Calibration of Parameter Fields Consisting of Multiple Statistical Populations

If a parameter field to be calibrated consists of more than one statistical population, usually not only the parameter values are uncertain, but the spatial distributions of the populations are uncertain as well. In this study, we demonstrate the potential of the multimodal calibration method we proposed recently for the calibration of such fields, as applied to real-world ground water models with several additional stochastic parameter fields. Our method enables the calibration of the spatial distribution of the statistical populations, as well as their spatially correlated parameterization, while honoring the complete prior geostatistical definition of the multimodal parameter field. We illustrate the implications of the method in terms of the reliability of the posterior model by comparing its performance to that of a "conventional" calibration approach in which the positions of the statistical populations are not allowed to change. Information from synthetic calibration runs is used to show how ignoring the uncertainty involved in the positions of the statistical populations not only denies the modeler the opportunity to use the measurement information to improve these positions but also unduly influences the posterior intrapopulation distributions, causes unjustified adjustments to the cocalibrated parameter fields, and results in poorer observation reproduction. The proposed multimodal calibration allows a more complete treatment of the relevant uncertainties, which prevents the abovementioned adverse effects and renders a more trustworthy posterior model.     

Percolation losses in paddy fields with a dynamic soil structure: model development and applications

The hydraulic characteristics of the plough pan of paddy fields provide continuous ponding conditions during the growing season and control the water use efficiency in wet rice production. Its saturated hydraulic conductivity K_s, however, exhibits a large spatiotemporal variability as a consequence of a highly dynamic soil structure involving temporary shrinkage cracks. Water flow through the earthen bunds surrounding the fields further contributes to the uncertainty in water flux calculations. The objective of this study was to develop a simple deterministic model with stochastic elements (‘PADDY‐FLUX’) for depiction of deep percolation, and to assess the effect of different water management scenarios on percolation in two channel command areas. Darcy's law is used as the fundamental equation for water flow calculations with the ponding water depth h as a time‐dependent variable. Flux uncertainty is estimated by a Monte‐Carlo‐type implementation. K_s is treated as a random variable of a bimodal probability density function (PDF), which is the weighted sum of two Gaussian PDFs (accounting for a matrix and a preferential flow domain). The weighing factor α is a function of h, reflecting an increasing risk for preferential flow situations after desiccation and the development of shrinkage cracks. Under‐bund percolation is calculated using transfer functions. The results demonstrate that percolation losses increase in the following order: continuous soil saturation < continuous flooding (CF) < mid‐season drainage and intermittent irrigation (MD + II) < mid‐season drainage and continuous flooding. The bunds contribute up to 54 and 17% to total fluxes under CF and MD + II, respectively. Preferential water fluxes are responsible for the major part of water losses as soon as desiccation causes the formation of shrinkage cracks. As a conclusion, continuous soil saturation should be promoted as the least water‐intensive irrigation regime, while intermittent irrigation is recommended only in case that irreversible shrinkage cracks have already developed. Copyright © 2010 John Wiley & Sons, Ltd.     

Impact of Berm Breaching on Hyperbenthic Macroinvertebrate Communities in Intermittently Closed Estuaries

Berm breaching of intermittently closed estuaries, either naturally or due to management practices, results in a physicochemical disturbance due to the flushing of water, material and biota into the ocean and the subsequent tidal influx. In 2007 and 2008 comparative and controlled studies were employed in three New Zealand estuaries: Sawmill Creek (46°04′ S 170°12′ E), Otokia Creek (45°57′ S 179°20′ E) and Kaikorai Lagoon (45°56′ S 170°23′ E), to investigate the impact of berm breaching on the hyperbenthic macroinvertebrate community in intermittently closed estuaries. Significant changes in community structure occurred in both the weekly comparative and the controlled studies (ANOSIM P < 0.01). Furthermore, the catch per unit effort of both total and key invertebrate taxa significantly increased after breaching (ANOVA P < 0.01). However, substantial numbers of individuals were expelled into the ocean (5,800 individuals, 20-min sample) while new taxa immigrated into the estuaries. Along with migration-related changes to community structure, berm breaches also resulted in the loss of ∼80% of inundated habitat and the concentration of existing populations. Consequently, the management of intermittent estuary connections to the ocean has implications for the ecology of individual, managed estuaries and also for regional coastal populations of epibenthic invertebrates.     

A Dutch multi-date land use database: Identification of real and methodological changes

Land cover and land use are important information sources for environmental issues. One of the most important changes at the Earth's surface concerns land cover and land use. Knowledge about the location and type of these changes is essential for environmental modeling and management. Remote sensing data in combination with additional spatial data are recognized as an important source of information to detect these land cover and land use changes.     

Occurrence and distribution of microplastics in marine sediments along the Belgian coast

Plastic debris is known to undergo fragmentation at sea, which leads to the formation of microscopic particles of plastic; the so called ‘microplastics’. Due to their buoyant and persistent properties, these microplastics have the potential to become widely dispersed in the marine environment through hydrodynamic processes and ocean currents. In this study, the occurrence and distribution of microplastics was investigated in Belgian marine sediments from different locations (coastal harbours, beaches and sublittoral areas).Particles were found in large numbers in all samples, showing the wide distribution of microplastics in Belgian coastal waters. The highest concentrations were found in the harbours where total microplastic concentrations of up to 390 particles kg⁻¹ dry sediment were observed, which is 15-50 times higher than reported maximum concentrations of other, similar study areas.The depth profile of sediment cores suggested that microplastic concentrations on the beaches reflect the global plastic production increase.     

Iron-hydroxide clogging of public supply wells receiving artificial recharge: near-well and in-well hydrological and hydrochemical observations

Clogging of water wells by iron-hydroxide incrustations due to mixing of anoxic and oxic groundwater is a common well-ageing problem. The relation between well operation (on and off), the spatial and temporal variations in hydrochemistry outside and inside a supply well, and the distribution of clogging iron-hydroxides were studied in an artificial recharge well field in the Netherlands. Camera inspection, high-resolution multi-level water sampling outside the well and detailed in-well pH/EC/O₂ profiles revealed remarkable patterns. During pumping, the top of the upper well screen abstracted oxic filtrate, although the larger part of the in-well water column was anoxic. The column rapidly turned oxic after shutdown due to a downward short-circuiting of oxic water via the well. Within 15 d it became anoxic due to the slow advance of anoxic lake filtrate created by local changes in flow direction as the neighboring wells continued to pump. Severe clogging occurred where the oxic filtrate entered the well, while half-clogging of the upper well screen occurred due to less inflow of oxic filtrate on the lake side. Transport of iron flocs and bacterial slimes after shutdown seemed to clog the lower part of the well screen. Frequent on/off switching should be avoided in iron-clogged wells.     

Effect of ENSO events on sediment production in a large coastal basin in northern Peru

Although the importance of ENSO on hydrological anomalies has been recognized, variations in sediment fluxes caused by these extreme events are poorly documented. The effect of ENSO is not limited to changes in sediment mobilization. Since ENSO events can affect terrestrial ecosystems, they may have important effects on sediment production and transport in river basins over time spans that are longer than the duration of the event itself. The Catamayo‐Chira basin is an interesting casestudy for investigating these geomorphic implications. The objectives were: (i) to study the effect of ENSO on stream flow and sediment yields in the basin, (ii) to investigate if ENSO events affect sediment yields in the post‐ENSO period and (iii) to understand which factors control the ENSO and post‐ENSO basin response. During strong negative ENSO periods, mean annual stream flow discharge at the inlet of the Poechos reservoir in the lower basin was 5.4 times higher than normal annual discharges, while average sediment fluxes exceeded those of normal years by a factor of about 11. In two heavily affected periods, 45.9% of the total sediment yield in the 29 years observation period was generated. Sediment fluxes in the post‐ENSO period are lower than expected, which proves post‐ENSO event dynamics are significantly different from pre‐event dynamics. Our analysis indicates the increase of vegetation growth in the lower basin is not the main reason explaining considerable sediment flux decrease in post‐ENSO periods. During strong ENSO events, sediment in alluvial stores in the lower part of the basin is removed due to enlarging and deepening of channels. In post‐ENSO periods, normal discharges and persisting sediment supplies from the middle/upper basin lead to river aggradation and storage of large amounts of sediment in alluvial plains. The decrease in sediment export will last for several years until the equilibrium is re‐established. Copyright © 2011 John Wiley & Sons, Ltd.     

Vegetation and topographic controls on sediment deposition and storage on gully beds in a degraded mountain area

Active gully systems developed on highly weathered or loose parent material are an important source of runoff and sediment production in degraded areas. However, a decrease of land pressure may lead to a return of a partial vegetation cover, whereby gully beds are preferred recolonization spots. Although the current knowledge on the role of vegetation on reducing sediment production on slopes is well developed, few studies exist on the significance of restoring sediment transport pathways on the total sediment budget of degraded mountainous catchments. This study in the Ecuadorian Andes evaluates the potential of vegetation to stabilize active gully systems by trapping and retaining eroded sediment in the gully bed, and analyses the significance of vegetation restoration in the gully bed in reducing sediment export from degraded catchments. Field measurements on 138 gully segments located in 13 ephemeral steep gullies with different ground vegetation cover indicate that gully bed vegetation is the most important factor in promoting short‐term (1–15 years) sediment deposition and gully stabilization. In well‐vegetated gully systems ( ≥ 30% of ground vegetation cover), 0.035 m³ m^–1 of sediment is deposited yearly in the gully bed. Almost 50 per cent of the observed variance in sediment deposition volumes can be explained by the mean ground vegetation cover of the gully bed. The presence of vegetation in gully beds gives rise to the formation of vegetated buffer zones, which enhance short‐term sediment trapping even in active gully systems in mountainous environments. Vegetation buffer zones are shown to modify the connectivity of sediment fluxes, as they reduce the transport efficiency of gully systems. First calculations on data on sediment deposition patterns in our study area show that gully bed deposition in response to gully bed revegetation can represent more than 25 per cent of the volume of sediment generated within the catchment. Our findings indicate that relatively small changes in landscape connectivity have the potential to create strong (positive) feedback loops between erosion and vegetation dynamics. Copyright © 2009 John Wiley & Sons, Ltd.