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

Abstract

Forests act as 'pumps' through their evapotranspiration (E tot ) and as 'sponges' by enhancing soil infiltration capacity and moisture retention. Tropical deforestation and poor post-forest land management generally result in lower E tot , but also reduce infiltration. Strongly diminished infiltration is typically accompanied by enhanced overland flow and can cause reduced groundwater recharge and baseflows. A grid-based land surface hydrological model (W3RA-LUM) was tailored to incorporate the trade-offs between the 'pump' and 'sponge' effects so to investigate where deforestation can be expected to have the greatest impacts on dry-season flows. Streamflow sensitivity analyses for scenarios with or without full forest cover and/or good versus poor surface infiltration were performed for: (i) selected tropical catchments with documented changes in streamflow after deforestation; and (ii) the tropics at large (23.5°N to 35°S, to include important seasonal montane forests). The catchment sensitivity analyses showed that W3A-LUM captured the streamflow response to imposed deforestation and changes in surface conditions reasonably well. For the tropics as a whole, an increase in mean annual streamflow of 18% was obtained for forest conversion (to pasture) only, versus 26% after an additional imposed reduction in surface infiltration. Much of the inferred flow increases concerned water-limited regions. A reduction in dry-season flows was predicted for nearly one-fifth of all grid cells (despite lower E tot after forest conversion) after impaired infiltration, potential 'hot spots' of hydrological change after deforestation and reduced infiltration. The affected grid cells shared the following key characteristics: (i) strong seasonality; (ii) high infiltration capacity under forested conditions;(iii) sufficient wet-season precipitation to recharge deep soil-and groundwater stores; (iv) sufficient soil water storage under forested conditions to 'carry over' infiltrated wet-season rainfall into the subsequent dry season; and (v) slow groundwater recession maintaining baseflow throughout the dry season.Our results demonstrate that forest removal in highly seasonal tropical catchments, whilst typically increasing mean annual water yield, can indeed decrease dry-season flows, depending on pre-and post-forest removal surface conditions and groundwater response times.

Precipitation Characteristics and Land Cover Control Wet Season Runoff Source and Rainfall Partitioning in Three Humid Tropical Catchments in Central Panama

“…Recent research suggests that tropical forests can behave like sponges; storing more wet season precipitation relative to pasture, which, in turn, leads to decreased wet‐season storm flows and increased dry‐season baseflow in comparison (Ogden et al., 2013; Peña‐Arancibia et al., 2019). This “forest sponge effect,” (similar to the “infiltration‐evaporation tradeoff hypothesis” observed by Krishnaswamy et al.…”

A Method for Performing Reforestation to Effectively Recover Soil Water Content in Extremely Degraded Tropical Rain Forests

“…This is not surprising, as 90% precipitation occur in the wet season (Luo et al, 2020), which should meet the water demand of the trees. Nevertheless, higher plant transpiration rates will result in higher soil water uptake, which will lead to overall lower soil water content (Krishnaswamy et al, 2013;Peña-Arancibia et al, 2019). Indeed, we found that mean soil water content around the dominant slow-growing species was 1.5 times higher than in soils surrounding the fast-growing species.…”

Impact of Changes in Land Use and Land Cover in the Contribution Basin of Doutor João Penido Water Supply Reservoir of Juiz de Fora, MG, Brazil

“…The areas of exposed land are those considered in this research as bare. According to Peña-Arancibia et al (2019), the deforestation of areas with vegetation, especially the riparian areas, as well as the incorrect use of the soil, reduce the infiltration rate, increase the dissipation of water from the surface and may cause the reduction of groundwater recharge. As a consequence of the removal of the vegetal covering from the spring areas, there is an increase in the volume and speed of the dissipation of water and erosion, which directly affects the quality and hydrodynamics of the bodies of water.…”