Estimating spatio-temporal trends of urban irrigation extent and rate using SPOT 5 imagery in a Mediterranean climate city

Urban irrigation is a crucial component that influences urban water use budgets, particularly in Mediterranean climate cities where water availability is limited during dry summers. The main function of urban irrigation is to maintain landscape vegetation greenness and growth. The objectives of this study were (1) to examine the relationships and distributions among urban vegetation greenness, irrigated areas, and irrigation rates within single-family residential (SFR) areas; and (2) to assess irrigation rates via landscape vegetation water demand to better understand trends of SFR water consumption. A time series of SPOT 5 satellite images (10-m resolution) was used to derive a Normalized Difference Vegetation Index (NDVI) thresholding approach for mapping irrigated areas in connection with monthly SFR water billing records for 2005-2007 in the City of Los Angeles, California. The water needs for landscape vegetation were estimated using a vegetation water demand model that included climate, vegetation, and irrigated areas variables. A water conservation ratio was calculated to elucidate the relationships between irrigation and vegetation water demand, and to assess spatio-temporal patterns of SFR outdoor water consumption. The lower vegetation greenness suggested that vegetation was enduring water-stress conditions even though estimated outdoor water consumption is typically highest in dry years. The distributions of irrigated areas, irrigation rates and vegetation greenness revealed distinct patterns of land lot sizes owned by SFR households within different areas. The results from the relationship between irrigation rates, vegetation water demand and vegetation greenness highlighted the potential applications of household irrigation systems and selection of landscape vegetation species. The irrigation rates were influenced by climate variability through different water years (i.e., precipitation and ETo). The water conservation ratio could also detect over-irrigation conditions spatially from different SFR land lot sizes, and incorporate a vegetation water demand approach.