Tomas Vogel

Effect of the shape of the soil hydraulic functions near saturation on variably-saturated flow predictions

Abstract Relatively small changes in the shape of the soil water retention curve near saturation can significantly affect the results of numerical simulations of variably saturated flow, including the performance of the numerical scheme itself in terms of stability and rate of convergence. In this paper, we use a modified form of the van Genuchten–Mualem (VGM) soil hydraulic functions to account for a very small, but non-zero minimum capillary height, hs, in the soil water retention curve. The modified VGM model is contrasted with the original formulation by comparing simulation results for infiltration in homogeneous soils assuming both constant pressure and constant flux boundary conditions. The two models gave significantly different results for infiltration in fine-textured soils, even for hs-values as small as −1 cm. Incorporating a small minimum capillary height in the hydraulic properties leads to less non-linearity in the hydraulic conductivity function near saturation and, because of this, to more stable numerical solutions of the flow equation. This study indicates an urgent need for experimental studies that assess the precise shape of the hydraulic conductivity curve near saturation, especially for relatively fine-textured soils. For one example we found considerable improvement in the predicted conductivity function when a value of −2 cm for hs was used in the modified VGM model.

On the reliability of unsaturated hydraulic conductivity calculated from the moisture retention curve

In comparison with direct measurements of unsaturated hydraulic conductivity, the methods of calculations from the moisture retention curve are attractive for their fast and simple use and low cost. These are the main reasons for their increasing use, mainly in spatial variability studies. On the other hand, it is known that their applicability is limited. The possibility of the use of the retention curve to indirectly determine hydraulic conductivities is analyzed as follows. The theoretical derivation of the relationK(h) − θ(h) is briefly discussed with regards to potential sources of inaccuracy. The sensitivity of the algorithm forK(h) calculation is studied as a response to possible inaccuracies in the retention curve determination. Conclusions about the usability of calculated hydraulic conductivities are drawn.

Changes of steady-state infiltration rates in recurrent ponding infiltration experiments

Abstract Ponding infiltration experiments on coarse acid brown soils (Cambisol) are described to show that the steady-state infiltration rate depends on the initial moisture content, which is in contrast with the theory. The effect is observed on three different scales: (1) in the field at randomly chosen places near grid points of a network for two recurrent days under various initial moisture content; (2) in the field, in cylinders installed at fixed places for several measurements under different initial moisture content; and (3) on undisturbed samples taken to the laboratory, where also the outflow of the sample was observed. The effect is most apparent in repetitive ponding infiltration experiments and is ascribed to trapped air which alters the volume available for the gravity dominated flow. Instead of piston-like flow, downward flow through macropores takes place. As a consequence, the ponding infiltration experiment does not supply theoretically assumed constants.

A review of CO2 and associated carbon dynamics in headwater streams: A global perspective

Terrestrial carbon export via inland aquatic systems is a key process in the global carbon cycle. It includes loss of carbon to the atmosphere via outgassing from rivers, lakes or reservoirs and carbon fixation in the water column as well as in sediments. This review focuses on headwater streams that are important because their stream biogeochemistry directly reflects carbon input from soils and groundwaters that becomes superimposed by additional inputs further downstream. Major drivers of carbon dioxide partial pressures (pCO2) in streams and mechanisms of terrestrial dissolved inorganic, organic and particulate organic carbon (DIC, DOC, and POC) influxes are summarized in this work. Our analysis indicates that the global river average pCO2 of 3,100 ppmV is more often exceeded by contributions from small streams when compared to rivers with larger catchments (>500 km2). Because of their large proportion in global river networks (>96 % of the total number of streams), headwaters contribute large – but still poorly quantified – amounts of CO2 to the atmosphere. Conservative estimates imply that globally 36 % (i.e. 0.93 Pg C yr-1) of total CO2 outgassing from rivers and streams originate from headwaters. We also discuss challenges in determination of CO2 sources, concentrations and fluxes. To overcome uncertainties of CO2 sources and its outgassing from headwater streams on the global scale, new investigations are needed that should include groundwater data. Such studies would also benefit from applications of integral CO2 outgassing isotope approaches and multi-scale geophysical imaging techniques.

Simulated cadmium transport in macroporous soil during heavy rainstorm using dual-permeability approach

Numerical modelling is used to analyze the transport of cadmium in response to an extreme rainfall event. The cadmium transport through the soil profile was simulated by the one-dimensional dual-permeability model, which assumes the existence of two mutually communicating domains: the soil matrix domain and the preferential flow domain. The model is based on Richards’ equation for water flow and advection-dispersion equation for solute transport. A modified batch technique allowed us to consider domain specific sorption, i.e. each of the domains has its own distribution coefficient. The dual-permeability model predicts that the cadmium can be transported substantially below the root zone after the storm. On the other hand, classical single permeability approach predicted that almost all applied cadmium stays retained near the soil surface.

Uncertainty Analysis of a Dual-Continuum Model Used to Simulate Subsurface Hillslope Runoff Involving Oxygen-18 as Natural Tracer

Uncertainty Analysis of a Dual-Continuum Model Used to Simulate Subsurface Hillslope Runoff Involving Oxygen-18 as Natural Tracer A one-dimensional dual-continuum model (also known as dual-permeability model) was used to simulate the lateral component of subsurface runoff and variations in the natural 18O content in hillslope discharge. Model predictions were analyzed using the GLUE generalized likelihood uncertainty estimation procedure. Model sensitivity was evaluated by varying two separate triplets of parameters. The first triplet consisted of key parameters determining the preferential flow regime, i.e., the volumetric proportion of the preferential flow domain, a first-order transfer coefficient characterizing soil water exchange between the two flow domains of the dual-continuum system, and the saturated hydraulic conductivity of the preferential flow domain. The second triplet involved parameters controlling exclusively the soil hydraulic properties of the preferential flow domain, i.e., its retention curve and hydraulic conductivity function. Results of the analysis suggest high sensitivity to all parameters of the first triplet, and large differences in sensitivity to the parameters of the second triplet. The sensitivity analysis also confirmed a significant improvement in the identifiability of preferential flow parameters when 18O content was added to the objective function. Analýza Nejistot Při Modelování Podpovrchového Odtoku ze Svahu Metodou Duálního Kontinua s Využitím Izotopu Kyslíku 18O Jako Přirozeného Stopovače K simulacím laterální složky podpovrchového proudění a změn koncentrace izotopu kyslíku 18O ve vodě vytékající ze svahu byl použit jednorozměrný model využívající přístupu duálního kontinua. Nejistota modelových předpovědí byla odhadnuta s využitím metody zobecněné věrohodnosti (GLUE). Citlivost modelu byla zjišťována pomocí variací dvou samostatných trojic parametrů. První trojice sestávala z klíčových parametrů pro určení režimu preferenčního proudění, tj. objemového podílu preferenční domény proudění, přenosového koeficientu charakterizujícího výměnu vody mezi oběma doménami duálního systému a nasycené hydraulické vodivosti preferenční domény. Druhá trojice zahrnovala výhradně parametry určující hydraulické charakteristiky preferenční domény proudění, tj. retenční křivku a funkci hydraulické vodivosti. Z výsledků analýzy vyplývá vysoká citlivost modelu na všechny parametry z první trojice a velké rozdíly v citlivostech parametrů druhé trojice. Analýza dále potvrdila významné zlepšení zjistitelnosti parametrů preferenční domény v případě, kdy je do cílové funkce přidána koncentrace izotopu kyslíku 18O.

Ponded infiltration into soil with biopores — field experiment and modeling

Preferential movement of water in macropores plays an important role when the process of ponded infiltration in natural porous systems is studied. For example, the detailed knowledge of water flow through macropores is of a major importance when predicting runoff responses to rainfall events. The main objectives of this study are to detect preferential movement of water in Chernozem soil and to employ numerical modeling to describe the variably saturated flow during a field ponded infiltration experiment. The infiltration experiment was performed at the Macov experimental station (Calcari-Haplic Chernozem in Danubian Lowland, Slovakia). The experiment involved single ring ponded infiltration. At the quasi steady state phase of the experiment dye tracer was added to the infiltrating water. Then the soil profile was excavated and the penetration pattern of the applied tracer was recorded. The abundance of biopores as a product of fauna and flora was found. To quantify the preferential flow effects during the infiltration experiment, three-dimensional axisymmetric simulations were carried out by a two-dimensional dual-continuum numerical model. The water flow simulations based on measured hydraulic characteristics without consideration of preferential flow effects failed to describe the infiltration experiment adequately. The 3D axisymmetric simulation based on dual-permeability approach provided relatively realistic space-time distribution of soil water pressure below the infiltration ring.

Field leaching of pesticides at five test sites in Hawaii: modeling flow and transport

BACKGROUND Physically based tier-II models may serve as possible alternatives to expensive field and laboratory leaching experiments required for pesticide approval and registration. The objective of this study was to predict pesticide fate and transport at five different sites in Hawaii using data from an earlier field leaching experiment and a one-dimensional tier-II model. As the predicted concentration profiles of pesticides did not provide close agreement with data, inverse modeling was used to obtain adequate reactive transport parameters. The estimated transport parameters of pesticides were also utilized in a tier-I model, which is currently used by the state authorities to evaluate the relative leaching potential. RESULTS Water flow in soil profiles was simulated by the tier-II model with acceptable accuracy at all experimental sites. The observed concentration profiles and center of mass depths predicted by the tier-II simulations based on optimized transport parameters provided better agreements than did the non-optimized parameters. With optimized parameters, the tier-I model also delivered results consistent with observed pesticide center of mass depths. CONCLUSION Tier-II numerical modeling helped to identify relevant transport processes in field leaching of pesticides. The process-based modeling of water flow and pesticide transport, coupled with the inverse procedure, can contribute significantly to the evaluation of chemical leaching in Hawaii soils.

The Infiltration-Outflow Experiment Used to Detect Flow Deviations

Simple recurrent infiltration-outflow experiments, together with dye tracing, show deviations from the theoretical assumptions on which classical soil-water flow modelling is based. The experimental set-up represents a well-defined system which enables the study of the dynamic character of soil properties expressed in the Richard’s-equation approach by soil hydraulic functions. The method may offer the possibility to develop an efficient classification of soils according to their dynamic behaviour. Results from four soils are presented.

Simulation of soil water dynamics in structured heavy soils with respect to root water uptake

In agricultural lands has the soil moisture uptake from the root system a significant effect on the water regime of the soil profile. In texturally heavy soils, where preferential pathways are present, infiltrated precipitation and irrigation water with diluted fertilizers quickly penetrate to a significant depth and often reach an under-root zone or even the ground-water level. Such a scenario is likely to happen during long summer periods without rain followed by heavy precipitation events, when a part of the water may flow through desiccated cracks.Since 2001 the effects of drip irrigation and nitrogen fertilization of potatoes (Solanum tuberosum L., cultivar Agria) have been monitored within the frame of a research project at the experimental site Valecov (Czech Republic). Based upon the measured data an attempt has been made to simulate the water regime of the soil profile at a selected experimental plot, considering the impact of preferential flow and root water uptake. The dual-permeability simulation model S_1D_Dual (VOGEL et al., 2000) was used for the simulation. The soil hydraulic parameters were inversely determined using Levenberg-Marquardt method. Measured and simulated pressure heads were utilized in the optimization criterion. The scaling approach was applied to simplify the description of the spatial variability of the soil profile.The results of simulations demonstrate that during particular rainfall events the water reaches significant depths of the soil profile via preferential pathways. The effect of the root zone is dominant during dry periods, when capillary water uptake from the layers below roots becomes important. This should be taken in account into the optimization of the drip irrigation and nitrogen fertilization schedule.