Editorial "Advances in Earth observation for water cycle science"

Since observing the Earth from space became possible more than forty years ago, satellite Earth Observation (EO) missions have become central to the monitoring and understanding of the Earth system, its different components and how they interact with each other. The continuous growth and improvements in the quality of the data and information provided by satellites has resulted in significant progress and advances in a broad range of scientific and application areas including the understanding and characterisation of the global water cycle hydrology and water management. The water cycle is a complex process driven mainly by solar radiation. The evaporation of water from open water and wet soil surfaces is controlled by energy and water availability and near-surface atmospheric conditions (air temperature, humidity and wind-speed), while transpiration of water is primarily controlled by vegetation. The result of evaporation and transpiration is the presence of water vapour in the atmosphere, a prerequisite for cloud formation. If cloud condensation nuclei are present and if the atmospheric state allows for condensation, clouds are formed which are then globally distributed by winds. In the presence of precipitating clouds, water returns back to the Earth’s surface where it accumulates in rivers, lakes and oceans. Surface water may also infiltrate into the soil, moistening the soil layers and accumulating as groundwater replenishing aquifers. Aquifers can store water for several (thousands of) years, provide water for human activities, or discharge it naturally to the surface or to the oceans. The response of the hydrological cycle to global warming is expected to be far reaching (Bengtsson, 2010), and because different physical processes control the change in water vapour and consequently evaporation and precipitation, a more extreme distribution of precipitation is expected leading to, in general, wet areas become wetter and dry areas become dryer (IPCC, 2008). In this context, relying on accurate and continuous observations of the long-term dynamics of the different key variables governing the energy and water cycle processes from global to local scale is essential to further increase not only our understanding of the different components of the water cycle both in its spatial and temporal variability, but also to characterise the processes and interactions between the terrestrial and atmospheric aspects of the energy and water cycle, and how this coupling may influence climate variability and predictability. Such global and continuous observations can only be secured by the effective use of Earth Observation (EO) satellites as a major complement to in-situ observation networks. In the years to come, EO technology will enter into a new era, where the increasing number of more sophisticated missions will provide scientists with an unprecedented capacity to observe and monitor the different components of the water cycle from the local to the global scales. Already today, global observations of several key parameters governing the global water cycle (e.g. precipitation, soil moisture, water vapor, evaporation and transpiration, water levels, gravityderived groundwater measurements, etc. ...) are feasible. In addition, significant progress has been made in the area of data assimilation enhancing the capabilities to integrate EObased products into suitable land surface and hydrological models; hence opening new opportunities for science and applications.