When are trees in deciduous forests extremely important in the water cycle

During prolonged dry periods the foliage is dropped to conserve water and prevent death from drought. When the leaves are shed, loos of water through evapotranspiration is reduced hence allow more rain to seep into the soil (rather than into roots and through the leaves into the air) and eventually flow downstream. In contrast, evergreen trees (like pines) retain their leaves year-round, and have been shown to have higher annual evapotranspiration and therefore reduce streamflows. Leaf drop is not seasonally dependent as it is in temperate climates, and can occur any time of year and varies by region of the world.