From a hydrological perspective, is specially necessary to face the challenge of obtaining a complete spatial arrangement of the drainage network on floodplains with extremely flat relief, impermeable soils and exposed to periodic flooding. Focusing on a sector of the Flooding Pampa in Argentina, our aim was to gain a better understanding of the ways in which vegetation along preferential superficial water flow paths across a floodplain landscape differs from the matrix. We selected three sites along these flow paths, spanning a groundwater salinity gradient. Sampling points were located along transects, transversally arranged to surface and groundwater flows direction. Considering that alternating flooding and drought is an intrinsic characteristic of the region, the drainage network shows expansion and contraction cycles in response to variations in water availability. In this dynamics, preferential superficial water flow paths emerge in the floodplain landscape or disappear. The degree of expression of these paths depends on their location on the topographic gradient, because they tend to lose their degree of channelling as the ground relative heights decrease. Although in the first instance the vegetation matrix seems to be relatively homogeneous, the analysis on a detailed scale allows us to distinguish certain heterogeneity in that pattern. Our analysis shows that there is a downstream variation in the spatial distribution of the vegetation following the topographic and groundwater salinity gradients and that the degree of channelling and soil alkalinity determines the type of plant community to be developed in a certain site of the floodplain landscape.

El uso del suelo y el grado de cobertura vegetal son parámetros importantes para estimar la potencialidad deescurrimiento superficial y erosión en una cuenca, y además para evaluar el desarrollo de cultivos,producción, etc. En este trabajo se evalúa la variación del uso del suelo y de la cobertura vegetal porcentual alo largo de un año (abril de 2005 hasta marzo de 2006) en una cuenca agrícola ganadera del centro de laprovincia de Buenos Aires, Argentina. Este ciclo anual, mientras no se produzcan modificaciones deestructura productiva de la zona, puede ser válido para muchos años más. Para lograr el objetivo buscado, serecurre a técnicas de procesamiento digital de imágenes satelitales y de sistemas de información geográfica.Así se determina el uso del suelo por medio de clasificación no supervisada de las imágenes. A cada usoluego se le asigna su cobertura porcentual estimada de manera conjunta según mediciones a campo y deacuerdo al estado fenológico, si se trata de cultivos. Se obtuvieron 12 categorías de uso a lo largo del año, alas que se agregan lagunas, afloramientos rocosos y montes. Los resultados muestran menores grados decobertura en invierno, y presentan un mínimo del 66 % en septiembre, para luego crecer hasta alcanzar unmáximo del 92 % en febrero y marzo, en el apogeo de los cultivos de soja y maíz.

From a hydrological point of view, the characteristic of the water behaviour in catchments so depressed as the Azul creek basin (centre of Buenos Aires province, Argentina) is water accumulation above the land surface. Thus, water on the ground does not have a single runoff direction, but moves in a disorderly, indefinite and unpredictable way. Considering that periodic floods are a typical disturbance of the region, the objective of this study is to analyse, under field conditions, the transformative effect of prolonged flooding on floristic composition, taking into account the different vegetation patches and their relative position over the relief, the chemical characteristics and the groundwater fluctuation, and some edaphic properties in each site. Vegetation samplings were performed during three consecutive springs, when the grassland was on different hydrological conditions due to very different rainfall precedent histories. A digital terrain model of the study area was built and a flow accumulation map was created from it. Pits were dug to describe edaphic variables and shallow wells were drilled for monitoring the groundwater characteristics. Flooding, in relation with surface and groundwater dynamics and soil characteristics, is the factor that determines and promotes the differentiation among sites that are relatively close, contiguous and even topographically in almost identical positions. So, some patches of vegetation get their differentiation through the limiting conditions of their soils, while others receive greater influence from the hydrodynamics to which they are subject. Thus, in this study it becomes evident how certain stands are floristically homogenised or differentiated over time according to their flooding conditions and, hence, according to the area from which they receive surface and groundwater flow. Also, results corroborate the way the water regime determines the structure and heterogeneity of plant communities in such environments.

Soil erosion is a global concern because of its consequences for the environment and the economy of countries. In the Argentine Pampas Region, soil erosion process is a priority issue, although there is little information about sediment concentration (SC) in agricultural catchments. The study aimed at assessing the factors that have a major influence on SC and discussing the dynamics of hydrological and sedimentological connectivity during 2012, a year with precipitation over the mean and significant erosive events. The study was conducted in a watershed of Buenos Aires province, Argentina. A linear regression model, that considered autocorrelation, was obtained. Maximum rainfall intensity in 30 minutes and peak flow were related to SC. An analysis of satellite images was carried out to discuss the hydrological connectivity, and a connectivity index was calculated to assess changes in sedimentological connectivity. The analyses suggested increments in hydrological and sedimentological connectivity, associated with the drainage area expansion and with water erosion rills. Hydrological connectivity is needed for sedimentological connectivity. However, increments in sedimentological connectivity may have been conditioned by the input of energy to detach and to transport particles. This may have been evidenced when flows exceeded a threshold runoff coefficient.