Report Number: 133
Year: 2011

Spatio-Temporal Dynamics of Badlands in Southern Guam: A Case Study of Selected Sites

Soil erosion is a major environmental problem in southern Guam affecting soil quality and water quality as well as the coral reef system. Badlands, the extreme form of soil erosion, appear throughout the savanna landscape and contribute large amounts of sediments into the waterways. This study investigated changes in badland extent over a 60-year period in three different sub-basins in southern Guam. Historical aerial photos from 1946 and 1994 and recent QuickBird satellite imagery from 2006 were analyzed in a Geographic Information System (GIS) to detect changes over time. In addition, GIS modeling was used to relate badland occurrence to slope, aspect, and elevation. Basic soil characteristics from field sampling were also investigated.

Results indicate an overall increase in badland cover in 2006 compared to 1946. Large badland patches (>500 m2) seen today in areas with little or no human activity generally existed before 1946. In contrast, areas extensively used by humans, mainly in the form of off-roading, experienced a drastic increase of large badland patches, indicating a large impact of human activities on badland expansion. New badland patches in the form of mass wasting were found in an area with no human impact, indicating natural factors, especially tropical cyclones, as a cause. Despite the general increase in the number of badlands and patch sizes, some badland areas were also able to re-vegetate naturally at all three selected study sites. Re-vegetation occurred rarely in an entire patch but rather on parts of it, hence decreasing the size of some badland patches. It was also found that badland occurrence is strongly influenced by topography (slope, aspect, and elevation) which controls the exposure of badlands to rain, wind, and sun and determines the degree of weathering and erosion. Soil properties of badlands and adjacent vegetative sites were very similar except for the organic matter content, which was almost completely absent in badlands indicating low fertility of the soil and associated lack of vegetation.

Overall, badland dynamics are complex and the result of various human and natural factors. Since it is unrealistic to stop badland expansion altogether and re-vegetate all existing badlands, erosion from badlands and associated sedimentation will continue to impact water quality. However, managing human activities, particularly off-roading, is essential to counteract the accelerated badland expansion caused by humans. This is especially important since human activities are likely to increase significantly with the anticipated military buildup on Guam.

Maria Kottermair
Mohammed H. Golabi
Shahram Khosrowpanah
Yuming Wen