Forests play an important role in maintaining water supply by supporting the water cycle through evapotranspiration and intercepting rainfall, maintaining water quality by acting as a natural filter, and keeping soil fertile by replenishing nutrients in the nutrient cycle. Deforestation in Kalimantan, Borneo is caused by activities like commercial logging, mining, farming, and settlements which have led to issues like soil erosion, loss of biodiversity, and increased carbon emissions. Solutions to deforestation include conservation through protected areas, reforestation, public awareness campaigns, and sustainable forest management practices.