The hydrologic interactions between the atmosphere, land cover and soil largely determine water availability to sustain ecosystems’and anthropogenic demands. Therefore, understanding how these interactions operate is required to design strategies to reduce or cope with the potential hydrological impacts of land-use and climate change. A particularly vulnerable ecosystem to changes in land use and climate is the Tropical Montane Cloud Forest (TMCF). The hydrology of this ecosystem is determined by the interactions between fog affected meteorology and land cover; both of which present a large spatio-temporal variability. This thesis aims to improve our understanding on TMCFs’ hydrological functioning in the seasonal Orinoco River basin headwaters located in the Colombian eastern Andes. This information is fundamental to anticipate the hydrological consequences of climate and land-use change.
Initially, the hydrometeorological spatio-temporal variability was evaluated in three neighboring catchments with contrasting forest cover and the corresponding hydrological responses of soil moisture and streamflow were assessed (Chapter 2). Results revealed a positive relation of fog/rainfall with elevation that steepened during the dry season. Therefore, hydrological responses were modulated by the interaction of seasonality, land cover and elevation. From these analyses, two key processes that could potentially explain the seasonal contrasts between the water budgets of the studied catchments, were identified.
The first was the water storage and release from organic soil layers. The hydrological role of these organic layers was assessed for different land-cover and climate conditions by combining field and laboratory data with modelling of vertical water flows under variable climate conditions (Chapter 3). From a land-use change perspective the storage loss caused by organic layer removal following deforestation largely explains the higher peak flows in the deforested catchment during the wet season. From the climate change perspective, simulated transpiration was limited by soil moisture during an observed severe dry season and continued to decline under expected climate change with the prolongation of dry spells. These results indicate that the organic layers have to be considered when evaluating hydrological impacts in TMCF catchments due to land-use and climate change.
The second identified key process was the potential fog influence on the catchments’ water budgets through its effect on the canopy water budget. Therefore, the fog’s effect on the canopy water budget was analyzed in three TMCF successional stages by combining hydrometeorological data on a high temporal resolution (i.e. 10 minutes) with modelling (Chapter 4). This approach helped to quantify, albeit a high uncertainty, the fog effects on the canopy-water balance. Even though additional water inputs by fog interception are not significant in relation to total rainfall, the fog’s effect in reducing potential evaporation plays a key role in sustaining canopy and overall system moisture. An improved understanding of the hydrological functioning in TMCFs in the Orinoco River basin, allowed to identify current and potential climate and land-use change impacts on the catchment water balances (Chapter 5). This information will be relevant in guiding future research on TMCF hydrological functioning and in supporting local and regional land-use planning policies and climate-change adaptation plans (Chapter 6).