remote sensing Review Satellite Remote Sensing of Precipitation and the Terrestrial Water Cycle in a Changing Climate Vincenzo Levizzani * and Elsa Cattani CNR-ISAC, via Gobetti 101, I-40129 Bologna, Italy; [email protected] * Correspondence: [email protected] Received: 13 September 2019; Accepted: 30 September 2019; Published: 2 October 2019 Abstract: The water cycle is the most essential supporting physical mechanism ensuring the existence of life on Earth. Its components encompass the atmosphere, land, and oceans. The cycle is composed of evaporation, evapotranspiration, sublimation, water vapor transport, condensation, precipitation, runoff, infiltration and percolation, groundwater flow, and plant uptake. For a correct closure of the global water cycle, observations are needed of all these processes with a global perspective. In particular, precipitation requires continuous monitoring, as it is the most important component of the cycle, especially under changing climatic conditions. Passive and active sensors on board meteorological and environmental satellites now make reasonably complete data available that allow better measurements of precipitation to be made from space, in order to improve our understanding of the cycle’s acceleration/deceleration under current and projected climate conditions. The article aims to draw an up-to-date picture of the current status of observations of precipitation from space, with an outlook to the near future of the satellite constellation, modeling applications, and water resource management. Keywords: water cycle; hydrological cycle; precipitation; water resources; climate change; satellite; remote sensing 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 [1–3], as detailed in Figure1. 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]. Remote Sens. 2019, 11, 2301; doi:10.3390/rs11192301 www.mdpi.com/journal/remotesensing Remote Sens. 2019, 11, 2301 2 of 41 Remote Sens. 2019, 11, x FOR PEER REVIEW 2 of 42 [6,7].Specifically, Specifically, it has beenit has suggested been suggested that thermodynamics that thermodynamics contribute contribute to an increase to inan globalincrease precipitation, in global 1 precipitation,with surface warming with surface at a ratewarming of about at a 1.5% rate K of− ;about whether 1.5% global K−1; whether precipitation global will precipitation increase at rateswill 1 increasecloser to at 1 orrates even closer to 3% to K1 −or willeven depend to 3% K on−1 will radiative depend changes on radi [8ative]. changes [8]. FigureFigure 1. 1. TheThe terrestrial terrestrial water water cycle cycle (or (or hydrological hydrological cycle) cycle) and and its components. CloudsClouds and and precipitation, along along with with water water vapor vapor mass mass exchanges, exchanges, play play a a substantial role role in climateclimate variability, variability, at both the global and regional levels. They They influence influence not not only climate, but also weather at all scales, and determine water availability. The temporal scales from weather to climate changechange processes are very widewide andand encompassencompass aa varietyvarietyof of phenomena phenomena that that are are outlined outlined in in Figure Figure2. 2.Aerosol Aerosol particles particles released released by by human human activities activities can can also also induce induce the the formation formation of brighterof brighter clouds clouds that that are areless less effi cientefficient at releasing at releasing precipitation, precipitation, thus thus leading leading to a to weaker a weaker hydrological hydrological cycle cycle [9]. [9]. VegetationVegetation covercover isis significantlysignificantly associated associated with with the the partitioning partitioning of of the the water water balance. balance. Changes Changes in inforest forest coverage coverage (especially (especially in the in tropics)the tropics) can havecan have short- short- and long-term and long-term effects effects on this on partitioning this partitioning [10,11]. [10,11].It seems It that seems shifts that from shifts forest from to savannahforest to savannah due to decreasing due to decreasing precipitation precipitation in the future in arethe morefuture likely are moreto occur likely in regionsto occur within regions a precursory with a precursory lower rainfall lower variability rainfall variability [12]. Moreover, [12]. Moreover, the impacts the of impacts forests ofand forests forestation and forestation on hydrological on hydrological services areservices now beingare now recognized being recognized as fundamental as fundamental in areas pronein areas to proneerosion to and erosion floods, and as floods, well as as those well in as need those of in adequate need ofwater adequate supply water management supply management [13]. Forest [13]. and Forestwater interactionsand water areinteractions now recognized are now as havingrecognized potentially as having positive potentially impacts positive on the supply impacts and on purity the supplyof available and waterpurity resources, of available on thewater cross-continental resources, on transportthe cross-continental of atmospheric transport moisture, of on atmospheric the cooling moisture,of terrestrial on surfaces,the cooling on of infiltration terrestrial and surfaces, groundwater on infiltration recharge, and on groundwater flood moderation, recharge, and on manyflood moderation,other processes and [ 14on, 15many]. Terrestrial other processes and aquatic [14,15]. ecosystems Terrestrial are and influenced aquatic ecosystems by, and influence, are influenced climate by,in many and influence, ways; thus climate climate in change many ways; mechanisms thus climate may impact change ecosystem mechanisms biodiversity, may impact structure, ecosystem and biodiversity,function [16]. structure, Note also and that function the global [16]. water Note cycle also maythat the also global have playedwater cycle a critical may rolealso in have the played Earth’s ahistory critical by role circulating in the Earth’s surface hist waterory by into circulating the deep surface mantle water and back into to the the deep surface mantle again and [17 back]. to the surface again [17]. Remote Sens. 2019, 11, 2301 3 of 41 Remote Sens. 2019, 11, x FOR PEER REVIEW 3 of 42 FigureFigure 2. Temporal 2. Temporal scales scalesof of weather and and climate climate processes. processes. A redesignA redesign of the of water the water planetary planetary boundaryboundary is es issential essential if we are if weto consider are to how consider climate and how climate living ecosystems respond to changes in the different forms of water on Earth: atmospheric water, and livingsoil ecosystems moisture, groundwater respond and to changesfrozen water, in and the surface different water forms [18]. The of water water cycle on is, Earth: in reality, atmospheric a water, soilhuman-water-coupled moisture, groundwater system, and thus, frozen hydrology water, is and increasingly surface forced water to [ 18deal]. with The socio- water cycle is, in reality,hydrological a human-water-coupled emergent issues [19] system, across different and thus, scales hydrology and places if isit is increasingly to devise tools forcedwith which to deal with socio-hydrologicalto predict emergentthe future issuestrajectories [19 ]of across system di ffbehaviorerent scalesunder andchanging places hydroclimatic if it is to devise and/or tools with socioeconomic conditions [20]. This includes the virtual water concept which was brought about by which to predictthe globalization the future of agriculture trajectories and trade of system [21] and behavior the exploitation under of changing the intimate hydroclimatic connections and/or socioeconomicbetween conditions ecohydrological [20]. processes This includes and water-relate the virtuald ecosystem water services