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Natural vegetation in arid and semi-arid environments is usually water-limited, while the urbanization process alters its phenology through anthropogenic activities such as irrigation and fertilization. In particular, urban irrigation has also been used as one of the strategies to alleviate heat stress during hot seasons. In this study, we aim to examine the impacts of irrigation and its scheduling on vegetation phenology and soil moisture dynamics in the Phoenix Metropolitan Area, a desert urban environment located in Arizona. We first investigate the plant phenology over different land cover types based on long-term remotely sensed data. Results show that precipitation is the primary determinant of natural vegetation growth, while irrigation controls urban greening and agricultural phenology. We then evaluate different irrigation schedules use a soil water balance model. Simulations suggest that irrigation substantially reduces plant water stress, but its effect varies with the scheduling. Seasonally varied daily irrigation is identified as the optimal irrigation practice to reduce plant water stress and improve evapotranspiration for both urban vegetation and crops. This study highlights the critical role irrigation plays in vegetation phenology in arid and semi-arid environments, and sheds new light on effectively using irrigation to mitigate heat stress in a desert city.

Keywords: Vegetation phenology; urban irrigation scheduling; soil water dynamics; plant water stress.

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