The unavailability of consistent accurate river flow data is a significant impediment to understanding water resources availability, and hydrological extremes. This is particularly true for remote, difficult to access, morphologically active and therefore rapidly changing rivers. The state of global river discharge monitoring with respect to water infrastructure and frequency of data collection has been on the decline over the past few decades. This is despite the significant importance of these data for river flow predictions. Fortunately, rapid advancements in technologies open up possibilities for water resource authorities to increase their ability to accurately, safely and efficiently establish river flow observation through remote and non-intrusive observation methods. Low-cost Unmanned Aerial Vehicles (UAVs) in combination with Global Navigation Satellite Systems (GNSS) can be used to collect geometrical information of the riverbed and floodplain. Such information, in combination with hydraulic modelling tools, can be used to establish physically based relationships between river flows and permanent proxies. This study attempts to monitor flow in volatile, dangerous and difficult to access rivers using only affordable and easy to maintain new technologies. This thesis consists of three main components: i) generating a workable framework for monitoring rivers using low-cost technologies; ii) establishment of river geometry using a combination of airborne photogrammetry and low-cost GNSS equipment iii) and physically based rating curve development through hydraulic modelling of surveyed river sections.

The first three chapters of this thesis provide an introduction in the form of a literature review, justification for the study and a description of the study area. In chapter 4, a framework is developed through an intensive review of traditional river monitoring processes. Uniquely effective and low-cost individual components are selected and placed within a framework. The ideal outcome is an interconnected framework which clearly presents the steps which are necessary for river monitoring in remote locations. The manner in which each critical step is related to the other is explained. Furthermore, the method by which modern technologies are assimilated into the method is described. Within the framework, critical thresholds are set up in order to signal the to the water manager whether the proposed model in its current state continues to perform as required.

Chapter 5 investigates how low-cost technologies such as UAVs in combination with low-cost GNSS devices can be used to generate river geometry for the purposes of application in a hydraulic model. Furthermore, performance of the open-source photogrammetry software substantiated the claim that, free and open-source available packages are capable of producing results which are as good as proprietary alternatives as shown by the RMSE analyses. A novel approach to generate a seamless bathymetry through merging and volumization was successfully tested. Results presented in this chapter encourage future studies to investigate the impact of variations in the number of Ground Control Points (GCPs) on discharge estimations in a hydraulic model with different hydrodynamic boundary conditions. This follow up was instituted in Chapter 6.

In this sixth chapter we accept that uncertainties in the data acquisition may propagate into uncertainties in the relationships found between discharge and state variables. This uncertainty prompts the need to understand the impact of varying geometries on hydraulic models. Specific attention is placed on variations caused by differing GCP numbers since the task of GCP placement is time consuming, potential dangerous and resource intensive in certain location and instances. We are successfully able to determine the minimum number of control points required to reproduce geometry. Overall, we successfully develop and test a workable method for water resources authorities to estimate river flows accurately through the application of advanced, low-cost technologies with minimal contact with measured variables.

The development and application of low-cost technologies for river flow monitoring has led to the following important conclusions:
• For the purpose of flow estimation, there is no need to use more than seven GCPs to establish accurate UAV-based geometry. Rather, it is more crucial to distribute the available markers to be maximally representative of the terrain elevations. Furthermore, it may be necessary to place more markers in close proximity to locations where one may expect the largest challenge for photogrammetry software (e.g.: water, thick forest/vegetation)
• In order to limit the impact of the “doming” effect on terrain geometry measurements, one of the most effective, yet easily implementable mechanisms is to measure a river line using Real Time Kinematic (RTK) Global Navigation Satellite Systems (GNSS) equipment. This data can then be used to correct the terrain post photogrammetry processing.
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Due to climate change, temperatures are rising worldwide resulting in sea level rise, higher evaporation and possible changes in precipitation patterns. These effects of climate change influence salt intrusion, particularly by affecting the freshwater supply. In The Gambia also a planned hydropower dam and possibly future irrigation development will affect the freshwater supply. The combined effects of climate change, the hydropower dam and irrigation development on salt intrusion are unknown. A river south of The Gambia, the Casamance, turned strongly hypersaline during the Sahelian droughts, which strongly affected the rivers discharge in the early 80s. The freshwater supply in the Casamance after these droughts was insufficient to flush away the salt and the estuary never restored. This led to the decay of the entire ecosystem that existed in and around the Casamance. The Gambia river is of major importance to the country, the agricultural sector relies on the freshwater it supplies for growing crops, the fishing industry relies on sufficient numbers of fish to catch and the forestry industry makes use of the mangroves growing around the river. All these industries combined account for ±75% of the working force in The Gambia. If the Gambia estuary turns strongly hypersaline such as the Casamance, this would have disastrous results. This thesis aims to clarify the effect that climate change, the hydropower dam and irrigation development have on salt intrusion in the Gambia river. A 1D model, SALNST, is used to model salt concentrations over the river length up to the year 2100. The effects of climate change, the hydropower dam and irrigation development are represented by four model parameters: climate projections, sea level rise, dam operations and irrigation development. Multiple parameter values, ranging from mild to strong conditions or impact are implemented, creating a set of 27 scenarios. These scenarios treat the parameters independently to distinguish the individual parameters effect on salt intrusion. It can be concluded that The Gambia is prone to climate change when considering salt intrusion. Due to the importance of the river, monitoring sea level rise and the development of climate change will be essential for planning and taking counter measures. Understanding the effects that climate change can have regarding salt intrusion can also be used in negotiations to reserve some freedom in the hydropower dam operations. In addition, solutions that limit the increase in evaporative surface for strong sea level rise (e.g., dikes along a part of the Gambia river) prevent extreme salt intrusion lengths and hyper salinity. The hydropower dam causes a reduced range of salt intrusion lengths, this creates easier to predict salt concentrations which is beneficial for aquaculture. Furthermore, the reduced maximum salt intrusion is also beneficial for agriculture that makes use of irrigation. ... Read more

Throughout the world, many people have been affected by water related issues in the past, some more extreme than others. In this context, hydrological models have often been used to gain more insight into the situation and to limit negative impacts as much as possible. There are many different types of hydrological models with each their strengths and weaknesses, but all models need a certain amount of reliable data. However, many river basins throughout the globe are poorly gauged which means there are only limited reliable ground observations available. That is why satellite observations provide many interesting opportunities to fill this gap of which many are not yet explored. Therefore the goal of this research was to answer the following main research question: What is the added value of satellite-based observations for hydrological modelling in a semi-arid, data-scarce river basin? This research focused on the Luangwa River in Zambia which is a large tributary of the Zambezi River. This semi-arid river basin is poorly gauged, mostly unregulated and sparsely populated. A process-based distributed hydrological model with sub-grid heterogeneity was developed in this research and modified step-wise when exploring the added value of different satellite observations for different aspects within hydrological modelling. This included analysing the information content of satellite-based river water level, i.e. altimetry, for model calibration, as well as evaporation and total water storage for step-wise model structure improvement and spatial-temporal model calibration. Overall, satellite-based observations have been used successfully to improve our understanding of the hydrological processes in the data-scarce Luangwa river basin, to improve the hydrological model structure and to allow for more reliable parameter identifications in the absence of reliable discharge data. This research focused on a selection of satellite-based observations and hydrological model applications. In other words, there remain many opportunities yet to be explored! ... Read more

Saltwater intrusion is a crucial issue in estuaries. The spread of salinity is described by the dispersion coefficient. A purely empirical equation which links the effective tidal average dispersion to the freshwater discharge was developed by Van der Burgh [1972]. Combining it with the salt balance equation, Savenije [1986] derived a one-dimensional model for salinity intrusion in estuaries. This Van der Burgh model has performed surprisingly well around the world. However, the physical basis of the empirical Van der Burgh coefficient (퐾) is still weak. This study provides a theoretical basis for the Van der Burgh method and presents alternative equations. MacCready [2004] presented a theoretical expression for the dispersion coefficient following a reductionist approach. Comparing the density-related parts of the equations of the dispersion coefficient developed by Savenije and MacCready, a predictive equation is obtained for the coefficient 퐾 using physical parameters. In addition, a new box-model has been developed considering the longitudinal densitydriven gravitational circulation and the lateral tide-driven horizontal circulation. The coefficient 퐾 (closely related to the Van der Burgh’s coefficient) is used as an index of the density-driven mixing mechanism while the tide-driven part is included by assuming that it is proportional to the longitudinal dispersion. This model is validated in sixteen alluvial estuaries worldwide by using calibrated 퐾 values (and the boundary conditions). These calibrated values correspond well with the predicted values from the theoretical derivation, revealing that 퐾 has smaller values when the tide is stronger. From a system perspective, alluvial estuaries are free to adjust dissipation processes to the energy sources that drive them. The potential energy of the river flow drives mixing by gravitational circulation. The maximum power concept assumes that the mixing takes place at the maximum power limit. To describe the complex mixing processes in estuaries holistically, different assumptions had to be made. The maximum power concept did not work satisfactorily when estuaries were assumed as isolated systems. However, by including the accelerating moment provided by the freshwater discharge, the open estuary system could be solved in analogy with Kleidon [2016] applying the maximum power concept. A new expression for the dispersion coefficient due to gravitational circulation has been derived and solved in combination with the advection-dispersion equation. This maximum power model works well in eighteen estuaries with a large convergence length, providing an alternative equation for the dispersion. These estuaries also have larger calibrated 퐾 values by the Van der Burgh method, revealing a relation between the empirical coefficient 퐾 and the geometry. All these models: the Van der Burgh model, the box-model, and the maximum power model, can describe the longitudinal salinity profiles. The comparison between these models implies that the empirical Van der Burgh coefficient is associated with the geometry and stratification conditions. Finally, new predictive equations have been obtained by regression with physical-based parameters which make the Van der Burgh salinity intrusion method predictive with a solid theoretical basis. ... Read more

Near-surface wind speed is typically only measured by point observations. The so-called Actively Heated Fiber-Optic (AHFO) technique, however, has the potential to provide high-resolution distributed observations, allowing for better understanding of different processes. However, before it can be widely used, its performance needs to be tested in a range of settings. Therefore, in this work, experimental results on this novel observational wind-probing technique are presented. We utilized a controlled wind-tunnel setup to assess both the accuracy and the precision of AHFO as well as its potential for outdoor atmospheric operation. The technique allows for wind speed characterization with a spatial resolution of 0.3 m on a 1 s time scale. The flow in the wind tunnel is varied in a controlled manner, such that the mean wind, ranges between 1 and 17 m/s. Comparison of the AHFO measurements with observations from a sonic anemometer shows a high overall correlation, ranging from 0.94-0.99. Also, both precision and accuracy are greater than 95 %. As such, it is concluded that the AHFO has potential to be employed as an outdoor observational technique in addition to existing techniques. In particular, it allows for characterization of spatial varying fields of mean wind in complex terrain, such as in canopy flows or in sloping terrain. In the future the technique could be combined with regular Distributed Temperature Sensing (DTS) for turbulent heat flux estimation in micrometeorological/hydrological applications. ... Read more

Conventional practices of rating curve computation fall short in many aspects. They are data-intensive and are notoriously inaccurate in high-flow regimes as a result of extrapolation of low-flow curve fitting. By making use of a simple commercial UAV, a physically-based rating curve is computed that is substantiated by a detailed representation of the section's geometry. Discharge data is still required for calibration of the roughness coefficient and determination of the stage at zero flow. The physically-based rating curve improves the accuracy of the rating curve in the high flows and allows for an easy separation between flow regimes. ... Read more

Direct measurement of river discharge is difficult, time consuming and costly. Therefore a rating curve is often used to estimate the river discharge. Limited measurements under extreme conditions result in extrapolation of the rating curve for high flow conditions. This induces uncertainties and errors in the stage-discharge relation. Recently there has been a gradual shift to more physically based rating curves, where the geometry of a river is included and no extrapolation is needed. This seems to be a promising shift to improve traditional river rating. However, the challenge now is to accurately determine the parameters bed roughness and hydraulic slope. The aim of this research is to develop and evaluate a method to better estimate the hydraulic parameters bed roughness and hydraulic slope. To do so a case study has been carried out in the Luangwa river catchment in Zambia. ... Read more

The direct measurement of discharges in rivers can be time-consuming and costly. The discharge is commonly estimated indirectly by means of a curve, relating water level to discharge. This so-called rating curve is traditionally determined by fitting a curve to a number of observations, which induces several uncertainties and difficulties: 1) the curve is approximated by a function type, 2) in general there are no observations for high flow conditions and 3) it needs to updated regularly due to cross sectional changes.

In this research a physics-based rating curve is developed and evaluated that is more reliable and easy to update. By the use of a photogrammetry techtnique 3D surface maps of river banks are generated by pictures obtained from an Unmanned Aerial Vehicle (UAV). The topography of the main channel is determined by using an Acoustic Doppler Current Profiler (ADCP) or it is approximated by the making use of expert judgement and the method developed by Lane (relates the water depth of the channel to the width and natural angle of repose). By knowing the geometric profile of a river reach, only the roughness coefficient and water surface slope are the unknown parameters to derive the rating curve, where the Manning’s formula acts as the basis of the rating curve. In this way, the rating curves can be made physically substantiated and the calibration parameter is the combination of roughness and water surface slope.

Instead of using the power law function as a approximate function of the rating curve, it is used as approximation of the conveyance-water level relationship. Therefore, the exponents in the power law function are physical substantiated and reduce uncertainties in the extrapolation zone of the traditional method, because the profile during high flow conditions is also known. Further more, the local invariabilities that are specific to single cross-section analyses are minimized by analysing river reaches. It is demonstrated that local invariabilities arises easily in natural rivers that are causing non-uniform flows, which make in general the the open-channel flow analysis much more complex. However, the average conveyance of a reach may led to the a valid assumption of a uniform flow (which is part of the assumption in the Manning’s formula), depending on how the local variabilities behave. The calibration of rating curves are much easier compared with the traditional way. Instead of collecting new discharge
measurements during dispersed flows, you just have to visit the area ones with a drone and surveying equipment. During this visit, the new new geometric profile can be captured and the rating curve can be updated, assuming that the roughness and water surface slope remains constant. ... Read more

This report introduces the use of Genetic Programming (GP) into hydrology by describing the results of GP using conceptual hydrological models as physical representation. First the possibilities of GP are tested on synthetic data, which results in a shortlist of good working objective functions and insight in the most important GP settings. The test on real data in the Belgium Ardennes showed that GP using the objective functions KG10, MM and Shafii performed better. Nevertheless all three models performed not well on simulating the low flows and high peaks. Furthermore GP using KG10 and MM both results in simple serial models which perform well overall, but bad on quick response runoff. Shafii resulted in parallel models which show quick response flow, however GP it is not able to capture the fast responses correctly (yet). GP has the potential to improve the understanding in the behaviour of catchments, however it still needs the human mind to observe, compare and analyse the modelling results. The main consideration with GP is to look for a balance between: model search space, objective function, randomness and (computational) time. The challenge is how to lead GP in an efficient way without removing the possibility of finding unknown patterns. ... Read more

Valdivia is a Chilean city located near an estuary system, 800 km south of Santiago. The navigation capacity of the Valdivia river mainly determines the present state and future possibilities for the welfare of the city. Since sedimentation problems arose in the river, the capacity for navigation became limited. The ministry of Valdivia therefore desires a solution for this problem in order to create possibilities for future growth. However, it is not known to what extent solutions for the sedimentation problem will actually contribute to an increase of welfare. By identifying the problem for the Ministry of Public Works of Chile, the boundaries and goal of the project could be set. In this research an attempt is made to answer the following research question: How can the surplus of sediment in the Valdivia river be remedied and to what extent will this contribute to the economic and social values of the city of Valdivia? In order to know how to remedy the surplus of sediment, a qualitative analysis of probable causes of the sedimentation was necessary. The main subjects which were researched by means of data and literature are tidal influence, river discharges, sediment composition and salt intrusion. With this knowledge it was concluded that the directions and magnitudes of flow lines in the river system can give good indications on what locations sedimentation can occur. The interaction between tidal currents and river discharges are the main drivers behind these flow characteristics. To verify the theoretical analysis, a basic Delft3D model was set up containing only tidal movement and river discharges. The pattern of the flow lines which was obtained from the Delft3D model supports the possibility that sedimentation occurs on the current identified sedimentation locations. Although other hydrological and morphological processes were found to possibly influence sedimentation rates in the river, qualitative data for studying these processes were missing. The current Delft3D model is therefore a good result regarding the available data and can be seen as a part of preliminary research in order to support further studies. Because the exact causes of the sedimentation were not identified in this research, it was not possible to come up with suitable solutions and to research how these solutions could affect the system. However, in order to gain insight in the effects of a river system without sedimentation problems, a fictitious scenario was studied by means of a social cost benefit analysis. The goal of the social costs and benefits analysis was to give insight in what factors should be taken into account when considering a project plan which solves the sedimentation problem. A dredging design was made in order to make the Valdivia river navigable for larger cargo vessels to increase the trading capacity of Valdivia. This dredging design comprises the deepening of the Valdivia river to a minimum water depth of 10 meters. Considering investment and maintenance costs of the dredging activities of this magnitude, it is concluded that the costs are not profitable compared to the social and economic effects of a higher traffic intensity on the river. With the social costs and benefits analysis all effects of solving the sedimentation problem are inventoried. When the Delft3D model can identify what the remedy of the sedimentation could be, the impact on the economic and social values of Valdivia can be determined. This research was not sufficient to answer the research question. Both the Delft3D model and the SCBA model were lacking accurate and reliable data. To expand this research, it is necessary to collect the data of which an overview has been made. Therefore, this research can serve as a preliminary study for further research. It is recommended that after data collection, solutions will be identified which can solve the sedimentation problem. These solutions need to be compared with a scenario analysis in order to decide on which is the most suitable solution for the sedimentation problem. Besides this, it is advised to research whether it is actually necessary to expand the river for transport when looking at all actors involved. ... Read more

Standard conceptual hydrological models can rarely accommodate stream tracer dynamics at the catchmentscale. They rely on the generation of runoff through the propagation of a pressure wave and do not accountfor the actual advective movement of particles. Over the last years different model frameworks have beendeveloped to account for this shortcoming. The difference between the frameworks lies in whether they arebased on mixing coefficients or storage age selection functions. Both methods have shown their ability tocapture the stream chemistry response. It is however not clear how these distinct approaches compare toeach other and to reality.To provide more clarification in this matter, this research applied both frameworks separately to model thehydrological and stream water chemistry response for a specific catchment. The results are analysed usingthe concept of transit times, where information on the fluxes and states in all model components is used togenerate distributions that describe the age structure of water. By comparing the distributions generated byboth methods and by evaluating the overall model performances, more insight is gained on the two frame-works and on their ability to capture subsurface mixing dynamics.From the results of this research it is concluded that both frameworks show similar performance. They arehowever conceptually different, which might have implications for their ability to capture specific dynamics.These implications depend on the characteristics of the catchment of interest. It is still concluded howeverthat performance for each frameworks is likely to be similar. Because the model description of the MixingCoefficient framework is less complex, it is recommended to continue research with this framework. ... Read more

Why do equal precipitation events not lead to equal discharge events across space and time? The easy answer would be because catchments are different, which then leads to the second question: Why do hydrologists often use the same rainfall-runoff model for different catchments? Probably because specifying and distributing hydrological processes across catchments is not straightforward. It requires catchment data and proper tools to evaluate the details and spatial representation of the modelled processes. However, making a model more specific and distributed can improve the performance and predictive power of the hydrological model. Therefore, this thesis evaluates the added value of including spatial characteristics in rainfall-runoff models.
Most model experiments in this thesis are carried out in the Ourthe catchment, a subcatchment of the Meuse basin. This catchment has a strong seasonal behaviour, responds quickly to precipitation and has a large influence on peak flows in the Meuse. It has a variety of landscapes, among which steep forested slopes and flat agricultural fields.
This thesis proposes a new evaluation framework (Framework to Assess Realism of Model structures (FARM)), based on different characteristics of the hydrograph (hydrological signatures). Key element of this framework is that it evaluates both performance (good reproduction of signatures) and consistency (reproduction of multiple signatures with the same parameter set). This framework is used together with various other model evaluation tools to evaluate models at three levels: internal model behaviour, model performance and consistency, and predictive power.
The root zone storage capacity (Sr) of vegetation is an important parameter in conceptual rainfall-runoff models. It largely determines the partitioning of precipitation into evaporation and discharge. Distribution of a climate derived Sr-value (i.e., based on precipitation and evaporation) was compared with Sr-values derived from soil samples in 32 New Zealand catchments. The comparison is based on spatial patterns and a model experiment. It is concluded that climate is a better estimator for Sr than soil, especially in wet catchments. Within the Meuse basin, climate derived Sr -values have been estimated as well; applying these newly derived storage estimates improved model results.
Two types of distribution have been tested for the Ourthe catchment: the distribution of meteorological forcing and the distribution of model structure. The distribution of forcing was based on spatially variable precipitation and potential evaporation. These were averaged at different levels within in the model, thereby creating four levels of model state distribution. The model structure was distributed by using two hydrological response units (HRUs), representing wetlands and hillslopes. Eventually, a lumped and a distributed model structure were compared, each with four levels of model state (forcing) distribution. From this, it is concluded that distribution of model structure is more important than distribution of forcing. However, if the model structure is distributed, the forcing should be distributed as well.
Knowing that distribution of model structure is relevant, more detailed process conceptualisations have been tested for the Ourthe Orientale, a subcatchment of the Ourthe. An additional agricultural HRU was introduced for which Hortonian overland flow and frost in the topsoil are assumed to be relevant. In addition, a degree-day based snow module has been added to all HRUs. Adding these process conceptualisations improved the performance and consistency of the model on an event basis. However, the implemented processes and the related signatures are sensitive to errors in forcing and model outliers and should therefore be implemented carefully.
This thesis finishes with two explorative comparisons; one comparing the newly developed model of the Ourthe Orientale catchment with other catchments; the second between the newly developed model and other models, including the HBV configuration currently used for operational forecasting in the Meuse basin. These comparisons were carried out based on visual inspections of parts of the hydrograph. The results show that the newly developed model can be applied in neighbouring catchments with similar performance. The comparison with other models demonstrates that a very quick overland flow component and a parallel configuration of fast and slow runoff generating reservoirs is important to reproduce the dynamics of the hydrograph related to different time scales. Both aspects are included in the newly developed model. As a results, the newly developed model is better able to reproduce most of the dynamics of the hydrograph than the operational HBV configuration, used at the moment of writing.
Distribution and detailed process conceptualisation are very beneficial for rainfall-runoff modelling of the Ourthe catchment. However, they should be applied with care. Conceptual models are a strong simplification of reality. When confronting them only with discharge data, there is a risk of misinterpreting other hydrological processes.
This thesis suggests two possible opportunities to further improve conceptual models. First, catchment understanding could be increased by adding more physical meaning to the models, such as the climate derived root zone storage capacity. And second, remote sensing and plot scale data could be combined to link hydrological processes at different scales. In this way conceptual models can probably be used to get more insight into scaling issues, which occur when moving from hillslope to catchment scale.
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Ungauged catchments can be found in many parts of the world, but particularly in sub-Saharan Africa. Information collected in a gauged catchment and its regionalisation to ungauged areas is crucial for water resources assessment. Especially farmers in semi-arid zones are in need of such information. Inter and Intra-seasonal rainfall variability is large in these areas, and farmers depend more and more on additional surface and groundwater resources for their crop production. As a result, understanding the key-hydrological processes, and determination of the frequencies and magnitudes of stream flows, is very important for local food production. This is particularly true for the ungauged Makanya catchment in Tanzania, which is the subject of this study. ... Read more