Wetland and water dynamics in small tropical headwater catchments of the Andes

Abstract

Small communities of the Colombian Andes have a large dependency on ecosystems for their water supply. Colombia has a policy of equitable access to water, but as a result of current institutional arrangements, rural communities have limited access to resources for the improvement of water infrastructure. A case study in the headwater region of the Barbas river catchment, examined the ecological and hydrological dynamics of providing a reliable flow of water to the community of Filandia, population 15,000. The catchment has numerous wetlands that are believed to be an important regulator of water flow especially in supplying baseflow during the dry season. Three wetlands were selected for detailed instrumentation and monitoring and were found to have different hydrological regimes but none of them contributed significant volumes to low flows during the dry season. The presence of an open water surface within a wetland, and a surface outflow, together with wetland size were found to be critical factors for the maintenance of low flows and the water yield of wetlands in relation to their host catchment. Antecedent precipitation conditions determined the discharge coefficient of wetlands, and lag time between peak in rainfall and discharge was influenced by wetland size and precipitation intensity. On the basis of isotopic signals from rain and wetland outflows, wetlands were found to prolong the response time of water in their catchments. Soils are andic with a low storage-discharge coefficient that differ depending on land use. Differences in soil water dynamics at the catchment scale were related to streamflow, and indicate that forests with the largest capacity to store and release water, reduced the fluctuation of baseflow in comparison with grasslands. The isotope analysis revealed a higher runoff coefficient for grasslands. Based on yield analysis, discharge coefficients, FDC, and two approaches of water isotope composition analysis, it was shown that although forests compete for water during the dry season, their soils contribute to sustaining baseflows. Results suggest that to maintain a regular and reliable flow of water for local rural communities, conservation of these upland headwaters is important, and that additional small scale storage structures are necessary.