# Satellite Remote Sensing of Precipitation and the Terrestrial Water Cycle in a Changing Climate
## 2 The Water Cycle
### Observations  
The investigation of the various climate processes requires the implementation and maintenance of a complex observation system with ground, airborne, and spaceborne components (Figure 4). Most of the sensors are already in place for meteorology, hydrology, and climatology, as documented by the Observing Systems Capability Analysis and Review (OSCAR, [[https://www.wmo-sat.info/oscar/](https://www.wmo-sat.info/oscar/)]([https://www.wmo-sat.info/oscar/](https://www.wmo-sat.info/oscar/)), last accessed 27 May 2019) tool of the World Meteorological Organization (WMO), which defines the requirements for the observation of physical variables. The Global Climate Observing System (GCOS, [[https://gcos.wmo.int/en/home](https://gcos.wmo.int/en/home)]([https://gcos.wmo.int/en/home](https://gcos.wmo.int/en/home)), last accessed 21 August 2019) maintains the definitions and observation needs [38] of the Essential Climate Variables (ECVs) required to systematically observe the Earth's changing climate. Ground-based observations produce datasets for estimates of changes in the hydrological cycle and for deriving trends [39], but this happens only in limited areas of the world, where the observing networks are dense enough and ensure adequate quality standards. In recent years, advances have taken place in our observational capabilities, especially in the remote sensing area, providing unprecedented contributions to advancing our understanding of the global hydrological cycle [40; 41; 42]. Moreover, initiatives and working groups have been established to take advantage of new observation technologies and to devise new observation methods with which to monitor the hydrological cycle [43; 44]. While recognizing the limitations of current satellite observations, especially related to the validation and long-term stability of the datasets, there is general consensus on the need to maintain and improve our space-borne observational capabilities in view of quantifying the human influence on the water cycle [45; 46]. In fact, recognition of the global nature of the water system comes with the awareness that human activities significantly and increasingly change the nature of the water cycle, thus threatening human health, economies, and biodiversity [47]. Figure 4: The global observing system of the water cycle. Note also that the hydrological cycle is potentially constrained by the carbon cycle and vice versa, and multiple observation sources can be used to constrain land surface models in order to mitigate bias from any single data type [48]. Closure experiments have been attempted, and are starting to produce convincing results based on satellite data [49; 50; 51], although closure has not yet been attained using satellite data alone. Rodell et al. [52] show that, in the majority of cases, the observed annual surface and atmospheric water budgets over the continents and oceans close with much less than a 10% residual, while observed residuals and optimized uncertainty estimates are considerably larger for monthly surface and atmospheric water budget closure, i.e., often \(\geq\)20% in North America, Eurasia, Australia and neighboring islands, and the Arctic and South Atlantic Oceans. Efforts to improve models integrating ground-based and remote sensing observations are critical to understand the behavior of the Earth system at the surface and sub-surface levels [53; 54; 55]. In this respect, it is essential that we fill observational gaps (e.g., due to the loss or replacement of satellite platforms) and discontinuities in the data records [49]. In particular, satellite observations are expected to significantly help in closing the water cycle in climatic hot spots where ground-based data are generally sparse. The following three examples are very significant. * _Mediterranean basin._ In depth studies would be beneficial for filling the gaps in our understanding of the common characteristics of Mediterranean-type climates around the world and their variability and change [56].