Revisiting irrigation efficiency before restoring ancient irrigation canals in multi-functional, nature-based water systems

Abstract

CONTEXT: In the Middle Ages, the Muslims introduced communal water management in the Iberian Peninsula. Some irrigation systems of medieval origin are still in operation in the mountainous areas of Southern Spain. Snowmelt runoff is diverted during spring from high-altitude streams into contoured recharge ditches that convey the water to areas of high infiltration (shallow aquifers). This regulates and delays discharge into the main river, from which downstream flow is diverted, during late spring and summer, to irrigation ditches that supply terraces and fields on river plains. The Busquístar irrigation ditch and its irrigation scheme comprise one of these ancient systems. OBJECTIVES: 1) To characterize the Busquístar system, its water source and regulation, its water users' association, its multi-functionality, and its quality as a nature-based solution for water security. 2) To review the irrigation efficiency concept applied to the restoration of ancient irrigation systems, taking into account their ecosystem services. METHODS: i) Semi-structured interviews with stakeholders to evaluate irrigation system operation and perceptions of multi-functionality; ii) field surveys for description of the irrigation ditch and its riparian flora; iii) satellite imagery for quantifying riparian vegetation; iv) water balance for irrigation efficiency computation. RESULTS AND CONCLUSIONS: Crop production is the main function of the Busquístar irrigation scheme but additional ecosystem services are creation of landscape and biodiversity. Eight riparian plant communities were distinguished along the irrigation ditch. The discharge measured at its head on 23 July and 3 September 2017 represented 14 and 50% of the mean river flow in July and September, respectively. The flow measured at the first bifurcation of the ditch was about half that at the head, revealing the proportion filtered along its 6470-m long conveyance reach. The remaining flow was then diverted to 4 branches along the 3140-m long distribution reach proportionally to the irrigable area in each branch. Return flows and irrigation efficiency were estimated for the two measuring dates considering riparian and crop evapotranspiration. The estimated efficiency was highly dependent on which water uses were considered beneficial and on the scale of analysis, field, entire system or watershed. SIGNIFICANCE: The study revealed the integrated approach behind the traditional water management in these ancient systems. The article proposes a review of the irrigation efficiency concept that allows reconciling the character of a nature-based solution with the current restoration of the system, responding to the needs of the emerging export horticulture in the area.