Flatland hydrology

General characteristics
The prevalence of vertical water flows (evapotranspiration – infiltration) over horizontal water flows (runoff – groundwater flow) is a characteristic of the hydrological process in a flatland. Moreover, in wet climate a strong interrelation between surface water and groundwater is recognized. These concepts are directly applied to most of the Buenos Aires Province The terms involved in the water budget (rainfall, evapotranspiration, runoff, groundwater flow) allow us to understand hydrologic behaviours. These water budgets are defined by the difference between water inflow and water outflow and the storage capacity variation (surface and subsoil) in a certain time. The slight slope (generally less than 0.1 per cent) causes a decrease in surface runoff, which implies a longer period of contact between the water and the surface, increasing infiltration and evapotranspiration. The objective of this short paper is to outline the main characteristics of the hydrological processes in the Buenos Aires Province flatland.

Regional hydrological processes
The combination of geomorphological and climate factors (humid climate) determines hydrological systems ranging from an integrated drainage network to a lack of drainage (Sala, 1983). In the first case (northeast of the Province of Buenos Aires), there is local runoff towards the streams, and regional runoff towards a final point of discharge. The water bodies (rivers, creeks and ponds) are effluent or gaining in relation to the groundwater. From a hydrological point of view, the large majority of the streams are autochthonous of the plain, because their sources are located in the same plain. Drainage networks of low density are recognized in the streams watershed. This is evidence of the importance of the infiltration that recharges the groundwater. The groundwater discharges into streams and this is the base flow of rivers, creeks and ponds. In other cases (northwest of the Province of Buenos Aires), when precipitation occurs, water fails to have enough energy to run off toward a specific point of discharge due to the low slope and the lack of streams. These areas of meager topographic slope are affected by periodical flooding. In these areas the outcrops of water table have a direct influence on the permanence and drainage of the water. Land use, groundwater over exploitation and effluent disposal lead to significant changes in the hydrologic cycle, both in terms of quantity and quality. The main consequences are a decrease in infiltration and an increase in runoff. In addition, groundwater and surface water contamination deteriorate the chemical water quality. Surface water is highly vulnerable to contamination. In general, the surface water in industrial areas shows presence of different contaminants, and therefore water quality is not acceptable for aquatic life and other uses. Groundwater vulnerability depends of groundwater occurrence, overlying lithology and groundwater depth. Although the physical environment may provide some degree of protection against the risk of groundwater contamination, industrial wastes, sewage discharges and pesticides affect groundwater quality particularly in urbanized and agricultural areas.