Hillslope Model Research Articles

Glassy dynamics of landscape evolution

Soil creeps imperceptibly downhill, but also fails catastrophically to create landslides. Despite the importance of these processes as hazards and in sculpting landscapes, there is no agreed-upon model that captures the full range of behavior. Here we examine the granular origins of hillslope soil transport by discrete element method simulations and reanalysis of measurements in natural landscapes. We find creep for slopes below a critical gradient, where average particle velocity (sediment flux) increases exponentially with friction coefficient (gradient). At critical gradient there is a continuous transition to a dense-granular flow rheology. Slow earthflows and landslides thus exhibit glassy dynamics characteristic of a wide range of disordered materials; they are described by a two-phase flux equation that emerges from grain-scale friction alone. This glassy model reproduces topographic profiles of natural hillslopes, showing its promise for predicting hillslope evolution over geologic timescales.

Predicting saturation‐excess runoff distribution with a lumped hillslope model: SWAT‐HS

AbstractSaturation‐excess runoff is the major runoff mechanism in humid well‐vegetated areas where infiltration rates often exceed rainfall intensity. Although the Soil and Water Assessment Tool (SWAT) is one of the most widely used models, it predicts runoff based mainly on soil and land use characteristics, and is implicitly an infiltration‐excess runoff type of model. Previous attempts to incorporate the saturation‐excess runoff mechanism in SWAT fell short due to the inability to distribute water from one hydrological response unit to another. This paper introduces a modified version of SWAT, referred to as SWAT‐Hillslope (SWAT‐HS). This modification improves the simulation of saturation‐excess runoff by redefining hydrological response units based on wetness classes and by introducing a surface aquifer with the ability to route interflow from “drier” to “wetter” wetness classes. Mathematically, the surface aquifer is a nonlinear reservoir that generates rapid subsurface stormflow as the water table in the surface aquifer rises. The SWAT‐HS model was tested in the Town Brook watershed in the upper reaches of the West Branch Delaware River in the Catskill region of New York, USA. SWAT‐HS predicted discharge well with a Nash‐Sutcliffe Efficiency of 0.68 and 0.87 for daily and monthly time steps. Compared to the original SWAT model, SWAT‐HS predicted less surface runoff and groundwater flow and more lateral flow. The saturated areas predicted by SWAT‐HS were concentrated in locations with a high topographic index and were in agreement with field observations. With the incorporation of topographic characteristics and the addition of the surface aquifer, SWAT‐HS improved streamflow simulation and gave a good representation of saturated areas on the dates that measurements were available. SWAT‐HS is expected to improve water quality model predictions where the location of the surface runoff matters.

Picturing and modeling catchments by representative hillslopes

Abstract. This study explores the suitability of a single hillslope as a parsimonious representation of a catchment in a physically based model. We test this hypothesis by picturing two distinctly different catchments in perceptual models and translating these pictures into parametric setups of 2-D physically based hillslope models. The model parametrizations are based on a comprehensive field data set, expert knowledge and process-based reasoning. Evaluation against streamflow data highlights that both models predicted the annual pattern of streamflow generation as well as the hydrographs acceptably. However, a look beyond performance measures revealed deficiencies in streamflow simulations during the summer season and during individual rainfall–runoff events as well as a mismatch between observed and simulated soil water dynamics. Some of these shortcomings can be related to our perception of the systems and to the chosen hydrological model, while others point to limitations of the representative hillslope concept itself. Nevertheless, our results confirm that representative hillslope models are a suitable tool to assess the importance of different data sources as well as to challenge our perception of the dominant hydrological processes we want to represent therein. Consequently, these models are a promising step forward in the search for the optimal representation of catchments in physically based models.

Rainfall‐triggered shallow landslides: infiltration dynamics in a physical hillslope model

AbstractIn this study, we investigate the triggering of shallow landslides through the analysis of physical experiments performed in an artificial hillslope. The physical model consists of a reinforced concrete box containing a soil prism with maximum height of 3.5 m, length of 6 m, and width of 2 m. In order to analyse and examine the factors leading to the failure and the triggering modes, the hillslope is equipped with sensors to monitor the pore water pressure and moisture content response to rainfall in a 60 cm thick sand layer overlying a sandy clay soil. Two experiments were performed with different degrees of the sand initial compaction, to investigate the role of porosity on the hydrologic response and the subsequent failure. The experimental results showed that, with initially loose sand, the failure occurred suddenly, without premonitory signs, the soil behaving like a viscous fluid. The collected data showed a rapid increase of the water pressure contextual to the failure of the sand layer. In the second experiment, with initially dense sand, three levels of instability were observed: (i) abundant runoff with limited erosion of the ground surface; (ii) local slip detachments involving a soil thickness of few centimetres; and (iii) a slow advancement of the entire sand layer volume. The hydrologic dynamics observed in the landslide experiments were simulated with numerical solutions of the Richards equation. The results of the simulations agree well with the experiments for the loose sand, while for the dense sand the comparison between experimental and numerical results shows some limitations related to the assumptions of single phase and rigid soil matrix implicit in the Richards equation. Copyright © 2016 John Wiley & Sons, Ltd.

Hillslope response to insect‐induced land‐cover change: an integrated model of end‐member mixing

AbstractCurrent understanding of streamflow composition in mountain watersheds is often limited by inherent uncertainties and collection limitations in field data and assumptions associated with modelling techniques. Additional complexity arises in catchments experiencing land‐cover change. Here, a hillslope model with fully integrated processes from the subsurface through the canopy is combined with Lagrangian particle tracking through the surface and subsurface domains to understand changes in flow paths and source waters with insect‐induced tree death. This approach explicitly simulates end‐member mixing by tracking parcels of water tagged as rain, snow, and pre‐simulation (‘old’) groundwater and provides a method of separating outflows from these sources. Model simulations identify increases in subsurface water availability resulting from transpiration loss and altered canopy processes that increase throughfall and land‐surface energy. Combined with changes in snowmelt timing, the shallower depth to saturation associated with tree death results in increased old groundwater contributions to streamflow. This shift in the source of outflow is consistent with prior field analysis of changing streamflow contributions with tree mortality from widespread insect infestation in the Rocky Mountains of North America. Model results also highlight mixing of old water and new precipitation within the groundwater end‐member. Mixed hillslope outflows indicate that combinations of topography and precipitation can drive a range of signatures in groundwater inputs over meaningful time periods. Ultimately, this work and analysis of field observations provide insight into hillslope hydrologic processes and can serve as a platform for more complex simulations of land‐cover perturbations to streamflow source partitioning. Copyright © 2016 John Wiley & Sons, Ltd.

Aggregated conceptual model of sediment transport for mountain basins in Antioquia- Colombia

An aggregated model to estimate mean denudation rates of tropical mountain basins with limited data is presented, which is calibrated for Antioquia (Colombia); based on the transport equation of Engelund-Hansen. Two complementary approaches are purposed: (1) alluvial river reach model, dependent on discharge, slope and hydraulic geometry and (2) hillslope model dependent on discharge minus base flow, slope, drainage density and flow convergence degree. Both models are closed with a regression for the Shields parameter, depending on sediment yield (t/km2/yr) and an erodibility factor, which is considered robust due to the diversity of scale, morphology and weather of the 23 basins analyzed. Both model approaches predicts inside a +/- 50% margin of error the sediment yields of 70% of the basins, those with less peculiarities; correcting it with a bed load portion, additional to the suspended transport measured, analytically estimated and proved with field data.

Analysis of subsurface storage and streamflow generation in urban watersheds

AbstractSubsurface storage as a regulator of streamflow was investigated as an explanation for the large proportion of pre‐event water observed in urban streams during storm events. We used multiple lines of inquiry to explore the relationship between pre‐event water proportion, subsurface storage, and streamflow under storm conditions. First, we used a three‐dimensional model of integrated subsurface and surface flow and solute transport to simulate an idealized hillslope to perform model‐based chemical hydrograph separation of stormflow. Second, we employed simple dynamical systems analysis to derive the relationship between subsurface storage and streamflow for three Baltimore, Maryland watersheds (3.8–14 km2 in area) along an urban‐to‐rural gradient. Last, we applied chemical hydrograph separation to high‐frequency specific conductance data in nested urban watersheds (∼50% impervious surface cover) in Dead Run, Baltimore County, Maryland. Unlike the importance of antecedent subsurface storage observed in some systems, we found that rainfall depth and not subsurface storage was the primary control on pre‐event water proportion in both field observations and hillslope numerical experiments. Field observations showed that antecedent stream base flow did not affect pre‐event water proportion or streamflow values under storm conditions. Hillslope model results showed that the relationship between streamflow values under storm conditions and subsurface storage was clockwise hysteretic. The simple dynamical systems approach showed that stream base flow in the most urbanized of three watersheds exhibited the largest sensitivity to changes in storage. This work raises questions about the streamflow generation mechanisms by which pre‐event water dominates urban storm hydrographs, and the shifts between mechanisms in rural and urban watersheds.

Quantification of Soil Moisture Effects on Runoff Formation at the Hillslope Scale

AbstractPre-rain-event soil moisture (preSM) plays a crucial role when evaluating runoff formation during heavy rainfall. Using sprinkling experiments and numerical modeling, this study investigates the impact of preSM on runoff formation at the small hillslope scale (≤100 m2). Sprinkling experiments were conducted on three sites in the Austrian Alps and observed hydrological behavior was simulated by using the hillslope model HILLFLOW. For each site, runoff was modeled with different preSM scenarios, covering the whole soil moisture range between permanent wilting point and saturation. Depending on the dominant runoff processes, the results showed impacts of preSM on both the maximum runoff and the total runoff. A predominant threshold was observed near field capacity (35 and 32% by volume). In general, the results clearly showed that the consideration of preSM is a prerequisite to reproduce runoff formation and total runoff amounts accurately. Field capacity turned out to be a good indicator (threshold...

Stable water isotope tracing through hydrological models for disentangling runoff generation processes at the hillslope scale

Abstract. Hillslopes are the dominant landscape components where incoming precipitation becomes groundwater, streamflow or atmospheric water vapor. However, directly observing flux partitioning in the soil is almost impossible. Hydrological hillslope models are therefore being used to investigate the processes involved. Here we report on a modeling experiment using the Catchment Modeling Framework (CMF) where measured stable water isotopes in vertical soil profiles along a tropical mountainous grassland hillslope transect are traced through the model to resolve potential mixing processes. CMF simulates advective transport of stable water isotopes 18O and 2H based on the Richards equation within a fully distributed 2-D representation of the hillslope. The model successfully replicates the observed temporal pattern of soil water isotope profiles (R2 0.84 and Nash–Sutcliffe efficiency (NSE) 0.42). Predicted flows are in good agreement with previous studies. We highlight the importance of groundwater recharge and shallow lateral subsurface flow, accounting for 50 and 16% of the total flow leaving the system, respectively. Surface runoff is negligible despite the steep slopes in the Ecuadorian study region.

An integral-balance nonlinear model to simulate changes in soil moisture, groundwater and surface runoff dynamics at the hillslope scale

We present a system of ordinary differential equations (ODEs) capable of reproducing simultaneously the aggregated behavior of changes in water storage in the hillslope surface, the unsaturated and the saturated soil layers and the channel that drains the hillslope. The system of equations can be viewed as a two-state integral-balance model for soil moisture and groundwater dynamics. Development of the model was motivated by the need for landscape representation through hillslopes and channels organized following stream drainage network topology. Such a representation, with the basic discretization unit of a hillslope, allows ODEs-based simulation of the water transport in a basin. This, in turn, admits the use of highly efficient numerical solvers that enable space–time scaling studies. The goal of this paper is to investigate whether a nonlinear ODE system can effectively replicate observations of water storage in the unsaturated and saturated layers of the soil. Our first finding is that a previously proposed ODE hillslope model, based on readily available data, is capable of reproducing streamflow fluctuations but fails to reproduce the interactions between the surface and subsurface components at the hillslope scale. However, the more complex ODE model that we present in this paper achieves this goal. In our model, fluxes in the soil are described using a Taylor expansion of the underlying storage flux relationship. We tested the model using data collected in the Shale Hills watershed, a 7.9-ha forested site in central Pennsylvania, during an artificial drainage experiment in August 1974 where soil moisture in the unsaturated zone, groundwater dynamics and surface runoff were monitored. The ODE model can be used as an alternative to spatially explicit hillslope models, based on systems of partial differential equations, which require more computational power to resolve fluxes at the hillslope scale. Therefore, it is appropriate to be coupled to runoff routing models to investigate the effect of runoff and its uncertainty propagation across scales. However, this improved performance comes at the expense of introducing two additional parameters that have no obvious physical interpretation. We discuss the implications of this for hydrologic studies across scales.

Exploring geochemical controls on weathering and erosion of convex hillslopes: beyond the empirical regolith production function

ABSTRACTLandscape curvature evolves in response to physical, chemical, and biological influences that cannot yet be quantified in models. Nonetheless, the simplest models predict the existence of equilibrium hillslope profiles. Here, we develop a model describing steady‐state regolith production caused by mineral dissolution on hillslopes which have attained an equilibrium parabolic profile. When the hillslope lowers at a constant rate, the rate of chemical weathering is highest at the ridgetop where curvature is highest and the ridge develops the thickest regolith. This result derives from inclusion of all the terms in the mathematical definition of curvature. Including these terms shows that the curvature of a parabolic hillslope profile varies with distance from the ridge. The hillslope model (meter‐scale) is similar to models of weathering rind formation (centimeter‐scale) where curvature‐driven solute transport causes development of the thickest rinds at highly curved clast corners. At the clast scale, models fit observations.Here, we similarly explore model predictions of the effect of curvature at the hillslope scale. The hillslope model shows that when erosion rates are small and vertical porefluid infiltration is moderate, the hill weathers at both ridge and valley in the erosive transport‐limited regime. For this regime, the reacting mineral is weathered away before it reaches the land surface: in other words, the model predicts completely developed element‐depth profiles at both ridge and valley. In contrast, when the erosion rate increases or porefluid velocity decreases, denudation occurs in the weathering‐limited regime. In this regime, the reacting mineral does not weather away before it reaches the land surface and simulations predict incompletely developed profiles at both ridge and valley. These predictions are broadly consistent with observations of completely developed element‐depth profiles along hillslopes denuding under erosive transport‐limitation but incompletely developed profiles along hillslopes denuding under weathering limitation in some field settings. Copyright © 2013 John Wiley & Sons, Ltd.

SAC-SMA a priori parameter differences and their impact on distributed hydrologic model simulations

Summary Deriving a priori gridded parameters is an important step in the development and deployment of an operational distributed hydrologic model. Accurate a priori parameters can reduce the manual calibration effort and/or speed up the automatic calibration process, reduce calibration uncertainty, and provide valuable information at ungauged locations. Underpinned by reasonable parameter data sets, distributed hydrologic modeling can help improve water resource and flood and flash flood forecasting capabilities. Initial efforts at the National Weather Service Office of Hydrologic Development (NWS OHD) to derive a priori gridded Sacramento Soil Moisture Accounting (SAC-SMA) model parameters for the conterminous United States (CONUS) were based on a relatively coarse resolution soils property database, the State Soil Geographic Database (STATSGO) (Soil Survey Staff, 2011) and on the assumption of uniform land use and land cover. In an effort to improve the parameters, subsequent work was performed to fully incorporate spatially variable land cover information into the parameter derivation process. Following that, finer-scale soils data (the county-level Soil Survey Geographic Database (SSURGO) ( Soil Survey Staff, 2011a , Soil Survey Staff, 2011b ), together with the use of variable land cover data, were used to derive a third set of CONUS, a priori gridded parameters. It is anticipated that the second and third parameter sets, which incorporate more physical data, will be more realistic and consistent. Here, we evaluate whether this is actually the case by intercomparing these three sets of a priori parameters along with their associated hydrologic simulations which were generated by applying the National Weather Service Hydrology Laboratory’s Research Distributed Hydrologic Model (HL-RDHM) ( Koren et al., 2004 ) in a continuous fashion with an hourly time step. This model adopts a well-tested conceptual water balance model, SAC-SMA, applied on a regular spatial grid, and links to physically-based kinematic hillslope and channel routing models. Discharge and soil moisture simulated using the different set of parameters are presented to show how the parameters affect the results and under what conditions one set of parameters works better than another. In total, 63 basins ranging in size from 30 km2 to 5224 km2 were selected for this study. Sixteen of them were used to study the effects of different a priori parameters on simulated flow. Simulated hourly flow time series from three cases were compared to hourly observed data to compute statistics. Although the overall statistics are similar for the three different sets of parameters, improvements in simulated flow are observed for small basins when SSURGO-based parameters are used. Fifty-seven basins covering different climate regimes were used to analyze differences in the modeled soil moisture. Results again showed that the use of SSURGO-based parameters generate better soil moisture results when compared to STATSGO-based results, especially for the upper soil layer of smaller basins and wet basins.

A novel explicit approach to model bromide and pesticide transport in connected soil structures

Abstract. The present study tests whether an explicit treatment of worm burrows and tile drains as connected structures is feasible for simulating water flow, bromide and pesticide transport in structured heterogeneous soils at hillslope scale. The essence is to represent worm burrows as morphologically connected paths of low flow resistance in a hillslope model. A recent Monte Carlo study (Klaus and Zehe, 2010, Hydrological Processes, 24, p. 1595–1609) revealed that this approach allowed successful reproduction of tile drain event discharge recorded during an irrigation experiment at a tile drained field site. However, several "hillslope architectures" that were all consistent with the available extensive data base allowed a good reproduction of tile drain flow response. Our second objective was thus to find out whether this "equifinality" in spatial model setups may be reduced when including bromide tracer data in the model falsification process. We thus simulated transport of bromide for the 13 spatial model setups that performed best with respect to reproduce tile drain event discharge, without any further calibration. All model setups allowed a very good prediction of the temporal dynamics of cumulated bromide leaching into the tile drain, while only four of them matched the accumulated water balance and accumulated bromide loss into the tile drain. The number of behavioural model architectures could thus be reduced to four. One of those setups was used for simulating transport of Isoproturon, using different parameter combinations to characterise adsorption according to the Footprint data base. Simulations could, however, only reproduce the observed leaching behaviour, when we allowed for retardation coefficients that were very close to one.

Instrumented failure of hillslope models with soil-pipes

Soil-pipes (porous pipes inside a hillslope) are often detected in collapsed slopes indicating their influence on slope failure processes. Only limited studies can be found regarding the impacts of soil-pipes on landslide mechanisms. Hillslope models prepared in a flume are experimented with different soil-pipe configurations: a) no pipe, b) closed pipe and c) open pipe. Pore-water pressures were measured at six different locations along a slope. Discharges at the outlet of soil-pipe and groundwater seepage were also recorded. For the above mentioned pipe configurations two types of experiments were conducted: a) rainfall-induced failure and b) seepage-induced failure.Experimental results show that a closed pipe accumulates water around its lower end and continuously increases pore-water pressure till a failure. An open pipe works as a means of hillslope drainage and reduces the pore-water pressure of an entire slope. However, if open pipe is blocked, pore-water pressure close to its lower end rises rapidly, leading to immediate soil mass movement. For both seepage and rainfall-induced failure experiments, the maximum pressure before the failure was larger at a slope with an open pipe (once it is closed) than a slope with a pipe closed from the beginning or that without a soil-pipe. This indicates that the blockage of soil-pipes makes a slope more susceptible to failure.Displacement vectors show that soil movement velocity close to the surface was highest at slopes with open pipes after closure and lowest at slopes without pipes because of a higher degree of saturation and pore-water pressure at the time of failure of the former. Before a large failure, small fluctuations in pore-water pressure were also observed which can be an indicator of impending failure.

Estimation of soil erosion rates in a subtropical mountain watershed using 137Cs radionuclide

The formulation of watershed management strategies to protect water resources threatened by soil erosion and sedimentation requires a thorough understanding of sediment sources and factors that drive soil movement in the watershed. This paper describes a study of medium-term water-driven soil erosion rates in a mountainous watershed of the Shihmen Reservoir in Taiwan. A total of 60 sampling sites were selected along a hillslope. At each sampling site, the inventory 137Cs activity was determined and then calculated with the diffusion and migration model to derive soil erosion rates. The rates are one to two orders of magnitude lower than estimates using the Universal Soil Loss Equation, a soil erosion model often used in Taiwan. Results of multiple regression analysis indicate that the spatial variability of soil erosion rates is associated with the relative position of a sampling site to the nearest ridge and soil bulk densities (r 2 = 0.33, p < 0.01). Finally, the patterns of soil redistribution rates on the hillslope follow the 137Cs hillslope model as soil erosion increases in the downslope direction. No deposition site is found at footslope because soil deposition is swept away by regular flooding along the stream channel. This study is an important first step in using 137Cs as a tracer of soil redistribution in mountainous watersheds of Taiwan.

Estimating changes in effective values of surface detention, depression storage and friction factor at the interrill scale, using a cheap and fast method to mold the soil surface micro-topography

Abstract Surface detention and depression storage constitute the two conceptual storages of water at the soil surface and depend on the spatial configuration of the micro-topography. To be able to measure directly those storages and investigate how they evolve with time and precipitation, it is necessary to isolate the impact of the micro-topography from the infiltration. Therefore we developed a fast and cheap in-situ molding method (+/−85 €/m 2 ) that combines alginate, plaster and lacquer. It allows creating stable and impermeable artificial micro-topographies that reproduce real field situations and that can be submitted to various laboratory runoff simulations. Both the surface of a specific soil and its artificial reproduction were measured with a laser scanner in order to assess the quality of the molding method. This method is shown to be precise with a standard deviation of 0.55 mm, which is also the spatial resolution of the laser scanner method. This novel molding method was applied to get ten footprints of the micro-topography of a loamy bare soil, at different levels of cumulative rainfall erosivity value (R), starting with a tilled soil, and letting it evolve with natural rain events. We studied the evolution of the bulk values of depression storage and surface detention at the interrill scale, considering that these bulk values may provide useful input to hillslope models for which the size of the single grid cell corresponds to the size of our entire molds. We related the hydraulic properties to R, assuming that R would help explain the change in hydraulic parameters of micro-topographies over time. We found a general decrease of maximum depression storage and steady state surface detention with R. We finally parametrized surface detention by an effective friction factor, using a classical equation for an equivalent laminar sheet flow. A simple model to compute the surface detention as a function of the discharge per unit width and the cumulative rainfall erosivity was proposed.

Gypsies in the palace: experimentalist's view on the use of 3‐D physics‐based simulation of hillslope hydrological response

AbstractAs a fundamental unit of the landscape, hillslopes are studied for their retention and release of water and nutrients across a wide range of ecosystems. The understanding of these near‐surface processes is relevant to issues of runoff generation, groundwater–surface water interactions, catchment export of nutrients, dissolved organic carbon, contaminants (e.g. mercury) and ultimately surface water health. We develop a 3‐D physics‐based representation of the Panola Mountain Research Watershed experimental hillslope using the TOUGH2 sub‐surface flow and transport simulator. A recent investigation of sub‐surface flow within this experimental hillslope has generated important knowledge of threshold rainfall‐runoff response and its relation to patterns of transient water table development. This work has identified components of the 3‐D sub‐surface, such as bedrock topography, that contribute to changing connectivity in saturated zones and the generation of sub‐surface stormflow. Here, we test the ability of a 3‐D hillslope model (both calibrated and uncalibrated) to simulate forested hillslope rainfall‐runoff response and internal transient sub‐surface stormflow dynamics. We also provide a transparent illustration of physics‐based model development, issues of parameterization, examples of model rejection and usefulness of data types (e.g. runoff, mean soil moisture and transient water table depth) to the model enterprise. Our simulations show the inability of an uncalibrated model based on laboratory and field characterization of soil properties and topography to successfully simulate the integrated hydrological response or the distributed water table within the soil profile. Although not an uncommon result, the failure of the field‐based characterized model to represent system behaviour is an important challenge that continues to vex scientists at many scales. We focus our attention particularly on examining the influence of bedrock permeability, soil anisotropy and drainable porosity on the development of patterns of transient groundwater and sub‐surface flow. Internal dynamics of transient water table development prove to be essential in determining appropriate model parameterization. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Dependence of effective hydraulic conductivity on rainfall intensity: loamy agricultural soils

AbstractEffective hydraulic conductivity (Ke) can be estimated with statistical models derived from datasets of field measured conductivities. Pedotransfer functions (PTFs) estimate constant Ke values, but suffer from large prediction errors because the functions usually do not account for soil structural heterogeneities. Rainfall‐runoff data have shown that the effective hydraulic conductivity, defined as final infiltration rate at steady state given ponding, is dependent on rainfall intensity. In this study a statistical approach to establish functions for rainfall intensity‐dependent Ke values is presented, including some specific functions for Western European loamy agricultural soils. Steady‐state rainfall experiments have been conducted with a drip‐type rainfall simulator at multiple intensities on small plots, installed on fields covered by four typical crops of the central part of the Belgian loess belt. A mixed linear model has been applied to temperature standardized and ln‐transformed values of apparent steady‐state infiltration rate and rainfall intensity data for the determination of significant explanatory variables and their parameters. Crop type was a necessary classification effect that accounts for much of the variation that could not be explained by continuous variables of soil and surface properties. However, soil surface bulk density, silt and sand content, tortuosity in surface microtopography, plant and residue cover (RC) were predictors that improved predictions combined with crop effect. Model efficiency (ME) values were between 0·81 and 0·97 for calibration and between 0·75 and 0·95 for validation. The application of the functions is limited to sloping, loamy agricultural soils in a Western European climate, during the spring and early summer growing phase, with not fully developed canopy cover and an RC of less than 20%. These limitations bound field conditions that are vulnerable to severe erosion events. A dynamic Ke could be applied in infiltration models, changing runoff and erosion response in hillslope models. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Prodrug Activation by Cryptosporidium Thymidine Kinase

Cryptosporidium spp. cause acute gastrointestinal disease that can be fatal for immunocompromised individuals. These protozoan parasites are resistant to conventional antiparasitic chemotherapies and the currently available drugs to treat these infections are largely ineffective. Genomic studies suggest that, unlike other protozoan parasites, Cryptosporidium is incapable of de novo pyrimidine biosynthesis. Curiously, these parasites possess redundant pathways to produce dTMP, one involving thymidine kinase (TK) and the second via thymidylate synthase-dihydrofolate reductase. Here we report the expression and characterization of TK from C. parvum. Unlike other TKs, CpTK is a stable trimer in the presence and absence of substrates and the activator dCTP. Whereas the values of k(cat) = 0.28 s(-1) and K(m)(,ATP) = 140 microm are similar to those of human TK1, the value of K(m)(thymidine) = 48 microm is 100-fold greater, reflecting the abundance of thymidine in the gastrointestinal tract. Surprisingly, the antiparasitic nucleosides AraT, AraC, and IDC are not substrates for CpTK, indicating that Cryptosporidium possesses another deoxynucleoside kinase. Trifluoromethyl thymidine and 5-fluorodeoxyuridine are good substrates for CpTK, and both compounds inhibit parasite growth in an in vitro model of C. parvum infection. Trifluorothymidine is also effective in a mouse model of acute disease. These observations suggest that CpTK-activated pro-drugs may be an effective strategy for treating cryptosporidiosis.

Modelling intercrop management impact on runoff and erosion in a continuous maize cropping system: Part I. Model description, global sensitivity analysis and Bayesian estimation of parameter identifiability

SummaryIn order to evaluate the long‐term impact of intercrop management on runoff and erosion in a continuous maize cropping system a new model was developed, based on the continuous plot‐scale runoff model of Laloy &amp; Bielders (2008). This model is called the Continuous Runoff and Erosion Hillslope model with DYnamic soil Surface properties (CREHDYS). This paper details the erosion and crop growth and decay model components and presents a thorough global variance‐based sensitivity analysis (GSA) with regard to the erosion prediction, followed by a Bayesian parameter identifiability assessment. As compared with the classical local univariate sensitivity analysis, a GSA is able to deal with the typical non‐linearity of process‐based hydrological and erosion models. The most influential parameters were the Manning's roughness coefficient followed by the saturated hydraulic conductivity of wheel track cells and the median particle size of the material. The soil aggregate stability and soil‐cohesion parameters were found to be almost non‐influential. Most of the results of the parameter identifiability procedure were in close agreement with the GSA. Indeed, the parameter uncertainty seemed to be proportional to the degree of influence, with Manning's coefficient being the most precisely identified, whereas soil and wheel track cohesion parameters showed the largest uncertainty. Exceptions were the soil aggregate stability and the Green‐Ampt soil matric potential. The uncertainty associated with the former was surprisingly low given its low level of influence whereas the uncertainty associated with the latter is partially explained by its negative correlation with the soil saturated hydraulic conductivities of overland flow and wheel track cells.