Land cover following rapid urbanization on the US-Mexico border: Implications for conceptual models of urban watershed processes

Land cover was mapped and used to test conceptual models of watershed processes in Tijuana, Mexico. The Wolman model of erosion in urban areas described three phases of the urban surface: (1) pre-urban vegetation, (2) construction phase, which exposes bare soil for 1-3 years and (3) mature phase with impervious surfaces and vegetation. The Wolman model was tested for Tijuana, Mexico using multiple endmember spectral mixture analysis (MESMA) on Landsat TM imagery from 2003. A time series of the urban boundary from 1938 to 2002 showed that, as predicted by the Wolman model, recently urbanized areas (1994-2002) had a high soil fraction (40%) and a low impervious surface fraction (20%) compared with areas urbanized prior to 1938 (17% soil, 62% impervious). Contrary to the Wolman model, the soil fraction decreased only slightly from newly urbanized areas to areas that had been urbanized for up to 40 years. A revised model based on the Griffin-Ford model of Latin American cities is proposed that describes areas of persistent soil cover, which instead of representing a construction phase as in the Wolman model, corresponded to a periphery and a zone of in situ accretion composed of informal settlements with permanent housing and dirt roads. The distinct pattern of urbanization in Tijuana resulted in high percentages of soil cover, and possibly high sediment production potential, for decades following urbanization. (C) 2010 Elsevier B.V. All rights reserved.