Influence Of Preferential Flow Research Articles

The influence of preferential flow on pressure propagation and landslide triggering of the Rocca Pitigliana landslide

The fast pore water pressure response to rain events is an important triggering factor for slope instability. The fast pressure response may be caused by preferential flow that bypasses the soil matrix. Currently, most of the hydro-mechanical models simulate pore water pressure using a single-permeability model, which cannot quantify the effects of preferential flow on pressure propagation and landslide triggering. Previous studies showed that a model based on the linear-diffusion equation can simulate the fast pressure propagation in near-saturated landslides such as the Rocca Pitigliana landslide. In such a model, the diffusion coefficient depends on the degree of saturation, which makes it difficult to use the model for predictions. In this study, the influence of preferential flow on pressure propagation and slope stability is investigated with a 1D dual-permeability model coupled with an infinite-slope stability approach. The dual-permeability model uses two modified Darcy-Richards equations to simultaneously simulate the matrix flow and preferential flow in hillslopes. The simulated pressure head is used in an infinite-slope stability analysis to identify the influence of preferential flow on the fast pressure response and landslide triggering. The dual-permeability model simulates the height and arrival of the pressure peak reasonably well. Performance of the dual-permeability model is as good as or better than the linear-diffusion model even though the dual-permeability model is calibrated for two single pulse rain events only, while the linear-diffusion model is calibrated for each rain event separately. In conclusion, the 1D dual-permeability model is a promising tool for landslides under similar conditions.

Estimation of Groundwater Recharge Using Tracers and Numerical Modeling in the North China Plain

Water resource shortage has been a serious problem since the 1980s in the North China Plain (NCP), resulting in plenty of environmental problems. Estimating the groundwater recharge rate accurately is vital for managing groundwater effectively. This study applied several methods, including chloride mass-balance, tracers (bromide and tritium) and numerical modeling (Hydrus-1D), to estimate groundwater recharge at three representative sites of the NCP: Zhengding (ZD), Luancheng (LC) and Hengshui (HS). The chloride concentration of the soil profile in the ZD site showed that the mean recharge was 3.84 mm/year with the residence time of 105 years for soil water transferring through the vadose area of 45.0 m in depth in the preferential flow model mainly. Considering the influence of preferential flow on the soil water movement in the field scale, the traditional methods (e.g., peak method of bromide and tritium tracers based on piston flow described in the literature) could be unsuitable to estimate groundwater recharge in the LC and HS sites, especially in areas with low recharge rates. Therefore, multi-region and mass balance methods were applied in this study. The results of this investigation showed that the mean values of recharge were 124.3 and 18.0 mm/year in the LC and HS sites, respectively, in 2010. Owing to complexity and uncertainty on the surface resulting from the measuring of evapotranspiration, the upper boundary of 1.4 m (under the ground where most of the plant roots did not reach) was chosen for the numerical modeling of Hydrus-1D, and the result showed that the mean recharge was 225 mm/year from 2003 to 2007, consistent with the result of tracers in the previous literature. The result also showed that the positive relation of groundwater recharge and the sum of irrigation and rainfall was presented in the spatial and temporal scale. Additionally, human activities promoted the recharge rate, and recharge rates increased with greater depths in the LC site generally. However, both cases did not appear clearly in the HS site, showing that the low penetrability of soil controlled the recharge rate in this site.

Characteristics of preferential flow paths and their effects on soil properties

Preferential flow plays an important role in solute transport in soils and it indirectly affects the quality of groundwater. The objectives of this study were to investigate the vertical distribution characteristics of preferential flow paths under different-aged forest types and to determine the influence of preferential flow on soil properties and soil metal content. Dye tracer experiments were conducted at six forest sites at three ages. Results show a >50% dye cover at a soil depth of 10 cm in all sites. Vertical distribution differed widely. Preferential flow paths reached deeper soils in mature stands than in younger stands. Soil organic matter (SOM) content, saturated hydraulic conductivity (SHC) and soil metal content in the preferential flow paths were different from those in the soil matrix. SOM was 5% to 20% higher in the preferential flow paths than in the soil matrix. Average value of the SHC was 39.3 mm per minute in the preferential flow paths compared with 20.1 mm per minute in the soil ma...

Phosphorus release from anaerobic peat soils during convective discharge — Effect of soil Fe:P molar ratio and preferential flow

Phosphorus (P) accumulation in drained agricultural lowlands causes a risk for P pollution to the aquatic environment following wetland restoration. While extensive knowledge is available on P sorption and desorption from anaerobic soils, very limited information is available on the interacting influence of soil geochemistry and local scale active flow volume. Combining batch incubation experiments and continuous column discharge experiments, we investigated iron (Fe) reduction and P release from 10 anoxic Fe-dominated (oxalate extractable Fe (Feox) from ~5500 to 50,000mgkg−1) lowland peat soils (TOC from 5 to 39%) with a gradient in Fe:P molar ratio (molar ratio between bicarbonate dithionite extractable Fe and P (FeBD:PBD) from 3 to 112) and degree of non-equilibrium (preferential) flow. Short-term batch incubation experiments (21days) indicated that concurrent Fe and P release was controlled by reductive Fe(III) dissolution, and was well predicted from the soil FeBD:PBD molar ratio. Continuous convective column discharge with oxygen-free deionised water at 1mmh−1 for 10 effluent pore volumes resulted in highly variable in situ redox potential (Eh from −200 to 300mV), effluent Fe(II) concentrations (23 to 2000μM) and effluent dissolved reactive phosphorus (DRP) concentrations (<6.5 to 316μM). Effluent P forms changed from dominantly particular P/dissolved organic P (PP/DOP) to dominantly DRP as Eh decreased in all soils. Total phosphorus (TP) release rates during convective discharge (3–66μmolkg−1day−1) were negatively non-linearly correlated with the soil FeBD:PBD molar ratio, which explained 71–73% of the variability, and with FeBD:PBD of 10 as a critical threshold ratio. Fe and P release rates from batch experiments were poorly correlated with convective discharge Fe and P release rates, indicating the overall influence of soil structure. Diffusion was found to be the rate-limiting step for P release after prolonged leaching. Although the soil FeBD:PBD molar ratio turned out as a key explanatory parameter in predicting P release rates following rewetting of these peat soils, the results did indicate the influence of preferential flow in decreasing P release rates. Although, sufficient available P is present for leaching in the initial phase after rewetting P enriched lowland soils, we do expect that soils with pronounced preferential flow will become more rapidly exhausted in available P, and hence limit P release after prolonged discharge.

Experimental and numerical simulation of water movement in soil

Preferential flow refers to a flow transport pattern where water and solute flows around the soil matrix, contributing to accelerating velocity of movement in the soil; soil recharge occurs over the whole inlet boundary. Preferential flow is produced by a large number of macropores and fractures in the soil, such as root or worm holes, and is very important to groundwater recharge owing to its rapid movement. Using the reaction of iodine turning blue on contact with starch, outdoor tracer experiments of heterogeneous flow in clay loam for different scales and boundary conditions are designed, so that the heterogeneous flow patterns can be studied. Using experimental image analysis, the soil is divided into a matrix and a preferential channel network, formed by the spatial structure of the flow channels. An irregular flow network is established through random generation of angles, lengths, aperture and location coordinates of flow channels, in order to achieve an accurate description of preferential flow channels. According to the soil moisture of each layer obtained by tracer experiments, soil water retention curve and saturated hydraulic conductivity, soil moisture variation was simulated. The distribution of the channel network and models of water movement in the soil are used to undertake a quantitative analysis of the influence of preferential flow on soil water migration.

Leaching requirement for soil salinity control: Steady-state versus transient models

Water scarcity and increased frequency of drought conditions, resulting from erratic weather attributable to climatic change or alterations in historical weather patterns, have caused greater scrutiny of irrigated agriculture's demand on water resources. The traditional guidelines for the calculation of the crop-specific leaching requirement (LR) of irrigated soils have fallen under the microscope of scrutiny and criticism because the commonly used traditional method is believed to erroneously estimate LR due to its assumption of steady-state flow and disregard for processes such as salt precipitation and preferential flow. An over-estimation of the LR would result in the application of excessive amounts of irrigation water and increased salt loads in drainage systems, which can detrimentally impact the environment and reduce water supplies. The objectives of this study are (i) to evaluate the appropriateness of the traditional steady-state method for estimating LR in comparison to the transient method and (ii) to discuss the implications these findings could have on irrigation guidelines and recommendations, particularly with respect to California's Imperial Valley. Steady-state models for calculating LR including the traditional model, which is an extension of the original U.S. Salinity Laboratory LR model, WATSUIT model, and water-production-function model were compared to transient models including TETrans and UNSATCHEM. The calculated LR was lower when determined using a transient approach than when using a steady-state approach. Transient conditions and the influence of preferential flow did not have as significant an effect on lowering the LR as salt precipitation for a representative study of the Imperial Valley using Colorado River water (EC = 1.23 dS/m) for irrigation. A valley-wide LR of 0.08 for a crop rotation of alfalfa/alfalfa/alfalfa/alfalfa/wheat/lettuce, as calculated by both WATSUIT and UNSATCHEM, was concluded to be the most reasonable estimate for the entire Imperial Valley as compared to a LR of 0.13 by the commonly used traditional method. The reduced LR for the Imperial Valley would result in a diminished drainage volume of approximately 1.23 × 10 8 m 3 (i.e., 100,000 ac-ft). The most significant conclusion derived from the comparison is that the use of the traditional steady-state model for estimating LR needs to be reevaluated.

Atrazine movement in a dark red latosol of the tropics

Atrazine displacement was studied in a soil profile as a function of water movement and time after herbicide application, taking into account possible influence of preferential flow on leaching. The experiment consisted of two 7 x 7m plots of a dark red latosol (Kanduidalfic Eutrudox), located at Piracicaba, SP, Brazil (22º 43'S and 47º 25'W), 250km inside continent, at an altitude of 580m. One plot was previously treated with 1,000 kg/ha of lime, in order to increase base saturation to 88%, and 500kg/ha of gypsum. Each plot was instrumented with tensiometers, neutron probe access tubes and soil solution extractors, in order to monitor water and atrazine flows. Atrazine was applied at the high rate of 6 kg/ha of active principle. Results showed intensive leaching of atrazine in the whole soil profile, up to the instrumented depth of 150cm, already ate the first sampling, seven days after herbicide application. The limed plot showed much higher atrazine leaching losses than the other plot. The atrazine adsorption capacity of the soil is very low, its maximum value being of the order of 10%, for the 0-15cm surface layer.

Tillage and Crop Rotation Effects on Subsurface Drainage Response to Rainfall

A field study was conducted to determine if tillage and crop rotation affected subsurface drainage response to rainfall. An instrumentation system collected subsurface drain flow data from thirty-six, 0.4 ha plots during the 1993, 1994 and 1995 growing seasons. Response time, time-to-peak drain flow rate, drainage volume, peak drain flow rate and percent preferential flow were compared between two tillage systems (no-till and chisel plow) and two crop rotations (continuous corn and corn-soybean) for 23 drainage events over the three-year study. The influence of preferential flow was estimated for each drainage event using a hydrograph separation procedure based on subsurface drain flow rate changes.