De Fraiture C.,UNESCO IHE |
Giordano M.,SRI International
Agricultural Water Management | Year: 2014
An increasing number of smallholder farmers engage in irrigation using their own resources. They buy or rent irrigation equipment and draw water from nearby sources without depending on or without interference from public agencies or water user associations. The individualization of Agricultural Water Management has been ongoing for several decades in South Asia where most irrigation now takes place from privately owned wells. Recently, small private irrigation is emerging also in sub Saharan Africa. It is farmer-driven, responds to a genuine demand from smallholders and has substantial potential for poverty alleviation and rural development. In many countries the area under privately managed and owned irrigation is larger than under public irrigation schemes. However, the individualization of irrigation and its spontaneous, unchecked spread pose challenges to equitable access to and sustainable management of water resources. Irrigation investments and research efforts have largely focused on the underperforming public irrigation sector, ignoring small private irrigation. This special issue describes and analyzes this thriving but overlooked sector, drawing from examples from five countries in sub-Saharan Africa and two states in India. The authors explore ways to enhance the potential of small private irrigation for all, without jeopardizing the sustainability of the available water resources. © 2013 Elsevier B.V.
Ranasinghe R.,UNESCO IHE |
Ranasinghe R.,Technical University of Delft |
Callaghan D.,University of Queensland |
Stive M.J.F.,Technical University of Delft
Climatic Change | Year: 2012
Accelerated sea level rise (SLR) in the twenty-first century will result in unprecedented coastal recession, threatening billions of dollars worth of coastal developments and infrastructure. Therefore, we cannot continue to depend on the highly uncertain coastal recession estimates obtained via the simple, deterministic method (Bruun rule) that has been widely used over the last 50 years. Furthermore, the emergence of risk management style coastal planning frameworks is now requiring probabilistic (rather than deterministic, single value) estimates of coastal recession. This paper describes the development and application of a process based model (PCR model) which provides probabilistic estimates of SLR driven coastal recession. The PCR model is proposed as a more appropriate and defensible method for determining coastal recession due to SLR for planning purposes in the twenty-first century and beyond. © 2011 The Author(s).
Van der Wegen M.,UNESCO IHE |
Jaffe B.E.,U.S. Geological Survey
Coastal Engineering | Year: 2013
Recent advances in the development of numerical, process-based models have led to remarkable performance in reproducing measured decadal morphodynamic developments. The advantage of this type of models is that they have a detailed output allowing for a close analysis of relevant processes. Drawback is that the output is associated with a high level of presumed uncertainty, because of the large number of processes involved and the high quality level of input data required.This study aims to explore possibilities to assess uncertainty levels associated with process-based morphodynamic modeling. In a probabilistic approach we consider the outcome of an ensemble of runs including variations of model input parameters and forcing schematizations. We propose to evaluate model performance by both a skill criterion (How well does the model reproduce observed patterns?), a confidence criterion (How sensitive are model results to uncertain input?) as well as a combination of these criteria. This methodology provides an objective assessment of the performance of process-based morphodynamic models. In addition, it can determine which input parameters cause largest uncertainty in the model outcome.The San Pablo Bay case study shows that 60% of the modeled volume meets the skill and confidence criteria for the depositional period (1856-1887) and 46% for the erosional period (1951-1983). Approximately 50% of the volume allocation meets the confidence criterion for a 30. year morphodynamic forecast (1983-2013). Model results are sensitive to model input variations only to a limited extend. We attribute this to the high impact of the San Pablo Bay plan form and bathymetry. The forecast shows continuous erosion of the channel and on the northern shoals and a continuous accretion of the channel slopes, albeit more concentrated in the western part of the channel than in preceding decades. © 2013 Elsevier B.V.
Van Der Wegen M.,UNESCO IHE
Journal of Geophysical Research: Earth Surface | Year: 2013
The morphod ynamic adaptation of estuaries to sea level fluctuations has been subject of geological studies based on sediment core analysis and qualitative modeling efforts. Limited attention has been paid to understanding bathymetric evolution based on a detailed process level. The current study aims to explore governing morphodynamic processes and timescales by application of a 2D, process-based modeling approach. The starting point of the analysis is an 80 km long and 2.5 km wide basin. Starting from a sandy flat bed, stable channel-shoal patterns emerge within a century under semidiurnal tidal forcing. We impose a gradual rise in sea level (up to 0.67 m per century) and compare the results with a run excluding sea level rise (SLR). Model results show that SLR drowns the basin so that intertidal area disappears. This process generates tidal asymmetry reflected by an emerging M4 tidal constituent. The basin shifts from exporting to importing sediment reflected by shoal patterns migrating in the landward direction. The landward sediment transport remains too limited to compensate for the loss in intertidal area and to restore equilibrium within a millennial time scale. Further sensitivity tests on initial bathymetry, tidal amplitude forcing, and rate of SLR show that shallow basins with limited tidal forcing are most vulnerable to SLR. Key Points Sea level rise is imposed on a schematized tidal embayment over 1600 years Morphodynamic system shifts from a sediment exporting to an importing system Disappearing intertidal area generates tidal asymmetry ©2013. American Geophysical Union. All Rights Reserved.
Schumann G.,University of Bristol |
Di Baldassarre G.,UNESCO IHE
Remote Sensing Letters | Year: 2010
A near-simultaneous satellite acquisition of a flood event on 12 December 2006 on theDee River in Wales (UK) is used to illustrate the potential to develop standalone space-borne flood risk mapping techniques. Both the ERS-2 and ENVISAT satellites recorded the event close to flood peak and only 28 min apart. This unique opportunity enables the creation of a very rare but extremely useful observed data set for flood inundation studies. This letter illustrates how this unique set of spaceborne radar images can be used for rapid flood risk mapping. An event-specific weighted hazard map was generated based on plausible flood area observations from an aggregation of widely applied image-processing techniques. This map can be further augmented to an event-specific fuzzy flood risk map by fusing the multialgorithm ensemble map with vulnerability-weighted land cover vector data in a geographic information system(GIS) environment. The technique presented is fairly flexible and leads to a potentially useful data set for direct use in flood management. © 2010 Taylor & Francis.