WRF-UCM Modeling of Urban Land-Atmosphere Interactions with a Focus on Landscape Irrigation in the Los Angeles Metropolitan Area

Abstract:

Urbanization is a demographic trend worldwide. Urban irrigation can exceed natural precipitation and is an important component of the water cycle in water stressed cities. 14-30% of municipal water consumption in California is used for irrigation. Understanding and quantifying the potential influence of urban anthropogenic soil moisture contribution on local and regional hydrological cycle is an imperative step toward sustainable and better-managed water resources in water scarce regions. In the current study we address the impact and feedback of urban irrigation by integrating a developed irrigation scheme within the coupled framework of the WRF-UCM (Urban Canopy Model) over the Los Angeles metropolitan area at 1 km spatial resolution. We focus on the impacts of irrigation on the urban water cycle and atmospheric feedback during the summer period. Our results demonstrate a significant sensitivity of WRF-UCM simulated surface turbulent fluxes to the incorporation of irrigation. Introducing anthropogenic moisture, the vegetated pixels show increased latent heat fluxes and decreased sensible and ground heat fluxes confirming irrigation induced shift in the energy partitioning toward elevated latent heat fluxes. The evaluation of the model performance against ground-based reference evapotranspiration (ET) observations indicates that WRF-UCM, after adding irrigation, performs reasonably during the course of the simulation, tracking day to day variability of ET. In the absence of irrigation, simulated ET values are significantly underestimated. This is due to fact that soil moisture is the only source of water in the absence of significant precipitation. In the course of model spin up, the moisture sorted in the soil layers is consumed, resulting in considerable decreases in the latent heat fluxes. Evaluating the model outputs against MODIS based land surface temperature illustrates that this ET reduction leads to reduced cooling effects of urban vegetation, particularly over fully vegetated and low intensity residential pixels. Results indicate the importance of accurate representation of urban irrigation in water scarce regions. Moreover, it is found that the initial soil moisture level plays a key role in dampening the impacts of irrigation in urban domains and extended spin up is recommended.