A simplified water transfer model of the reservoir–ebb tide system, including preferential flow, in the semi-arid region in Northeastern Brazil
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.
Section snippets
Water flow
The small reservoir–ebb tide system is considered as a set of different interacting water storage reservoirs (Fig. 1). The first one is the small reservoir, which represents the main water storage, accumulated during the rainy season. Considering that no major rain event occurs during the dry season, the water balance in this reservoir during the ebb tide cultivation, for a time interval Δt may be written aswhere ΔhA is the decrease of the water level in the small reservoir
Study area and region
The study has been performed in the ebb tide zone of the basin of Flocos dam, municipality of Tuparetama, Pernambuco State (7°36′S and 37°18′W), in the semi-arid region in the Northeast of Brazil.
The rainfall regime in the semi-arid region is highly irregular, with precipitation concentrated in three to four consecutive months per year, between November and June. The total area of Pernambuco State is of 98,937.8 km2, with 86.6% in the semi-arid region. In the region of the Tuparetama
Calibration and validation of the model
The calibration was performed using a 70 days of the monitoring period (23rd July–1st October 1998). The results for the four calibrated parameters (KEq,aΣ, pΣ,nΣ) are presented in Table 4. The resulting values are realistic as compared to experimental observations. For example, for the parameters aΣ, pΣ and nΣ the resulting characteristic curve of water storage agree well with the behavior of a single layer 80–100 cm presented in Fig. 5. The calibrated value for the equivalent hydraulic
Conclusions
A simplified water balance model has been developed aiming to characterize field situations and to simulate different scenarios, in order to help the decision-making process for the adequate management of small reservoir systems. This simplified model allows precise quantification of the water flow from the small reservoir towards the water table and finally towards the vadose zone in the ebb tide area. It predicts the evolution of both water table depth and water level in the reservoir, from
Acknowledgements
We thank the CNPq (National Research Council, Brazil), the IRD (Research Institute for Development, France) and the cooperation project CAPES-COFECUB (no. 350/01) for providing financial support to this study.
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