Influence of Land Cover on Stream Water Chemistry in a Mountainous Headwater Catchment in Southern Ecuador

Abstract:

The Andean páramo has suffered considerable changes during the last decades. The conversion of natural grassland into pine forest, pasture or cropland exerts a high pressure on this ecosystem. Possibly, the change of vegetation cover has an impact on the hydro-biogeochemical cycles within the ecosystem. Therefore our objective was to understand how land cover might influence the stream water chemistry in this ecosystem.

During three sampling campaigns, we took snapshot samples every 100 m along the Quinuas River (14 km) which drains a headwater catchment of 91 km² in south Ecuador. The samples were analyzed for nitrate, BOD, COD, and electric conductivity. We then calculated the subcatchment of each of the sampling points based on a 3 m resolution DEM. Land cover classes were derived from a Pléiades satellite image with a 0.5 m spatial resolution. Finally, we calculated the correlation between stream water concentrations and land cover for the individual sampling campaigns.

First results reveal a negative correlation between páramo grassland and stream nitrate concentrations, but a positive correlation between nitrate and forest and nitrate and pasture. BOD showed a dependency on flow conditions. Páramo areas ensured a continuous supply of BOD to the stream during low flow conditions. But during storm flow events, BOD leaching from páramo areas decreased and increased with a growing share of pasture and forest.

Our preliminary results indicate that hydro-biogeochemical processes, namely nitrogen and carbon cycling and runoff generation, differ between natural páramo grassland and forest and pasture. A change of vegetation cover may not only alter considerably stream water chemistry, but the functioning of the páramo ecosystem as a whole.