TROPICAL SOIL AND REGOLITH PROPERTIES AND THEIR INFLUENCE ON RAINFALL /RUNOFF PROCESSES IN A LOWLAND WATERSHED, AGUA SALUD, PANAMA

Vegetation changes are a significant process in many tropical landscapes, with the most common being a change from forest to grassland. These changes in vegetation communities have been associated with changes in the infiltration/runoff processes and ultimately to the water supply in streams. It has been postulated that macroporosity and bulk density are two soil properties strongly impacted by the changing vegetation communities. We evaluated the soils and regolith on a convex slope where a sharp boundary between forest and grassalnd had been in existence for at least 80 years. Soils were described at two slope positions, upper shoulder and lower backslope. The regolith was subdivided into saprolite, strongly weathered bedrock and weakly weathered bedrock. The thickness of each of these regolith types varied with slope position. In upper slope positions, the saprolite was > 10 m. but thinned towards the midslope and was absent in the lower backslope position adjacent to the stream channels. Similarly the upper slope soils were forming in saprolite with soils in the lower slope positions, forming in recent colluvium overlying weathered bedrock. Soils forming in saprolite are characterized by high kaolinitic clay contents that nevertheless produced significant surface cracking and vertical macropores during the dry seasons. Surface and subsurface macroporosity decreased in the rainy season as the clays expanded closing many of the macropores.

Rainfall/runoff relations are different between forested and grassland sites with significantly more runoff being measured from grassland sites than forested areas. Greater runoff in the grassland sites was attributed to decreased infiltration due to increased compaction form grazing animals. However in the rainy season the grassland soil was saturated to within 20 cm of the surface with the upper soil horizon having well developed structure and high macroporosity. The forest soil in contrast, was unsaturated to at least 4 m and had significantly greater moisture storage. Increased runoff from grassland soils would appear to be driven by saturation excess overland flow.