of such a transition grows by about 1.7-5.8 times in size compared to the immediate lower-warming scenario (e.g. SSP2-4.5 compared to SSP1-2.6). In contrast, the risk growth in the Congo is less substantial, ranging from 0.7-1.7 times. These insights underscore the urgent need to limit the rise in global surface temperature below the Paris Agreement to conserve rainforest ecosystems and associated ecosystem services
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
The fate of land evaporation - A global dataset
Get PDFThis work provides a global dataset on the fate of land evaporation for a fine-meshed grid of source and receptor cells. The dataset was created through a global run of the numerical moisture tracking model WAM-2layers. The tracking was conducted on a 1.5° * 1.5° grid and was based on reanalysis data from the ERA-Interim database. Climatic input data were incorporated in 3 respectively 6-hour time steps and represent the time period from 2001 to 2018. The tracking direction was forwards in time and the geographical borders of the model were located at +/- 79.5° latitude. As a result of the model run, the annual as well as the monthly average fate of evaporation was determined for 8684 land grid cells (all land cells except those located within Greenland and Antarctica) and provided via source-receptor matrices. The gained dataset was complemented via an aggregation to country and basin scales in order to highlight possible usages for areas of interest larger than grid cells. This resulted in data for 265 countries and 8223 basins. Finally, five types of source-receptor matrices for average moisture transfers were chosen to build the core of the dataset: land grid cell to grid cell, country to grid cell, basin to grid cell, country to country, basin to basin. Besides providing results averaged over the considered period (basic dataset), inter-annual results on a grid cell basis (inter-annual dataset) were added in addition. The dataset is the first ready-to-download dataset providing the overall fate of evaporation for land-cells of a global fine-meshed grid in monthly resolution. At the same time, information on the sources of precipitation can be extracted from it. It could be used for investigations into average annual, seasonal and inter-annual sink and source regions of atmospheric moisture from land masses for most of the regions in the world and comes along with example scripts for the readout and plotting of the data
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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