Closing the Water Cycle from Observations across Scales: Where Do We Stand?

Abstract

Life on Earth vitally depends on the availability of water. Human pressure on freshwater resources is increasing, as is human exposure to weather-related extremes (droughts, storms, floods) caused by climate change. Understanding these changes is pivotal for developing mitigation and adaptation strategies. The Global Climate Observing System (GCOS) defines a suite of Essential Climate Variables (ECVs), many related to the water cycle, required to systematically monitor the Earth's climate system. Since long-term observations of these ECVs are derived from different observation techniques, platforms, instruments, and retrieval algorithms, they often lack the accuracy, completeness, resolution, to consistently to characterize water cycle variability at multiple spatial and temporal scales.Here, we review the capability of ground-based and remotely sensed observations of water cycle ECVs to consistently observe the hydrological cycle. We evaluate the relevant land, atmosphere, and ocean water storages and the fluxes between them, including anthropogenic water use. Particularly, we assess how well they close on multiple temporal and spatial scales. On this basis, we discuss gaps in observation systems and formulate guidelines for future water cycle observation strategies. We conclude that, while long-term water-cycle monitoring has greatly advanced in the past, many observational gaps still need to be overcome to close the water budget and enable a comprehensive and consistent assessment across scales. Trends in water cycle components can only be observed with great uncertainty, mainly due to insufficient length and homogeneity. An advanced closure of the water cycle requires improved model-data synthesis capabilities, particularly at regional to local scales.

Global Recharge Data Set Indicates Strengthened Groundwater Connection to Surface Fluxes

“…A strong connection of groundwater with surface water and plant transpiration remains absent from most diagrams of the global water cycle (Abbott et al., 2019; Dorigo et al., 2021; Oki & Kanae, 2006). Although such water cycle diagrams may not be intended as complete representations of the hydrological cycle, they often play an important role in teaching, research, communication, and policymaking (Abbott et al., 2019).…”

A novel approach to partitioning evapotranspiration into evaporation and transpiration in flooded ecosystems

“…Evapotranspiration (ET) is the combined water loss from terrestrial ecosystems via transpiration ( T ), that is, water lost by plants during the process of carbon assimilation, and evaporation ( E ), that is, water lost via direct evaporation of soil and surface water (including evaporation of intercepted precipitation; NB: There is some discussion in the community around the correct use of the terms evapotranspiration versus evaporation (Miralles et al, 2020); We have opted to follow the common use of the term evapotranspiration throughout this manuscript to describe the total biosphere–atmosphere water flux, including transpiration as well as direct evaporation from soil and surface waters). Through these processes, ET adds on the order of 65–75 thousand km 3 of water to the atmosphere every year (Dorigo et al, 2021; Jung et al, 2018; Oki & Kanae, 2006; Trenberth et al, 2009) and constitutes an important component of the terrestrial water cycle. Despite its importance to the global water cycle, ET is, however, currently poorly constrained in global land surface models (LSMs), and although there is general consensus that ET will increase under climate change, the processes and trends are unclear, which creates large uncertainties in climate predictions (Brutsaert & Parlange, 1998; Pascolini‐Campbell et al, 2021; Zeng et al, 2018).…”

Detection of Changes in Evapotranspiration on a Catchment Scale Under Changing Climate Conditions in Selected River Basins of Slovakia

“…When computing a hydrological balance, it is crucial to integrate data on the hydrological cycle into a consistent set of figures, which must adequately represent the processes that must be considered on the given temporal and spatial scales. Integrating datasets from different sources requires a careful selection of the available data products of each variable, taking into account the appropriate spatial and temporal resolutions (Dorigo et al, 2021). The data selection determines whether and how the closure of the hydrological balance is ultimately achieved.…”