Runoff response to rainfall variance in managed and unmanaged lands in Mo'orea, French Polynesia
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Background. Increased precipitation variance is a predicted effect of climate change, with the highest variance expected in tropical areas. Increased rainfall can lead to increased runoff, which can cause soil erosion, eutrophication, and other forms of environmental degradations, in both terrestrial and marine environments. Mo’orea, a small volcanic island in French Polynesia with an extremely rugged mountainous landscape, is likely to be affected by increased rainfall in coming decades. This puts the island’s surrounding coral reef at risk of stress from increased runoff. This study was designed to compare the responses of soil from differently managed lands when subjected to different amounts of rainfall.
Methods. Fieldwork consisted of sampling soil from three differently managed areas in the ‘Opunohu Valley in Mo’orea: rangeland, grassland, and forest. Soil was measured for bulk density and water content. The soil was then used in a simulated rainfall experiment to test for time it took for water to flow off of the soil surface and amount of runoff collected.
Results. Results showed the grassland soil to have the highest bulk density and water content. The grassland soil took the longest for water to runoff in the rainfall experiments. A strong positive relationship between increasing soil bulk density and increasing time in seconds for the simulated rainfall to runoff the soil samples was found. All other differences tested were found to be insignificant.
Discussion. The differences in bulk density and water content found in soil between study sites suggest that the way land is used and managed affects soil physical properties, which in turn affects soil response to different amounts of rain. Insignificant results may reveal differences with further experimental trials. Additionally, the soil water content results from this study support previous findings that the soil in ‘Opunohu Valley are loose, moist alluvial deposits.