Quantifying and Predicting Outdoor Water Use in Los Angeles

Residential water consumption is the largest urban water user and represents the largest potential for conservation according to a peer-reviewed study by the Pacific Institute in California (2003). Outdoor water use represents a large percentage of the residential water budget but significant uncertainties are associated with current estimates and predictive models. The objectives of the current study are to analyze the spatial and temporal trends in outdoor use, determine correlations to climate and vegetation patterns, and establish key drivers of outdoor use in Los Angeles. Monthly individual water use records were acquired from the Los Angeles Department of Water and Power (LADWP) for the 2000 to 2010 period. Indoor use was estimated at the census tract level using a range of established models; with outdoor use then calculated as the residual between total and indoor use. The comparison of different estimates at the census tract level reveals significant variability between commonly-used outdoor use methods. Two of the Pacific Institute methods show that outdoor use percentages vary from 7%-10% to 60%-67% of total residential water use at the tract level across the City. A third tested method, based on average household size to model indoor use, presents a broader range of results, with outdoor use ranging from 2% to 93% of total water use. Climate variables, property characteristics as well as remotely-sensed vegetation indices and evapotranspiration estimates were also collected and aggregated at the census tract level for the same period. A linear regression model was developed using these variables to identify the key predictors of outdoor use for the study area. The residual and regression model estimates will serve to validate the development of a biophysical model including tree and grass cover areas, climate variables and high resolution evapotranspiration estimates. Ultimately, models will be used for predictions for a range of future climate and landscape scenarios. Finally, project results will inform water managers to implement efficient landscaping irrigation conservation strategies part of an integrated water resources management plan for sustainable regional water supply system in Southern California.