Boud Verbeiren

A System-based Paradigm of Drought Analysis for Operational Management

Conventionally droughts are studied in terms of their dimensions (severity, duration and areal extent), without specifying the affected system. The paper presents an innovative system-based approach for drought analysis, which can lead to rational decisions for combating drought. Concepts of water scarcity (drought, water shortage, aridity and desertification) are viewed within the perspective of this new approach. The paper focuses also on operational water management in the presence of drought. Starting from the needs for such management, the affected system is defined and the related quantities are identified. Also, sub-systems are considered which allow the establishment of the link between specific variables and drought. Some drought characterisation methods are particularly suited for the systemic approach. Finally drought is considered as a natural hazard phenomenon and its consequences are discussed. Each physical sub-system can be improved by a variety of measures aiming at decreasing its vulnerability towards drought, so that the drought risk is mitigated. It is concluded that the clear definition of the affected system on the spatial and temporal scales can significantly contribute to the rational management for combating drought.

Assessing urbanisation effects on rainfall-runoff using a remote sensing supported modelling strategy

Abstract This paper aims at developing a methodology for assessing urban dynamics in urban catchments and the related impact on hydrology. Using a multi-temporal remote sensing supported hydrological modelling approach an improved simulation of runoff for urban areas is targeted. A time-series of five medium resolution urban masks and corresponding sub-pixel sealed surface proportions maps was generated from Landsat and SPOT imagery. The consistency of the urban mask and sealed surface proportion time-series was imposed through an urban change trajectory analysis. The physically based rainfall-runoff model WetSpa was successfully adapted for integration of remote sensing derived information of detailed urban land use and sealed surface characteristics. A first scenario compares the original land-use class based approach for hydrological parameterisation with a remote sensing sub-pixel based approach. A second scenario assesses the impact of urban growth on hydrology. Study area is the Tolka River basin in Dublin, Ireland. The grid-based approach of WetSpa enables an optimal use of the spatially distributed properties of remote sensing derived input. Though change trajectory analysis remains little used in urban studies it is shown to be of utmost importance in case of time series analysis. The analysis enabled to assign a rational trajectory to 99% of all pixels. The study showed that consistent remote sensing derived land-use maps are preferred over alternative sources (such as CORINE) to avoid over-estimation errors, interpretation inconsistencies and assure enough spatial detail for urban studies. Scenario 1 reveals that both the class and remote sensing sub-pixel based approaches are able to simulate discharges at the catchment outlet in an equally satisfactory way, but the sub-pixel approach yields considerably higher peak discharges. The result confirms the importance of detailed information on the sealed surface proportion for hydrological simulations in urbanised catchments. In addition a major advantage with respect to hydrological parameterisation using remote sensing is the fact that it is site- and period-specific. Regarding the assessment of the impact of urbanisation (scenario 2) the hydrological simulations revealed that the steady urban growth in the Tolka basin between 1988 and 2006 had a considerable impact on peak discharges. Additionally, the hydrological response is quicker as a result of urbanisation. Spatially distributed surface runoff maps identify the zones with high runoff production. It is evident that this type of information is important for urban water management and decision makers. The results of the remote sensing supported modelling approach do not only indicate increased volumes due to urbanisation, but also identifies the locations where the most relevant impacts took place.

Improved DisTrad for Downscaling Thermal MODIS Imagery over Urban Areas

Spaceborne thermal sensors provide important physical parameters for urban studies. However, due to technical constraints, spaceborne thermal sensors yield a trade-off between their spatial and temporal resolution. The aims of this study are (1) to downscale the three originally low spatial resolution (960 m) Moderate Resolution Imaging Spectroradiometer (MODIS/Terra) land surface temperature image products (MOD11_L2, MOD11A1 and MOD11A2) to resolutions of 60, 90, 120, 240 and 480 m; and (2) to propose an improved version of the DisTrad method for downscaling the MODIS/Terra land surface temperature products over urban areas. The proposed improved DisTrad is based on a better parameterization of the original DisTrad residuals in urban areas. The improved resampling technique is based on a regression relationship between the residuals of the temperature estimation and the impervious percentage index. Validation of the improved DisTrad, the original DisTrad, and the uniformly disaggregated MODIS land surface temperature images (UniTrad) are performed by comparative analysis with a time-coincident Landsat 7 ETM+ thermal image. Statistical results indicate that the improved DisTrad method shows a higher correlation (R2 = 0.48) with the observed temperatures than the original DisTrad (R2 = 0.43) and a lower mean absolute error (MAE = 1.88 °C) than the original DisTrad (MAE = 2.07 °C). It is concluded that the improved DisTrad method has a stronger capability to downscale land surface temperatures in urban areas than the original DisTrad.

Location- and Time-Specific Hydrological Simulations with Multi-Resolution Remote Sensing Data in Urban Areas

A major challenge in hydrologic modeling remains the mapping of vegetation dynamics in an urban landscape. The impact of vegetation on interception storage varies over time and needs to be quantified in order to enable proper management of water resources in urban areas. However, the heterogeneity and complexity of the urban landscape makes it challenging to monitor urban vegetation. A more detailed spatial and temporal scale is needed. To characterize surface cover at a high spatial resolution, a hyperspectral APEX image (2 m) is used, while a time series of Proba-V images (daily, 100 m) allows a detailed characterization of the seasonal variation of urban greenness. For this study, we use and validate the leaf area index (LAI) maps derived from APEX and Proba-V data for a selected pixel in the Watermaelbeek catchment in Brussels (Belgium). The ground-truthing of the Proba-V pixels includes a detailed mapping of land cover characteristics and more specifically vegetation cover throughout the seasons. LAI values calculated based on the APEX image agree with the LAI values measured from the ground (n = 106, R 2 = 0.68). Further, the aggregated APEX pixels correlate with the Proba-V pixels ( R 2 = 0.79), and the Proba-V data can be used to monitor vegetation dynamics. As the seasonal LAI measurements correspond with the Proba-V dynamics, we conclude that Proba-V images allow the characterization of vegetation dynamics at a high spatial resolution in heterogeneous areas. We create a time series of LAI maps at a high resolution (2 m), which allows a location- and time-specific simulation of interception storage and thus contributes to managing water resources in urban areas.

Identification of the influencing factors on groundwater drought and depletion in north-western Bangladesh

Groundwater drought is a specific type of hydrological drought that concerns groundwater bodies. It may have a significant adverse effect on the socio-economic, agricultural, and environmental conditions. Investigating the effect of different climatic and anthropogenic factors on groundwater drought provides essential information for sustainable planning and management of (ground) water resources. The aim of this study is to identify the influencing factors on groundwater drought in north-western Bangladesh, to understand the forcing mechanisms. A multi-step methodology is proposed to achieve this objective. The standardised precipitation index (SPI) and reconnaissance drought index (RDI) have been used to quantify the aggregated deficit between precipitation and the evaporative demand of the atmosphere, i.e. meteorological drought. The influence of land-cover patterns on the groundwater drought has been identified by calculating spatially distributed groundwater recharge as a function of land cover. Groundwater drought is defined by a threshold method. The results show that the evapotranspiration and rainfall deficits are determining meteorological drought, which shows a direct relation with groundwater recharge deficits. Land-cover change has a small effect on groundwater recharge but does not seem to be the main cause of groundwater-level decline (depletion) in the study area. The groundwater depth and groundwater-level deficit (drought) is continuously increasing with little correlation to meteorological drought or recharge anomalies. Overexploitation of groundwater for irrigation seems to be the main cause of groundwater-level decline in the study area. Efficient irrigation management is essential to reduce the growing pressure on groundwater resources and ensure sustainable water management.RésuméLa sécheresse en eaux souterraines est un type spécifique de sécheresse hydrologique qui concerne les systèmes aquifères. Elle peut avoir un effet nuisible significatif sur les conditions socio-économiques, agricoles, et environnementales. L’étude de l’effet de différents facteurs climatiques et anthropiques sur la sécheresse en eaux souterraines fournit une information essentielle pour la planification et la gestion durable de ces ressources. Le but de cette étude est d’identifier les facteurs influençant la sécheresse en eau souterraine au nord-ouest du Bangladesh, pour comprendre les mécanismes de forçage. On propose une méthodologie en plusieurs étapes pour atteindre cet objectif. L’indice des précipitations normalisées (SPI) et l’indice de reconnaissance de la sècheresse (RDI) ont été employés pour mesurer le déficit agrégé entre la précipitation et la demande évaporatoire de l’atmosphère, c.-à-d. la sécheresse météorologique. L’influence de l’occupation du sol sur la sécheresse en eaux souterraines a été identifiée en calculant la distribution spatiale de la recharge en fonction de l’occupation du sol. La sécheresse en eaux souterraines est définie par une méthode de seuil. Les résultats prouvent que les déficits d’évapotranspiration et de précipitations déterminent la sécheresse météorologique qui montre une relation directe avec les déficits de recharge d’eaux souterraines. Les changements dans l’occupation du sol ont un léger effet sur la recharge d’eaux souterraines mais ils ne semblent pas être la cause principale du déclin du niveau (rabattement) des eaux souterraines dans le secteur d’étude. La profondeur des eaux souterraines et le déficit de niveau des eaux souterraines (sécheresse) augmente sans interruption montrant peu de corrélation avec les anomalies de sècheresse météorologique ou de recharge. La surexploitation des eaux souterraines pour l’irrigation semble être la cause principale du déclin du niveau des eaux souterraines dans le secteur d’étude. Une gestion efficace de l’irrigation est essentielle pour réduire la pression croissante sur les ressources en eaux souterraines et pour assurer la gestion durable de l’eau.ResumenLa sequía del agua subterránea es un tipo específico de sequía hidrológica que afecta a los cuerpos de agua subterránea. Puede tener un efecto adverso significativo en las condiciones socioeconómicas, agrícolas y ambientales. La investigación del efecto de los diferentes factores climáticos y antropogénicos sobre la sequía del agua subterránea proporcionan información esencial para la planificación y gestión sostenible de los recursos hídricos subterráneos. El objetivo de este estudio es identificar los factores que influyen en la sequía del agua subterránea en el noroeste de Bangladesh, para comprender los mecanismos forzantes. Para alcanzar este objetivo se propone una metodología en varias etapas. El índice de precipitación estandarizado (SPI) y el índice de reconocimiento de la sequía (RDI) se han utilizado para cuantificar el déficit total entre la precipitación y la demanda de evaporación de la atmósfera, es decir, la sequía meteorológica. La influencia de los patrones de cobertura del suelo en la sequía del agua subterránea se han identificado mediante el cálculo de la recarga de agua subterránea espacialmente distribuida en función de la cobertura del suelo. La sequía del agua subterránea se define mediante un método umbral. Los resultados muestran que la evapotranspiración y el déficit pluviométrico están determinando una sequía meteorológica que muestra una relación directa con los déficit de recarga del agua subterránea. El cambio en la cobertura del suelo tiene un pequeño efecto sobre la recarga del agua subterránea, pero no parece ser la principal causa de disminución (agotamiento) del nivel del agua subterránea en el área de estudio. La profundidad del agua subterránea y el déficit a nivel del agua subterránea (sequía) está aumentando continuamente con poca correlación con las sequías meteorológicas o las anomalías de recarga. La sobreexplotación del agua subterránea para el riego parece ser la principal causa de la disminución del nivel del agua subterránea en el área de estudio. La gestión eficaz del riego es esencial para reducir la creciente presión sobre los recursos hídricos subterráneos y garantizar una gestión sostenible del agua.摘要地下水干旱是一种特别的有关地下水体的水干旱。它可对社会-经济、农业和环境状况产生负面影响。调查不同气候和人为因素对地下水干旱的影响能够为(地)下水资源的可持续规划和管理提供基本信息。本研究的目的就是确定影响孟加拉西北部地下水干旱的因素,了解其机理。为了达到这个目标,提出了一个多步骤方法。采用标准化的降水指数(SPI)及勘测干旱指数(RDI)对降水和大气蒸发需求,也就是气象干旱之间的聚合赤字进行了量化。通过计算作为土地盖层函数的空间分布的地下水补给,确定了土地覆盖层模式对地下水干旱的影响。通过阈值方法定义了地下水干旱。结果显示,蒸发蒸腾和降雨赤字决定了气象干旱,而气象干旱显示和地下水补给赤字有直接关系。土地盖层变化对地下水补给有很小的影响,但似乎不是研究区地下水位下降(枯竭)的主要原因。地下水深度和地下水位赤字(干旱)持续增加,与气象干旱和补给异常相关性很小。用于灌溉的地下水超采似乎是研究区地下水位下降的主要原因。有效灌溉管理对于降低地下水资源日益增长的压力以及确保可持续水管理至关重要。ResumoA estiagem das águas subterrâneas é um tipo específico de estiagem hidrológica que se refere aos aquíferos. Isso pode ter um significativo efeito adverso em condições socioeconômicas, agrícolas e ambientais. Investigar os efeitos de diferentes fatores climáticos e antropogênicos na estiagem das águas subterrâneas prove informações essenciais para um planejamento e gerenciamento sustentável dos recursos hídricos (subterrâneos). O objetivo desse estudo é identificar os fatores de influência na estiagem das águas subterrâneas no noroeste de Bangladesh, para entender os mecanismos forçantes. Uma metodologia multipasso é proposta para atingir esse objetivo. O índice de precipitação padronizado (SPI) e o índice de reparação de seca (RDI) foram usados para quantificar o déficit agregado entre a precipitação e a demanda evaporativa da atmosfera, p. ex. seca meteorológica. A influência dos padrões de cobertura da terra na estiagem das águas subterrâneas foi identificada pelo cálculo da recarga distribuída das águas subterrâneas como uma função da cobertura da terra. A estiagem das águas subterrâneas é definida por um método de intervalos. Os resultados mostram que os déficits na evapotranspiração e precipitação são secas meteorológicas determinantes que mostram uma relação direta com os déficits da recarga das águas subterrâneas. Mudanças na cobertura da terra tiveram um pequeno efeito na recarga das águas subterrâneas, mas não parecem ser a principal causa do declínio (depleção) das águas subterrâneas na área de estudo. A profundidade das águas subterrâneas e o déficit nos níveis das águas subterrâneas (estiagem) estão aumentando continuamente com uma pequena correlação com a seca meteorológica ou anomalias na recarga. Superexploração das águas subterrâneas para irrigação parece ser a principal causa do declínio dos níveis das águas subterrâneas na área de estudo. Gestão eficiente da irrigação é essencial para reduzir a pressão crescente nos recursos hídricos subterrâneos e garantir um gerenciamento sustentável da água.

A distributed monthly water balance model: formulation and application on Black Volta Basin

Recharge assessment is of critical importance for groundwater resources evaluation in arid/semiarid areas, as these have typically limited surface water resources. There are several models for water balance evaluation. One of them is WetSpass, which has the ability to simulate spatially distributed recharge, surface runoff, and evapotranspiration for seasonally averaged conditions. This paper presents a modified methodology and model, WetSpass-M, in which the seasonal resolution is downscaled to a monthly scale. A generalized runoff coefficient was introduced, enabling runoff estimation for different land-use classes. WetSpass-M has been calibrated and validated with observed streamflow records from Black Volta. Base-flow from simulated recharge was compared with base-flow derived via a digital filter applied to the observed streamflow and has shown to be in agreement. Previous studies have concluded that for this basin, small changes in rainfall could cause a large change in surface runoff, and here a similar behavior is observed for recharge rates. An advantage of the new model is that it is applicable to medium- and large-sized catchments. It is useful as an assessment tool for evaluating the response of hydrological processes to the changes in associated hydrological variables. Since monthly data for streamflow and climatic variables are widely available, this new model has the potential to be used in regions where data availability at high temporal resolution is an issue. The spatial–temporal characteristics of the model allow distributed quantification of water balance components by taking advantage of remote sensing data.

Doode Bemde CASI-SWIR 2002: Hyperspectral sensing of moisture gradients--set-up and first results of a combined field and airborne campaign

Determination and description of groundwater systems is essential for the management and development of ecological values, especially in the valley parts of river basins. At the land surface, groundwater systems appear as infiltration (relatively dry) and discharge zones (relatively wet). Groundwater discharge zones offer a high potential for nature values because of their constant moisture presence and their specific water quality. Current methods for the determination of discharge and infiltration zones use either detailed time-consuming fieldwork or data intensive numerical simulation models. Consequently, there is a direct need for repeatable, area covering, mapping possibilities for the determination of moisture gradients and more specifically discharge and infiltration zones. Within the framework of the CASI-SWIR measuring campaign 2002, the Department of Hydrology and Hydraulic Engineering of the Vrije Universiteit Brussel (VUB) executed an airborne hyperspectral remote sensing and field campaign in the Doode Bemde to analyze moisture gradients in the Doode Bemde, a riparian nature reserve. The main objective of the study is to test the best hyperspectral analysis method, using the hyperspectral CASI-SWIR data, for the known, based upon field and simulation data, moisture gradients in the Doode Bemde area. Simultaneously with the airborne hyperspectral campaign, field measurements of soil moisture, groundwater levels, vegetation temperature and spectral characteristics of some key vegetation species (phreatophytes) were performed. The method of analysis consists of statistical comparison of moisture gradients, obtained from measurements and simulations, with individual bands, a combination of bands and multivariate derivatives. The paper describes the set-up of the field and airborne measurement campaign, the methodology of analysis as well as first analysis results.

FloodCitiSense: Early Warning Service for Urban Pluvial Floods for and by Citizens and City Authorities

FloodCitiSense aims at developing an urban pluvial flood early warning service for, but also by citizens and city authorities, building upon the state-of-the-art knowledge, methodologies and smart technologies provided by research units and private companies. FloodCitiSense targets the co-creation of this innovative public service in an urban living lab context with all local actors. This service will reduce the vulnerability of urban areas and citizens to pluvial floods, which occur when heavy rainfall exceeds the capacity of the urban drainage system. Due to their fast onset and localized nature, they cause significant damage to the urban environment and are challenging to manage. Monitoring and management of peak events in cities is typically in the hands of local governmental agencies. Citizens most often just play a passive role as people negatively affected by the flooding, despite the fact that they are often the ‘first responders’ and should therefore be actively involved. The FloodCitiSense project aims at integrating crowdsourced hydrological data, collaboratively monitored by local stakeholders, including citizens, making use of low-cost sensors and web-based technologies, into a flood early warning system. This will enable ‘citizens and cities’ to be better prepared for and better respond to urban pluvial floods. Three European pilot cities are targeted: Brussels – Belgium, Rotterdam – The Netherlands and Birmingham – UK.

Developing a New Modelling Tool to Allocate Low Impact Development Practices in a Cost Optimized Method

Nowadays there is a need to overcome the effects caused by rapid urbanisation with more innovative methods. Recently, source control approaches, known as Low Impact Development (LID), are being used by urban planners to cope with water related problems and any other environmental issues due to their cost-effectiveness and reliability. To meet the needs of decision makers, the effects of these practices should be analysed at catchment scale. To do this, allocation of LID techniques in most suitable locations is essential. In this research a new modelling tool called LID locator is added to the WetSpa-Urban software package for more accurate placement of these techniques. The maximum area that can be covered by different types of LIDs are defined by finding the potential areas prone to generate runoff in combination with suitability maps calculated based on size limitation and implementation restriction for each LID measures. Then, the new cost-optimization tool is added through new procedure. This study is testified in the Watermaelbeek catchment situated in Brussels capital region.

The importance of city trees for reducing net rainfall: comparing measurements and simulations

An in-situ tree interception experiment was conducted to determine the hydrological impact of a solitary standing Norway maple and small leaved lime in an urban environment. During the two-year experiment, rainfall data was collected 10 and divided into interception, throughfall and stemflow. With approximately 38 % of the gross precipitation intercepted by both trees, the interception storage was higher than for similar studies done in Mediterranean regions. A regression analyses for the Norway maple found rainfall duration, rainfall amount and the tree’s leaf area index to be the most important variables influencing interception storage. The regression analysis and the tree interception models by Gash and Rutter, as well as an adapted version of the Water and Energy Transfer between Soil, Plants and Atmosphere model (WetSpa), were tested for their 15 accuracy in modelling the measured interception storage. The models in general overestimated interception storage for small interception events (< interception storage) and underestimated interception storage for bigger interception events (> interception storage). The regression analysis wasn’t stable throughout seasons, event sizes and trees, making it unsuitable for generic use. The method of Gash slightly overperformed WetSpa and Rutter for all events throughout seasons and trees. However, WetSpa showed a better performance for rainfall events > 10 mm. A scenario analysis, featuring the construction of 20 student houses on a university campus, demonstrated the potential of urban trees to retain rainfall water. Even though trees alone could not restore the natural hydrological balance, they could partly mitigate the increased runoff volume and peak discharge caused by sealing of the natural surface through decreasing the net rainfall that reaches the ground. This study highlights the role of solitary trees in an urban environment where natural hydrological processes are severely altered. 1.Introduction 25 1.1 The context Currently 54 % of the population is living in an urban environment, with an expected increase to 66 % by 2050 (United Nations, Departement of Economic and Social Affairs, 2014). The migration of the growing population towards cities gives rise to a whole new set of challenges. An urban environment exhibits built-up areas that significantly alter the natural processes (Grimm 30 et al., 2008). This leads to problems such as the urban heat island effect (UHI) (Lauwaet et al., 2015) and the increased density of particle matter (Zhang et al., 2015). Another prominent problem modern cities face is the increase in runoff during and after rain events (Paul & Meyer, 2001). Due to urban expansion and the use of impervious materials such as concrete and asphalt, the hydrological cycle is altered and natural processes such as infiltration and interception are impeded. This results in an increased runoff that causes significant economic losses, especially during heavy rainfall events. Flood frequency in Western 35 Europe has increased six fold since 1970 (Barredo, 2006) and affected over 3 million people between 1998 and 2009, resulting in an economic loss of 60 billion euro (EAA, 2016). For Western-Europe, the IPCC predicts that the amount and intensity of precipitation will increase considerably in the coming decades, leading to more extreme events, and concludes that an efficient water regulation policy will be the most important challenge of the 21 century (IPCC, 2013). Hydrol. Earth Syst. Sci. Discuss., https://doi.org/10.5194/hess-2018-417 Manuscript under review for journal Hydrol. Earth Syst. Sci. Discussion started: 4 September 2018 c