Weekly and Monthly Groundwater Recharge Estimation in A Rural Piedmont Environment using the Water Table Fluctuation Method
Abstract
La Colacha basin (Córdoba province, Argentina) is a typical piedmont rural area where the unconfined aquifer is used for agricultural activities. The objective of this work is to show the estimation of the recharge (R) rate in the unconfined aquifer, using the water table fluctuation method (WTF). Furthermore, considerations in relation to monthly and weekly recharge rhythms and to the aquifer discharge (D) were perform ed. The aquifer shows a typical behavior of groundwater recharge areas with an important and quick answer of water table to the arrival of precipitations (P). After that, a recession curve is observed, representing the groundwater discharge to the local ba se level (the main stream of the basin). The monthly estimation resulted in an annual average R value of 14.3 % of total P. Although the major amounts of recharge occur in full summer, according to the major total amounts of P, the correlation between mont hly R and P was low (r2 < 0.2) as a result of the high quantities of rainfall water that are converted into runoff. The regression coefficient is higher (r2 = 0.6) for the end of summer and autumn when rainfalls diminish and have low intensities. This situ ation provides less water to the aquifer, but the recharge process is more efficient. The ratio R/D for the 3 year series was positive, whic h means that the aquifer recharge was dominant. In the weekly recharge analysis, the annual average R is slightly lo wer than in the monthly one, that is, 12.4 % of the total P. Thus, it may be concluded that, in this case, the change from monthly to a weekly time step, did not much improve the final value. However, the information obtained with the weekly estimation is much more useful to interpret the aquifer detailed behavior.
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Introduction
Although groundwater is the most used resource worldwide it is not only involved in the water abstraction for several uses, but also in numerous ecosystems behavior. The analysis and quantification of the aquifer dynamics and recharge is a vital requirement for an efficient management of groundwater resources [1, 2, 3, 4], particularly in semi-arid regions and areas where there is overexploitation, i.e., the water extraction from the aquifer is higher than its natural replenishment [5, 6]. Thus, the identification and the definition of a conceptual model of groundwater flow and recharge processes are of major significance. The complexity of flow within aquifers may require extensive data and detailed modeling to answer development questions. However, relatively simple data, such as specific water levels in a carefully designed network of monitoring wells, can be combined with estimates of rainfall input to provide key indications of groundwater dynamics and recharge. Thus, geological and groundwater data are essential to elaborate numerical models to test and improve the conceptual model and the aquifer management. The estimation of aquifer recharge is difficult since it varies in time and space and its rhythms are difficult to measure in a direct way. Even though accurate estimations of the recharge are greatly desirable, uncertainty in estimates generated by current methods remains as well as the difficulty in assessing the uncertainty associated with any given estimate [2]. Recharge is defined as the downward flow of water reaching the water table, adding to groundwater storage [2]. Groundwater recharge occurs through diffuse and focused mechanisms as can be seen in Figure 1. Specifically, recharge processes in unconfined aquifers of the Argentinian Pampean plain must be described and quantified to evaluate the groundwater resources to be used and the replenishment from rainfalls. Furthermore, the evaluation is required because groundwater is susceptible to the arrival of contaminants, which are able to reach the water table justly through recharge water [7]. In La Colacha basin (Córdoba province, Argentina), a typical piedmont rural area (Fig. 1) groundwater studies were carried out due to the importance of the unconfined aquifer for human consumption and, especially, for rural water supply (mainly cattle). Some results of these studies are presented in this paper whose main objective is to show the estimation of the recharge rate in the unconfined aquifer of La Colacha basin, using the water table fluctuation method (WTF). Several considerations in relation to monthly and weekly recharge rhythms and to the aquifer discharge were also included in this paper.
Conclusion
The evaluated unconfined aquifer in the piedmont environment shows a typical behavior of recharge areas with an important and quick answer of water table to the arrival of precipitations and then the discharge to the base level, the main stream of the basin. A delay of the rises of the water table level (days) was observed in relation to the main storms, a phenomenon attributable to the water table depth. Using monthly estimation, an annual average recharge value of 14.3 % of total precipitation was obtained. The correlation between monthly R and P was low (r2 < 0.2), a situation that may be related to the high quantities of rainfall water that are converted into runoff and do not arrive to the aquifer. Nevertheless, the major amounts of recharge (in the order of 50 mm) Page | 112 International Journal of Environmental & Agriculture Research (IJOEAR) ISSN:[2454-1850] [Vol-2, Issue-5, May- 2016] occurred in full summer, according to the major total amounts of rainfalls. The regression coefficient is higher (r2 = 0.6) for the end of the summer and autumn as a consequence of minor rainfall quantities and intensities. Thus, less water is provided to the aquifer, but the recharge process is more efficient and clearly linked to rainfalls. The 2007-2008 hydrological year shows low recharge successive peaks during autumn and winter which may be the consequence of delayed wetting fronts coming from minor intensity rainfalls. The ratio Recharge/Discharge for the whole series was positive which means that the aquifer recharge was dominant, a fact that is coherent with the higher position of the water table at the end of the studied series. In the weekly recharge analysis, the aquifer recharge in the 3 years is slightly lower than the monthly estimation, that is, 12.4 % of the total precipitation. It may be concluded that, in this case, the change from monthly to a weekly time step, did not improve the general information. Thus, the final result was similar and even a little lower, despite having made the calculations with a laborious weekly time step. However, the information obtained with the weekly estimation is much more useful to interpret the aquifer detailed behavior. Taking into account the easy management of the computer code, the calculus may be done for different periods of hydrological interest, including daily time steps. Nevertheless, and even though it is recommended to have values with weekly or major frequencies, the monthly time step can be very useful to analyze long periods to have a first approach to the aquifer behavior.
