No abstract

No abstract

The study of global change impacts on nitrogen dynamics in mesoscale catchments remains an important topic. The primary mechanisms of change can be grouped broadly into those focused on land management, land use, climatic change, as well as on N deposition patterns. The current state of mesoscale studies of N dynamics is outlined in an effort to present the potential tools and methods available to researchers, as well as to outline future directions for additional research. This review focuses on a comparison of the common model approaches that are used to simulate the N cycle in catchments. The review is not meant as an exhaustive list of all models that might include N cycling, but instead outlines a classification framework as a means of better understanding key differences between common modelling strategies. We conclude with a blueprint of what hydro-biogeochemical models should be capable of, and which additional efforts should be considered in the course of model development and verification.Key words agriculture; mesoscale; modelling; nitrogen; process description; spatial resolution Concepts actuels relatifs à la dynamique de l'azote dans les bassins versants de méso-échelleRésumé L'étude des impacts du changement global sur la dynamique de l'azote dans les bassins versants de méso-échelle reste un sujet important. Les principaux mécanismes de changement relèvent globalement de la gestion du territoire, de l'occupation du sol, du changement climatique et des modalités de dépôt d'azote. L'état actuel des études de la dynamique de l'azote à la méso-échelle est souligné avec une attention particulière pour présenter les outils potentiels et les méthodes disponibles pour les chercheurs, et pour identifier les directions futures pour des recherches complémentaires. Cette revue est focalisée sur une comparaison des approches communes de modélisation qui sont utilisées pour simuler le cycle de l'azote dans les bassins versants. La revue ne prétend pas fournir une liste exhaustive des modèles qui intègrent le cycle de l'azote, mais fournit plutôt un cadre de classification permettant de mieux comprendre les différences clefs qui existent entre les stratégies de modélisation courantes. Nous concluons avec une synthèse de ce que les modèles hydro-biogéochimiques devraient permettre, et des efforts additionnels qui devraient être considérés en termes de développement et de vérification de modèle.

No abstract

Land‐use change is one of the main drivers of watershed hydrology change. The effect of forestry related land‐use changes (e.g. afforestation, deforestation, agroforestry) on water fluxes depends on climate, watershed characteristics and spatial scale. The Soil and Water Assessment Tool (SWAT) model was calibrated, validated and used to simulate the impact of agroforestry on the water balance in the Mara River Basin (MRB) in East Africa. Model performance was assessed by Nash–Sutcliffe Efficiency (NSE) and Kling–Gupta Efficiency (KGE). The NSE (and KGE) values for calibration and validation were: 0.77 (0.88) and 0.74 (0.85) for the Nyangores sub‐watershed, and 0.78 (0.89) and 0.79 (0.63) for the entire MRB. It was found that agroforestry in the watershed would generally reduce surface runoff, mainly because of enhanced infiltration. However, it would also increase evapotranspiration and consequently reduce baseflow and overall water yield, which was attributed to increased water use by trees. Spatial scale was found to have a significant effect on water balance; the impact of agroforestry was higher at the smaller headwater catchment (Nyangores) than for the larger watershed (entire MRB). However, the rate of change in water yield with an increase in area under agroforestry was different for the two and could be attributed to the spatial variability of climate within the MRB. Our results suggest that direct extrapolation of the findings from a small sub‐catchment to a larger watershed may not always be accurate. These findings could guide watershed managers on the level of trade‐offs that might occur between reduced water yields and other benefits (e.g. soil erosion control, improved soil productivity) offered by agroforestry. Copyright © 2016 John Wiley & Sons, Ltd.

Abstract:The objective of this study was to analyze patterns, dynamics and processes of land-use/cover changes in the transboundary Mara River Basin in East Africa. We specifically focused on deforestation and expansion of agriculture in the watershed. The intensity analysis approach was used to analyze data from satellite imagery-derived land-use/cover maps. Results indicate that swap change accounted for more than 50% of the overall change, which shows a very dynamic landscape transformation. Transition from closed forest to open forest was found to be a dominant landscape change, as opposed to a random change. Similarly, transition from open forest to small-scale agriculture was also found to be a dominant transition. This suggests a trend (pathway) of deforestation from closed forest to small-scale agriculture, with open forest as a transitional land cover. The observed deforestation may be attributed to continuous encroachment and a series of excisions of the forest reserve. Transition from rangeland to mechanized agriculture was found to be a dominant land-use change, which was attributed to change in land tenure. These findings are crucial for designing strategies and integrated watershed management policies to arrest further deforestation in the forest reserves as well as to sustainably control expansion of agriculture.

No abstract

No abstract