# Satellite Remote Sensing of Precipitation and the Terrestrial Water Cycle in a Changing Climate
## 2 The Water Cycle
### Observations
Specifically, observational datasets [49; 50] are providing new insights on long-term changes in the Mediterranean basin, in support of model projections predicting increasing temperatures and decreasing evapotranspiration and precipitation over the region by the middle of this century [57; 58]. The most recent datasets are contributing to addressing the contribution of the Mediterranean Sea to climatological precipitation on one side, and extreme precipitation on the other [59]. * _Arctic._ Gaps in observations are particularly evident in the Arctic, where rapid changes in the hydrological cycle challenge our process understanding. Observations show that runoff is systematically larger (smaller) than precipitation increases (decreases), and thus, that quality observations need to resolve changes in evapotranspiration, and groundwater and permafrost storage [60]. Enhanced process understanding and modeling capabilities are starting to be able to better quantify the role of the atmosphere in the Arctic water cycle changes [61]. Uncertainties are still high in the determination of the large-scale freshwater cycle because of the sparseness of hydrographic data and insufficient information on sea-ice volume [62], as well as inadequate monitoring of precipitation, evapotranspiration, and river discharge fluxes [60; 63]. Coordinated efforts in monitoring, modeling, and process studies on various scales are thus desirable at the interface between hydrology, atmosphere, ecology, resources, and oceans [64]. * _High mountains._ The melting of glaciers, and consequent intensification of the water cycle with greening ecosystems and increasing frequency of hazards, is closely linked to recent warming, especially over the Asian Third Pole, requiring investigations of every major component in the system, especially through improved observations [65]. Recent research efforts have attempted to evaluate the uncertainty of terrestrial water budget components over High Mountain Asia, which is significantly impacted by the uncertainty on the driving meteorology [66], and is of the utmost importance for the assimilation of the frozen components in land surface models [67]. Exploring water cycle mechanisms through remote sensing prompts us to take up the challenges presented by water resource management for sustainable development in areas with sparse in situ monitoring networks. Unprecedented opportunities are now available thanks to satellite-based services, although issues need to be addressed concerning accuracy, sampling, and continuity, together with the massive amount of new data [68]. Along with opportunities, the use of high-resolution satellite remote sensing data brings possible privacy concerns at the individual, institutional, and disciplinary levels that need adequate consideration [69]. The prevalence of the application of remote sensing to the study of water processes has increased substantially in recent times: Cui et al. [70] showed that the number of publications on this subject rose at an average of about 16% per year over the past 19 years. This means that most of the disciplines in this highly interdisciplinary field have strongly augmented their interest in remote sensing, from water resources to environmental science, geology, engineering, ecology, and agriculture. A vast amount of data has become available, opening de facto a new era of treating big data for the global water cycle: retrievals, time and space reconstruction, and scale conversion are foremost science topics that will contribute to advancing global hydrology and water security [71].