Ad hoc irrigation piloting for the urban vegetation: How to find the contextually relevant sensors and criteria?

The irrigation of urban vegetation in planters aims to keep the vegetation healthy and control the cooling effect from evapotranspiration during overheatingdays. The choice of sensors that characterise the water status of the plant, the soil and/or relevant criteria for triggering irrigation are crucial and far from obvious. This descriptive and analytical paper targets the identification of effective system properties, on which irrigation strategies should rely from a properly thought trial-and-error procedure relying on observational evidence. The study takes advantage of a real-scale pilot in Mediterranean climate (Montpellier, France), which simulates an east-west street canyon with three planters with shrubs on the north side and three planters with climbing plants on the south side, and a dense mesh of sensors in the substrate monitoring the condition of the planters. The provided guidelines explain how to comprehend and qualify any given context (plant species, planter dimensions, type and location of the sensors), before establishing ad hoc irrigation decision rules. This study demonstrates the developed methodology, by testing different irrigation doses, frequencies and criteria based on soil water content and tension information, which proves efficient for understanding and managing irrigation, even with limited information on plant stress tolerance and soil characteristics. Moreover, it allows for periodic reassessment of optimization criteria, which is crucial in addressing current challenges posed by sustainable cities without compromising water resources.