Forests as ‘sponges’ and ‘pumps’: Modelling the impact of deforestation on dry season flows across the tropics

Abstract:

Metaphorically speaking, forests act as pumps through evapotranspiration and as sponges by enhancing infiltration capacity and moisture retention. Ongoing tropical deforestation and post-forest land use will likely result in a reduction in evapotranspiration (the pump effect), but potentially also a reduction in soil surface infiltration (the sponge effect). Greatly reduced infiltration capacity may translate into overland flow enhancement and possibly a concurrent reduction in groundwater recharge and baseflow. This may have a negative impact on water availability if dry season flows (generally composed mostly of baseflow) are used for productive activities such as irrigation and/or to maintain ecosystems, particularly in catchments dominated by a marked seasonal precipitation regime (wet-dry cycles).   A grid-based land surface hydrological model (W3RA-LUM) was tailored to incorporate the pump and sponge trade-offs in order to examine impacts on hydrology, particularly on dry season flows. Sensitivity analyses for scenarios with or without full forest cover and/or good or poor surface infiltration conditions were performed for: (i) selected catchments in the tropics with documented changes in dry season flows as a result of deforestation and (ii) across the tropics. The catchment sensitivity analyses showed that several relevant hydrological processes (particularly for the seasonal tropics) as well as the response to imposed deforestation and changes in soil conditions were reasonably captured by the model. Across the tropics, results showed an increase in mean annual streamflow of 18% if only vegetation changes were taken into account. The increase in mean annual streamflow was of 26% if there were also reductions in soil surface infiltration. Much of the increase occurred in water-limited regions. Areas that showed a reduction in streamflow for some of the driest months despite the increases in evapotranspiration associated with the conversion from forest to grassland and allowing the development of poor soil conditions to occur included: Central America, northern South America, the Andes, Bolivia, Brazil, the Caribbean, Congo, Gabon, Tanzania, Ethiopia, South Africa, Madagascar, India, Bangladesh, several countries in Southeast Asia and northern Australia. These areas had climatic and biophysical characteristics (as represented in the model) that suggested which areas are more vulnerable to reductions in dry season flows after deforestation and severe reductions in infiltration capacity.