Chloride profile technique to estimate water movement through unsatured zone in a cropped area in subhumid climate (Po Valley - NW Italy)

Two methods based on a chloride concentration profile were applied to evaluate the annual groundwater recharge in a subhumid area cropped with maize where chloride anthropogenic inputs were greater than the natural ones. The site is located in the alluvial Po plain (NW Italy). The two methods were a steady-state model and an approximate diffusive movement equation. They were applied to the Cl- content of retention water extracted from porous cups on 24 sampling dates through one year. The sampling depth ranged from 0.2 to 2.6 m, and the concentration was steady in time below 1.6 m. Considering all the approximations introduced (homogeneous and non-dispersive medium, constant diffusion coefficient with depth, all the liquid phase in movement, no macroporosity, steady state conditions), the results were consistent with those obtained with a long-term mass-balance method. The mean annual recharge assessed using the steady-state chloride profile method was 205 mm yr-1, using the approximate diffusive movement equation it was 216 mm yr-1, while using the mass-balance method the calculated mean recharge was 174 mm yr-1. The estimate showed a moderate dependence with the sampling date. Chloride inputs were wet and dry atmospheric deposition, irrigation water and fertilizers. The incorporation of chloride fertilizers to the soil is one of the more unusual aspects of the chloride approach in this study.