Hydrology, erosion and nutrient transfers over a transition from semi-arid grassland to shrubland in the South-Western USA: A modelling assessment
Introduction
Recent research on changes in runoff and erosion over a grassland to shrubland transition (Turnbull et al., 2010, Turnbull et al., in preparation-a, Turnbull et al., in preparation-b) has provided new insights into changes in ecosystem structure and function. The observed changes in ecosystem structure and function over a semi-arid, grass–shrub ecotone demonstrate that changes in ecosystem structure and function do not necessarily follow a linear continuum across a grassland to shrubland transition. As the distribution and percentage cover of vegetation decreased over the grassland to shrubland transition, concurrent changes in the connectivity of these surface properties have profound effects on hydrological function (Turnbull et al., 2010). Determining more fully the interactions between ecosystem structure and function, and interactions/feedbacks between biotic and abiotic components of the ecosystem is imperative in order to ascertain the effects that climate and land-use change will have on these ecosystems in the future (Turnbull et al., 2008).
Modelling-based studies provide a potential means of expanding further our understanding of interactions and feedbacks between different components of an ecosystem, under a multitude of climatic and land-use conditions, that would not be feasible to investigate using field-based experimentation alone. However, the limitation at present of carrying out such modelling-based studies is that the transfers of matter and energy among units are rarely included in models of vegetation change, but are critically important in arid ecosystems in which resource redistribution plays a significant role (Peters and Herrick, 2001). Therefore, the development of existing modelling approaches is imperative in order to understand further the dynamics of land degradation in semi-arid areas. The aim of this study is to develop a process-based model to understand better the dynamics of dissolved and particulate-bound nutrient transport processes in runoff, with particular emphasis on the testing of the model algorithms. The specific model to be developed is Mahleran (Model for Assessing Hillslope to Landscape Erosion, Runoff And Nutrients; Müller et al., 2007a, Müller et al., 2007b, Wainwright et al., 2008a, Wainwright et al., 2008b).
Section snippets
Modelling approach
In the following sections the key components of Mahleran are outlined: (i) the hydrology component (Wainwright and Parsons, 2002), (ii) the erosion component (Wainwright et al., 2008b), (iii) the dissolved nutrient component (Müller et al., 2007a, Müller et al., 2007b), and (iv) a new particulate-bound nutrient component of Mahleran which is developed here, based on the improved understanding of particulate-bound nutrient fluxes outlined in Turnbull et al. (in preparation-b).
Parameterizing Mahleran for Grassland–Shrubland Plots at the Sevilleta LTER Site and methods of model testing
In order to evaluate the different model components in the context of changing vegetation structures, Mahleran is parameterized for four sites over a transition from black grama (Bouteloua eriopoda) grassland to creosotebush (Larrea tridentata) shrubland at the Sevilleta Long-Term Ecological Research site in central New Mexico, USA, which is lies on the northern margin of the Chihuahuan desert. Sites were located within approximately 1 km of each other and have similar slopes and aspects. The
Results and discussion
In the following sections the hydrological, sediment transport, dissolved nutrient and particulate-nutrient-transport components of the model are evaluated against data collected on water, sediment and nutrient fluxes over the grassland to shrubland transition at the Sevilleta (Turnbull et al., 2010, Turnbull et al., in preparation-b). Suitable field-based observations could not be collected to evaluate quantitatively the within-plot, spatial predictions of erosion and particulate-bound
Conclusions
This paper has outlined the key components of Mahleran and detailed the development of the nutrient-transport-modelling component. Details of methods and data used to parameterize Mahleran were presented for a case study relating to the replacement of grassland by shrubland at the Sevilleta LTER site, New Mexico. Model testing was carried out against detailed field measurements of water, nutrient and sediment fluxes over four plots with different types and amounts of vegetation cover, under
Acknowledgements
The authors are grateful for support from the Sevilleta LTER (NSF award DEB-0217774) and in particular, help from Eva Müller, Scott Collins, Jennifer Johnson, Jim Elliot, Mike Friggens, Renee Robichaud, The University of Sheffield, and the Worshipful Company of Farmers, and the Royal Society Dudley Stamp Memorial Fund Award for providing travel bursaries that made data collection possible. Two anonymous reviewers made invaluable comments which led to significant improvements of this paper, for
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