Hydropower Construction and Deforestation: Linking Forest Cover to Changes in Water Balance

As of November 2015, 237 dams were planned or under construction in the Brazilian Amazon, with the Tapajos River Basin accounting for one third of this total. The estimated deforested area in the Tapajos River Basin is expected to be between 42% and 105% greater than background rates of deforestation by 2030. Deforestation has several effects on the local and regional water balance. Rainfall is dependent on both the volume of water and the energy required to move this water from soil to the atmosphere through evapotranspiration. Deforestation increases the speed and volume of water discharge while reducing the amount of energy and water returning to the atmosphere. This increases seasonal variability in water flow and changes the ecological function of rivers and streams. The forest provides a slow release of water into streams during the dry season, which is essential for consistent hydroelectric power generation. The removal of forests for pasture or crop production also affects the energy balance, as pastures and crop fields reflect energy from the sun back to the atmosphere rather than releasing it through evapotranspiration. This effect has measurable impacts on rainfall, leading to a decrease in rainfall at local and regional scales. This could negatively affect rainfall-dependent crop production, resulting in less product and/or increased irrigation costs. Land-surface modeling results from the IBIS model quantify the effect of deforestation on the magnitude of change in water balance characteristics. The results show that water discharge and evapotranspiration are more strongly affected by deforestation during the wet season. Net energy, which is the rainfall potential, is also affected by deforestation, with a stronger impact observed in the Tapajos River Basin. The study recommends policy strategies to avoid or offset the effects of deforestation. These include the use of alternative energy strategies, such as solar and wind energy, which maintain the proper functioning of the water cycle. Deforestation in one place can affect rainfall hundreds of miles away, so policies that reduce the costs of forest restoration and incentivize the maintenance of forested areas should be prioritized. Government-funded construction projects should also bear some cost for funding forest restoration to maintain a functioning water cycle for both river discharge and rainfall.