Influence of irrigation water management on the quantity and quality of irrigation return flows

Irrigated agriculture is working towards environmental sustainability but more study is needed regarding the agro-systems’ response to alterations imposed on them. This work analyzed the repercussions that alternative irrigation management can cause on water use and on the quality of irrigation return flows. The case of Bardenas Canal Irrigation District no. V (ID-V) was studied, by analyzing the change of salinity and nitrate concentration in the drainage network through monthly samplings, before (2000) and after (2007) ID-V implanted alternative flood irrigation management. The results showed that the electric conductivity (25 °C) and nitrate concentration in the drainage ditches increased in 2007 (0.99 dS/m and 62 mg/l) with respect to 2000 (0.86 dS/m and 57 mg/l). Nevertheless, the decrease in irrigation drainage in 2007 (88% lower in 2007 when compared to that of 2000) was a result of the decrease in water requirement (594 mm in 2007 against 752 mm in 2000) and of the increase in irrigation efficiency (93% in 2007 and 67% in 2000), causing the Riguel River to present a lower flow (13% inferior when compared with 2000), lower salinity (1.08 dS/m in 2007 and 1.18 dS/m in 2000), and lower nitrate concentration (29 mg/l in 2007 and 33 mg/l in 2000) when exiting ID-V in 2007. In summary, simple alternatives in irrigation management achieved an increment of 26% in water use, decreasing by 20% and 24% the salt and nitrate masses exported, respectively, ameliorating the quality of the system receiving the irrigation return flows (Riguel River).