Estimation of soil hydraulic properties and their uncertainty through the Beerkan infiltration experiment

The Beerkan method based on in situ single‐ring water infiltration experiments along with the relevant specific Beerkan estimation of soil transfer parameters (BEST) algorithm is attractive for simple soil hydraulic characterization. However, the BEST algorithm may lead to erroneous or null values for the saturated hydraulic conductivity and sorptivity especially when there are only few infiltration data points under the transient flow state, either for sandy soil or soils in wet conditions. This study developed an alternative algorithm for analysis of the Beerkan infiltration experiment referred to as BEST‐generalized likelihood uncertainty estimation (GLUE). The proposed method estimates the scale parameters of van Genuchten water retention and Brooks–Corey hydraulic conductivity functions through the GLUE methodology. The GLUE method is a Bayesian Monte Carlo parameter estimation technique that makes use of a likelihood function to measure the goodness‐of‐fit between modelled and observed data. The results showed that using a combination of three different likelihood measurements based on observed transient flow, steady‐state flow and experimental steady‐state infiltration rate made the BEST‐GLUE procedure capable of performing an efficient inverse analysis of Beerkan infiltration experiments. Therefore, it is more applicable for a wider range of soils with contrasting texture, structure, and initial and saturated water content. Copyright © 2015 John Wiley & Sons, Ltd.     

Steady state flow with hydraulic conductivity change around large diameter wells

Darcian flow law in aquifers assumes that the aquifer hydraulic conductivity is constant and the groundwater movement is due only to the piezometric level changes through hydraulic gradient. In practice, after the well development the aquifer just around the well has comparatively larger hydraulic conductivity and gradient. Patchy aquifer solutions in the literature consider sudden hydraulic conductivity changes with distance for the steady state flow. The change of transmissivity is demonstrated by the application of slope‐matching procedure to actual field data. It is the main purpose of this paper to derive simple analytical expressions for aquifer parameter evaluations with steadily decreasing hydraulic conductivity around the well. Spatial nonlinear hydraulic conductivity changes around a large‐diameter well within the depression cone of a confined aquifer are considered as exponentially decreasing functions of the radial distance. Copyright © 2010 John Wiley & Sons, Ltd.     

A coupled analysis of cavity and pore volume changes for pulse tests conducted in soft clay deposits

This paper presents a new interpretation method for pulse tests, a field permeability test that allows for rapid measurements of hydraulic conductivity k in aquitards. This new method applies to soft clay deposits. To initiate a pulse test, a known volume of water is injected into the sand filter of a monitoring well isolated by a packer. The resulting pressure increase yields an outward movement of the sand filter cavity wall. After presenting the usual interpretation methods and their limits, this paper proposes a new interpretation method based on a coupled analysis of the pressure and displacement fields in the soil using the Biot–Darcy formulation. A series of analytical and numerical non‐dimensional velocity graphs, normalized plots of the mean hydraulic head difference versus its rate of change, are given. For a linear elastic material, these type curves show relatively small variations with the sand filter aspect ratio and the Poisson ratio of the tested clay. The type curves are also found to be independent of the clay compressibility (m_v) and k, an important result. A series of pulse tests conducted in a soft marine clay deposit near Montreal, Canada, are interpreted with the proposed method. The hydraulic conductivity values calculated from these tests are closely correlated with independent estimates obtained using long‐duration variable‐head tests. Compared with previous interpretation methods, the proposed method allows soil volume changes to be reconciled with cavity expansion phenomena and the range of type curves available for the interpretation of test data to be constrained. Copyright © 2013 John Wiley & Sons, Ltd.     

Fire decreases near‐surface hydraulic conductivity and macropore flow in blanket peat

Many peatlands have been subjected to wildfire or prescribed burning, but it is not known how these fires influence near‐surface hydrological processes. Macropores are important flowpaths in the upper layers of blanket peat and were investigated through the use of tension disc infiltrometers, which also provide data on saturated hydraulic conductivity. Measurements were performed on unburnt peat (U), where prescribed burning had taken place 2 years (B2), 4 years (B4) and >15 (B15+) years prior to sampling, and where a wildfire (W) had taken place 4 months prior to sampling. Where there had been recent burning (B2, B4 and W), saturated hydraulic conductivity was approximately three times lower than where there was no burning (U) or where burning was last conducted >15 years ago (B15+). Similarly, the contribution of macropore flow to overall infiltration was significantly lower (between 12% and 25% less) in the recently burnt treatments compared to B15+ and U. There were no significant differences in saturated hydraulic conductivity or macropore flow between peat that had been subject to recent wildfire (W) and those that had undergone recent prescribed burning (B2 and B4). The results suggest that fire influences the near‐surface hydrological functioning of peatlands but that recovery in terms of saturated hydraulic conductivity and macropore flow may be possible within two decades if there are no further fires. Copyright © 2013 John Wiley & Sons, Ltd.     

Flood‐loss modelling for assessing impacts of flood‐frequency adjustment,Middle Mississippi River,USA

This study modelled flood losses (economic damages) along the Middle Mississippi River (MMR) (1) using current US government estimates of flow frequencies and (2) using frequencies based on the original, unaltered discharge measurements. The official flood frequencies were quantified in the Upper Mississippi River System Flow Frequency Study (UMRSFFS), but as a last step in that study, early discharges along the MMR were reduced by up to 54% to reflect a purported bias in early measurements. Subsequently, early discharge measurements were rigorously tested, and no such bias was found. Here, flood damages were quantified using a combination of one‐dimensional hydraulic modelling and flood‐loss modelling. For all recurrence intervals, damages were much less using the UMRSFFS flow frequencies compared with the frequencies based on the original discharge measurements, with differences ranging up to 79% (100‐year event) and $2.9bn (200‐year event). Annualized losses in the study area based on the UMRSFFS frequencies were just $41.6m versus $125.6m using the raw frequencies (an underestimation of 67%). These totals do not include flood losses elsewhere along the MMR, including in metropolitan St Louis. In summary, a seemingly small methodological adjustment – in this case, a single hidden adjustment, not documented anywhere within the UMRSFFS – can have dramatic societal impacts in terms of underestimation of flood probabilities and flood risk. Copyright © 2012 John Wiley & Sons, Ltd.     

Physical-hydraulic properties of tropical sandy-loam soil in response to rice-husk dust and cattle dung amendments and surface mulching

ABSTRACT

The effects of topsoil addition of rice-husk dust (RHD) and cattle dung (CD), alongside surface mulching with dry grasses/legume, on the infiltration characteristics and intrinsic structural properties of a deep, well-drained soil in southeastern Nigeria are assessed. Treatments are RHD-amended, CD-amended and “unamended”, each plot being either surface-mulched or left bare, with the unamended-bare plots as control. Amendments and mulch were applied at 20 t/ha equivalents. Their effects on the soil’s infiltration characteristics 7 months later were not evident; however, there was a tendency for differences: CD-amended ≥ RHD-amended ≥ unamended and surface-mulched ≥ bare-surface. By contrast, saturated hydraulic conductivity (K_s ) differed thus: CD-mulched ≥ unamended-mulched > the rest. Similar values were recorded for K_s (50.89 cm/h) and final infiltration rate (50.74 cm/h) only under CD-amended plots, which also showed the highest values (43.50 cm/h) for transmissivity of the soil. Soil penetrometer resistance was lowest in CD-amended plots (113.44 kPa) and highest in unamended plots (166.78 kPa). Topsoil addition of cattle dung and surface mulching could increase infiltration, though marginally, and permeability of coarse-textured tropical soils beyond the season of their application when their effects on soil structure have almost waned.     

Infiltration mechanisms in a herbaceous peat: results of a controlled infiltration experiment/Mécanismes d'infiltration dans une tourbière herbacée: résultats d'une expérience d'infiltration contrôlée

Abstract

The theoretical spatial distribution of hydraulic head during infiltration is used to interpret the results of infiltration experiments made in the field on a single, isolated, column of herbaceous peat in a flood-plain wetland in central England. Crusts of different hydraulic resistance were applied to the column surface. These regulated the water influx enabling the hydraulic conductivity of the peat to be estimated at between 1 and 19.5 m day^-1. It is inferred that, when the hydraulic gradient changes, water may follow different pathways through the peat. Water moves rapidly through macropores in proportion to the applied hydraulic gradient, and infiltrates the peat matrix from the macropore walls. The results indicate the significance of hydraulic conductivity variations with depth, and the importance of precipitation intensity.     

In situ quantification of spatial and temporal variability of hyporheic exchange in static and mobile gravel‐bed rivers

Seepage meters modified for use in flowing water were used to directly measure rates of exchange between surface and subsurface water in a gravel‐ and cobble bed river in western Pennsylvania, USA (Allegheny River, Q_mean = 190 m³/s) and a sand‐ and gravel‐bed river in Colorado, USA (South Platte River, Q_mean = 9·7 m³/s). Study reaches at the Allegheny River were located downstream from a dam. The bed was stable with moss, algae, and river grass present in many locations. Median seepage was + 0·28 m/d and seepage was highly variable among measurement locations. Upward and downward seepage greatly exceeded the median seepage rate, ranging from + 2·26 (upward) to ? 3·76 (downward) m/d. At the South Platte River site, substantial local‐scale bed topography as well as mobile bedforms resulted in spatial and temporal variability in seepage greatly in exceedence of the median groundwater discharge rate of 0·24 m/d. Both upward and downward seepage were recorded along every transect across the river with rates ranging from + 2·37 to ? 3·40 m/d. Despite a stable bed, which commonly facilitates clogging by fine‐grained or organic sediments, seepage rates at the Allegheny River were not reduced relative to those at the South Platte River. Seepage rate and direction depended primarily on measurement position relative to local‐ and meso‐scale bed topography at both rivers. Hydraulic gradients were small at nearly all seepage‐measurement locations and commonly were not a good indicator of seepage rate or direction. Therefore, measuring hydraulic gradient and hydraulic conductivity at in‐stream piezometers may be misleading if used to determine seepage flux across the sediment‐water interface. Such a method assumes that flow between the well screen and sediment‐water interface is vertical, which appears to be a poor assumption in coarse‐grained hyporheic settings. Copyright © 2011 John Wiley & Sons, Ltd.     

A conceptual–numerical model to simulate hydraulic head in aquifers that are hydraulically connected to surface water bodies

In this paper, we present a conceptual‐numerical model that can be deduced from a calibrated finite difference groundwater‐flow model, which provides a parsimonious approach to simulate and analyze hydraulic heads and surface water body–aquifer interaction for linear aquifers (linear response of head to stresses). The solution of linear groundwater‐flow problems using eigenvalue techniques can be formulated with a simple explicit state equation whose structure shows that the surface water body–aquifer interaction phenomenon can be approached as the drainage of a number of independent linear reservoirs. The hydraulic head field could be also approached by the summation of the head fields, estimated for those reservoirs, defined over the same domain set by the aquifer limits, where the hydraulic head field in each reservoir is proportional to a specific surface (an eigenfunction of an eigenproblem, or an eigenvector in discrete cases). All the parameters and initial conditions of each linear reservoir can be mathematically defined in a univocal way from the calibrated finite difference model, preserving its characteristics (geometry, boundary conditions, hydrodynamic parameters (heterogeneity), and spatial distribution of the stresses). We also demonstrated that, in practical cases, an accurate solution can be obtained with a reduced number of linear reservoirs. The reduced computational cost of these solutions can help to integrate the groundwater component within conjunctive use management models. Conceptual approximation also facilitates understanding of the physical phenomenon and analysis of the factors that influence it. A simple synthetic aquifer has been employed to show how the conceptual model can be built for different spatial discretizations, the parameters required, and their influence on the simulation of hydraulic head fields and stream–aquifer flow exchange variables. A real‐world case was also solved to test the accuracy of the proposed approaches, by comparing its solution with that obtained using finite‐difference MODFLOW code. Copyright © 2011 John Wiley & Sons, Ltd.     

An automated routing methodology to enable direct rainfall in high resolution shallow water models

Recent high profile flood events have highlighted the need for hydraulic models capable of simulating pluvial flooding in urban areas. This paper presents a constant velocity rainfall routing scheme that provides this ability within the LISFLOOD‐FP hydraulic modelling code. The scheme operates in place of the shallow water equations within cells where the water depth is below a user‐defined threshold, enabling rainfall‐derived water to be moved from elevated features such as buildings or curbstones without causing instabilities in the solution whilst also yielding a reduction in the overall computational cost of the simulation. Benchmarking against commercial modelling packages using a pluvial and point‐source test case demonstrates that the scheme does not impede the ability of LISFLOOD‐FP to match both predicted depths and velocities of full shallow water models. The stability of the scheme in conditions unsuitable for traditional two‐dimensional hydraulic models is then demonstrated using a pluvial test case over a complex urban digital elevation model containing buildings. Deterministic single‐parameter sensitivity analyses undertaken using this test case show limited sensitivity of predicted water depths to both the chosen routing speed within a physically plausible range and values of the depth threshold parameter below 10 mm. Local instabilities can occur in the solution if the depth threshold is >10 mm, but such values are not required even when simulating extreme rainfall rates. The scheme yields a reduction in model runtime of ~25% due to the reduced number of cells for which the hydrodynamic equations have to be solved. Copyright © 2012 John Wiley & Sons, Ltd.