Using a spatio-temporal dynamic state-space model with the EM algorithm to patch gaps in daily riverflow series

A spatio-temporal linear dynamic model has been developed for patching short gaps in daily river runoff series. The model was cast in a state-space form in which the state variable was estimated using the Kalman smoother (RTS smoother). The EM algorithm was used to concurrently estimate both parameter and missing runoff values. Application of the model to daily runoff series in the Volta Basin of West Africa showed that the model was capable of providing good estimates of missing runoff values at a gauging station from the remaining time series at the station and at spatially correlated stations in the same sub-basin

Using a spatio-temporal dynamic state-space model with the EM algorithm to patch gaps in daily riverflow series, with examples from the Volta Basin, West Africa

International audienceA spatio-temporal linear dynamic model has been developed for patching short gaps in daily river runoff series. The model was cast in a state-space form in which the state variable was estimated using the Kalman smoother (RTS smoother). The EM algorithm was used to concurrently estimate both parameter and missing runoff values. Application of the model to daily runoff series in the Volta Basin of West Africa showed that the model was capable of providing good estimates of missing runoff values at a gauging station from the remaining series at the station and at spatially correlated stations in the same sub-basin

Impacts of Climate Change on Stream Inflows into the Volta Lake

The study examined the impact of climate change on future water availability in the Volta Lake from climate downscaled data using ensemble projections of two Global Climate Models (MPEH5 and HADCM3) and two emission scenarios (A1B and A2) used in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. The SWAT hydrological model was calibrated and validated and then used with the downscaled climate change data to assess the impacts of climate change on the inflows to the Lake. Results from the impact assessment showed that future mean annual streamflow into the Volta Lake could increase by about 17 and 16 percent under the A1B and A2 scenarios, respectively. However, streamflow from Geoere, Boromo and Dapola in the Black Volta basin and Nawuni in the White Volta basin could decrease. The projected increase in total annual streamflow in the basin is consistent with the increase in annual rainfall in the basin under both the A1B and A2 scenarios of between 2.0 and 8.0 percent obtained in a separate climate downscaling study. Since streamflow in some of the sub-basins were found to decrease under the climate change scenarios investigated, integrated and prudent management of the basin's water resources would be necessary to ensure sustainability in water use

Suspended sediment transport into a water supply reservoir in southern Ghana

Weija reservoir on the Densu river serves as a source of water supply to parts of Greater Accra and Central regions of Ghana. The Densu river basin is characterised by accelerated land degradation. A number of Integrated Water Resources Management (IWRM) interventions have been implemented in the basin. Reports suggest there has been an improvement of physicochemical parameters of the water resources. However, there is no recent study on fluvial sediment transport in the basin. This paper assessed sediment transport into the Weija reservoir via the Densu River. Weekly suspended sediment concentration and river discharge were monitored over a 1 year period to quantify sediment yield into the reservoir. The results indicated that total annual suspended sediment yield, and annual specific suspended sediment yield were 5375 t yr-1 and 2.0 t km-2 yr-1, respectively. These were relatively low compared to results obtained by past studies on the Densu basin at upstream Mangoase (6146 t yr-1 and 2.49 t km-2 yr-1) and other river basins in Ghana. Parameters established from a yield rating curve indicated that parts of the basin were degraded and sediment transport in the basin was largely due to the availability of sediment in the catchment and not just due to high river discharges. The relatively low sediment transport at Weija may be indicative of the success of the IWRM interventions. It is recommended that these be sustained and/or intensified

The water resource implications of changing climate in the Volta River Basin

The Volta River is one of the major rivers in Africa. A transboundary basin, which is the principal water source for approximately 24 million people in six riparian states, it is likely to experience increasing stress in the near future as a consequence of both greater water demand and climate change. In a study to ascertain the joint impacts of changes in demand and supply within the basin, a dynamic regional climate model (CCLM), a hydrological model (SWAT) and a water resource model (WEAP) were used to provide an assessment of the possible implications of one downscaled ‘middle impact’ (i.e., lying between extremes) climate change scenario on the performance of existing and planned irrigation and hydropower schemes. The models were used to simulate the climate change in tandem with four scenarios, each reflecting different levels of water resources development as indicated in the plans of the riparian states. It is not possible to quantify the error arising from the models in combination and the results should be considered indicative rather than absolute. Nonetheless, they provide a useful indicator of possible future change and have important implications for water resource planning. The results indicate that, by the middle of the twenty-first century, basin-wide average annual rainfall, mean annual runoff and mean groundwater recharge, will all decline. These changes significantly undermine the technical performance of existing and planned reservoirs, which, in turn, affects development outcomes. In the ‘intermediate development’ scenario, climate change is anticipated to reduce average annual hydropower generation by approximately 30% and increase average annual unmet irrigation demand four-fold by the middle of the century. By the end of the century and in the ‘full development’ scenario, the reduction in technical performance of reservoirs is even greater. Therefore, even though investment in reservoirs brings benefits, these benefits are significantly reduced in comparison to those that would accrue in the absence of climate change. The changes are likely to have dire consequences for economic development, food security and poverty in the region. Against this background, water resources development in the basin requires interventions that bolster resilience and water security. This necessitates much more systematic planning of water storage, greater cooperation between the riparian states and consideration of innovative approaches to water storage, such as managed aquifer recharge

Discrimination of water quality monitoring sites in River Vouga using a mixed-effect state space model

The surface water quality monitoring is an important concern of public organizations due to its relevance to the public health. Statistical methods are taken as consistent and essential tools in the monitoring procedures in order to prevent and identify environmental problems. This work presents the study case of the hydrological basin of the river Vouga, in Portugal. The main goal is discriminate the water monitoring sites using the monthly dissolved oxygen concentration dataset between January 2002 and May 2013. This is achieved through the extraction of trend and seasonal components in a linear mixed-effect state space model. The parameters estimation is performed with both maximum likelihood method and distribution-free estimators in a two-step procedure. The application of the Kalman smoother algorithm allows to obtain predictions of the structural components as trend and seasonality. The water monitoring sites are discriminated through the structural components by a hierarchical agglomerative clustering procedure. This procedure identified different homogenous groups relatively to the trend and seasonality components and some characteristics of the hydrological basin are presented in order to support the results