An iterative pressure‐stabilized fractional step algorithm in saturated soil dynamics

This paper presents an iterative, incremental pressure‐stabilized fractional step algorithm for coupled hydro‐mechanical problems with mixed formulations of the displacement–pressure ( u –p) model in saturated soil dynamics that allows the use of finite elements with equal low order of interpolation approximation of u and p. In comparison with the original fractional step algorithm, the distinct features of the proposed algorithm lie in its enhanced stability owing to the introduction of both an iteration procedure and a finite increment calculus (FIC) process into the algorithm. The introduction of the iterative procedure makes the velocity term satisfy the momentum conservation equation in an implicit sense and allows much larger time step sizes to be used than those limited in existing explicit and semi‐implicit versions of the algorithm. The introduction of the FIC process removes the dependence of the stability of the proposed algorithm on the time step size, as a result it allows to using the incremental version of the algorithm and evades the minimum time step size requirement presented in the existing versions of the fractional step algorithm that restricts the application of the algorithm to saturated soil dynamics problems with high frequencies. Numerical experiments demonstrate the effectiveness and improved performance of the proposed iterative pressure‐stabilized fractional step algorithm. Copyright © 2009 John Wiley & Sons, Ltd.     

Time‐scale analysis in unsaturated porous media under external wave loads

Extreme waves caused by tsunamis and storm surges can lead to soil failures in the near‐shore region, which may have severe impact on coastal environments and communities. Multiphase flows in deformable porous media involve several coupled processes and multiple time scales, which are challenging for numerical simulations. The objective of this study is to investigate the roles of the various processes and their interactions in multiphase flows in unsaturated soils under external wave loading, via theoretical time‐scale analysis and numerical simulations. A coupled geomechanics–multiphase flow model based on conservation laws is used. Theoretical analysis based on coupled and decoupled models demonstrates that transient and steady‐state responses are governed by pore pressure diffusion and saturation front propagation, respectively, and that the two processes are essentially decoupled. Numerical simulations suggest that the compressibility of the pore fluids and the deformation of the soil skeleton are important when the transient responses of the media are of concern, while the steady‐state responses are not sensitive to these factors. The responses obtained from the fully coupled numerical simulations are explained by a simplified time‐scale analysis based on coupled and decoupled models. Copyright © 2010 John Wiley & Sons, Ltd.     

Effects of the Soil Heat Flux Estimates on Surface Energy BalanceClosure over a Semi-Arid Grassland

Soil heat flux is important for surface energy balance (SEB), and inaccurate estimation of soil heat flux often leads to surface energy imbalance. In this paper, by using observations of surface radiation fluxes and soil temperature gradients at a semi-arid grassland in Xilingguole, Inner Mongolia, China from June to September 2008, the characters of the SEB for the semi-arid grassland were analyzed. Firstly, monthly averaged diurnal variations of SEB components were revealed. A 30-min forward phase displacement of soil heat flux (G) observed by a fluxplate at the depth of 5-cm below the soil surface was conducted and its effect on the SEB was studied. Secondly, the surface soil heat flux (Gs) was computed by using harmonic analysis and the effect of the soil heat storage between the surface and the fluxplate on the SEB was examined. The results show that with the 30-min forward phase displacement of observed G, the slope of the ordinary linear regression (OLR) of turbulent fluxes (H+LE) against available energy (Rn–G) increased from 0.835 to 0.842, i.e., the closure ratio of SEB increased by 0.7%, yet energy imclosure of 15.8% still existed in the SEB. When Gs, instead of G was used in the SEB equation, the slope of corresponding OLR of (H+LE) against (Rn–Gs) reached 0.979, thereby the imclosure ratio of SEB was reduced to only 2.1%.     

A broad‐scale assessment of the effect of improved soil management on catchment baseflow index

Although the importance of sustainable soil management is recognized, there are many threats to soils including widespread soil structural degradation. This reduces infiltration through the soil surface and/or the percolation of water through the soil profile, with important consequences for crop yields, nutrient cycling and the hydrological response of catchments. This article describes a broad‐scale modelling approach to assess the potential effect that improved agricultural soil management, through reduced soil structural degradation, may have on the baseflow index (BFI) of catchments across England and Wales. A daily soil–water balance model was used to simulate the indicative BFI of 45 696 thirty‐year model runs for different combinations of soil type, soil/field condition, land cover class and climate which encapsulate the variability across England and Wales. The indicative BFI of catchments was then calculated by upscaling the results by spatial weighting. WaSim model outputs of indicative BFI were within the 95% confidence intervals of the national‐average BFI values given for the Hydrology of Soil Type (HOST ? ) classes for 26 of the 28 classes. At the catchment scale, the concordance correlation coefficient between the BFI from the WaSim model outputs and those derived from HOST was 0·83. Plausible improvements in agricultural soil/field condition produced modest simulated increases of up to 10% in the indicative BFI in most catchments across England and Wales, although for much of southern and northern England the increases were less than 5%. The results suggest that improved soil management might partially mitigate the expected adverse effects of climate change on baseflow to rivers. Healthy, well‐functioning soils produce many additional benefits such as better agricultural yields and reduced pollutant movement, so improved soil management should provide win‐win opportunities for society, agricultural systems and the environment and provide resilience to some of the expected environmental impacts of climate change. Copyright © 2011 John Wiley & Sons, Ltd.     

Implementing quality control on a random number stream to improve a stochastic weather generator

For decades, stochastic modellers have used computerized random number generators to produce random numeric sequences fitting a specified statistical distribution. Unfortunately, none of the random number generators we tested satisfactorily produced the target distribution. The result is generated distributions whose mean even diverges from the mean used to generate them, regardless of the length of run. Non‐uniform distributions from short sequences of random numbers are a major problem in stochastic climate generation, because truly uniform distributions are required to produce the intended climate parameter distributions. In order to ensure generation of a representative climate with the stochastic weather generator CLIGEN within a 30‐year run, we tested the climate output resulting from various random number generators. The resulting distributions of climate parameters showed significant departures from the target distributions in all cases. We traced this failure back to the uniform random number generators themselves. This paper proposes a quality control approach to select only those numbers that conform to the expected distribution being retained for subsequent use. The approach is based on goodness‐of‐fit analysis applied to the random numbers generated. Normally distributed deviates are further tested with confidence interval tests on their means and standard deviations. The positive effect of the new approach on the climate characteristics generated and the subsequent deterministic process‐based hydrology and soil erosion modelling are illustrated for four climatologically diverse sites. Copyright © 2007 John Wiley & Sons, Ltd.     

Spatio‐temporal distribution of near‐surface and root zone soil moisture at the catchment scale

Soil moisture is highly variable both spatially and temporally. It is widely recognized that improving the knowledge and understanding of soil moisture and the processes underpinning its spatial and temporal distribution is critical. This paper addresses the relationship between near‐surface and root zone soil moisture, the way in which they vary spatially and temporally, and the effect of sampling design for determining catchment scale soil moisture dynamics. In this study, catchment scale near‐surface (0–50 mm) and root zone (0–300 mm) soil moisture were monitored over a four‐week period. Measurements of near‐surface soil moisture were recorded at various resolutions, and near‐surface and root zone soil moisture data were also monitored continuously within a network of recording sensors. Catchment average near‐surface soil moisture derived from detailed spatial measurements and continuous observations at fixed points were found to be significantly correlated (r² = 0·96; P = 0·0063; n = 4). Root zone soil moisture was also found to be highly correlated with catchment average near‐surface, continuously monitored (r² = 0·81; P < 0·0001; n = 26) and with detailed spatial measurements of near‐surface soil moisture (r² = 0·84). The weaker relationship observed between near‐surface and root zone soil moisture is considered to be caused by the different responses to rainfall and the different factors controlling soil moisture for the soil depths of 0–50 mm and 0–300 mm. Aspect is considered to be the main factor influencing the spatial and temporal distribution of near‐surface soil moisture, while topography and soil type are considered important for root zone soil moisture. The ability of a limited number of monitoring stations to provide accurate estimates of catchment scale average soil moisture for both near‐surface and root zone is thus demonstrated, as opposed to high resolution spatial measurements. Similarly, the use of near‐surface soil moisture measurements to obtain a reliable estimate of deeper soil moisture levels at the small catchment scale was demonstrated. Copyright © 2007 John Wiley & Sons, Ltd.     

A multi‐year study of rainfall and soil water controls on Scots pine transpiration under Mediterranean mountain conditions

This study is focused on the analysis of the relationship between sap‐flow‐derived transpiration measured in a Scots pine stand in the Vallcebre research catchments (NE Iberian Peninsula) and meteorological and rainfall data. The first part of the study is focused on the analysis of temperature and rainfall anomalies. Then, the Scots pine transpiration response to inter‐annual rainfall variability, soil water stress and water table depth variations during the period 1997–2000 is analysed. This period includes the extremely dry year of 1998, which allows us to infer the response of Scots pine transpiration to severe droughts. Scots pine transpiration during the summer presented a high inter‐annual variability, largely related to rainfall amounts. Daily transpiration during dry summers was 40% of the transpiration of a summer day with average rainfall. Moreover, during dry summers, transpiration rates were not fully recovered even after significant rainfall events. The analysis of the dependence of Scots pine transpiration on available water indicated the strong limitation on transpiration induced by water content in the whole soil profile as well as by water table position. Under these drought conditions, a reduction of runoff and deep water stores was observed at the catchment scale, suggesting that the predicted increase in the frequency of severe summer droughts may threaten the current role of Mediterranean mountain catchments as suppliers of water resources for lowland areas. Copyright © 2010 John Wiley & Sons, Ltd.     

Flood forecasting with a watershed model: a new method of parameter updating

Abstract

Flood forecasting is of prime importance when it comes to reducing the possible number of lives lost to storm-induced floods. Because rainfall-runoff models are far from being perfect, hydrologists need to continuously update outputs from the rainfall-runoff model they use, in order to adapt to the actual emergency situation. This paper introduces a new updating procedure that can be combined with conceptual rainfall-runoff models for flood forecasting purposes. Conceptual models are highly nonlinear and cannot easily accommodate theoretically optimal methods such as Kalman filtering. Most methods developed so far mainly update the states of the system, i.e. the contents of the reservoirs involved in the rainfall-runoff model. The new parameter updating method proves to be superior to a standard error correction method on four watersheds whose floods can cause damage to the greater Paris area. Moreover, further developments of the approach are possible, especially along the idea of combining parameter updating with assimilation of additional data such as soil moisture data from field measurements and/or from remote sensing.     

Factor of safety in a partially saturated slope inferred from hydro‐mechanical continuum modeling

Rainfall weakens an earth slope and triggers mass movement. Relevant triggering mechanisms are complex and include reduction of capillary pressure due to increased saturation and frictional drag on the sediment induced by fluid flow. Physics‐based continuum models utilizing modern computational tools are useful for understanding the mechanisms of deformation in partially saturated slopes; however, they do not provide a scalar indicator called ‘factor of safety’ that measures the potential of a given slope for mass movement. In the present work, we employ sequential calculations consisting of a physics‐based finite element model that couples solid deformation with fluid flow to quantify the stress and deformation fields in a steep hillside slope subjected to rainfall infiltration. This is followed by a limit‐equilibrium calculation based on the method of slices that evaluates the desired factor of safety. The field condition investigated is similar to the steep experimental catchment CB1 near Coos Bay, Oregon, which failed as a large debris flow from heavy rainfall. Copyright © 2011 John Wiley & Sons, Ltd.