Modelling surface water/groundwater interaction at large scale on the Lower Salado River Basin

RODRÍGUEZ LB; GIAMPIERI RC; VIONNET CA; PAZ R; STORTI M; IDELSOHN RC; KROHLING D; IRIONDO M

Foz de Iguazu

Congreso; VII Scientific Assembly of the IAHS; 2005

IAHS - International Association of Hydrological Science

During the past 30 years the State of Santa Fe, Argentina, has experienced significant climatic fluctuations due to localized, highly intense precipitation events. For the period 1940-70, annual isohyets were comprised between 800 and 1,100 mm, whereas for the period 1971-2000 those values were between 900 and 1,200 mm. Higher precipitation induced groundwater level rises over vast areas of the State, causing a decrease on the buffering capacity of the unsaturated zone and, in turn, an increase of surface runoff. Consequently, the state economy suffered the devastating effects of recurrent flooding of productive lands and the damage to the road infrastructure. Extremely high precipitation over most of the State saturated the lower basin of the Salado River during the last quarter of 2002 and the first quarter of 2003. During the month of April 2003, a frontal storm added unusually heavy rains. More than 1500 mm of rain were registered for the first four months of 2003 at some rain gages. On April 29, the State Capital, Santa Fe City, suffered the most catastrophic flood on records of the Salado River. Thousands of inhabitants were displaced from their homes and forced to seek shelter on higher grounds. In rural areas, bridges and roads were severely damaged and more than 2 millions hectares were under water. Therefore, there is a clear need to integrate data capture processes and predicting tools in order to analyse past and future hydrologic scenarios. This work summarizes the preliminary findings of the FLAGS project, which brings together a group of geologists, hydrologists, engineers, and numerical modellers in an effort to better understand the dynamics of surface water and its interrelationship with groundwater on a regional scale. The study area covers about 32,000 km2 on the lower basin of the Salado River in the west-central region of the Santa Fe State. Some of the issues the FLAGS project is attempting to address include: 1) the natural boundary conditions (recharge and discharge zones) that drive the dynamics of surface water/groundwater at very large scales, 2) the potential impact of changing hydrology on groundwater levels (e.g., is the observed rising of water levels over the state a natural process and thus reversible, or is due to climate change?), 3) the potential response of the surface water/groundwater system to land use changes. Consequently, some of the technological objectives of the project are: a) improve the knowledge of the stratigraphy and sedimentary structures of the study area, b) deploy a groundwater/rainfall monitoring network, c) develop appropriate conventional and parallel numerical models, d) develop distributed water balance models to estimate the net recharge to the aquifer. To those aims, numerical models are being developed and implemented. While MODFLOW, based on a conventional modelling approach, uses approximately 200,000 nodes on a regular, 500 m x 500 m grid, the FLAGS code under development can withstand up to several millions of nodes. This code, parallelized on a cluster of 14 PCs, is based on a finite element discretization of the ground-water flow equation and the kinematic wave approximation to the Saint Venant equations for streamflow routing. Thus, the FLAGS code seeks to solve the coupling between surface and subsurface water at very large scale while keeping the required representation of the processes involved at small scales. As such, the FLAGS code should lead to improve forecasting capabilities for the region.