# Satellite Remote Sensing of Precipitation and the Terrestrial Water Cycle in a Changing Climate
## 1 Introduction  
The common description of the water cycle, or hydrological cycle, as consisting of evaporation, condensation, and precipitation is too simple to account for the high degree of complexity of the involved phenomena. Several physical processes contribute significantly to determining the final balance (or local imbalance), such as water vapor transport, sublimation, surface runoff, soil moisture, infiltration, percolation, plant uptake, and groundwater flow [1; 2; 3], as detailed in Figure 1. Moreover, evaporation happens not only from the surface of water bodies, but also from clouds and falling precipitation. However, another element is completely overlooked in this description: human influence, a key factor which is imposing a change of perspective if we want to quantify water cycle changes in the changing climate of the Anthropocene age [4]. The processes behind the land-atmosphere-ocean system are strictly interconnected so that changes in one will induce changes on, and feedback from, the others [5]. The Intergovernmental Panel on Climate Change (IPCC) aims to quantify changes due to human and/or natural causes. Such a quantification is very complex and calls for adequate observations and modeling, since a complete and satisfactory appraisal of climate change cannot be reached without an in depth understanding of climate variability, which has not yet been comprehensively achieved in spite of the substantial progress made over the past few decades. Global climate drivers have a direct or indirect influence on the balance between incoming solar shortwave radiation (SWR) and outgoing longwave radiation (OLR). Thus, a correct understanding of global climate changes cannot be attained without knowing how energy is distributed and exchanged in the Earth's atmosphere, ocean, land, and snow surfaces [6; 7]. Specifically, it has been suggested that thermodynamics contribute to an increase in global precipitation, with surface warming at a rate of about 1.5% K\({}^{-1}\); whether global precipitation will increase at rates closer to 1 or even to 3% K\({}^{-1}\) will depend on radiative changes [8]. Clouds and precipitation, along with water vapor mass exchanges, play a substantial role in climate variability, at both the global and regional levels. They influence not only climate, but also weather at all scales, and determine water availability. The temporal scales from weather to climate change processes are very wide and encompass a variety of phenomena that are outlined in Figure 2. Aerosol particles released by human activities can also induce the formation of brighter clouds that are less efficient at releasing precipitation, thus leading to a weaker hydrological cycle [9]. Vegetation cover is significantly associated with the partitioning of the water balance. Changes in forest coverage (especially in the tropics) can have short- and long-term effects on this partitioning [10; 11]. It seems that shifts from forest to savannah due to decreasing precipitation in the future are more likely to occur in regions with a precursory lower rainfall variability [12]. Moreover, the impacts of forests and forestation on hydrological services are now being recognized as fundamental in areas prone to erosion and floods, as well as those in need of adequate water supply management [13]. Forest and water interactions are now recognized as having potentially positive impacts on the supply and purity of available water resources, on the cross-continental transport of atmospheric moisture, on the cooling of terrestrial surfaces, on infiltration and groundwater recharge, on flood moderation, and on many other processes [14; 15]. Terrestrial and aquatic ecosystems are influenced by, and influence, climate in many ways; thus climate change mechanisms may impact ecosystem biodiversity, structure, and function [16]. Note also that the global water cycle may also have played a critical role in the Earth's history by circulating surface water into the deep mantle and back to the surface again [17].