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 performed. 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 base 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 monthly 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 situation provides less water to the aquifer, but the recharge process is more efficient. The ratio R/D for the 3 year series was positive, which means that the aquifer recharge was dominant. In the weekly recharge analysis, the annual average R is slightly lower 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.