There is a distinct poleward zonation of climate defined by gradations of progressively colder annual mean temperature in tropical, subtropical, temperate, boreal, arctic, and polar latitudes. Additional climate zones are defined based on annual precipitation and the seasonality of temperature and rain. The climate at large spatial scales extending over thousands of kilometers is known as the macroclimate. It is determined by geographic variation in solar heating of the planet, which sets in motion large-scale atmospheric circulations that transport heat poleward from the tropics, and also by proximity to oceans, which similarly transport heat in ocean currents. Mountains and large lakes create a regional climate that can deviate from the macroclimate. Climate at this scale, generally up to a several hundred kilometers , is referred to as mesoclimate. Variation in topography, soils, and vegetation creates local climates at a spatial scale ranging from a few to tens of kilometers, known as microclimates. A south-facing slope has a different microclimate than a north-facing slope. Forests have a different microclimate compared with open land.

This chapter delves into the science of forests and streamflow. The amount of water that runs off the land into a stream is known as water yield. It is the water available for human uses. Evapotranspiration is a loss of water that reduces streamflow. Forests increase annual evapotranspiration and reduce annual streamflow compared with grasslands and other types of vegetation. The science, however, is not precise and our understanding is more qualitative than quantitative. Water yield and the climate services of forests represent conflicting demands for water. Forests cool the surface climate through evapotranspiration and remove carbon dioxide from the atmosphere during growth, but in doing so they consume water and reduce water yield for human usage. Consideration of spatial scale further muddies the policy implications because any precipitation benefits of forests occur at large spatial scales covering vast regions of land or entire continents, while the water cost of forests is felt at the scale of the watersheds that supply towns and cities.

Freshwater biodiversity is threatened by growing human consumption and contamination of fresh water - a globally scarce resource. As human populations increase, the quality and quantity available for freshwater biodiversity declines.The result is a tragedy of the freshwater commons with increasing competition among groups of humans – evident from the hydropolitics of transboundary rivers - and between humans and nature.Humans may even be approaching the planetary boundary for freshwater use.Pollution and contamination are widespread, with emerging threats from microplastics and pharmaceuticals.Dams, drainage-basin disturbance, climate change, alien species, and overexploitation of aquatic animals pose additional threats.Their synergistic effects are evident from a global analysis of rivers: both biodiversity and human water security are at risk in many parts of the world while, in others, investments in infrastructure have enhanced water security although biodiversity remains under threat. Everywhere on Earth where there are substantial human populations, freshwater biodiversity is threatened.In many of these places, human water security is at risk also.