83 research outputs found
Traveling liquid bridges in unsaturated fractured porous media
Interplay between capillary, gravity and viscous forces in unsaturated fractures gives rise to a range of complex flow phenomena. Evidence of highly intermittent fluxes, preferential and sustainable flow pathways lead to potentially significant flow focusing of concern for regulatory and management of water resources in fractured rock formations. In previous work[Ghezzehei TA,Or D.: Water Resour. Res. In Review(2005)] we developed mechanistic models for formation, growth and detachment of liquid bridges in geometrical irregularities within fractures. Such discrete and intermittent flows present a challenge to standard continuum theories. Our focus here is on predicting travel velocities of detached liquid elements and their interactions with fracture walls. The scaling relationships proposed by Podgorski etal. [Podgorski, T., etal.: Phys. Rev. Lett. 8703(3), 6102-NIL_95 (2001)] provide a general framework for processes affecting travel velocities of discrete liquid elements in fractures, tubes, and in coarse porous media. Comparison of travel velocity and distance by discrete bridges relative to equivalent continuous film flow reveal significantly faster and considerably larger distances traversed by liquid bridges relative to liquid films. Coalescence and interactions between liquid bridges result in complex patterns of travel times and distances. Mass loss on rough fracture surfaces shortens travel distances of an element; however, results show that such retardation provides new opportunities for coalescence of subsequent liquid elements traveling along the same path, resulting in mass accumulation and formation of larger liquid elements traveling larger distances relative to smooth fracture surfaces. Such flow focusing processes may be amplified considering a population of liquid bridges within a fracture plane and mass accumulation in fracture intersection
Root uptake under mismatched distributions of water and nutrients in the root zone
Most plants derive their water and nutrient needs from soils where the resources are often scarce, patchy, and ephemeral. It is not uncommon for plant roots to encounter mismatched patches of water-rich and nutrient-rich regions in natural environments. Such an uneven distribution of resources necessitates plant reliance on strategies for exploring and acquiring nutrients from relatively dry patches. We conducted a laboratory study that elucidates the biophysical mechanisms that enable this adaptation. The roots of tomato (Solanum lycopersicum) seedlings were laterally split and grown in two adjacent, hydraulically disconnected pots, which permitted precise control of water and nutrient applications to each compartment. We observed that the physical separation of water-rich and nutrient-rich compartments (one received 90% water and 0% nutrients and the other received 10% water and 100% nutrients) does not significantly stunt plant growth and productivity compared to two control treatments (control 1: 90% water and 100% nutrients versus 10% water and 0% nutrients; control 2: 50% water and 50% nutrients in each compartment). Specifically, we showed that soil dryness does not reduce nutrient uptake, vegetative growth, flowering, and fruiting compared to control treatments. We identified localized root proliferation in nutrient-rich dry soil patches as a critical strategy that enabled nutrient capture. We observed nocturnal rewetting of the nutrient-rich but dry soil zone (10% water and 100% nutrients) but not in the nutrient-free and dry zone of the control experiment (90% water and 100% nutrients). We interpreted the rewetting as the transfer of water from the wet to dry zones through roots, a process commonly known as hydraulic redistribution (HR). The occurrence of HR likely prevents the nutrient-rich soil from drying due to permanent wilting and the subsequent decline of root functions. Sustaining rhizosphere wetness is also likely to increase nutrient mobility and uptake. Lack of HR in the absence of nutrients suggests that HR is not entirely a passive, water-potential-gradient driven flow. The density and size of root hairs appeared to be higher (qualitative observation) in the nutrientrich and dry compartments than in the nutrient-free and dry compartments. We also observed organic coating on sand grains in the rhizosphere of the nutrient-rich and dry compartments. The observations are consistent with prior observations that root hairs and rhizodeposition aid rhizosphere wetting. These findings were synthesized in a conceptual model that explains how plants of dry regions may be adapted to mismatched resources. This study also suggests that separating the bulk of applied nutrients from the frequently irrigated soil region can increase nutrient use efficiency and curtail water pollution from intensive agricultural systems
Dos métodos para estimar las propiedades hidráulicas del suelo a partir de: un proceso de (I) humectación por capilaridad más evaporación, y (II) humectación por capilaridad con multitensión: análisis teórico
6 Pags.- 1 Tabl.- 7 Figs. Trabajo originalmente presentado en las XII Jornadas de Investigación en la Zona No Saturada del Suelo (Alcalá de Henares, 18-20 de nov. de 2015). © de los textos: sus autores[ES] La determinación de la curva de retención
θ(h) y conductividad hidráulica saturada (Ks) del suelo es
fundamental para caracterizar la zona no saturada. Esta
comunicación presenta dos métodos para estimar Ks y los
parámetros α y n de θ(h) a partir de: (i) un proceso de
humectación por capilaridad a saturación seguido de una
sobre-presión, más un proceso de evaporación,
considerando el fenómeno de histéresis, y (ii) un proceso
de humectación por capilaridad a tensión negativa seguida
de una tensión a saturación. El análisis inverso se realizó
sobre un cilindro de 5 cm de altura y 5 cm de diámetro con
suelo franco, utilizando el programa HYDRUS-2D. Los
mapas de error de la función objetivo (Ks, α, n) para los
planos Ks-α, α-n y Ks-n obtenidos para ambos métodos
mostraron un único mínimo, lo que indica que estos
métodos permiten estimar de forma precisa los parámetros
hidráulicos del suelo.[EN] The determination of the soil water retention
curve, θ(h), and the saturated hydraulic conductivity, Ks,
are of paramount importance to correctly characterize the
vadose zone. This communication presents two methods to
estimate Ks and the water retention curve α and n
parameters from: (i) a capillary rise wetting process at
saturation followed by an overpressure step, plus an
evaporation process, taking into account the hysteresis
phenomena; and (ii) a capillary wetting process at negative
tension plus a saturation step. The theoretical analysis was
performed on a 5-cm diameter and 5 cm high cylinder of
loam soil, using the HYDRUS-2D software. The responses
surfaces of the objective function (Ks, α, n) for the planes
Ks-α, α-n and Ks-n obtained with both methods showed a
unique and well defined minimum, which indicates these
methods allow accurate estimates of the soil hydraulic
properties.Este trabajo ha sido financiado por el Ministerio de
Economía y Competitividad de España (AGL2010-22050-C03-02).Peer reviewe
Vulnerability of Physically Protected Soil Organic Carbon to Loss Under Low Severity Fires
Soil aggregate degradation during medium and high severity fires is often identified as the main mechanism that leads to loss of soil organic matter (SOM) due to fire. Low severity fires, however, are considered not to cause aggregate degradation assuming that temperatures <250°C, as occurring during low-severity burns, have only limited effects on the stability of the soil organic binding agents. Recent studies suggest that low severity burns may cause soil aggregate degradation due to rapid vaporization of soil pore water that can induce pressure on the soil aggregates beyond their yield stress. Such pressure-driven degradation of soil aggregates may expose physically protected organic carbon to decomposition. Our study investigated the effect of a low-severity fire on soil organic matter (SOM), water extractable organic C, and N as well as respiration for two initial soil moisture conditions undergoing three “heating regimes” using aggregates from a California forest and a Nevada shrubland soil. We found that initially moist soil aggregates that were rapidly heated up degraded the most, showing increased cumulative carbon mineralization when compared to aggregates that were not heated, aggregates that were dry before being heated, and initially moist soil aggregates that were slowly heated. Our results suggest that exposure of previously physically protected organic carbon within the soil aggregates to oxidative conditions was the most likely cause of increased rates of decomposition of organic matter after low-severity burns. Additionally, we show that for a shrubland soil, aggregates with relatively low organic carbon content, low severity burns increased cumulative carbon mineralization. We hypothesized that this was due to decomposition of cytoplasmic material from lysed microbes. Our results suggest that low severity burns can accelerate decomposition of soil organic carbon (SOC) protected in soil aggregates
Errors in determination of soil water content using time domain reflectometry caused by soil compaction around waveguides
Using Wastewater in Irrigation: The Effects on Infiltration Process in a Clayey Soil
Soil water infiltration is a critical process in the soil water cycle and agricultural practices, especially when wastewater is used for irrigation. Although research has been conducted to evaluate the changes in the physical and chemical characteristics of soils irrigated by treated wastewater, a quantitative analysis of the effects produced on the infiltration process is still lacking. The objective of this study is to address this issue. Field experiments previously conducted on three adjacent field plots characterized by the same clayey soil but subjected to three different irrigation treatments have been used. The three irrigation conditions were: non-irrigated (natural conditions) plot, irrigated plot with treated wastewater for two years, and irrigated plot with treated wastewater for five years. Infiltration measurements performed by the Hood infiltrometer have been used to estimate soil hydraulic properties useful to calibrate a simplified infiltration model widely used under ponding conditions, that were existing during the irrigation stage. Our simulations highlight the relevant effect of wastewater usage as an irrigation source in reducing cumulative infiltration and increasing overland flow as a result of modified hydraulic properties of soils characterized by a lower capacity of water drainage. These outcomes can provide important insights for the optimization of irrigation techniques in arid areas where the use of wastewater is often required due to the chronic shortage of freshwater
Synergy between compost and cover crops in a Mediterranean row crop system leads to increased subsoil carbon storage
Subsoil carbon (C) stocks are a prime target for efforts to increase soil C storage for climate change mitigation. However, subsoil C dynamics are not well understood, especially in soils under long-term intensive agricultural management. We compared subsoil C storage and soil organic matter (SOM) composition in tomato-corn rotations after 25 years of differing C and nutrient management in the California Central Valley: CONV (mineral fertilizer), CONV+WCC (mineral fertilizer and cover crops), and ORG (composted poultry manure and cover crops). The cover crop mix used in these systems is a mix of oat (Avena sativa L.), faba bean (Vicia faba L.), and hairy vetch (Vicia villosa Roth). Our results showed a ∼19Mgha-1 increase in soil organic C (SOC) stocks down to 1m under ORG systems, no significant SOC increases under CONV+WCC or CONV systems, and an increased abundance of carboxyl-rich C in the subsoil (60-100cm) horizons of ORG and CONV+WCC systems. Our results show the potential for increased subsoil C storage with compost and cover crop amendments in tilled agricultural systems and identify potential pathways for increasing C transport and storage in subsoil layers. Copyright
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Heterogeneous Seepage at the Nopal I Uranium Mine,Chihuahua, Mexico
Correspondence of the Gardner and van Genuchten-Mualem relative permeability function parameters
Effect of cover crop on carbon distribution in size and density separated soil aggregates
Increasing soil organic carbon (SOC) stocks in agricultural soils can contribute to stabilizing or even lowering atmospheric greenhouse gas (GHG) concentrations. Cover crop rotation has been shown to increase SOC and provide productivity benefits for agriculture. Here we used a split field design to evaluate the short-term effect of cover crop on SOC distribution and chemistry using a combination of bulk, isotopic, and spectroscopic analyses of size-and density-separated soil aggregates. Macroaggregates (\u3e250 µm) incorporated additional plant material with cover crop as evidenced by more negative δ13C values (−25.4%∘ with cover crop compared to −25.1%∘without cover crop) and increased phenolic (plant-like) resonance in carbon NEXAFS spectra. Iron EXAFS data showed that the Fe pool was composed of 17–21% Fe oxide with the remainder a mix of primary and secondary minerals. Comparison of oxalate and dithionite extractions suggests that cover crop may also increase Fe oxide crystallinity, especially in the dense (\u3e2.4 g cm−3) soil fraction. Cover crop δ13C values were more negative across density fractions of bulk soil, indicating the presence of less processed organic carbon. Although no significant difference was observed in bulk SOC on a mass per mass basis between cover and no cover crop fields after one season, isotopic and spectroscopic data reveal enhanced carbon movement between aggregates in cover crop soil
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