Spatial average aquifer recharge through atmospheric chloride mass balance and its uncertainty in continental Spain

The atmospheric chloride mass balance (CMB) method allows spatial evaluations of the average diffuse aquifer recharge by rainfall ((R) over bar) in large and varied territories when long-term steady conditions can be assumed. Often, the distributed average CMB variables necessary to calculate (R) over bar have to be estimated from the available variable-length data series, which may be of suboptimal quality and spatial coverage. This paper explains the use of these data and the reliability of the results in continental Spain, chosen as a large and varied territory. The CMB variables have been regionalized by ordinary kriging at the same 4976 nodes of a 10 km x 10 km grid. Nodal (R) over bar values vary from 14 to 810 mm year(-1), 90% ranging from 30 to 300 mm year(-1). The recharge-to-precipitation ratios vary from 0.03 in low-permeability formations and semiarid areas to 0.65 in some carbonate massifs. Integrated average results for the whole of continental Spain yield a potential aquifer recharge of 64 km(3) year (1), the net recharge over permeable formations (40% of the territory) being 32 km(3) year(-1). Two main sources of uncertainty affecting (R) over bar (given by the coefficient of variation, CV), induced by the inherent natural variability of the variables (CVR) and from mapping (CVRK), have been segregated. The average CVR is 0.13 and could be improved with longer data series. The average CVRK is 0.07 and may be decreased with better data coverage. The (R) over bar estimates were compared with other regional and local recharge estimates, (R) over bar being 4% and 1% higher, respectively. Copyright (C) 2012 John Wiley & Sons, Ltd.