A simplified water transfer model of the reservoir–ebb tide system, including preferential flow, in the semi-arid region in Northeastern Brazil

A simplified model representing the small reservoir–ebb tide system by a set of different interacting water storage reservoirs has been proposed. The first one is the small reservoir, which represents the main water storage, accumulated during the rainy season. The second reservoir is the ebb tide zone. The link between the two reservoirs, the saturated zone, represents a buffer reservoir for water transfer. The water transfer in the system is described by a set of two balance equations, with two unknowns: water level in the reservoir and piezometric levels in the ebb tide area, both time-dependent. The solution for the set of equations depends on the equivalent saturated hydraulic conductivity of the medium and on atmospheric conditions. In order to evaluate the preferential flow throughout the network of fissures located in the deepest part of the reservoir two approaches have been considered, denominated the equivalent medium and the new model formulation. Four parameters were calibrated by inverse method using data of 70 days during a the monitoring period (23rd July–1st October 1998) and the model was validated for a 243 days period (17th April–16th December 1998) with different measured initial conditions and evaporation flux variation in time. Both the approaches were capable of reproducing the water levels in the reservoir and in the aquifer. In spite of differences in the estimated daily values, the two approaches predicted well-cumulative water flow and preferential flow. This study allowed a better understanding of the water transfer process in the reservoir–ebb tide system during the evaporation period.