67 research outputs found
Operational tools and applications of EO satellite data to retrieve surface fluxes in semi-arid countries
Get PDFThe overall objective of the thesis is to develop and evaluate useful tools and applications of Earth Observation (EO) satellite data to estimate surface fluxes in semiarid countries. The first specific objective (Chapter 4) is to assess the performance and robustness of the triangle method, a remote sensing (RS) based method to retrieve and map land evapotranspiration (ET). Emphasis is put on the estimation of soil heat flux (G), which represents an important sink of energy in these regions. Estimating accurately G is still very problematic, because of its dependence on both vegetation cover fraction and soil physical properties. To that aim, the partial goals are: - Proposal of a new parameterisation scheme for G, based on a linear relationship with the evaporative fraction (EF) - Assessment of the ET retrieval triangle method with the new Gparameterisation As a second specific objective (Chapter 5), the triangle method was compared with predictions supplied by a land surface model (JULES) with the purpose of illustrating how these two different approaches can be combined to improve the estimations of surface fluxes. The third objective is to demonstrate the potential of RS-based information for actual operational applications related to the monitoring of surface fluxes in semi-arid countries (Chapter 6). The first one is the application of the triangle method to obtain maps of actual ET in western Africa. The second one is the application of the same method to analyse the annual evolution of surface energy balance (SEB) components and to study drought climatology over a 12-year period in the Segura river basin in South-East Spain. The third is the use of sea surface temperature (SST) coupled to an energy balance model of a water body to characterise the thermal behaviour of coastal lagoon (Mar Menor) located in South-East Spain.Universidad Politécnica de Cartagen
Hydrological outlook UK - September 2023
Get PDFThe Hydrological Outlook provides an insight into future hydrological conditions across the UK. Specifically, it describes likely trajectories for river flows and groundwater levels on a monthly basis, with a particular focus on the next three months.
Well established monitoring programmes provide the current status of both river flows and groundwater levels at many sites across the UK, and data from these programmes provide the starting point for the Outlook. A number of techniques are used to project forwards from the current state and results from these are used to produce a summary that includes a highlights map
Hydrological outlook UK - March 2022
Get PDFThe Hydrological Outlook provides an insight into future hydrological conditions across the UK. Specifically, it describes likely trajectories for river flows and groundwater levels on a monthly basis, with a particular focus on the next three months.
Well established monitoring programmes provide the current status of both river flows and groundwater levels at many sites across the UK, and data from these programmes provide the starting point for the Outlook. A number of techniques are used to project forwards from the current state and results from these are used to produce a summary that includes a highlights map
Historical gridded reconstruction of potential evapotranspiration for the UK
Get PDFPotential evapotranspiration (PET) is a necessary input data for most hydrological models and is often needed at a daily time step. An accurate estimation of PET requires many input climate variables which are, in most cases, not available prior to the 1960s for the UK, nor indeed most parts of the world. Therefore, when applying hydrological models to earlier periods, modellers have to rely on PET estimations derived from simplified methods. Given that only monthly observed temperature data is readily available for the late 19th and early 20th century at a national scale for the UK, the objective of this work was to derive the best possible UK-wide gridded PET dataset from the limited data available.
To that end, firstly, a combination of (i) seven temperature-based PET equations, (ii) four different calibration approaches and (iii) seven input temperature data were evaluated. For this evaluation, a gridded daily PET product based on the physically based PenmanâMonteith equation (the CHESS PET dataset) was used, the rationale being that this provides a reliable âground truthâ PET dataset for evaluation purposes, given that no directly observed, distributed PET datasets exist. The performance of the models was also compared to a ânaĂŻve methodâ, which is defined as the simplest possible estimation of PET in the absence of any available climate data. The ânaĂŻve methodâ used in this study is the CHESS PET daily long-term average (the period from 1961 to 1990 was chosen), or CHESS-PET daily climatology.
The analysis revealed that the type of calibration and the input temperature dataset had only a minor effect on the accuracy of the PET estimations at catchment scale. From the seven equations tested, only the calibrated version of the McGuinnessâBordne equation was able to outperform the ânaĂŻve methodâ and was therefore used to derive the gridded, reconstructed dataset. The equation was calibrated using 43 catchments across Great Britain.
The dataset produced is a 5âŻkm gridded PET dataset for the period 1891 to 2015, using the Met Office 5âŻkm monthly gridded temperature data available for that time period as input data for the PET equation. The dataset includes daily and monthly PET grids and is complemented with a suite of mapped performance metrics to help users assess the quality of the data spatially.
This dataset is expected to be particularly valuable as input to hydrological models for any catchment in the UK.
The data can be accessed at https://doi.org/10.5285/17b9c4f7-1c30-4b6f-b2fe-f7780159939c
Hydrological outlook UK - December 2022
Get PDFThe Hydrological Outlook provides an insight into future hydrological conditions across the UK. Specifically, it describes likely trajectories for river flows and groundwater levels on a monthly basis, with a particular focus on the next three months.
Well established monitoring programmes provide the current status of both river flows and groundwater levels at many sites across the UK, and data from these programmes provide the starting point for the Outlook. A number of techniques are used to project forwards from the current state and results from these are used to produce a summary that includes a highlights map
Hydrological outlook UK - April 2023
Get PDFThe Hydrological Outlook provides an insight into future hydrological conditions across the UK. Specifically, it describes likely trajectories for river flows and groundwater levels on a monthly basis, with a particular focus on the next three months.
Well established monitoring programmes provide the current status of both river flows and groundwater levels at many sites across the UK, and data from these programmes provide the starting point for the Outlook. A number of techniques are used to project forwards from the current state and results from these are used to produce a summary that includes a highlights map
Hydrological outlook UK - November 2023
Get PDFThe Hydrological Outlook provides an insight into future hydrological conditions across the UK. Specifically, it describes likely trajectories for river flows and groundwater levels on a monthly basis, with a particular focus on the next three months.
Well established monitoring programmes provide the current status of both river flows and groundwater levels at many sites across the UK, and data from these programmes provide the starting point for the Outlook. A number of techniques are used to project forwards from the current state and results from these are used to produce a summary that includes a highlights map
Benchmarking ensemble streamflow prediction skill in the UK
Get PDFSkilful hydrological forecasts at sub-seasonal to seasonal lead times would be extremely beneficial for decision-making in water resources management, hydropower operations, and agriculture, especially during drought conditions. Ensemble streamflow prediction (ESP) is a well-established method for generating an ensemble of streamflow forecasts in the absence of skilful future meteorological predictions, instead using initial hydrologic conditions (IHCs), such as soil moisture, groundwater, and snow, as the source of skill. We benchmark when and where the ESP method is skilful across a diverse sample of 314 catchments in the UK and explore the relationship between catchment storage and ESP skill. The GR4J hydrological model was forced with historic climate sequences to produce a 51-member ensemble of streamflow hindcasts. We evaluated forecast skill seamlessly from lead times of 1 day to 12 months initialized at the first of each month over a 50-year hindcast period from 1965 to 2015. Results showed ESP was skilful against a climatology benchmark forecast in the majority of catchments across all lead times up to a year ahead, but the degree of skill was strongly conditional on lead time, forecast initialization month, and individual catchment location and storage properties. UK-wide mean ESP skill decayed exponentially as a function of lead time with continuous ranked probability skill scores across the year of 0.75, 0.20, and 0.11 for 1-day, 1-month, and 3-month lead times, respectively. However, skill was not uniform across all initialization months. For lead times up to 1 month, ESP skill was higher than average when initialized in summer and lower in winter months, whereas for longer seasonal and annual lead times skill was higher when initialized in autumn and winter months and lowest in spring. ESP was most skilful in the south and east of the UK, where slower responding catchments with higher soil moisture and groundwater storage are mainly located; correlation between catchment base flow index (BFI) and ESP skill was very strong (Spearman's rank correlation coefficientâŻ=â0.90 at 1-month lead time). This was in contrast to the more highly responsive catchments in the north and west which were generally not skilful at seasonal lead times. Overall, this work provides scientific justification for when and where use of such a relatively simple forecasting approach is appropriate in the UK. This study, furthermore, creates a low cost benchmark against which potential skill improvements from more sophisticated hydro-meteorological ensemble prediction systems can be judged
Distribution, trends, and drivers of flash droughts in the United Kingdom
Get PDFFlash droughts have been the subject of a great deal of scientific attention in the last decade, but the greatest emphasis has been on relatively dry climates. Here, we characterise this type of rapid-onset drought events in a more humid setting, the United Kingdom (UK), for the period 1969â2021. Our results show that flash droughts have affected both the wetter regions of the north-west and the drier regions of the south-east in every season over the last 5 decades. Nevertheless, the spatio-temporal distribution of flash droughts is highly variable in the UK, with important regional and seasonal contrasts. Central and northern regions are generally the most frequently affected by flash droughts in comparison to the south-eastern region. Overall, there are non-significant trends in the frequency of flash droughts in winter, summer, and autumn. However, we found a significant increase in the number of flash droughts recorded in spring. We also analysed the relative contribution of the atmospheric evaporative demand (AED) and precipitation to flash drought development in the UK. Our findings show that flash drought occurrence responds primarily to precipitation variability in all seasons but particularly in winter and autumn. In spring and summer, the AED is important as a secondary driver for triggering flash droughts, especially in the drier regions of the south-east. Furthermore, the trends observed in AED contribution evidence that its relevance is rising significantly in spring, also in the south-eastern UK in summer. The atmospheric and oceanic conditions related to flash drought development were also examined. Remarkable anomalies in sea level pressure and 500âhPa geopotential height associated with the presence of high-pressure systems are noted in all seasons during the development of the main flash droughts affecting the UK. Similarly, flash droughts typically develop under the negative phase of the North Atlantic Oscillation in winter and autumn, while in summer and spring the positive phase is dominant. We also found positive anomalies in sea surface temperature during the development of flash droughts in spring and summer, while mixed anomalies are observed in winter and autumn. This study presents a detailed characterisation of the flash drought phenomenon in the UK, providing useful information for drought assessment and management and a climatology of flash droughts that can be used as a baseline against which future changes in flash drought occurrence can be assessed
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