The objective of this study is the evaluation of the spatial variability and intrinsic connectivity features of model input parameters for the parameterization of process-based, spatially distributed overland flow models. Parameter scaling tools based on the statistical and geostatistical properties of an extensive field data set were developed. These allowed the reproduction of the spatial heterogeneity of model parameters associated with the soil- and vegetation related properties of semi-arid shrubland environments to a varying degree. The outcome of the study emphasizes that connectivity plays a fundamental role in the modeling of water fluxes within semi-arid catchments. The larger the degree to which connected features are represented, the better the model performance. In contrast, the parameterization approaches that did not contain connected patterns of parameter values performed comparatively poorly. A spatially connected overland flow model therefore enabled the generation of realistic overland flow patterns that qualitatively resembles field surveys of overland flow generation not only at the outlet of the model domains, but also within the catchments without the need of calibration.

%B Water Resources Research %V 43 %8 2007 %G eng %U files/bibliography/JRN00485.pdf %M JRN00485 %L 00913 %) In File (11/20/2007) %R 10.1029/2006WR005006 %F 1395 %0 Journal Article %J Earth Surface Processes and Landforms %D 2006 %T A bed-load transport model for rough turbulent open-channel flows on plane beds %A Athol D. Abrahams %A Gao, Peng %K article %K hydrology, bed-load transport %K hydrology, saltation %K hydrology, sediment transport %K hydrology, sheet flow %K journal %K model %K model, bed-load transport %K model, hydrology %XData from flume studies are used to develop a model for predicting bed-load transport rates in rough turbulent two-dimensional open-channel flows moving well sorted non-cohesive sediments over plane mobile beds. The object is not to predict transport rates in natural channel flows but rather to provide a standard against which measured bed-load transport rates influenced by factors such as bed forms, bed armouring, or limited sediment availability may be compared in order to assess the impact of these factors on bed-load transport rates. The model is based on a revised version of Bagnold’s basic energy equation *ibsb *= *eb*ù, where *ib *is the immersed bed-load transport rate, ù is flow power per unit area, *eb *is the efficiency coefficient, and *sb *is the stress coefficient defined as the ratio of the tangential bed shear stress caused by grain collisions and fluid drag to the immersed weight of the bed load. Expressions are developed for *sb *and *eb *in terms of *G*, a normalized measure of sediment transport stage, and these expressions are substituted into the revised energy equation to obtain the bed-load transport equation *ib *= ù *G *3·4. This equation applies regardless of the mode of bed-load transport (i.e. saltation or sheet flow) and reduces to *ib *= ù where *G *approaches 1 in the sheet-flow regime. That *ib *= ù does not mean that all the available power is dissipated in transporting the bed load. Rather, it reflects the fact that *ib *is a transport rate that must be multiplied by *sb *to become a work rate before it can be compared with ù. It follows that the proportion of ù that is dissipated in the transport of bed load is *ibsb*/ù, which is approximately 0·6 when *ib *= ù. It is suggested that this remarkably high transport efficiency is achieved in sheet flow (1) because the ratio of grain-to-grain to grain-to-bed collisions increases with bed shear stress, and (2) because on average much more momentum is lost in a grain-to-bed collision than in a grain-to-grain one. Copyright © 2006 John Wiley & Sons, Ltd.

Current estimates of rates of soil erosion by water derived from plots are incompatible with estimates of long-term lowering of large drainage basins. Traditional arguments to reconcile these two disparate rates are flawed. The flux of sediment leaving a specified area cannot be converted to a yield simply by dividing by the area, because there is no simple relationship between flux and area. Here, we develop an approach to the determination of erosion rates that is based upon the entrainment rates and travel distances of individual particles. The limited available empirical data is consistent with the predictions of this approach. Parameterization of the equations to take account of such factors as gradient and sediment supply is required to proceed from the conceptual framework to quantitative measurements of erosion. However, our conceptual model solves the apparent paradox of the sediment delivery ratio, resolves recent discussion about the validity of erosion rates made using USLE erosion plots, and potentially can reconcile erosion rates with known lifespans of continents. Our results imply that previous estimates of soil erosion are fallacious. Copyright © 2004 John Wiley & Sons, Ltd.

%B Earth Surface Processes and Landforms %V 29 %P 1293-1302 %8 2004 %@ DOI: 10.1002/esp.1096 %G eng %U files/bibliography/JRN00404.pdf %M JRN00404 %L 00935 %) Not in File (added 7/29/2005) %R 10.1002/esp.1096 %F 1327 %0 Thesis %D 2004 %T An investigation into the effects of the transition from grassland to shrubland on hydrological connectivity in the Jornada Basin, New Mexico %A Turnbull, Laura %K dissertation %K dissertations %K hydrology, grassland %K hydrology, model %K hydrology, runoff %K hydrology, runon %K hydrology, shrubland %K model, hydrology %K theses %K thesis %X The effect of the transition from grassland to shrubland on hydrological connectivity is investigated over a semi-arid rangeland. A cellular model is developed to simulate hydrological connectivity for individual rain events. The model is based upon the probability of transfer of runoff to downslope cells. Simulations were based upon surveyed elevation data over 30 x 30m grassland and shrubland plots. Results show that the transition from grassland to shrubland causes a profound increase in hydrological connectivity, attributed to interactions between surface characteristics that propagate flow to the plot outlet, though primarily to the shrubland associated microtopography which concentrates runoff. %I University of London %C King's College London %P 88 %8 2004 %G eng %9 M.S. Dissertationpp %M JRN00405 %L 00841 %F 1283 %0 Journal Article %J Journal of Hydrologic Engineering %D 2003 %T Bed-load transport equation for sheet flow %A Athol D. Abrahams %K article %K geomorphology, bedload transport %K geomorphology, sheet flow %K hydrology, bedload %K hydrology, channels %K hydrology, processes %K hydrology, sediment transport %K hydrology, sheet flow %K journal %K model, bedload transport rate %K model, hydrology %XWhen open-channel flows become sufficiently powerful, the mode of bed-load transport changes from saltation to sheet flow. Where there is no suspended sediment, sheet flow consists of a layer of colliding grains whose basal concentration approaches that of the stationary bed. These collisions give rise to a dispersive stress that acts normal to the bed and supports the bed load. An equation for predicting the rate of bed-load transport in sheet flow is developed from an analysis of 55 flume and closed conduit experiments. The equation is i(b) = omega where i(b) = immersed bed-load transport rate; and omega = flow power. That i(b) = omega implies that e(b) = tan alpha = u(b)/u, where e(b) = Bagnold's bed-load transport efficiency; u(b) = Mean grain velocity in the sheet-flow layer; and tan alpha = dynamic internal friction coefficient. Given that tan alpha approximate to 0.6 for natural sand, u(b) approximate to 0.6u, and e(b)approximate to 0.6. This finding is confirmed by an independent analysis of the experimental data. The value of 0.60 for e(b) is much larger than the value of 0.12 calculated by Bagnold, indicating that sheet flow is a much more efficient mode of bed-load transport than previously thought.

%B Journal of Hydrologic Engineering %V 129 %P 159-163 %8 2003 %@ 0733-9429/2003/2-159-163 %G eng %U files/bibliography/JRN00378.pdf %M JRN00378 %L 00883 %) In File (8/8/2006) %R 10.1061/(ASCE)0733-9429(2003)129:2(159) %F 1286 %0 Journal Article %J Global Change Biology %D 2003 %T Historical shrub-grass transitions in the northern Chihuahan Desert: modeling the effects of shifting rainfall seasonality and event size over a landscape gradient %A Q. Gao %A J. F. Reynolds %K article %K climate, shrub-grass transition %K grassland, model %K grassland, shrub transition %K journal %K model, Bouteloua %K model, desertification %K model, grassland %K model, hydrology %K model, landscape %K model, Larrea %K model, Prosopis %K model, rainfall %K model, resource allocation %K model, runoff %K model, runon %K model, shrub %K model, shrub-grass %K rainfall, model %K shrub, grass transition %K shrub, model %XWe use a spatially explicit landscape model to investigate the potential role of rainfall on shrub-grass transitions in the Jornada Basin of southern New Mexico during the past century. In long-term simulations (1915 - 1998) along a 2700m transect running from a dry lake bed to the foothills of a small mountain, we test two hypotheses: (i) that wetter winters and drier summers may have facilitated shrub encroachment in grasslands, and (ii) that increases in large precipitation events may have increased soil water recharge at deeper layers, thus favoring shrub establishment and growth. Our model simulations generally support the hypothesis that wetter winters and drier summers may have played a key role, but we are unable to reproduce the major shifts from grass- to shrubdomination that occurred in this landscape during the early part of the 1900s; furthermore, the positive shrub response to wetter winters and drier summers was only realized subsequent to the drought of 1951 - 1956, which was a relatively short ‘window of opportunity’ for increased shrub establishment and growth. Our simulations also generally support the hypothesis that an increase in the number of large precipitation events may also have favored shrub establishment and growth, although these results are equivocal, depending upon what constitutes a ‘large’ event and the timing of such events. We found complex interactions among (i) the amount/seasonality of rainfall, (ii) its redistribution in the landscape via run-on and runoff, (iii) the depth of the soil water recharge, and (iv) subsequent water availability for the growth and reproduction of shrubs vs. herbaceous plants at various landscape positions. Our results suggest that only a mechanistic understanding of these interactions, plus the role of domestic cattle grazing, will enable us to elucidate fully the relative importance of biotic vs. abiotic factors in vegetation dynamics in this semiarid landscape.

%B Global Change Biology %V 9 %P 1475-1493 %8 2003 %G eng %U files/bibliography/JRN00386.pdf %M JRN00386 %L 00844 %) In File (10/24/2005) %R 10.1046/j.1365-2486.2003.00676.x %F 1337 %0 Journal Article %J Earth Surface Processes and Landforms %D 2001 %T A sediment transport equation for interrill overland flow on rough surfaces %A Athol D. Abrahams %A Gary Li %A Krishnan, Chitra %A Atkinson, Joseph F. %K article %K articles %K hydrology, bedload %K hydrology, hillslopes %K hydrology, interrill flow %K hydrology, overland flow %K hydrology, sediment transport %K hydrology, soil erosion %K journal %K journals %K model, hydrology %K model, interrill overland flow %K model, sediment transport %XA model for predicting the sediment transport capacity of turbulent interrill flow on rough sur4faces is developed from 1295 flume experiments with flow depths ranging from 3.4 to 43.4 mm, flow velocities from 0.09 to 0.65 m s^{-1}, Reynolds numbers from 5000 to 26949, Froude numbers from 0.23 to 2.93, bed slopes from 2.7° to 10°, sediment diameters from 0.098 to 1.16 mm, volumetric sediment concentrations from 0.002 to 0.304, roughness concentrations from 0 to 0.57, roughness diameters from 1.0 to 91.3 mm, rainfall intensities from 0 to 159 mm h^{-1}, flow densities from 1002 to 1501 kg m^{-3}, and flow kinematic viscosities from 0.913 to 2.556 x 10^{-6} m^{2} s^{-1}. Stones, cylinders and miniature ornamental trees are used as roughness elements. Given the diverse shapes, sizes and concentrations of these elements, the transport model is likely to apply to a wide range of ground surface morphologies. Using dimensional analysis, a total-load transpot equation is developed for open-channel flows, and this equation is shown to apply to interrill flows both with and without rainfall. The euation indicates that the dimensionless sediment transport rate Ø is a function of, and therefore can be predicted by, the dimensionless shear stress è, its critical value è_{c}, the resistance coefficient u/u_{*}, the inertial settling velocity of the sediment w_{i}, the roughness concentration C_{r}, and the roughness diameter D_{r}. Testing reveals that the model gives good unbiased predictions of Ø in flows with sediment concentrations less than 0.20. FLows with higher concentrations appear to be hyperconcentrated and to have sediment transport rates higher than those predicted by the model. Copyright © 2001 John Wiley & Sons, Ltd.