Abstract:
Bahía Blanca case study area comprises three watersheds that discharge in the estuary, near
Bahía Blanca city, from west to east: Sauce Chico, Saladillo and Napostá Grande. The total area is
3,915 km2.
The study area is included in the Colorado basin that is characterized by faulted bedrock which
affects the Paleozoic substratum. Since tertiary age no significant faults occur and the overlying
sediments were deposited with a gentle slope towards the centre of the basin in a syneclise structure.
The pedologic description is based on the Soil Map of the Buenos Aires Province of the Argentine
Republic. From the pedologic description of the soils and their properties, the information required to run
the sequential daily water balance models and to characterize the S parameter of the DRASTIC index of
groundwater vulnerability to pollution was derived.
Land use is of major importance to characterize the depth of the soil subject to the
evapotranspiration and to define the ability of a determined area to produce direct runoff. Both these
informations are also needed for the sequential daily water balance models. These models allow the
computation of the direct runoff, real evapotranspiration and deep percolation processes. Deep
percolation is an estimator of groundwater recharge.
A detailed description of these sequential daily water balance models is presented. Two methods
were applied, (A) one, included in the BALSEQ_MOD program, that computes direct runoff using the
soil properties and the real evapotranspiration using the dual crop coefficient approach, and (B) another,
included in the BALSEQ program, which computes direct runoff using the land use/soil properties and
the real evapotranspiration assuming a constant crop coefficient. Both methods require the computation
of the potential evapotranspiration. The potential evapotranspiration is computed using the reference
evapotranspiration and the crop coefficients. Daily climate data recorded in Bahía Blanca Aerodrome is
used. This data was processed and missing data were filled. Also the precipitation from this climate
station was used. For each land cover unit the crop coefficients were determined and presented.
Another method used to estimate groundwater recharge is the separation of the surface flow
hydrograph. This method, included in the DECHIDR_VB program, is described and is applied to the
Cerro del Águila stream flow gauge station, in the upper Napostá Grande watershed.
From the applied methodologies, the BALSEQ method provided the best results: average
precipitation = 723 mm.year-1, real evapotranspiration = 631 mm.year-1, average direct runoff =
27 mm.year-1, average recharge = 61 mm.year-1.
Finally, groundwater discharge to the estuary, as determined in Heffner (2003), is only
2,000 m3.d-1, a value that represents less than 1 mm.year-1 if the total watershed area is considered.
