UC Santa Barbara

Irrigation is a large component of urban water budgets in semi-arid regions and is critical for the management of landscape vegetation and water resources. This is particularly true for Mediterranean climate cities such as Los Angeles, where water availability is limited during dry summers. These interactions were examined by using 10-m resolution satellite imagery and a database of monthly water use records for all residential water customers in Los Angeles in order to map vegetation greenness, the extent and distribution of irrigated areas, and irrigation rates. A water conservation ratio between rates of irrigation and vegetation water demand was calculated to assess over-irrigation. The analyses were conducted for the water years (WY) 2005–2007, which included wet, average, and dry extremes of annual rainfall. Although outdoor water usage was highest in the dry year, vegetation greenness could not be maintained as well as in wetter years, suggesting that lower greenness was due to water stress. However, annual rainfall from WY 2005 to 2007 did not significantly influence the variability in the magnitude and spatial pattern of irrigation, with mean irrigated rates ranging only from 81 to 86 mm. The water conservation ratio showed that 7 % of the postal carrier routes across the city were over-irrigated in the dry year, but 43 % were over-irrigated in the wet year. This was largely because the climatic demand for water by vegetation decreased in wet years, but irrigation rates changed little from year-to-year. This overwatering can be addressed by water conservation, planning and public education, especially in the current California drought. The approach demonstrated here should be transferable to other cities in semi-arid climates.