Hydrosaline balance evolution of an irrigated zone: The case of Lerma basin (Spain, 2004-2020).

Geologically saline zones with scarce pluviometry are areas susceptible to salinization of their natural drainage. However, the salinization of the receiving water systems can be accelerated with the implementation of irrigation. This work aims to analyze the effects of irrigation on some zones transformed into irrigation land, from the beginning of the process until its complete consolidation. To this end, salt balances are evaluated as a whole and for each significant chemical element. The study zone is the irrigable area of the Lerma basin (Spain), where hydrosaline balances have been carried out since the hydrological year 2004 (before the implementation of irrigation) until 2020 (after the consolidation of irrigation). The implementation of irrigation in the area has doubled the mass of exported salts up to an average of 3177 kg/ha irrigable·year, for the entire study period. 55 % of that amount results from a global mineral dissolution, although this process seems to decrease with time as these minerals are being flushed from the soil. Before irrigation was implemented, the general global dissolution pattern produced more concentration of most ions (SO, Cl, Mg, Na, and K) in the water outputs than in the water inputs. After the implementation of irrigation, there were more water inputs than outputs in the balance and that was shown by the decrease in the dissolved HCO and Ca.These results indicate that the consolidation of irrigation progressively decreases the induced salinization in the water systems that receive the irrigation return flows. Further studies are required to expand the general understanding of the process and its effects, quantify the different geochemical processes involved, and identify possible additional environmental issues induced by irrigation.