Dry Initial Conditions Research Articles

Hydrometeorological controls of and social response to the 22 October 2019 catastrophic flash flood in Catalonia, north-eastern Spain

Abstract. On 22 October 2019, the Francolí River basin in Catalonia, north-eastern Spain, experienced a heavy precipitation event that resulted in a catastrophic flash flood, causing six fatalities. Few studies comprehensively address both the physical and human dimensions and their interrelations during extreme flash flooding. This research takes a step forward towards filling this gap in knowledge by examining the alignment among all these factors. The hydrometeorological factors are investigated using the new Triangle-based Regional Atmospheric Model, radar-derived precipitation estimates, post-flood field and gauge observations, and the Kinematic Local Excess Model. The social dimension is assessed by examining the relationship between catchment dynamics and warning response times and by quantifying human behaviour during the course of the flash flood through a post-event citizen science campaign. Results reveal that a persistent south-easterly airflow brought low-level moisture and established convective instability in the region, while local orography was instrumental to triggering deep moist convection. A convective train promoted intense, copious, and prolonged precipitation over the north-western catchment headwaters. Basin response was significantly modulated by the very dry initial soil moisture conditions. After the long-lasting rainfall, an acute burst of precipitation resulted in extreme flash flooding. Fast and abrupt increases in streamflow affect small spatial scales and leave limited time for the effective implementation of protective measures. The institutional organization–protection–prevention cycle unfolded at the spatial and temporal scales typically dominated by the meteorological rather than hydrological scales. Although the citizen science campaign reveals the effectiveness of the warnings in reaching the population living in the most affected areas, a significant proportion of the respondents expressed a lack of adequate information or were unfamiliar with the intended meaning. In addition, a majority of the interviewees did not perceive any significant threat to life or property. In view of these results, this study identifies potential areas for improving social preparedness for similar natural hazards in the future.

Responses of soil moisture at different topographic positions to rainfall events along a steep subtropical forested hillslope

AbstractUnderstanding the dynamic response of soil moisture to rainfall is crucial for describing hydrological processes at the hillslope scale. However, because of sparse monitoring coupled with the complexity of water movement and steep topography, the findings of rainfall‐related soil moisture dynamics have not always been consistent, indicating a need for systematic investigations of soil moisture dynamics and infiltration patterns following rainfall inputs at multiple topographic positions along a hillslope. This study aimed to examine the nature of these responses by characterizing and quantifying the response amplitude, rate and time for 37 large rainfall events at 25 combinations of topographic positions and soil depths along a steep forested hillslope. Our results showed that soil moisture responses under different rainfall patterns could be attributed to one or the other rainfall characteristics, such as rainfall intensity and amount. However, soil moisture dynamics at different hillslope positions after rainfall varied widely due to the controls of soil properties, topography, and non‐equilibrium flow. Preferential flow was more evident under dry initial soil conditions than under wet initial soil conditions. Findings of this study reveal that the dynamic response patterns of soil moisture to rainfall do not always follow topographic controls, which can improve our understanding of water cycling related to the infiltration process at the hillslope scale, and support water resources management in subtropical mountain ecosystems.

Spectral imaging of grass species in arid ecosystems of Namibia

Grasslands across the African continent are under pressure from climate change and human activities, particularly in arid ecosystems. From a remote sensing perspective, these ecosystems have not received much scientific attention, especially in Namibia. To address this knowledge gap, various remote sensing methods were implemented using new generation spaceborne imaging spectrometers amongst others. Therefore, this research provides a first methodological approach aimed at mapping and evaluating the distribution of grasslands within two private nature reserves, namely, the NamibRand Nature Reserve (NRNR) and ProNamib Nature Reserve (PNNR) with surrounding farmlands on the edge of Namib Sand Sea. The multi-sensor approach utilizes Mixture Tuned Matched Filtering (MTMF) and incorporated spectral information collected in the field to analyze grasslands. The research involves a sensor comparison of multispectral Sentinel-2 and PlanetScope data, hyperspectral data from Environmental Mapping and Analysis Programme (EnMAP) and PRecursore IperSpettrale della Missione Applicativa (PRISMA) and an additional data fusion product derived from Sentinel-2 and EnMAP imagery based on a Smoothing Filter-based Intensity Modulation Hypersharpening method (SFIM-HS). Additionally, a unique spectral library of collected field spectra was established and inter-species spectral separability and intra-species spectral homogeneity was analyzed. This library presents newly published spectra of individual species. Due to dry initial conditions, the calculated spectral separability of individual grasses is limited, making only a mean endmember feasible for partial unmixing. The validation results of satellite comparison show that data fusion products (R2 = 0.51 with Normalized Difference Vegetation Index (NDVI); R2 = 0.66 with Soil Adjusted Vegetation Index (SAVI)) are more suitable for mapping arid grasslands than multispectral or hyperspectral data (all R2 < 0.35). More research is required and potential methodological adjustments are discussed to further investigate the spatio-temporal dynamics of arid grasslands and to aid conservation efforts in the Greater Sossusvlei-Namib Landscape in line with the United Nations Decade of Restoration.

Estimation of Infiltration Parameters: The Role of Pedotransfer Functions and Initial Moisture Conditions

Infiltration is one of the key components of rainfall-runoff processes at small catchment scale and is therefore a vital component of hydrological models. However, small catchments typically lack direct infiltration measurements and associated soil hydraulic characteristics, which are being substituted with soil databases and pedotransfer functions built upon these databases. We utilized two sets of pedotransfer functions Rosetta 3 and EUPTFv2 derived from two well known soil databases: UNSODA, based on US and global soil data, and EU-HYDI, based on EU soil data. Our study evaluates the impact of choosing one of these pedotransfer functions on modeled infiltration in relation to variable initial moisture content- It investigates the behavior of the simplified infiltration models of Green-Ampt and Philip under different initial conditions, in comparison with Richards’ complex solution, which is used as a benchmark. Our evaluation was conducted using soil property data available in the Czech Republic, covering ten USDA soil textures that are prevalent within the European Union. The applied initial moisture conditions were derived for specific suction pressure values that correspond to hydrological initial conditions ranging from near saturation to near the permanent wilting point. Variation in the derived parameters for the Green-Ampt and Philip models across the pedotransfer function sources results in considerable variation in the modeled infiltration. Our results imply different sensitivities of the simplified infiltration models to the initial moisture for different pedotransfer functions. The Green-Ampt and Philip infiltration models demonstrate better alignment with Richards’ complex solution with a low and consistent RMSE when using the Rosetta 3 pedotransfer function. By contrast, the EUPTFv2 model scenarios, show a clear trend of increasing RMSE towards dry initial conditions. As the European soil database is not an open data source, we only hypothesize that the distributions of soil characteristics in the European soil database differ systematically from those in UNSODA, and that this would cause different results of pedotransfer functions for different initial moisture conditions. The findings of this study are important for selecting appropriate infiltration models and, appropriate pedotransfer functions for estimating model parameters, and for assessing uncertainties connected with varying initial conditions.

Utility of remotely operated underwater vehicle in flood inundation mapping for dam failure: A case study of Lake Tuscaloosa Dam

AbstractA remotely operated underwater vehicle (ROV) is a tethered underwater mobile device that can conduct a bathymetric survey cost‐efficiently. Assessment of the utility of ROV‐based bathymetric surveys in flood inundation mapping remains limited. This study aims to examine the utility of ROV‐based bathymetric surveys in high‐resolution flood inundation mapping for a hypothetical case study of the Lake Tuscaloosa Dam breach in the state of Alabama, USA. This study conducted the sensitivity test of flood inundation mapping to the river channel depth (ROV‐based vs. digital elevation model [DEM]‐based) and initial flow condition (e.g., wet vs. dry), via six different simulations of the parallelized diffusion hydrodynamic model (pDHM). This study found that the ROV‐based pDHM runs had higher maximum water depths over the flood‐inundated areas, ranging from +73% to +166% of the simulated depths of the DEM‐based pDHM runs. However, the impact of initial streamflow condition on the maximum depths was limited. This study also found that the pDHM runs with a dry initial flow condition delayed the time to reach the maximum depth after the dam breach by 2 h relative to the pDHM runs with a wet initial streamflow condition. This study suggests that ROV‐based bathymetry surveys improve flood inundation mapping by emphasizing the influence of river channel depth, initial streamflow conditions, and bathymetry, thereby bolstering community resilience to a potential human‐made hazard such as dam failure.

Classification of Floods in Europe and North America with Focus on Compound Events

Compound events occur when multiple drivers or hazards occur in the same region or on the same time scale, hence amplifying their impacts. Compound events can cause large economic damage or endanger human lives. Thus, a better understanding of the characteristics of these events is needed in order to protect human lives. This study investigates the drivers and characteristics of floods in Europe and North America from the compound event perspective. More than 100 catchments across Europe and North America were selected as case study examples in order to investigate characteristics of floods during a 1979–2019 period. Air temperature, precipitation, snow thickness, snow liquid water equivalent, wind speed, vapour pressure, and soil moisture content were used as potential drivers. Annual maximum floods were classified into several flood types. Predefined flood types were snowmelt floods, rain-on-snow floods, short precipitation floods and long precipitation floods that were further classified into two sub-categories (i.e., wet and dry initial conditions). The results of this study show that snowmelt floods were often the dominant flood type in the selected catchments, especially at higher latitudes. Moreover, snow-related floods were slightly less frequent for high altitude catchments compared to low- and medium-elevation catchments. These high-altitude areas often experience intense summer rainstorms that generate the highest annual discharges. On the other hand, snowmelt-driven floods were the predominant flood type for the lower elevation catchments. Moreover, wet initial conditions were more frequent than the dry initial conditions, indicating the importance of the soil moisture for flood generation. Hence, these findings can be used for flood risk management and modelling.

Hydrometeorological analysis of the 12 and 13 September 2019 widespread flash flooding in eastern Spain

Abstract. On 12 and 13 September 2019, a long-lasting heavy precipitation episode (HPE) affected the València, Murcia and Almería regions in eastern Spain. Observed rainfall amounts were close to 500 mm in 48 h, being the highest cumulative precipitation registered in some rain gauges for the last century. Subsequent widespread flash flooding caused seven fatalities and estimated economical losses above EUR 425 million. High-resolution precipitation estimates from weather radar observations and flood response from stream gauges are used in combination with a fully distributed hydrological model to examine the main hydrometeorological processes within the HyMeX programme. This HPE was characterized by successive, well-organized convective structures that impacted a spatial extent of 7500 km2, with rainfall amounts equal to or larger than 200 mm. The main factors driving the flood response were quasi-stationarity of heavy precipitation, very dry initial soil moisture conditions and large storage capacities. Most of the examined catchments exhibited a dampened and delayed hydrological response to cumulative precipitation: until runoff thresholds were exceeded, infiltration-excess runoff generation did not start. This threshold-based hydrological behaviour may impact the shape of flood peak distributions, hindering strict flood frequency statistical analysis due to the generally limited lengths of data records in arid and semi-arid catchments. As an alternative, a power-law relationship is analytically derived to specifically link flood magnitude to rainfall amount, with the parameters depending on basin and precipitation features. This power law could aid in developing empirical envelope curves so as to assess hydrologic risk in arid and semi-arid catchments with a threshold-based hydrological behaviour.

Impact of Soil Moisture Initializations on WRF‐Simulated North American Monsoon System

AbstractThis study investigates the influence of soil moisture initialization on the North American Monsoon System (NAMS) using the Weather Research and Forecasting (WRF) model coupled with Noah‐MP Land Surface Model. Five sets of experiments using different soil moisture initializations were conducted from May 1 to October 1 in 2015–2017. The model forecast for each set of experiments consists of five ensemble members. The simulation using North American Regional Reanalysis is the control run (WRF‐Ctrl). Four sets of sensitivity experiments were conducted with extremely wet (WRF‐Wet) and dry (WRF‐dry) initial conditions and using different soil moisture products from the Global Land Data Assimilation System (WRF‐GLDAS) and NASA Soil Moisture Active Passive L4 (WRF‐SMAP). Results show that the WRF model can capture the key features of the NAMS, but it exhibits a wet bias in the higher elevation regions and a dry bias in the lower elevation regions. Our analysis of extremely wet and dry cases reveals that initial soil moisture conditions can affect precipitation through both modulating surface energy partitioning and influencing large‐scale pressure and wind patterns. When different soil moisture products are used to initialize the model, it can lead to substantial changes in spatial pattern of the NAM precipitation. Initializing the model with SMAP and GLDAS can reduce the overestimation of precipitation over high terrain areas. The results indicate that high‐quality soil moisture products have the potential to improve model representation of the NAMS, especially with future improvements in model convective parameterization.

Patterns of plant rehydration and growth following pulses of soil moisture availability

Abstract. Plant hydraulic and photosynthetic responses to individual rain pulses are not well understood because field experiments of pulse behavior are sparse. Understanding individual pulse responses would inform how rainfall intermittency impacts terrestrial biogeochemical cycles, especially in drylands, which play a large role in interannual global atmospheric carbon uptake variability. Using satellite-based estimates of predawn plant and soil water content from the Soil Moisture Active Passive (SMAP) satellite, we quantify the timescales of plant water content increases following rainfall pulses, which we expect bear the signature of whole-plant mechanisms. In wetter regions, we find that plant water content increases rapidly and dries along with soil moisture, which we attribute to predawn soil–plant water potential equilibrium. Global drylands, by contrast, show multi-day plant water content increases after rain pulses. Shorter increases are more common following dry initial soil conditions. These are attributed to slow plant rehydration due to high plant resistances using a plant hydraulic model. Longer multi-day dryland plant water content increases are attributed to pulse-driven growth, following larger rain pulses and wetter initial soil conditions. These dryland responses reflect widespread drought recovery rehydration responses and individual pulse-driven growth responses, as supported by previous isolated field experiments. The response dependence on moisture pulse characteristics, especially in drylands, also shows ecosystem sensitivity to intra-annual rainfall intensity and frequency, which are shifting with climate change.

Numerical investigation of flow instabilities using fully unstructured discretization for variably saturated flow problems

Richards equation for variably saturated flow can exhibit stability problems due to its nonlinearity. The challenge is to resolve sharp wetting fronts without introducing spurious oscillations, especially for simulations with very dry initial conditions. Flow instabilities at sharp wetting fronts may arise particularly in anisotropic and heterogeneous media, leading to oscillations or convergence problems. The focus of this work is to evaluate and minimize instability problems for simulations with unstructured meshes. To this end, numerical experiments were performed to investigate the accuracy, monotonicity and convergence behavior of numerical solutions for variably saturated flow based on different control volume methods, piecewise gradient reconstruction methods, and flux approximation methods. In particular, nonphysical oscillations at the wetting front were investigated. A novel multi-point flux approximation with multi-point upstream weighting based on piecewise gradient reconstruction was developed. Numerical simulations in both homogeneous and heterogeneous domains with isotropic and anisotropic conductivity tensors demonstrate that the proposed multi-point flux approximation with multi-point upstream weighting avoids spurious oscillations and improves the results for challenging sharp wetting front problems using general unstructured meshes as well as meshes with distortion. The revised Voronoi-dual control volume method has also been found to provide more flexibility than commonly used center-dual and median-dual control volume methods.

Spatio-temporal relevance and controls of preferential flow at the landscape scale

Abstract. The spatial and temporal controls of preferential flow (PF) during infiltration are still not fully understood. As soil moisture sensor networks allow us to capture infiltration responses in high temporal and spatial resolution, our study is based on a large-scale sensor network with 135 soil moisture profiles distributed across a complex catchment. The experimental design covers three major geological regions (slate, marl, sandstone) and two land covers (forest, grassland) in Luxembourg. We analyzed the responses of up to 353 rainfall events for each of the 135 soil moisture profiles. Non-sequential responses (NSRs) within the soil moisture depth profiles were taken as one indication of bypass flow. For sequential responses maximum porewater velocities (vmax⁡) were determined from the observations and compared with velocity estimates of capillary flow. A measured vmax⁡ higher than the capillary prediction was taken as a further indication of PF. While PF was identified as a common process during infiltration, it was also temporally and spatially highly variable. We found a strong dependence of PF on the initial soil water content and the maximum rainfall intensity. Whereas a high rainfall intensity increased PF (NSR, vmax⁡) as expected, most geologies and land covers showed the highest PF under dry initial conditions. Hence, we identified a strong seasonality of both NSR and vmax⁡ dependent on land cover, revealing a lower occurrence of PF during spring and increased occurrence during summer and early autumn, probably due to water repellency. We observed the highest fraction of NSR in forests on clay-rich soils (slate, marl). vmax⁡ ranged from 6 to 80 640 cm d−1 with a median of 120 cm d−1 across all events and soil moisture profiles. The soils in the marl geology had the highest flow velocities, independent of land cover, especially between 30 and 50 cm depth, where the clay content increased. This demonstrates the danger of treating especially clay soils in the vadose zone as a low-conductive substrate, as the development of soil structure can dominate over the matrix property of the texture alone. This confirms that clay content and land cover strongly influence infiltration and reinforce PF, but seasonal dynamics and flow initiation also have an important impact on PF.

NUMERICAL EVALUATION OF UNSATURATED FLOW IN POROUS MEDIA USING ITERATIVE TECHNIQUES FOR VARIOUS SOILS

Unrelenting difficulties that arise in forming numerical solutions of water flow in variably saturated problems include controlling the stability of the nonlinear equation solver. Besides, the accuracy of solutions, also of concern is the required computational effort, especially when nonlinear soil hydraulic properties and dry initial conditions are involved. A general numerical algorithm in the context of finite element formulation and Picard and Newton iteration schemes are described to solve Richards’ equation, in which a mass-conservative, head based formulation is proposed to approximate the governing equation, and mass-lumping techniques is employed to improve solution convergence and stability behavior. A difficult one-dimensional test case presented emphasizes different aspects of the performance of the iterative methods and the different issues that can influence their convergence and efficiency. The result is validated with extensive average values of soil hydraulic parameters for 12 soil textural groups.

A Comprehensive Model for Single Ring Infiltration I: Initial Water Content and Soil Hydraulic Properties

Core Ideas New single ring infiltration model accounts for a wide range of experimental conditions. Soil capillarity as function of initial water content was explored. Model closely matched HYDRUS‐3D simulations for thirty infiltration scenarios. Model was tested with both Brooks and Corey and van Genuchten–Mualem hydraulic parameters. Brooks and Corey parameters gave more consistent predictions for cumulative infiltration. Single ring infiltration tests are commonly used to understand soil hydraulic properties. While a number of models have been developed to describe three‐dimensional infiltration from single ring sources, these expressions are often restricted to cases where the soil is initially dry and the source pressure head is negligible. These conditions may restrict the use of these expressions in real field settings. In this study we modified a set of infiltration models to explicitly account for variations in soil hydraulic properties, initial conditions, and experimental setups. The resultant expressions allowed us to explore how soil capillary length (a measure of the capillary force acting through the wetted zone) and three‐dimensional wetting profiles vary under different initial matric heads. Specifically, as initial matric head increases, infiltration becomes increasingly one‐dimensional due to a decrease of the soil capillary force. However, for most realistic situations (i.e., moderately wet to very dry initial conditions) the model can be simplified by assuming a constant capillary length. The resulting expressions compared well with numerical results from HYDRUS‐3D for five different soils, including both dry and wet initial conditions, two ring insertion depths, and two ponding depths. Overall, the model provides further insight on the variations in soil capillarity (as sorptivity) and infiltration across a range of soil types and initial conditions, and can be used to describe single ring infiltration under nearly every scenario.

Quantity and Quality of Dispersed Fine Particles after the Low‐Energy Water‐Dispersible Soil Test: Impact of the Initial Soil Matrix Potential

Core Ideas Moist initial conditions allow better discrimination of particle dispersion among soils. The nature of dispersed <2‐μm particles varies with the initial soil matrix condition. The low‐energy water‐dispersible clay test at two different initial conditions allows soil functioning analysis. Transfer of <2‐µm soil particles is a key process in the dispersion of pollutants and the textural differentiation of soil. Dispersion tests involving low‐energy water‐dispersible clay (LEWDC) are commonly used to indicate the stock of particles <2‐µm that can be mobilized. However, notable differences in LEWDC protocols exist. We discuss the impact of the initial soil matrix potential on the dispersion sensitivity of <2‐µm particles and the nature (mineralogy and particle size) of the dispersed particles. The dispersion sensitivity was more differentiated among soils with a high initial water content, which allows better soil discrimination. The nature of dispersed <2‐µm particles was closer to that of the bulk <2‐µm soil when moist initial conditions were considered, whereas enrichment in coarse clay minerals (with a particle‐size mode of around 2 µm) was observed with dry initial conditions. Performing the LEWDC test under both moist and dry initial conditions can be used to interpret soil functioning.

Exploration of warm-up period in conceptual hydrological modelling

One of the important issues in hydrological modelling is to specify the initial conditions of the catchment since it has a major impact on the response of the model. Although this issue should be a high priority among modelers, it has remained unaddressed by the community. The typical suggested warm-up period for the hydrological models has ranged from one to several years, which may lead to an underuse of data. The model warm-up is an adjustment process for the model to reach an ‘optimal’ state, where internal stores (e.g., soil moisture) move from the estimated initial condition to an ‘optimal’ state. This study explores the warm-up period of two conceptual hydrological models, HYMOD and IHACRES, in a southwestern England catchment. A series of hydrologic simulations were performed for different initial soil moisture conditions and different rainfall amounts to evaluate the sensitivity of the warm-up period. Evaluation of the results indicates that both initial wetness and rainfall amount affect the time required for model warm up, although it depends on the structure of the hydrological model. Approximately one and a half months are required for the model to warm up in HYMOD for our study catchment and climatic conditions. In addition, it requires less time to warm up under wetter initial conditions (i.e., saturated initial conditions). On the other hand, approximately six months is required for warm-up in IHACRES, and the wet or dry initial conditions have little effect on the warm-up period. Instead, the initial values that are close to the optimal value result in less warm-up time. These findings have implications for hydrologic model development, specifically in determining soil moisture initial conditions and warm-up periods to make full use of the available data, which is very important for catchments with short hydrological records.

Validation of COBRA-SFS with Measured Temperature Data from Spent-Fuel Storage Casks

The COBRA-SFS thermal-hydraulic code has been incorporated into the Used Nuclear Fuel-Storage, Transportation & Disposal Analysis Resource and Data System tool as a module devoted to spent-fuel-package thermal analysis. COBRA-SFS has been extensively validated and widely applied to thermal-hydraulic analysis of a large range of spent-fuel storage systems. Instead of recapping that long and detailed history, this paper summarizes the most significant and unique verification and validation of COBRA-SFS, which consists of comparison of code temperature predictions to experimental data obtained in the Test Area North Facility at the Idaho National Laboratory in the 1980s and early 1990s. These data were obtained as part of a program undertaken by the U.S. Department of Energy Office of Civilian Radioactive Waste Management for thermal performance testing of commercial spent-fuel storage cask designs. In total, four casks were tested, and all tests were performed with Westinghouse 15×15 pressurized water reactor spent fuel from the Surry or Turkey Point reactors. COBRA-SFS code results and experimental data comparisons are shown only for the CASTOR-V/21 and the TN-24P casks. CASTOR-V/21 was loaded with the highest decay heat load tested in this program, with individual assembly decay heat values up to 1.83 kW. This effectively bounds storage conditions currently contemplated for high-heat-load systems with test conditions reaching fuel cladding temperatures that approached and in some cases exceeded 400°C, the current regulatory limit for peak cladding temperature in dry storage. TN-24P, with a decay heat load of 20.5 kW, provides comparisons with experimental data that represent a realistic upper bound on typical dry storage initial conditions in independent spent fuel storage installations around the country. The consistency and accuracy of the COBRA-SFS temperature predictions in comparison to the measured data from these casks show that the code appropriately predicts the thermal-hydraulic and heat transfer behavior of these systems. The results presented here provide an excellent illustration of the capability of the COBRA-SFS code to correctly capture all three modes of heat transfer (thermal radiation, conduction, and convection) and the internal circulation of the backfill gas within a spent-fuel package in horizontal or vertical orientation.

A MASS LUMPING AND DISTRIBUTING FINITE ELEMENT ALGORITHM FOR MODELING FLOW IN VARIABLY SATURATED POROUS MEDIA

The Richards equation for water movement in unsaturated soil is highly nonlinear partial differential equations which are not solvable analytically unless unrealistic and oversimplifying assumptions are made regarding the attributes, dynamics, and properties of the physical systems. Therefore, conventionally, numerical solutions are the only feasible procedures to model flow in partially saturated porous media. The standard Finite element numerical technique is usually coupled with an Euler time discretizations scheme. Except for the fully explicit forward method, any other Euler time-marching algorithm generates nonlinear algebraic equations which should be solved using iterative procedures such as Newton and Picard iterations. In this study, lumped mass and distributed mass in the frame of Picard and Newton iterative techniques were evaluated to determine the most efficient method to solve the Richards equation with finite element model. The accuracy and computational efficiency of the scheme and of the Picard and Newton models are assessed for three test problems simulating one-dimensional flow processes in unsaturated porous media. Results demonstrated that, the conventional mass distributed finite element method suffers from numerical oscillations at the wetting front, especially for very dry initial conditions. Even though small mesh sizes are applied for all the test problems, it is shown that the traditional mass-distributed scheme can still generate an incorrect response due to the highly nonlinear properties of water flow in unsaturated soil and cause numerical oscillation. On the other hand, non oscillatory solutions are obtained and non-physics solutions for these problems are evaded by using the mass-lumped finite element method.

Biocatalytic behavior of a new Aspergillus niger whole-cell biocatalyst with high operational stability during the synthesis of green biosolvent isopropyl esters

Using microbial cell-surface displayed enzymes to produce chemicals is a promising green chemistry procedure, but few studies evaluated filamentous fungi cell-surface display systems. Here, a novel Aspergillus niger whole-cell biocatalyst with excellent operational stability was constructed using a heterologous anchor protein, and the details about its practical catalytic characteristics during the synthesis of green biosolvent isopropyl esters (IPEs) were investigated. Candida antarctica lipase B (CALB) was chosen as a model enzyme, fused to a Saccharomyces cerevisiae glycosylphosphatidylinositol protein, SED1, and displayed on the A. niger surface, the hydrolytic activity of the A. niger mycelium-surface displayed CALB reached 440.30U/g dry cell. In solvent-free esterifications of medium-long chain fatty acids and isopropanol, the A. niger/CALB-Flag-SED1 whole-cell biocatalyst showed great organic substrate tolerance, high thermostability, no significant lag time in a dry initial condition and no synthetic activity decrease after five batch reactions at 65°C. The highest yields of isopropyl laurate, isopropyl myristate, and isopropyl palmitate, 79.21, 81.62, and 81.41%, respectively, were obtained after a 6-h reaction. Acetone washing helped to remove the excessive water accumulating during esterification and benefitted the operational stability. The relative acid conversions of the three IPEs in batch 5 were 99.79, 99.10, and 96.83%, respectively, of those in batch 1. The A. niger/CALB-Flag-SED1 whole-cell biocatalyst is a promising alternative to commercial immobilized products, and SED1 can serve as an efficient anchor module to display industrial enzymes in an A. niger expression system.

Effect of Surface Fluxes versus Radiative Heating on Tropical Deep Convection

Abstract The effects of turbulent surface fluxes and radiative heating on tropical deep convection are compared in a series of idealized cloud-system-resolving simulations with parameterized large-scale dynamics. Two methods of parameterizing the large-scale dynamics are used: the weak temperature gradient (WTG) approximation and the damped gravity wave (DGW) method. Both surface fluxes and radiative heating are specified, with radiative heating taken as constant in the vertical in the troposphere. All simulations are run to statistical equilibrium. In the precipitating equilibria, which result from sufficiently moist initial conditions, an increment in surface fluxes produces more precipitation than an equal increment of column-integrated radiative heating. This is straightforwardly understood in terms of the column-integrated moist static energy budget with constant normalized gross moist stability. Under both large-scale parameterizations, the gross moist stability does in fact remain close to constant over a wide range of forcings, and the small variations that occur are similar for equal increments of surface flux and radiative heating. With completely dry initial conditions, the WTG simulations exhibit hysteresis, maintaining a dry state with no precipitation for a wide range of net energy inputs to the atmospheric column. The same boundary conditions and forcings admit a rainy state also (for moist initial conditions), and thus multiple equilibria exist under WTG. When the net forcing (surface fluxes minus radiative heating) is increased enough that simulations that begin dry eventually develop precipitation, the dry state persists longer after initialization when the surface fluxes are increased than when radiative heating is increased. The DGW method, however, shows no multiple equilibria in any of the simulations.

CONSEQUENCE OF BACKWARD EULER AND CRANK-NICOLSOM TECHNIQUES IN THE FINITE ELEMENT MODEL FOR THE NUMERICAL SOLUTION OF VARIABLY SATURATED FLOW PROBLEMS

Modeling water flow in variably saturated, porous media is important in many branches of science and engineering. Highly nonlinear relationships between water content and hydraulic conductivity and soil-water pressure result in very steep wetting fronts causing numerical problems. These include poor efficiency when modeling water infiltration into very dry porous media, and numerical oscillation near a steep wetting front. A one-dimensional finite element formulation is developed for the numerical simulation of variably saturated flow systems. First order backward Euler implicit and second order Crank?Nicolson time discretization schemes are adopted as a solution strategy in this formulation based on Picard and Newton iterative techniques. Five examples are used to investigate the numerical performance of two approaches and the different factors are highlighted that can affect their convergence and efficiency. The first test case deals with sharp moisture front that infiltrates into the soil column. It shows the capability of providing a mass-conservative behavior. Saturated conditions are not developed in the second test case. Involving of dry initial condition and steep wetting front are the main numerical complexity of the third test example. Fourth test case is a rapid infiltration of water from the surface, followed by a period of redistribution of the water due to the dynamic boundary condition. The last one-dimensional test case involves flow into a layered soil with variable initial conditions. The numerical results indicate that the Crank?Nicolson scheme is inefficient compared to fully implicit backward Euler scheme for the layered soil problem but offers same accuracy for the other homogeneous soil cases.