Behind the efficiency of border irrigation: Lesson learned in Northern Italy

Border irrigation is one of the most ancient but, at the same time, still widely applied irrigation methods across the world. Due to the intrinsic characteristics of the method, the water volumes involved are often difficult to measure and this implies a large uncertainty in the estimation of water use efficiency. In this study, we analyze the results of an extensive monitoring campaign, which was carried out over the agricultural season 2021 and was aimed at detecting the actual performance of border irrigation in different experimental fields located in the Padana plain (i.e. the largest irrigated plain in the EU). In particular, three maize fields characterized by different soil textures, irrigation scheduling and rainfall conditions were considered. The number of irrigation events, their duration, and the water amounts applied were examined and compared with the actual crop water requirements assessed by using the AquaCrop-OS agro-hydrological model. The performance of border irrigation, evaluated through a water use efficiency indicator, was assessed for the monitored agricultural season, as well as simulated over the last 28 years (1993–2020) to detect potential effects of different weather conditions. The main results show that the flow rate applied during each irrigation event ranges from 11 to 18 l s−1 per unit width (m) of the irrigated strip, whereas the irrigation event duration varies between 80 and 130 min ha−1 and substantially reflects the irrigation flow rate and the plot slope, the latter being variable between 2‰ and 7‰. The water use efficiency is highly dependent on the irrigation management, namely the flowrate and the scheduling of the irrigation events, with a minimum value of 16% and a maximum of 56%. The same range of efficiency was obtained for the last 28 years, mainly as a consequence of the application of a rigid irrigation scheduling determined by the rotation period. These findings provide useful information for improving the management of border irrigation in practice, and, more specifically, to understand what margin of improvement can be obtained in terms of water use efficiency, both under current conditions and in view of future increasing freshwater scarcity.