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Conventional freshwater resources are currently under pressure, while the freshwater demand is expected to increase further due to the growing world population, urbanization, and climate change. Re-using wastewater for irrigation in agriculture may alleviate the pressure on freshwater resources.  However, the contaminants present in wastewater may pose a risk to the public health and the environment. Several aspects regarding contaminant transport in the environment are considered in this thesis. In current sustainability frameworks regarding wastewater irrigation, only one or several contaminant pathways or transport processes are considered. However, all potential pathways should be considered to accurately predict the associated public health and environmental risks. Therefore, we developed such an integrated framework for the sustainability assessment of wastewater irrigation. The framework highlights the need for accurate parametrization of contaminant behaviour in the environment, which is lacking especially for contaminants of emerging concern. The effect of degradation concepts for adsorbing contaminants under unsteady water flow conditions was also investigated. When contaminants do not degrade while adsorbed, adsorption does not affect the steady state fluxes and concentration, only the timing. When contaminants do degrade while adsorbed, the steady state fluxes and concentration are affected for nonlinear adsorption, but not for linear adsorption. Thus, it is necessary to experimentally determine the appropriate degradation concept to assess the role of adsorption on the long-term concentration and fluxes in the subsurface. Wastewater often contains high sodium concentrations, which may lead to soil structure degradation of clayey and loamy soils. The step from changes of the microscopic soil structure to changes of the macroscopic soil hydraulic properties is challenging. A methodology for upscaling flow and transport effects in such charged porous, Conventionele zoetwaterbronnen staan onder druk, terwijl de vraag naar zoetwater naar verwachting verder zal toenemen door klimaatverandering, verstedelijking en bevolkingsgroei. Daarom is het gebruik van alternatieve waterbronnen noodzakelijk. Gereinigd afvalwater voldoet misschien niet aan de kwaliteitseisen voor drinkwater, maar kan mogelijk wel gebruikt worden voor irrigatie in de landbouw. Dit kan een groot deel van de druk op bestaande zoetwaterbronnen wegnemen, aangezien de landbouw verantwoordelijk is voor 70% van het watergebruik wereldwijd. In droge gebieden in de wereld wordt al jaren geïrrigeerd met afvalwater. Recentelijk is er ook meer aandacht voor irrigatie met afvalwater in landen met een meer gematigd klimaat. Ten eerste, omdat er meer extreme weerspatronen worden verwacht in de toekomst met het oog op klimaatverandering. Daarnaast wordt gereinigd afvalwater in deze landen nu direct geloosd in oppervlaktewateren. Irrigatie met afvalwater kan daarom een milieuvriendelijker alternatief zijn, omdat een deel van de verontreinigingen uit het water worden gefilterd door de bodem. Echter kleven er ook nadelen aan het hergebruik van afvalwater voor irrigatie. De verontreinigingen in het afvalwater kunnen schade toebrengen aan de volksgezondheid en het milieu. Hoewel er normen bestaan voor enkele traditionele verontreinigingsgroepen (bijvoorbeeld zware metalen of bepaalde bacteriën), is dit niet het geval voor veel andere verontreinigingen, waaronder de zogenaamde opkomende stoffen als geneesmiddelen en verzorgingsproducten. In dit proefschrift worden meerdere uitdagingen beschouwd die gerelateerd zijn aan de verontreinigingen in afvalwater en hun schadelijke effecten op de volksgezondheid en het milieu. Dit proefschrift is opgebouwd uit twee delen. In het eerste deel focussen we op algemene analyses die toepasbaar zijn op vele adsorberende en afbreekbare verontreinigingen, zoals veel opkomende stoffen. Het tweede deel van het proefschrift richt zich op bod

ProblemGully erosion is one of the most severe land degradation processes in Ethiopia's highlands, posing a significant threat to people and the environment. It destroys the environment and the economy by disrupting various soil and land functions. Gully conservation efforts did better in semi-arid regions than in humid ones. Due to a lack of understanding of the erosion process, which results in a poor technical quality of soil and water conservation measures, and a lack of integrated approach, gully restoration efforts in sub-humid Ethiopia were unsuccessful. Understanding the soil and hydrological processes that control gully erosion and identifying gully hotspot areas is critical for successfully implementing gully reclamation measures.MethodIn the 2017 and 2018 Ethiopian rainy seasons, the geomorphic adjustment of two gullies, one eroding on Nitisol and the other on Vertisol, representing the dominant valley bottom soils in the Minzir catchment, was studied. Both gullies cut through communal grazing land in the valley bottom and are actively eroding. This research aims to understand the VBG erosion process using field data and modeling approaches. A literature review across different agroclimatic conditions supports the identification of parameters for further field investigation. Soil hydrological, meteorological, and gully morphological parameters were monitored to assess the impact of various interacting factors on gully erosion. Secondary data such as digital elevation model (DEM), soil, and land cover were used to locate gully erosion hotspots and rank factors based on their importance to gully erosion. Finally, we used a Python programming tool to develop a landscape gully erosion model called Landscape Evolution by Gully Erosion.FindingsIn Chapter 2, a literature review identified the causes and controlling factors of valley bottom gullies. It also found deficiencies in existing rehabilitation measures. Valley bottom gullies appear as incisions in the lo

Water resource scarcity, food security, and environmental pollution are three major issues, that threaten the sustainability of the natural environment and the lives and livelihoods of people around the globe, especially the marginalized and those who live in developing regions. Despite increasing efforts by scientists and policymakers to resolve these issues, there is still no end in sight, partly because of the lack of conviction, and partly due to inability. With currently available technology, it is often necessary to make sacrifices regarding one of the issues in order to effect an improvement in another. For example, attempts to tackle food shortages by increasing agricultural production would necessitate the use of more water resources for irrigation, and agricultural chemicals for soil fertilization, which in turn may exacerbate water scarcity and environmental pollution, resulting in no net benefit to sustainability. It is therefore imperative that methods to resolve such issues that have minimal trade-offs are developed.A new method of agricultural irrigation that has the potential to alleviate the above three issues is currently being tested at an experimental agricultural site in the Netherlands. This new method involves a newly developed subsurface irrigation and drainage system, irrigated with treated wastewater. To justify the use and development of this new method, Chapter 1 provides some historical context and a brief technical description of the method, along with an introductory discussion on its risks, benefits, shortcomings, and advantages in terms of environmental sustainability and crop contamination. Wastewater, including treated wastewater as no treatment technique can fully remove all impurities from wastewater without consuming large amounts of energy, contains residual biological and chemical substances. Some of these substances are known as contaminants of emerging concern (CECs), and are toxic or ecotoxic substances that are poorly unde

The recovery efficiency of aquifer storage systems with radial flow fields are studied for heterogeneous aquifers. Macrodispersion, arising from spatially heterogeneous hydraulic conductivity, is modeled as a scale-dependent mechanical dispersion process. Approximate solutions for the recovery efficiency as a function of local dispersion and macrodispersion parameters, the injection-extraction rate (Formula presented.) and duration (Formula presented.), and storage cycle count, are derived and validated against numerical simulations. If macrodispersion dominates and the macrodispersion coefficient scales linearly with distance, the recovery efficiency is independent of both (Formula presented.). For sublinear and superlinear scalings, recovery increases and decreases respectively if (Formula presented.) increases. However, if local dispersion dominates, increasing (Formula presented.) always increases recovery. As macrodispersion becomes increasingly dominant with scale, the recovery efficiency may be a nonmonotonic function of (Formula presented.), with a maximum. In homogeneous aquifers, nonmonotonicity does not occur for 1D and 2D radial flow, but occurs for 3D radial flow fields only as a function of (Formula presented.), not (Formula presented.). These methods may also be used for fitting local dispersion and macrodispersion parameters with push-pull tests using recovery data, with advantages in scope of applicability and ease of data acquisition and interpretation, compared to existing push-pull test methods, which fit to breakthrough curves and do not consider macrodispersion. Furthermore, characterizing macrodispersion with push-pull tests may be advantageous over methods that use observation wells, as observation well placement may be challenging in highly heterogeneous aquifers. The results show that the macrodispersion parameters are not innate aquifer hydraulic properties, as their values vary with flow field geometry.

The dataset includes both source code (in R) and data underlying the related publication, which is used to make analytical and numerical approximations of the groundwater level midway between the drains in a tile-drained field under controlled drainage. Both steady and transient simulations can be made, and active management of the drainage base is possible., This dataset is related to https://doi.org/10.1029/2021WR029623. The dataset includes both source code (in R) and data underlying the related publication, which is used to make analytical and numerical approximations of the groundwater level midway between the drains in a tile-drained field under controlled drainage. Both steady and transient simulations can be made, and active management of the drainage base is possible. When using the data, cite it as follows: van de Craats, D., van der Zee, S.E.A.T.M., & Leijnse, A. (2021). Anticipatory drainage base management for groundwater level optimization [Data Set]. Zenodo. https://doi.org/10.5281/zenodo.4428869 Please also cite the related publication if this data or source code is used.

Controlled drainage is normally operated on a seasonal timescale to increase availability of water and to limit nutrient loss to surface water. It is conceivable that with more frequent, or even daily, management of the drainage base, controlled drainage can be both more efficient and effective. With the analytical successive steady state model presented here, rapid simulations of groundwater level and discharge as a function of time for a field under either regular or controlled drainage are feasible. The model includes effects of sudden changes in drainage base as well as damping and attenuation of dynamics by the unsaturated zone. It is validated against a numerical model and shows good agreement with numerical results. We demonstrate how this analytical model may facilitate anticipatory drainage base management based on ensemble weather forecasts, such that groundwater levels can be maintained at a theoretical optimal level.

Artesian outflows are important for local populations as pumping is not required for them to get access to such a groundwater resource. However, in many world’s artesian basins, free-flow cessations have occurred due to artesian head-drops. In this PhD thesis, a characterization and numerical modelling of head changes were performed for a south artesian basin of Benin so as to capture plausible artesian heads evolution under population growth and climate change. Sensitivity of the simulated heads to hydraulic and storage properties as well as recharge was analyzed so as to understand the properties that are most important in such modelling endeavors. We found that the artesian basin is structured into peripheral and central parts, which have different reactions to external stresses. The peripheral artesian zone was found adjacent to groundwater recharge areas and is more subjected to higher head-fluctuations and hence to temporal free-flow cessation compared to central artesian zones.

Phosphorus (P) is a finite resource and critical to plant growth and therefore food security. Regional‐ and continental‐scale studies propose how much P would be required to feed the world by 2050. These indicate that Sub‐Saharan Africa soils have the highest soil P deficit globally. However, the spatial heterogeneity of the P deficit caused by heterogeneous soil chemistry in the continental scale has never been addressed. We provide a combination of a broadly adopted P‐sorption model that is integrated into a highly influential, large‐scale soil phosphorus cycling model. As a result, we show significant differences between the model outputs in both the soil‐P concentrations and total P required to produce future crops for the same predicted scenarios. These results indicate the importance of soil chemistry for soil‐nutrient modeling and highlight that previous influential studies may have overestimated P required. This is particularly the case in Somalia where conventional modeling predicts twice as much P required to 2050 as our new proposed model.

Upscaling the microscopic processes in charged porous media which are responsible for pore pressure evolution and swelling is a topic of ongoing research. Current theories assume homogeneous media with uniform pore sizes and the impact of microscopic heterogeneity is neglected. This is a preliminary study to determine the significance of such pore-scale heterogeneity on the pressure evolution in charged porous media, where we neglect deformation of the solid phase. We present a pore-network model to simulate salt transport and pressure evolution in a charged porous medium. Results show that, for pore radii following a log-normal distribution, the average pressure in heterogeneous networks are significantly lower than in homogeneous networks with the same mean pore size. This is expressed by lower average pressures, as well as lower streaming potentials and faster ion transport rates in heterogeneous networks. These results indicate that heterogeneity in charged porous media should be investigated further.

The Dutch lowlands may be subject to salinization, through saline upward seepage and increased salt concentrations in the surface water, which may affect agriculture and nature areas with floating fens. A simple approach was proposed to estimate root zone salinization in areas with fresh water lenses that float on upward seeping saline groundwater, based on analytical solutions and approximations, which could be used for prioritization. The hydrology of floating fens was investigated and important processes and parameters were identified, which could be used to estimate root zone exposure to salinity. The flow processes are mostly controlled by the root mat geometry, which determines which part of the root mat floats within the surface water. A greenhouse experiment showed that salinity may affect the growth of some plant species from floating fens, which implies that salinization may affect species composition in these nature areas.

Measurements of soil water storage are hard to obtain on scales relevant for water management and policy making. Therefore, this research develops a new measurement methodology for soil water storage estimation in clay containing soils. The proposed methodology relies on the specific property of clay soils to shrink when drying and to swell when (re-)wetted, and the capabilities of a remote sensing technique called satellite based radar interferometry (InSAR) to measure centimetre to millimetre scale displacements of the soil surface. The objective of this thesis was to develop the application of InSAR for soil water storage change estimation on the field scale to regional scale. Two relations are investigated: 1) the relation between water storage change and surface elevation change as a result of swelling and shrinkage of a clayey soil; and 2) the relation between these surface elevation changes and InSAR phase observations. The shrinkage potential of the soil is very important for successful application of radar interferometry to measure vertical deformation as a result of swelling and shrinkage of clay. Therefore, the shrinkage potential and the water storage change-volume change relation (called the soil shrinkage characteristic, SSC) have been quantified in the laboratory for clay aggregates from the study area in the Purmer, the Netherlands. The clay content of the sampled soil ranged from 3.4 to 23.6%. The aggregates had moderate shrinkage potential over the soil moisture content range from saturation to air-dryness. Shrinkage phases were distinguished based on the portion of water content change that was compensated by volume change. Approximately 40-50% of water was released in the normal shrinkage phase, where loss of water is fully compensated by volume change. However, the residual shrinkage phase, where volume change is smaller than water content change, started at approx. 50% normalized soil moisture content (actual moisture content with respect to the m

Land degradation is a large problem worldwide, especially in agricultural areas. Between 1-6 billion ha of land worldwide is affected by land degradation. With an increasing world population, more food production is needed and, therefore, more land is converted into agricultural areas. This conversion of land to agricultural areas, in turn, leads to more land degradation. Some common forms of land degradation are desertification, salinization and soil erosion by water. The negative effects of soil erosion have been recognized for a long time. Since the early 20th century, researchers have tried to quantify soil displaced due to water, and to measure and model the efficiency of management strategies. The implications of problems with upscaling, wrong process representation and equifinality include the difficulty to properly predict sediment sources, pathways and sinks within catchments. These problems then can translate into the implementation of sub-optimal management strategies. To deal with these non-linear processes and the lack of proper representation of water and sediment sources, pathways and sinks, the concept of connectivity was developed. Currently, many definitions of connectivity have been proposed, although the definition most used is that of hydrological connectivity by Pringle (2003): ‘Hydrologic connectivity is the water-mediated transport of matter, energy and organisms within or between elements of the hydrologic cycle’. A unified theory on what constitutes connectivity and how connectivity should be measured or inferred remains one of the biggest challenges within catchment science. In addition, it is unclear whether connectivity should be an output or an input of a model and if an input, whether this should be added explicitly or implicitly. The main objective of this thesis was, therefore, to assess and quantify hydrological and sediment connectivity in a meaningful way, which can further our understanding of hydrological and sediment transport

In coastal-zone fields with a high groundwater level and sufficient rainfall, freshwater lenses are formed on top of saline or brackish groundwater. The fresh and the saline water meet at shallow depth, where a transition zone is found. This study investigates the mixing zone that is characterized by this salinity change, as well as by cation exchange processes, and which is forced by seepage and by rainfall which varies as a function of time. The processes are first investigated for a one-dimensional (1D) stream tube perpendicular to the interface concerning salt and major cation composition changes. The complex sequence of changes is explained with basic cation exchange theory. It is also possible to show that the sequence of changes is maintained when a two-dimensional field is considered where the upward saline seepage flows to drains. This illustrates that for cation exchange, the horizontal component (dominant for flow of water) has a small impact on the chemical changes in the vertical direction. The flow’s horizontal orientation, parallel to the interface, leads to changes in concentration that are insignificant compared with those that are found perpendicular to the interface, and are accounted for in the 1D flow tube. Near the drains, differences with the 1D considerations are visible, especially in the longer term, exceeding 100 years. The simulations are compared with field data from the Netherlands which reveal similar patterns.

In winter, saturation excess (SE) ponding is observed regularly in temperate lowland regions. Surface runoff dynamics are controlled by small topographical features that are unaccounted for in hydrological models. To better understand storage and routing effects of small-scale topography and their interaction with shallow groundwater under SE conditions, we developed a model of reduced complexity to investigate SE runoff generation, emphasizing feedbacks between shallow groundwater dynamics and mesotopography. The dynamic specific yield affected unsaturated zone water storage, causing rapid switches between negative and positive head and a flatter groundwater mound than predicted by analytical agrohydrological models. Accordingly, saturated areas were larger and local groundwater fluxes smaller than predicted, leading to surface runoff generation. Mesotopographic features routed water over larger distances, providing a feedback mechanism that amplified changes to the shape of the groundwater mound. This in turn enhanced runoff generation, but whether it also resulted in runoff events depended on the geometry and location of the depressions. Whereas conditions favorable to runoff generation may abound during winter, these feedbacks profoundly reduce the predictability of SE runoff: statistically identical rainfall series may result in completely different runoff generation. The model results indicate that waterlogged areas in any given rainfall event are larger than those predicted by current analytical groundwater models used for drainage design. This change in the groundwater mound extent has implications for crop growth and damage assessments.

In winter, saturation excess (SE) ponding is observed regularly in temperate lowland regions. Surface runoff dynamics are controlled by small topographical features that are unaccounted for in hydrological models. To better understand storage and routing effects of small scale topography and their interaction with shallow groundwater under SE conditions, we developed a model of reduced complexity to investigate SE runoff generation, emphasizing feedbacks between shallow groundwater dynamics and mesotopography. The dynamic specific yield affected unsaturated zone water storage, causing rapid switches between negative and positive head and a flatter groundwater mound than predicted by analytical agro-hydrological models. Accordingly, saturated areas were larger and local groundwater fluxes smaller than predicted, leading to surface runoff generation. Mesotopographic features routed water over larger distances, providing a feedback mechanism that amplified changes to the shape of the groundwater mound. This in turn enhanced runoff generation, but whether it also resulted in runoff events depended on the geometry and location of the depressions. Whereas conditions favourable to runoff generation may abound during winter, these feedbacks profoundly reduce the predictability of SE runoff: statistically identical rainfall series may result in completely different runoff generation. The model results indicate that waterlogged areas in any given rainfall event are larger than those predicted by current analytical groundwater models used for drainage design. This change in the groundwater mound extent has implications for crop growth and damage assessments.

Whereas irrigation and drainage are intended to address the shortage and surplus of soil water, respectively, an important aspect to address is also the management of salinity. Plants have a limited tolerance for soil water salinity, and despite significant gaps in our practical knowledge, an impression of acceptable salinities is available for many crops. To manage soil salinity, the Leaching Requirement is an old, yet useful, concept. In this chapter, we extend this concept for soils with shallow groundwater. Particularly if shallow groundwater is saline, management is needed to avoid capillary rise of this water into the root zone. One of the tools to do so is Climate Adaptive Drainage (CAD), for which many practical gaps in knowledge remain. Also, soil mulching, of which a special case is considered in more detail, i.e., using plastic covers, may be beneficial for many purposes, including improving the water and salt balances of the root zone. However, use of plastics may have significant adverse effects. Due to water shortage, also wastewater may be re-used for irrigation. For this reason, the hazard of sodicity due to elevated Na concentrations in domestic wastewater is highlighted.