Irrigation return flows in a mediterranean aquifer inferred from combined chloride and stable isotopes mass balances

Abstract

The supply of irrigation water often overcomes crop evapotranspiration, and the resulting return flow may infiltrate and significantly contribute to an aquifer water budget. Despite its crucial importance for water resource management, the proportion of irrigation water that contributes to groundwater recharge, namely the return flow coefficient, often remains difficult to assess. Here, a chloride mass balance is combined with an isotopic mixing model (δ18O and δD) to quantify return flow coefficients, in the Crau alluvial-type aquifer (Southern France), characterized by a long-term traditional practice of flood irrigation. Local groundwater compositions are interpreted in terms of average recharge along different flow paths. The high isotopic contrast between irrigation water and regional precipitation allows the partitioning of recharge between rainfall infiltration and irrigation return flows. Isotopic mixing proportions are then used to decipher the chloride concentration of groundwater purely recharged by return flow. This allows an original application of the chloride mass balance approach to estimate return flow coefficients, which doesn't rely on any atmospheric chloride survey. Values around 0.53 ± 0.16 were found for well defined stream lines averaging the functioning of the upstream aquifer, which leads to a return flow rate of 1190 ± 140 mm yr−1. These results are consistent with a local daily time series of recharge fluxes derived from the water-table fluctuation method over the 2003-2009 period, and in line with the spatial average previously quantified over the whole aquifer. This study confirms the ability of geochemical tracers to provide recharge rates fully independent from flux measurements. They can be further used to assess the irrigation efficiency in other similar systems, or to monitor the variations of irrigation return flow, which will result from the future modifications of land use, irrigation practices and climate.