# Satellite Remote Sensing of Precipitation and the Terrestrial Water Cycle in a Changing Climate
## 1 Introduction
Figure 1: The terrestrial water cycle (or hydrological cycle) and its components. A redesign of the water planetary boundary is essential if we are to consider how climate and living ecosystems respond to changes in the different forms of water on Earth: atmospheric water, soil moisture, groundwater and frozen water, and surface water [18]. The water cycle is, in reality, a human-water-coupled system, and thus, hydrology is increasingly forced to deal with socio-hydrological emergent issues [19] across different scales and places if it is to devise tools with which to predict the future trajectories of system behavior under changing hydroclimatic and/or socioeconomic conditions [20]. This includes the virtual water concept which was brought about by the globalization of agriculture and trade [21] and the exploitation of the intimate connections between ecohydrological processes and water-related ecosystem services [22]. Recent studies have demonstrated that humans modify the global water cycle, creating a water crisis for billions of individuals and many ecosystems worldwide [4; 23]. However, such an influence is mostly absent from current depictions and perceptions. Major challenges are associated with gathering sufficient knowledge of ecological systems for the identification of extreme ecological responses and the attribution of climate extremes as drivers of extreme ecological responses, defined as extreme climatic events (ECE) [24]. Consequently, modern hydrology is undergoing profound changes, becoming an Earth system science and adopting new holistic approaches, as opposed to the physical, fluid mechanics-based approaches of the past [25]. In fact, as Abbott et al. [4] point out, the fluid dynamics perspective of hydrological sciences adopted till now has generated great understanding, but has failed to protect ecosystems. Hydrological, geomorphological, ecological, and pedological processes, each operating at a different rate, are progressively more investigated to reach this goal. New questions are asked about the urban part of the water cycle [26] and about the role of vegetated land cover and associated ecohydrological processes in moderating the so-called urban dry island and maintaining a stable environment while planning massive urbanization [27]. These issues challenge our modeling capabilities, in view of using model outputs as decision-making tools [28]. Better observations and models are thus required to quantify the water cycle components and determine their changes, in particular accelerations and decelerations at the global and regional scales. In other words, improved observations, models, and data assimilation systems will help to close the scientific gaps in the understanding of water storage in the climate system, fluxes, and spatial-temporal variability [29], and will lead the way to improved hydrological predictions [30]. Modern sensors on satellite platforms are capable of observing some of the components of the water cycle, offering, for the first time, a quantitative picture of the mechanisms that are interlaced and often very complex in  
Figure 2: Temporal scales of weather and climate processes. nature. Precipitation is one of the climate variables that greatly benefits from satellite observations. It is a fundamental component of the water cycle, and its intensity, duration, frequency, and phase are equally important as the total amounts [31; 32]. The response of precipitation characteristics to global warming is a key research topic because of its influence on stress, due to an increase in dry and wet extremes and changes in interannual variability, with consequences on predictability of precipitation events [33]. A better understanding of this response will, in turn, provide feedback to the fundamental behavior of precipitation, especially in the tropics [34], and will help confirm the thermodynamic and dynamic mechanisms for the intensification of the hydrological cycle, which has been suggested by the models [35].