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

Abstract

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.

Introduction

The problem of water resources is of major concern in the Northeast of Brazil, especially as it is related to agricultural water management on dry-land, ebb tide and irrigated cultivation. The availability of water in this semi-arid area, is mainly linked to the topography, where natural depressions collect water from the catchment area, through intermittent rivers or underground flows. In these areas, small dams are traditionally built to retain water and create small reservoirs. At the end of the rainy season, when the reservoir empties, ebb tide agriculture is performed on the margins of these reservoirs, on the recently emerged and water saturated soil. This traditional cultivation technique is commonly used, but with different regional characteristics. During the ebb tide cultivation, the crop (forage grass, sweet potato, corn, bean, water-melon) generally hardly consumes the total amount of water available to complete its growing cycle. In this area of Brazil, however, only few studies have been performed on these systems, and little is known on their water balance, on the characterization of unsaturated soil hydraulic parameters, and consequently, few models describing this local specific condition are available. Antonino and Audry (2001) performed a complete water budget in a small reservoir–ebb tide system, and pointed out the importance of preferential water flow through fissures in the bottom of the reservoir. In this context, it is essential to be able to quantify preferential flow phenomena, which contribute to rapid mass transport into soils, and their interaction with water table and with atmosphere.

In the last decades increasing efforts have been put into the study of preferential flow phenomena in soils (Bouma, 1981, Gish and Shirmohammadi, 1991). Rapid drainage, preferential or by-pass flow, referring to a rapid infiltration response to water input, through a specific path in soil, is still a scientific question. Preferential flow phenomena can sometimes govern the water transfer into soils. The porous network heterogeneity (macropores, roots, macrofauna porosity, swelling–shrinkage cracks) mainly contributes to preferential flow (Beven and Germann, 1982). The impact of this phenomenon, on the efficiency of agricultural water use and management, and deep water and pollutant percolation, is well established (Germann and Beven, 1985, Beven and Clarke, 1986). The quantification of this process is of major importance for conservation of the quality of water resources.

This paper is focused on the proposal of a simplified flow model representing the water transfer process in the small reservoir–ebb tide system. The paper also deals with the assessment of preferential flow throughout the network of fissures at the bottom of the reservoir using two approaches, as denominated, the EM and the new model formulation.