Land cover changes and climate fluctuations in the upper San Pedro River Basin in Sonora, Mexico

The San Pedro River is a transboundary river that originates in Sonora, Mexico, and flows north into Arizona in the United States. Previous studies using satellite imagery obtained for different years from 1973 through 1997 showed significant changes in land cover in the upper San Pedro region in Mexico. Here, we examine whether these land cover changes over the past few decades are related to climatic changes, to human impacts on the landscape, or both. In particular, this study examines rainfall fluctuations during the past 50 years for the San Pedro Basin, and contrasts those changes with land cover changes in the Upper San Pedro. The results showed that natural vegetation types underwent significant reduction, especially during the period 1973-86. The most dramatic changes were the conversion of grasslands into mesquite and gallery forest into agricultural land. The precipitation record indicates the occurrence of a prolonged period of below average rainfall before the early 1980s, but then generally above average rainfall since then. It is possible that observed changes in land cover composition-mainly gallery forest, desert scrub, grassland, and mesquite-are at least partly related to a prolonged and severe dry period in the San Pedro River Basin, representing a delayed response to the multiyear episode of below normal precipitation. The higher rainfall in the past 20 years may have helped to eliminate negative climatic pressures on the landscape even though human impacts have continued.