Diffuse and concentrated recharge evaluation using physical and tracer techniques: results from a semiarid carbonate massif aquifer in southeastern Spain

In the high-permeability, semiarid carbonate aquifer in the Sierra de Gador Mountains (southeastern Spain), some local springs draining shallow perched aquifers were of assistance in assessing applicability of the atmospheric chloride mass balance (CMB) for quantifying total yearly recharge (R (T)) by rainfall. Two contrasting hydrological years (October through September) were selected to evaluate the influence of climate on recharge: the average rainfall year 2003-2004, and the unusually dry 2004-2005. Results at small catchment scale were calibrated with estimated daily stand-scale R (T) obtained by means of a soil water balance (SWB) of rainfall, using the actual evapotranspiration measured by the eddy covariance (EC) technique. R (T) ranged from 0.35 to 0.40 of rainfall in the year, with less than a 5% difference between the CMB and SWB methods in 2003-2004. R (T) varied from less than 0.05 of rainfall at mid-elevation to 0.20 at high elevation in 2004-2005, with a similar difference between the methods. Diffuse recharge (R (D)) by rainfall was quantified from daily soil water content field data to split R (T) into R (D) and the expected concentrated recharge (R (C)) at catchment scale in both hydrological years. R (D) was 0.16 of rainfall in 2003-2004 and 0.01 in 2004-2005. Under common 1- to 3-day rainfall events, the hydraulic effect of R (D) is delayed from 1 day to 1 week, while R (C) is not delayed. This study shows that the CMB method is a suitable tool for yearly values complementing and extending the more widely used SWB in ungauged mountain carbonate aquifers with negligible runoff. The slight difference between R (T) rates at small catchment and stand scales enables results to be validated and provides new estimates to parameterize R (T) with rainfall depth after checking the weight of diffuse and concentrated mechanisms on R (T) during moderate rainfall periods and episodes of marked climatic aridity.