bluuebunny/crossref_metadata_2025 huggingface dataset:
Less rain, more water in ponds: a remote sensing study of the dynamics of surface waters from 1950 to present in pastoral Sahel (Gourma region, Mali)
Authors: Gardelle J. , Hiernaux P. , Kergoat L. , Grippa M.
Published: April 2010

Abstract. Changes in the flooded area of ponds in the Gourma region from 1950 to present are studied by remote sensing, in the general context of the current multi-decennial Sahel drought. The seasonal and interannual variations of the areas covered by surface water are assessed using multi-date and multi-sensor satellite images (SPOT, FORMOSAT, LANDSAT-MSS, –TM, and -ETM, CORONA, and MODIS) and aerial photographs (IGN). Water body classification is adapted to each type of spectral resolution, with or without a middle-infrared band, and each spatial resolution, using linear unmixing for mixed pixels of MODIS data. The high-frequency MODIS data document the seasonal cycle of flooded areas, with an abrupt rise early in wet season and a progressive decrease in the dry season. They also provide a base to study the inter-annual variability of the flooded areas, with sharp contrasts between dry years such as 2004 (low and early maximal area) and wetter years such as 2001 and 2002 (respectively high and late maximal area).The highest flooded area reached annually greatly depends on the volume, intensity and timing of rain events. However, the overall reduction by 20% of annual rains during the last 40 years is concomitant with an apparently paradoxical large increase in the area of surface water, starting from the 1970's and accelerating in the mid 1980's. Spectacular for the two study cases of Agoufou and Ebang Mallam, for which time series covering the 1954 to present period exist, this increase is also diagnosed at the regional scale from LANDSAT data spanning 1972–2007. It reaches 108% between September 1975 and 2002 for 91 ponds identified in central Gourma. Ponds with turbid waters and no aquatic vegetation are mostly responsible for this increase, more pronounced in the centre and north of the study zone. Possible causes of the differential changes in flooded areas are discussed in relation with the specifics in topography, soil texture and vegetation cover over the watersheds that feed each of the ponds. Changes in rain pattern and in ponds sedimentation are ruled out, and the impact of changes in land use, limited in the area, is found secondary, as opposed to what has often been advocated for in southern Sahel. Instead, major responsibility is attributed to increased runoff triggered by the lasting impact of the 1970–1980's droughts on the vegetation and on the runoff system over the shallow soils prevailing over a third of the landscape.
Global spatial optimization with hydrological systems simulation: application to land-use allocation and peak runoff minimization
Authors: Yeo I.-Y. , Guldmann J.-M.
Published: April 2010

Abstract. A general methodology is presented to integrate complex simulation models of hydrological systems into optimization models, as an alternative to scenario-based approaches. A gradient-based hill climbing algorithm is proposed to reach locally optimal solutions from distinct starting points. The gradient of the objective function is estimated numerically with the simulation model. A statistical procedure based on the Weibull distribution is used to build a confidence interval for the global optimum. The methodology is illustrated by an application to a small watershed in Ohio, where the decision variables are related to land-use allocations and the objective is to minimize peak runoff. The results suggest that this specific runoff function is convex in terms of the land-use variables, and that the global optimum has been reached. Modeling extensions and areas for further research are discussed.
Parameterization of a coupled CO2 and H2O gas exchange model at the leaf scale of Populus euphratica
Authors: Zhu G. F. , Li X. , Su Y. H. , Huang C. L.
Published: April 2010

Abstract. The following two models were combined to simultaneously predict CO2 and H2O gas exchange at the leaf scale of Populus euphratica: a Farquhar et al. type biochemical sub-model of photosynthesis (Farquhar et al., 1980) and a Ball et al. type stomatal conductance sub-model (Ball et al., 1987). The photosynthesis parameters [including maximum carboxylation rate allowed by ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) carboxylation rate (Vcmax), potential light-saturated electron transport rate (Jmax), triose phosphate utilization (TPU) and day respiration (Rd)] were determined by using the genetic algorithm (GA) method based on A/Ci data. Values of Vcmax and Jmax standardized at 25 °C were 75.09±1.36 (mean ± standard error), 117.27±2.47, respectively. The stomatal conductance sub-model was calibrated independently. Prediction of net photosynthesis by the coupled model agreed well with the validation data, but the model tended to underestimate transpiration rates. Overall, the combined model generally captured the diurnal patterns of CO2 and H2O exchange resulting from variation in temperature and irradiation.
Towards understanding hydroclimatic change in Victoria, Australia – preliminary insights into the "Big Dry"
Authors: Kiem A. S. , Verdon-Kidd D. C.
Published: April 2010

Abstract. Since the mid-1990s the majority of Victoria, Australia, has experienced severe drought conditions (i.e. the "Big Dry") characterized by streamflow that is the lowest in approximately 80 years of record. While decreases in annual and seasonal rainfall totals have also been observed, this alone does not seem to explain the observed reduction in flow. In this study, we investigate the large-scale climate drivers for Victoria and demonstrate how these modulate the regional scale synoptic patterns, which in turn alter the way seasonal rainfall totals are compiled and the amount of runoff per unit rainfall that is produced. The hydrological implications are significant and illustrate the need for robust hydrological modelling, that takes into account insights into physical mechanisms that drive regional hydroclimatology, in order to properly understand and quantify the impacts of climate change (natural and/or anthropogenic) on water resources.
Accessible integration of agriculture, groundwater, and economic models using the Open Modeling Interface (OpenMI): methodology and initial results
Authors: Bulatewicz T. , Yang X. , Peterson J. M. , Staggenborg S. , Welch S. M. , Steward D. R.
Published: April 2010

Abstract. Policy for water resources impacts not only hydrological processes, but the closely intertwined economic and social processes dependent on them. Understanding these process interactions across domains is an important step in establishing effective and sustainable policy. Multidisciplinary integrated models can provide insight to inform this understanding, though the extent of software development necessary is often prohibitive, particularly for small teams of researchers. Thus there is a need for practical methods for building interdisciplinary integrated models that do not incur a substantial development effort. In this work we adopt the strategy of linking individual domain models together to build a multidisciplinary integrated model. The software development effort is minimized through the reuse of existing models and existing model-linking tools without requiring any changes to the model source codes, and linking these components through the use of the Open Modeling Interface (OpenMI). This was found to be an effective approach to building an agricultural-groundwater-economic integrated model for studying the effects of water policy in irrigated agricultural systems. The construction of the integrated model provided a means to evaluate the impacts of two alternative water-use policies aimed at reducing irrigated water use to sustainable levels in the semi-arid grasslands overlying the Ogallala Aquifer of the Central US. The results show how both the economic impact in terms of yield and revenue and the environmental impact in terms of groundwater level vary spatially throughout the study region for each policy. Accessible integration strategies are necessary if the practice of interdisciplinary integrated simulation is to become widely adopted.
Evaluation of alternative formulae for calculation of surface temperature in snowmelt models using frequency analysis of temperature observations
Authors: Luce C. H. , Tarboton D. G.
Published: April 2010

Abstract. The snow surface temperature is an important quantity in the snow energy balance, since it modulates the exchange of energy between the surface and the atmosphere as well as the conduction of energy into the snowpack. It is therefore important to correctly model snow surface temperatures in energy balance snowmelt models. This paper focuses on the relationship between snow surface temperature and conductive energy fluxes that drive the energy balance of a snowpack. Time series of snow temperature at the surface and through the snowpack were measured to examine energy conduction in a snowpack. Based on these measurements we calculated the snowpack energy content and conductive energy flux at the snow surface. We then used these estimates of conductive energy flux to evaluate formulae for the calculation of the conductive flux at the snow surface based on surface temperature time series. We use a method based on Fourier frequency analysis to estimate snow thermal properties. Among the formulae evaluated, we found that a modified force-restore formula, based on the superimposition of the force-restore equation capturing diurnal fluctuations on a gradually changing temperature gradient, had the best agreement with observations of heat conduction. This formula is suggested for the parameterization of snow surface temperature in a full snowpack energy balance model.
Coupled hydrogeophysical parameter estimation using a sequential Bayesian approach
Authors: Rings J. , Huisman J. A. , Vereecken H.
Published: April 2010

Abstract. Coupled hydrogeophysical methods infer hydrological and petrophysical parameters directly from geophysical measurements. Widespread methods do not explicitly recognize uncertainty in parameter estimates. Therefore, we apply a sequential Bayesian framework that provides updates of state, parameters and their uncertainty whenever measurements become available. We have coupled a hydrological and an electrical resistivity tomography (ERT) forward code in a particle filtering framework. First, we analyze a synthetic data set of lysimeter infiltration monitored with ERT. In a second step, we apply the approach to field data measured during an infiltration event on a full-scale dike model. For the synthetic data, the water content distribution and the hydraulic conductivity are accurately estimated after a few time steps. For the field data, hydraulic parameters are successfully estimated from water content measurements made with spatial time domain reflectometry and ERT, and the development of their posterior distributions is shown.
Frozen soil parameterization in a distributed biosphere hydrological model
Authors: Wang L. , Koike T. , Yang K. , Jin R. , Li H.
Published: April 2010

Abstract. In this study, a frozen soil parameterization has been modified and incorporated into a distributed biosphere hydrological model (WEB-DHM). The WEB-DHM with the frozen scheme was then rigorously evaluated in a small cold area, the Binngou watershed, against the in-situ observations from the WATER (Watershed Allied Telemetry Experimental Research). First, by using the original WEB-DHM without the frozen scheme, the land surface parameters and two van Genuchten parameters were optimized using the observed surface radiation fluxes and the soil moistures at upper layers (5, 10 and 20 cm depths) at the DY station in July. Second, by using the WEB-DHM with the frozen scheme, two frozen soil parameters were calibrated using the observed soil temperature at 5 cm depth at the DY station from 21 November 2007 to 20 April 2008; while the other soil hydraulic parameters were optimized by the calibration of the discharges at the basin outlet in July and August that covers the annual largest flood peak in 2008. With these calibrated parameters, the WEB-DHM with the frozen scheme was then used for a yearlong validation from 21 November 2007 to 20 November 2008. Results showed that the WEB-DHM with the frozen scheme has given much better performance than the WEB-DHM without the frozen scheme, in the simulations of soil moisture profile at the cold regions catchment and the discharges at the basin outlet in the yearlong simulation.
Calibration analysis for water storage variability of the global hydrological model WGHM
Authors: Werth S. , Güntner A.
Published: April 2010

Abstract. The aim of this study is to provide an improved global simulation of continental water storage variations by calibrating the WaterGAP Global Hydrology Model (WGHM) for 28 of the largest river basins worldwide. Five years (January 2003–December 2007) of satellite-based estimates of the total water storage changes from the GRACE mission were combined with river discharge data in a multi-objective calibration framework that uses the most sensitive WGHM model parameters. The uncertainty and significance of the calibration results were analysed with respect to errors in the observation data. An independent simulation period (January 2008–December 2008) was used for validation. The contribution of single storage compartments to the total water budget before and after calibration was analysed in detail. A multi-objective improvement of the model states was obtained for most of the river basins, with mean error reductions of up to 110 km3/month for discharge and up to 24 mm of a water mass equivalent column for total water storage changes, such as for the Amazon basin. Errors in the phase and signal variability of seasonal water mass changes were reduced. The calibration is shown to primarily affect soil water storage in most river basins. The variability of groundwater storage variations was reduced on a global scale after calibration. Structural model errors were identified from a small contribution of surface water storage including wetlands in river basins with large inundation areas, such as the Amazon or the Mississippi. Our results demonstrate the value of both the GRACE data and the multi-objective calibration approach for improving large-scale hydrological simulations, and they provide a starting-point for improving model structures. The integration of complimentary observation data to further constrain the simulation of single storage compartments is encouraged.
Process identification of soil erosion in steep mountain regions
Authors: Konz N. , Baenninger D. , Konz M. , Nearing M. , Alewell C.
Published: April 2010

Abstract. Mountainous soil erosion processes were investigated in the Urseren Valley (Central Switzerland) by means of measurements and simulations. The quantification of soil erosion was performed on hill slope scale (2·20 m) for three different land use types: hayfields, pastures with dwarf shrubs and pastures without dwarf shrubs with three replicates each. Erosion rates during growing season were measured with sediment traps between June 2006 and November 2007. Long-term soil erosion rates were estimated based on Cs- 137 redistribution. In addition, soil moisture and surface flow were recorded during the growing season in the field and compared to model output. We chose the WEPP model (Water Erosion Prediction Project) to simulate soil erosion during the growing season. Model parameters were determined in the field (slope, plant species, fractional vegetation cover, initial saturation level), by laboratory analyses (grain size, organic matter) and by literature study. The WEPP model simulates sheet erosion processes (interrill and splash erosion processes, please note that no rill erosion occurs at our sites). Model output resulted in considerable smaller values than the measured erosion rates with sediment traps for the same period. We attribute the differences to observed random gravity driven erosion of soil conglomerates. The Cs-137 measurements deliver substantially higher mean annual erosion rates, which are most likely connected to snow cover related processes such as snow gliding and avalanche activities.
Evaluation of a bias correction method applied to downscaled precipitation and temperature reanalysis data for the Rhine basin
Authors: Terink W. , Hurkmans R. T. W. L. , Torfs P. J. J. F. , Uijlenhoet R.
Published: April 2010

Abstract. In many climate impact studies hydrological models are forced with meteorological data without an attempt to assess the quality of these data. The objective of this study was to compare downscaled ERA15 (ECMWF-reanalysis data) precipitation and temperature with observed precipitation and temperature and apply a bias correction to these forcing variables. Precipitation is corrected by fitting it to the mean and coefficient of variation (CV) of the observations. Temperature is corrected by fitting it to the mean and standard deviation of the observations. It appears that the uncorrected ERA15 is too warm and too wet for most of the Rhine basin. The bias correction leads to satisfactory results, precipitation and temperature differences decreased significantly, although there are a few years for which the correction of precipitation is less satisfying. Corrections were largest during summer for both precipitation and temperature. For precipitation alone large corrections were applied during September and October as well. Besides the statistics the correction method was intended to correct for, it is also found to improve the correlations for the fraction of wet days and lag-1 autocorrelations between ERA15 and the observations. For the validation period temperature is corrected very well, but for precipitation the RMSE of the daily difference between modeled and observed precipitation has increased for the corrected situation. When taking random years for calibration, and the remaining years for validation, the spread in the mean bias error (MBE) becomes larger for the corrected precipitation during validation, but the overal average MBE has decreased.
Thermal conductivity of unsaturated clay-rocks
Authors: Jougnot D. , Revil A.
Published: April 2010

Abstract. The parameters used to describe the electrical conductivity of a porous material can be used to describe also its thermal conductivity. A new relationship is developed to connect the thermal conductivity of an unsaturated porous material to the thermal conductivity of the different phases of the composite, and two electrical parameters called the first and second Archie's exponents. A good agreement is obtained between the new model and thermal conductivity measurements performed using packs of glass beads and core samples of the Callovo-Oxfordian clay-rocks at different saturations of the water phase. We showed that the three model parameters optimised to fit the new model against experimental data (namely the thermal conductivity of the solid phase and the two Archie's exponents) are consistent with independent estimates. We also observed that the anisotropy of the effective thermal conductivity of the Callovo-Oxfordian clay-rock was mainly due to the anisotropy of the thermal conductivity of the solid phase.
Combining remotely sensed data using aggregation algorithms
Authors: Shuttleworth W. J.
Published: April 2010

Abstract. This paper describes a strategic approach for providing documentation of the surface energy exchange for heterogeneous land surfaces via the simultaneous, four-dimensional assimilation of several streams of remotely sensed data into a coupled land surface-atmosphere model. The basic concepts and underlying theory behind this proposed approach are presented with the intent that this will guide, facilitate, and stimulate future research focused on its practical implementation when appropriate data from the Earth Observing System (EOS) become available. The theoretical concepts that underlie the approach are derived from relationships between the values of parameters which control surface exchanges at pixel (or patch) scale and the area-average value of equivalent parameters applicable at larger, grid scale. A three-step implementation method is proposed which involves (a) estimating grid-average surface radiation fluxes from appropriate remotely sensed data; (b) absorbing these radiation flux estimates into a four-dimensional data assimilation model in which grid-average values of vegetation-related parameters are calculated from pertinent remotely sensed data using the equations that link pixel and grid scales; and (c) improving the resulting estimate of the surface energy balance-again using scale-linking equations by estimating the effect of soil-moisture availability, perhaps assuming that cloud-free pixels are an unbiased subsample of all the pixels in the grid square.
Evaluation of a conceptual rainfall forecasting model from observed and simulated rain events
Authors: Dolciné L. , Andrieu H. , French M. N.
Published: April 2010

Abstract. Very short-term rainfall forecasting models designed for runoff analysis of catchments, particularly those subject to flash-floods, typically include one or more variables deduced from weather radars. Useful variables for defining the state and evolution of a rain system include rainfall rate, vertically integrated rainwater content and advection velocity. The forecast model proposed in this work complements recent dynamical formulations by focusing on a formulation incorporating these variables using volumetric radar data to define the model state variables, determining the rainfall source term directly from multi-scan radar data, explicitly accounting for orographic enhancement, and explicitly incorporating the dynamical model components in an advection-diffusion scheme. An evaluation of this model is presented for four rain events collected in the South of France and in the North-East of Italy. Model forecasts are compared with two simple methods: persistence and extrapolation. An additional analysis is performed using an existing mono-dimensional microphysical meteorological model to produce simulated rain events and provide initialization data. Forecasted rainfall produced by the proposed model and the extrapolation method are compared to the simulated events. The results show that the forecast model performance is influenced by rainfall temporal variability and performance is better for less variable rain events. The comparison with the extrapolation method shows that the proposed model performs better than extrapolation in the initial period of the forecast lead-time. It is shown that the performance of the proposed model over the extrapolation method depends essentially on the additional vertical information available from voluminal radar.
An analytical solution for the estimation of the critical available soil water fraction for a single layer water balance model under growing crops
Authors: Brisson N.
Published: April 2010

Abstract. In the framework of simplified water balance models devoted to irrigation scheduling or crop modelling, the relative transpiration rate (the ratio of actual to maximal transpiration) is assumed to decrease linearly when the soil dries out below a critical available water value. This value is usually expressed as a fraction, F, of the maximal available soil water content. The present work aims to use the basic laws governing water transfer through the plants at a daily time step to compute F dynamically as the crop grows. It can be regarded as an expansion of Slabbers' (1980) approach to crop growing conditions. Starting from the mathematical representation given by single-root models (Gardner, 1960), an analytical expression for F is derived, using simplified hypotheses. This expression accounts for plant attributes such as the mean root radius, the critical leaf water potential for stomatal closure and the root length density profile growing with the crop. Environmental factors such as soil type and atmospheric demand also influence F. The structural influence of soil comes from the required introduction of the bulk soil hydraulic conductivity in the single-root model. The shape of the root length density profile is assumed to be sigmoidal and a new profile is calculated at each value of the rooting depth. A sensitivity analysis of F to all those factors is presented. The first general result is that F decreases as the root system grows in depth. Differences in the shape of the root profile can be responsible for differential water stress sensitivity in the early stages of growth. Yet, low critical leaf water potential can compensate partially for a poor root profile. Conversely, F is relatively insensitive to the average root radius. F sensitivity to soil type seems somewhat artificial: given the bulk soil hydraulic conductivity formula, the soil sensitivity results from F being expressed as a fraction of the maximal available soil water content. The atmospheric demand together with the rooting depth appear as the most important factors. However, when assuming predictable climatic and crop evolution, compensation occurs between those two effects leading to a relative stability of F when the crop is fully developed. Though relying on well-known physical laws, the present approach remains in the framework of single layer models with the same limitations.
Non-linearity and spatial resolution in a cellular automaton model of a small upland basin
Authors: Coulthard T. J. , Kirkby M. J. , Macklin M. G.
Published: April 2010

Abstract. The continuing development of computational fluid dynamics is allowing the high resolution study of hydraulic and sediment transport processes but, due to computational complexities, these are rarely applied to areas larger than a reach. Existing approaches, based upon linked cross sections, can give a quasi two-dimensional view, effectively simulating sediment transport for a single river reach. However, a basin represents a whole discrete dynamic system within which channel, floodplain and slope processes operate over a wide range of space and time scales. Here, a cellular automaton (CA) approach has been used to overcome some of these difficulties, in which the landscape is represented as a series of fixed size cells. For every model iteration, each cell acts only in relation to the influence of its immediate neighbours in accordance with appropriate rules. The model presented here takes approximations of existing flow and sediment transport equations, and integrates them, together with slope and floodplain approximations, within a cellular automaton framework. This method has been applied to the basin of Cam Gill Beck (4.2 km2 ) above Starbotton, upper Wharfedale, a tributary of the River Wharfe, North Yorkshire, UK. This approach provides, for the first time, a workable model of the whole basin at a 1 m resolution. Preliminary results show the evolution of bars, braids, terraces and alluvial fans which are similar to those observed in the field, and examples of large and small scale non-linear behaviour which may have considerable implications for future models.
The impacts of conifer harvesting on runoff water quality: a regional survey for Wales
Authors: Neal C. , Reynolds B. , Wilkinson J. , Hill T. , Neal M. , Hill S. , Harrow M.
Published: April 2010

Abstract. Major, minor and trace element chemistry of runoff at stormflow and baseflow from 67 catchments (2 to 5 ha in area) has been determined to investigate the effects of clear felling and replanting of conifers on stream water quality across Wales. Samples, collected by local forestry workers (Forest Enterprise staff) on a campaign basis on up to eight occasions, were for 16 mature first rotation standing forest: the remainder represented areas completely clear felled from less than one to up to forty years previously. As the waters drain acidic and acid sensitive soils, acidic runoff is often encountered. However, higher pH values with associated positive alkalinities and base cation enrichments are observed due to the influence of weathering reactions within the bedrock. There is little systematic variation in water quality between baseflow and stormflow for each site indicating a complex and erratic contribution of waters from the soil and underlying parent material. 80% or more of the data points show hardly any changes with felling time, but there are a few outlier points with much higher concentrations that provide important information on the processes operative. The clearest outlier felling response is for nitrate at five of the more recently felled sites on brown earth, gley and podzolic soil types. ANC, the prime indicator of stream acidity, shows a diverse response from both high to low outlier values (>+400 to -300 μEq/l). In parallel to nitrate, aluminium, potassium and barium concentrations are higher in waters sampled up to 4 years post felling, but the time series response is even less clear than that for nitrate. Cadmium, zinc and lead and lanthanides/actinides show large variations from site to site due to localized vein ore-mineralization in the underlying bedrock. The survey provides a strong indication that forest harvesting can have marked local effects on some chemical components of runoff for the first four years after felling but that this is confined to a small number of sites where nitrate production and aluminium leaching are high. In general, deforestation leads to a reversal of acidification when the nitrate pulse is low. The variability in water quality from catchment to catchment is too high for generalized conclusions to be made over the extent of the potential changes from site to site. The value of an organised campaign of opportunistic sampling using an infrastructure of enthusiastic staff from regionally dispersed organisations associated with environmental matters (in this case the forestry industry) is highlighted.
Effects of nitrogen deposition and climate change on nitrogen runoff at Norwegian boreal forest catchments: the MERLIN model applied to Risdalsheia (RAIN and CLIMEX projects)
Authors: Wright R. F. , Beier C. , Cosby B. J.
Published: April 2010

Abstract. The catchment scale-experiments of the RAIN and CLIMEX projects conducted on boreal forest ecosystems at Risdalsheia, southernmost Norway, provide a unique set of data on the flux of nitrogen (N) in runoff following changes in N deposition, carbon dioxide (CO2) level and temperature. MERLIN (Model of Ecosystem Retention and Loss of Inorganic Nitrogen), a recently-developed model that focuses on N leaching, provides a means by which these data can be placed into a quantitative framework. The features of the N flux in runoff at Risdalsheia to be explained include (1) leaching of about 30-50 mmol m-2 yr-1 (30-40% of N deposition) during the period 1985-1997 at reference catchments, (2) rapid and dramatic reduction in N leaching following experimental reduction in N deposition in 1985 at KIM catchment, (3) increased flux of about 5 mmol m-2 yr-1 following onset of 3-5°C warming and increased CO2 in 1995 at KIM catchment, and (4) increased flux of about 12 mmol m-2 yr-1 following 3-5°C warming of soil in 1995 at EGIL catchment. One set of calibrated model parameters is sufficient to simulate the changes in N runoff at both experimental catchments for both of the manipulations. The model support the conceptual picture of the soil as the major sink for N inputs from deposition with N accumulating in both the forest floor (labile organic matter LOM) and the bulk soil (refractory organic matter ROM). As the molar carbon/nitrogen (C/N) ratio of LOM decreases to below 23, progressively less N is immobilised and more goes to runoff. The model also supports the conceptual picture of increased rate of decomposition of old soil organic matter in response to higher temperature. An increase of 5% is sufficient to produce the 5-12 mmol m-2 yr-1 increase in N flux in runoff observed at the 2 experimental catchments. The MERLIN simulations are consistent with measurements of increase in net mineralisation rates (per catchment area by 70 mmol m-2 yr-1) and N contents in foliage in treated and reference areas before and after onset of treatment. Runoff provides a very sensitive indicator of changes in N cycling within the ecosystem. Small changes in key processes such as N mineralisation give rise to large relative changes in N flux. Uncertainties in measurements are generally much larger than changes indicated by the model calibration.
Soil moisture gradients and controls on a southern Appalachian hillslope from drought through recharge
Authors: Yeakley J. A. , Swank W. T. , Swift L. W. , Hornberger G. M. , Shugart H. H.
Published: April 2010

Abstract. Soil moisture gradients along hillslopes in humid watersheds, although indicated by vegetation gradients and by studies using models, have been difficult to confirm empirically. While soil properties and topographic features are the two general physio-graphic factors controlling soil moisture on hillslopes, studies have shown conflicting results regarding which factor is more important. The relative importance of topographic and soil property controls was examined in an upland forested watershed at the Coweeta Hydrologic Laboratory in the southern Appalachian mountains. Soil moisture was measured along a hillslope transect with a mesic-to-xeric forest vegetation gradient over a period spanning precipitation extremes. The hillslope was transect instrumented with a time domain reflectometry (TDR) network at two depths. Soil moisture was measured during a severe autumn drought and subsequent winter precipitation recharge. In the upper soil depth (0-30 cm), moisture gradients persisted throughout the measurement period, and topography exerted dominant control. For the entire root zone (0-90 cm), soil moisture gradients were found only during drought. Control on soil moisture was due to both topography and storage before drought. During and after recharge, variations in soil texture and horizon distribution exerted dominant control on soil moisture content in the root zone (0-90 cm). These results indicate that topographic factors assert more control over hillslope soil moisture during drier periods as drainage progresses, while variations in soil water storage properties are more important during wetter periods. Hillslope soil moisture gradients in southern Appalachian watersheds appear to be restricted to upper soil layers, with deeper hillslope soil moisture gradients occurring only with sufficient drought.
Modelling the response of soil and runoff chemistry to forest harvesting in a low deposition area (Kangasvaara, eastern Finland)
Authors: Kämäri J. , Rankinen K. , Finér L. , Piirainen S. , Posch M.
Published: April 2010

Abstract. A simple dynamic soil model developed to analyse the effects of atmospheric deposition and nutrient cycling on terrestrial ecosystems, SMART 2, was applied to the Kangasvaara catchment in eastern Finland. Given the historical deposition and forest growth patterns and reasonable values for the input parameters, SMART 2 was calibrated successfully to reproduce present-day soil and Kangasvaara catchment on the soil and runoff water chemistry under a future deposition scenario (GRP scenario). These impacts were also compared to the effects of further reducing the deposition of sulphur and nitrate under the maximum feasible reduction (MFR) scenario. The model demonstrates the consequences of breaking the nutrient cycle, and predicts that final cutting results in increased leaching of inorganic nitrogen and base cations from the cut part of the catchment for about 10 years. The resulting concentrations in the stream will depend on the ability of the buffer zones surrounding the stream to capture and utilize these nutrients.
A comparison of methods for estimating soil characteristics in regional acidification models; an application of the MAGIC model to Scotland
Authors: Helliwell R. C. , Ferrier R. C. , Evans C. D. , Jenkins A.
Published: April 2010

Abstract. The results from an application of MAGIC (Model of Acidification of Groundwater In Catchments) to 733 Scottish catchments are presented. The availability of representative, good quality soil data is frequently limiting factor for biogeochemical modelling, particularly those involving modelling at various spatial scales. This study tests the sensitivity of MAGIC to soil input data derived from two different methodologies; the "nearest neighbour method" considers the closest representative soil profile to a catchment, and the "spatial weighting method" of all soil types identified in a catchment, based on a soil physico-chemical classification of Scotland. Soil data (soil depth, density, cation exchange capacity and base saturation) calculated using the "nearest neighbour method" and the "spatial weighting method" were highly variable, although the range of upper and lower limits were greater for soil data produced using the nearest neighbour method. In contrast to the predominantly organic soil data calculated by the nearest neighbour method, the spatially weighted soil parameters included a greater proportion of mineral soils. With regard to simulated surface water Acid Neutralising Capacity (ANC) for 1851, 1997 and 2050, MAGIC predictions were similar irrespective of the methodology used to determine soil input parameters. However, soil input data derived from both methods resulted in variable base saturation predictions. It is concluded that the "nearest neighbour" methodology is most appropriate if the objective is to determine the predicted response of the most acid- sensitive sites within a region in line with the approach used in Critical Laod mapping. On the other hand, "spatial weighting" integrates catchment soils and represents a more robust methodology by which to determine changes in median soil and water response in a regional context. The anticipated reductions in S emissions associated with the Second S Protocol are predicted to have a marginal beneficial effect on the recovery of soils and surface waters of Scotland.
A regional application of the MAGIC model in Wales: calibration and assessment of future recovery using a Monte-Carlo approach
Authors: Sefton C. E. M. , Jenkins A.
Published: April 2010

Abstract. A survey and resurvey of 77 headwater streams in Wales provides an opportunity for assessing changes in streamwater chemistry in the region. The Model of Acidification of Groundwater In Catchment (MAGIC) has been calibrated to the second of two surveys, taken in 1994-1995, using a Monte-Carlo methodology. The first survey, 1983-1984, provides a basis for model validation. The model simulates a significant decline of water quality across the region since industrialisation. Agreed reductions in sulphur (S) emissions in Europe in accordance with the Second S Protocol will result in a 49% reduction of S deposition across Wales from 1996 to 2010. In response to these reductions, the proportion of streams in the region with mean annual acid neutralising capacity (ANC) > 0 is predicted to increase from 81% in 1995 to 90% by 2030. The greatest recovery between 1984 and 1995 and into the future is at those streams with low ANC. In order to ensure that streams in the most heavily acidified areas of Wales recover to ANC zero by 2030, a reduction of S deposition of 80-85% will be required.
Predicting regional recovery from acidification; the MAGIC model applied to Scotland, England and Wales
Authors: Evans C. D. , Jenkins A. , Helliwell R. C. , Ferrier R.
Published: April 2010

Abstract. A dynamic, process-based model of surface water acidification, MAGIC, has been applied to over a thousand sites across the UK. The model is calibrated to surface water samples collected during a survey for the Critical Loads programme, and utilises the best available and consistent estimates of soil physical and chemical properties, rainfall and runoff volumes, and deposition chemistry. A total of 698 sites were calibrated successfully. At these sites, surface water chemistry was reconstructed from 1850 to the present day, and forecast to 2050 based on future decreases in sulphur (S) deposition in response to the Second S Protocol. Model outputs capture distinct regional patterns of acidification and recovery. the most acidic present-day conditions are found in acid-sensitive regions of Northern England (the Pennines, Lake District and North York Moors). Although a significant proportion of sites in these areas failed to calibrate, those that did are predicted to have experienced severe historic decreases in acidic neutralising capacity (ANC) in response to high levels of acidic deposition. The model also indicates significant acidification in the moderate deposition areas of Wales and Galloway, whereas in the low deposition region of northern Scotland, acidification has been minor even in areas of acid-sensitive geology. ANC is forecast to recover at virtually all sites, with the greatest recovery predicted for areas currently subject to high deposition. The model indicates that the Second S Protocol, however, will not be sufficient to produce full recovery, with average ANC increases to 2050 counteracting just 27% of the simulated decline from 1850 to present day. Acidic conditions (ANC < 0) are predicted to persist until 2050 at a significant number of sites in Northern England, Wales and Galloway.
Solute dispersion along unsteady groundwater flow in a semi-infinite aquifer
Authors: Kumar N. , Kumar M.
Published: April 2010

Abstract. Analytical and numerical solutions are obtained for dispersion of pollutants along unsteady groundwater flow in a longitudinal direction through semi-infinite aquifers the permeability of which is either uniform or varies with position. Sources of pollution are both a concentrated point input at the origin and a spatially distributed background source. One expression chosen to represent the seasonal pattern of the time dependent velocity is sinusoidal behaviour over a year. The solutions obtained predict the time and distance from the location at which an input concentration is introduced at which the pollution concentration becomes harmless. Also, the time period for rehabilitating a polluted aquifer for human use can also be assessed.
Combined modelling of shortwave and thermal radiation for one-imensional SVATs
Authors: Pearson D. , Daamen C. C. , Gurney R. J. , Simmonds L. P.
Published: April 2010

Abstract. Expressions for the upwelling and downwelling fluxes of optical and thermal radiation between soil, vegetation and the sky are derived, under certain simple assumptions. These are that interception of radiation by the vegetation is a purely geometric effect, while scattering is isotropic, with a strength given by a single-scattering albedo in the optical part of the spectrum, and by Kirchhoff's Law in the thermal. The soil is assumed to be a lambertian reflector, also scattering according to an albedo and Kirchhoff's Law. The model, called RM, conserves energy exactly. As part of a SVAT, it is driven by measured insolation instead of radiation, with little increase in computational cost and number of parameters.
The FORGEX method of rainfall growth estimation I: Review of requirement
Authors: Stewart E. J. , Reed D. W. , Faulkner D. S. , Reynard N. S.
Published: April 2010

Abstract. A growth factor is the ratio of the T-year extreme value to an index extreme value such as the mean of annual maxima. Whereas a record length of ten or more years may suffice to estimate the index variable, it is generally necessary to blend data from several sites if estimates of exceptional extreme values are to be obtained. Methods of rainfall growth estimation are reviewed, including traditional methods which extend frequency curves to long return period by a distributional assumption, and methods which study spatial dependence in extreme rainfalls. It is desirable that estimates at neighbouring sites, and across different durations and return periods, are internally consistent. The review concludes that rather special techniques may be required if this goal of estimation extreme rainfall depth consistently is to be met. The motivation of the Focused Rainfall Growth Extension (FORGEX) method is presented.
Hydrological processes and water resources management in a dryland environment IV: Long-term groundwater level fluctuations due to variation in rainfall
Authors: Butterworth J. A. , Schulze R. E. , Simmonds L. P. , Moriarty P. , Mugabe F.
Published: April 2010

Abstract. To evaluate the effects of variations in rainfall on groundwater, long-term rainfall records were used to simulate groundwater levels over the period 1953-96 at an experimental catchment in south-east Zimbabwe. Two different modelling methods were adopted. Firstly, a soil water balance model (ACRU) simulated drainage from daily rainfall and evaporative demand; groundwater levels were predicted as a function of drainage, specific yield and water table height. Secondly, the cumulative rainfall departure method was used to model groundwater levels from monthly rainfall. Both methods simulated observed groundwater levels over the period 1992-96 successfully, and long-term simulated trends in historical levels were comparable. Results suggest that large perturbations in groundwater levels area a normal feature of the response of a shallow aquifer to variations in rainfall. Long-term trends in groundwater levels are apparent and reflect the effect of cycles in rainfall. Average end of dry season water levels were simulated to be almost 3 m higher in the late 1970s compared to those of the early 1990s. The simulated effect of prolonged low rainfall on groundwater levels was particularly severe during the period 1981-92 with a series of low recharge years unprecedented in the earlier record. More recently, above average rainfall has resulted in generally higher groundwater levels. The modelling methods described may be applied in the development of guidelines for groundwater schemes to help ensure safe long-term yields and to predict future stress on groundwater resources in low rainfall periods; they are being developed to evaluate the effects of land use and management change on groundwater resources.
Spatial variability of nitrate concentration in lakes in Snowdonia, North Wales, UK
Authors: Kernan M. R. , Allott T. E. H.
Published: April 2010

Abstract. At a regional scale, high nitrate (NO3‾) concentrations in upland surface waters generally occur in tandem with high nitrogen (N) deposition levels. However, significant differences in the patterns of surface water NO3‾ concentration have been observed within areas of similar N deposition yet relatively few studies have been undertaken which examine within-region variation of NO3‾ concentrations. A study of 76 lakes in Snowdonia, north Wales, an area of high deposition and sensitive catchments, was undertaken to assess variation in surface water NO3‾ concentration across a 20 x 20 km grid square and to identify catchments vulnerable to NO3‾ leaching. Nitrate concentrations vary considerably, particularly during winter when values range from 0.7 to 70 μeq l-1. Although retention by vegetation and soil microbes in summer reduces the amount of NO3‾ reaching the lakes, 37 % of sites are characterised by NO3‾ concentrations greater then 4 μeq l-1. The elevated concentrations occurring in summer suggests that N breakthrough has occurred. By examining the ratio of NO3‾ to total strong acid anions, it is shown that NO3‾ contributes significantly to freshwater acidity, particularly during the winter. Redundancy analysis shows that NO3‾ leaching is greatest, both in winter and summer, in catchments with high proportions of bare rock where soil and vegetation cover is limited. Nitrogen cycling in these catchments, generally at higher altitudes, may no longer be governed by seasonal biological controls. They are likely to be saturated with respect to nitrogen (i.e. incapable of further N retention) as a result of the elevated deposition levels in the area. The contribution of NO3‾ to acidity is also greatest at these sites. In winter, significant positive relationships are also evident between NO3‾ concentration and soil pH and coniferous woodland. The study demonstrates the importance of catchment factors in modifying the relationship between N deposition and N leaching in upland catchments.