Relative humidity gradients as a key constraint on terrestrial water and energy fluxes

Abstract. Earth's climate and water cycle are highly dependent on terrestrial evapotranspiration and the associated flux of latent heat. Although it has been hypothesized for over 50 years that land dryness becomes embedded in atmospheric conditions through evaporation, underlying physical mechanisms for this land–atmosphere coupling remain elusive. Here, we use a novel physically based evaporation model to demonstrate that near-surface atmospheric relative humidity (RH) fundamentally coevolves with RH at the land surface. The new model expresses the latent heat flux as a combination of thermodynamic processes in the atmospheric surface layer. Our approach is similar to the Penman–Monteith equation but uses only routinely measured abiotic variables, avoiding the need to parameterize surface resistance. We applied our new model to 212 in situ eddy covariance sites around the globe and to the FLUXCOM global-scale evaporation product to partition observed evaporation into diabatic vs. adiabatic thermodynamic processes. Vertical RH gradients were widely observed to be near zero on daily to yearly timescales for local as well as global scales, implying an emergent land–atmosphere equilibrium. This equilibrium allows for accurate evaporation estimates using only the atmospheric state and radiative energy, regardless of land surface conditions and vegetation controls. Our results also demonstrate that the latent heat portion of available energy (i.e., evaporative fraction) at local scales is mainly controlled by the vertical RH gradient. By demonstrating how land surface conditions become encoded in the atmospheric state, this study will improve our fundamental understanding of Earth's climate and the terrestrial water cycle.

Download Full-text

Relative humidity gradients as a key constraint on terrestrial water and energy fluxes

10.5194/hess-2020-643 ◽

2020 ◽

Author(s):

Yeonuk Kim ◽

Monica Garcia ◽

Laura Morillas ◽

Ulrich Weber ◽

T. Andrew Black ◽

...

Keyword(s):

Relative Humidity ◽

Latent Heat ◽

Land Surface ◽

Water Cycle ◽

Near Surface ◽

Surface Conditions ◽

New Model ◽

Terrestrial Water ◽

Atmospheric State ◽

Earth’S Climate

Abstract. Earth's climate and water cycle are highly dependent on terrestrial evapotranspiration and the associated flux of latent heat. Despite its pivotal role, predictions of terrestrial evapotranspiration remain uncertain due to highly dynamic and spatially heterogeneous land surface dryness. Although it has been hypothesized for over 50 years that land dryness becomes embedded in atmospheric conditions, underlying physical mechanisms for this land-atmospheric coupling remain elusive. Here, we use a novel physically-based evaporation model to demonstrate that near-surface atmospheric relative humidity (rh) fundamentally coevolves with rh at the land surface. The new model expresses the latent heat flux as a combination of thermodynamic processes in the atmospheric surface layer. Our approach is similar to the Penman-Monteith equation but uses only routinely measured abiotic variables, avoiding the need to parameterize surface resistance. We applied our new model to 212 in-situ eddy covariance sites around the globe and to the FLUXCOM global-scale evaporation product. Vertical rh gradients were widely observed to be near zero on daily to yearly time scales for local as well as global scales, implying an emergent land-atmosphere equilibrium. This equilibrium allows for accurate evaporation estimates using only the atmospheric state and radiative energy, regardless of land surface conditions and vegetation controls. Our results also demonstrate that the latent heat portion of available energy (i.e., evaporative fraction) at local scales is mainly controlled by the vertical rh gradient. By demonstrating how land surface conditions become encoded in the atmospheric state, this study will improve our fundamental understanding of Earth's climate and the terrestrial water cycle.

Download Full-text

Diagnosing hydrological limitations of a land surface model: application of JULES to a deep-groundwater chalk basin

Hydrology and Earth System Sciences ◽

10.5194/hess-20-143-2016 ◽

2016 ◽

Vol 20 (1) ◽

pp. 143-159 ◽

Cited By ~ 11

Author(s):

N. Le Vine ◽

A. Butler ◽

N. McIntyre ◽

C. Jackson

Keyword(s):

Land Surface ◽

Expert Knowledge ◽

Water Cycle ◽

Land Surface Model ◽

Surface Model ◽

Deep Groundwater ◽

Resolution Model ◽

Terrestrial Water ◽

Water Redistribution ◽

Types Of Information

Abstract. Land surface models (LSMs) are prospective starting points to develop a global hyper-resolution model of the terrestrial water, energy, and biogeochemical cycles. However, there are some fundamental limitations of LSMs related to how meaningfully hydrological fluxes and stores are represented. A diagnostic approach to model evaluation and improvement is taken here that exploits hydrological expert knowledge to detect LSM inadequacies through consideration of the major behavioural functions of a hydrological system: overall water balance, vertical water redistribution in the unsaturated zone, temporal water redistribution, and spatial water redistribution over the catchment's groundwater and surface-water systems. Three types of information are utilized to improve the model's hydrology: (a) observations, (b) information about expected response from regionalized data, and (c) information from an independent physics-based model. The study considers the JULES (Joint UK Land Environmental Simulator) LSM applied to a deep-groundwater chalk catchment in the UK. The diagnosed hydrological limitations and the proposed ways to address them are indicative of the challenges faced while transitioning to a global high resolution model of the water cycle.

Download Full-text

Intercomparison of terrestrial water budgets in EURO-CORDEX and TSMP evaluation runs

10.5194/egusphere-egu2020-5745 ◽

2020 ◽

Author(s):

Mohamed Eltahan ◽

Klaus Goergen ◽

Carina Furusho-Percot ◽

Stefan Kollet

Keyword(s):

Land Surface ◽

Water Budget ◽

Climate Models ◽

Water Cycle ◽

Weather Prediction ◽

Regional Climate ◽

Shallow Groundwater ◽

Surface Model ◽

Frequency Distributions ◽

Terrestrial Water

Water is one of Earth&#8217;s most important geo-ecosystem components. Here we present an evaluation of water cycle components using 12 EURO-CORDEX Regional Climate Models (RCMs) and the Terrestrial Systems Modeling Platform (TSMP) from ERA-Interim driven evaluation runs. Unlike the other RCMs, TSMP provides an integrated representation of the terrestrial water cycle by coupling the numerical weather prediction model COSMO, the land surface model CLM and the surface-subsurface hydrological model ParFlow, which simulates shallow groundwater states and fluxes. The study analyses precipitation (P), evapotranspiration (E), runoff (R), and terrestrial water storage (TWS=P-E-R) at a 0.11degree spatial resolution (about 12km) on EUR-11 CORDEX grid from 1996 to 2008. As reference datasets, we use ERA5 reanalysis to represent the complete terrestrial water budget, as well as the E-OBS, GLEAM and E-Run datasets for precipitation, evapotranspiration and runoff, respectively. The terrestrial water budget is investigated for twenty catchments over Europe (Guadalquivir, Guadiana, Tagus, Douro, Ebro, Garonne, Rhone, Po, Seine, Rhine, Loire, Maas, Weser, Elbe, Oder, Vistuala, Danube, Dniester, Dnieper, and Neman). Annual cycles, seasonal variations, empirical frequency distributions, spatial distributions for the water cycle components and budgets over the catchments are assessed. The analysis demonstrates the capability of the RCMs and TSMP to reproduce the overall characteristics of the water cycle over the EURO-CORDEX domain, which is a prerequisite if, e.g., climate change projections with the CORDEX RCMs or TSMP are to be used for vulnerability, impacts, and adaptation studies.

Download Full-text

The AmazonWater Cycle: Perspectives fromWater Budget Closure and Ocean Salinity

Journal of Climate ◽

10.1175/jcli-d-20-0309.1 ◽

2020 ◽

pp. 1-44

Author(s):

J. E. Jack Reeves Eyre ◽

Xubin Zeng

Keyword(s):

Water Vapor ◽

Land Surface ◽

Water Cycle ◽

Water Storage ◽

River Mouth ◽

Small Subset ◽

Terrestrial Water Storage ◽

Crucial Component ◽

Ocean Salinity ◽

Terrestrial Water

AbstractGlobal and regional water cycle includes precipitation, water vapor divergence, and change of column water vapor in the atmosphere, and land surface evapotranspiration, terrestrial water storage change, and river discharge which is linked to ocean salinity near the river mouth. The water cycle is a crucial component of the Earth system, and numerous studies have addressed its individual components (e.g., precipitation). Here we assess, for the first time, if remote sensing and reanalysis datasets can accurately and self consistently portray the Amazon water cycle. This is further assisted with satellite ocean salinity measurements near the mouth of the Amazon River. The widely-used practice of taking the mean of an ensemble of datasets to represent water cycle components (e.g., precipitation) can produce large biases in water cycle closure. Closure is achieved with only a small subset of data combinations (e.g., ERA5 reanalysis precipitation and evapotranspiration plus GRACE satellite terrestrial water storage), which rules out the lower precipitation and higher evapotranspiration estimates, providing valuable constraints on assessments of precipitation, evapotranspiration and their ratio. The common approach of using the Óbidos stream gauge (located hundreds of kilometres from the river mouth) multiplied by a constant (1.25) to represent the entire Amazon discharge is found to misrepresent the seasonal cycle, and this can affect the apparent influence of Amazon discharge on tropical Atlantic salinity.

Download Full-text

Creation of the WATCH Forcing Data and Its Use to Assess Global and Regional Reference Crop Evaporation over Land during the Twentieth Century

Journal of Hydrometeorology ◽

10.1175/2011jhm1369.1 ◽

2011 ◽

Vol 12 (5) ◽

pp. 823-848 ◽

Cited By ~ 596

Author(s):

G. P. Weedon ◽

S. Gomes ◽

P. Viterbo ◽

W. J. Shuttleworth ◽

E. Blyth ◽

...

Keyword(s):

Twentieth Century ◽

Land Surface ◽

Water Cycle ◽

Potential Evapotranspiration ◽

Reanalysis Data ◽

Meteorological Forcing ◽

Global Average ◽

Murray Darling Basin ◽

Terrestrial Water ◽

Reference Crop

Abstract The Water and Global Change (WATCH) project evaluation of the terrestrial water cycle involves using land surface models and general hydrological models to assess hydrologically important variables including evaporation, soil moisture, and runoff. Such models require meteorological forcing data, and this paper describes the creation of the WATCH Forcing Data for 1958–2001 based on the 40-yr ECMWF Re-Analysis (ERA-40) and for 1901–57 based on reordered reanalysis data. It also discusses and analyses model-independent estimates of reference crop evaporation. Global average annual cumulative reference crop evaporation was selected as a widely adopted measure of potential evapotranspiration. It exhibits no significant trend from 1979 to 2001 although there are significant long-term increases in global average vapor pressure deficit and concurrent significant decreases in global average net radiation and wind speed. The near-constant global average of annual reference crop evaporation in the late twentieth century masks significant decreases in some regions (e.g., the Murray–Darling basin) with significant increases in others.

Download Full-text

Diagnosing hydrological limitations of a Land Surface Model: application of JULES to a deep-groundwater chalk basin

Hydrology and Earth System Sciences Discussions ◽

10.5194/hessd-12-7541-2015 ◽

2015 ◽

Vol 12 (8) ◽

pp. 7541-7582

Author(s):

N. Le Vine ◽

A. Butler ◽

N. McIntyre ◽

C. Jackson

Keyword(s):

Land Surface ◽

Expert Knowledge ◽

Water Cycle ◽

Land Surface Model ◽

Surface Model ◽

Deep Groundwater ◽

Resolution Model ◽

Terrestrial Water ◽

Water Redistribution ◽

Types Of Information

Abstract. Land Surface Models (LSMs) are prospective starting points to develop a global hyper-resolution model of the terrestrial water, energy and biogeochemical cycles. However, there are some fundamental limitations of LSMs related to how meaningfully hydrological fluxes and stores are represented. A diagnostic approach to model evaluation is taken here that exploits hydrological expert knowledge to detect LSM inadequacies through consideration of the major behavioural functions of a hydrological system: overall water balance, vertical water redistribution in the unsaturated zone, temporal water redistribution and spatial water redistribution over the catchment's groundwater and surface water systems. Three types of information are utilised to improve the model's hydrology: (a) observations, (b) information about expected response from regionalised data, and (c) information from an independent physics-based model. The study considers the JULES (Joint UK Land Environmental Simulator) LSM applied to a deep-groundwater chalk catchment in the UK. The diagnosed hydrological limitations and the proposed ways to address them are indicative of the challenges faced while transitioning to a global high resolution model of the water cycle.

Download Full-text

UniFHy v0.1: A community framework for the terrestrial water cycle in Python

10.5194/gmd-2021-419 ◽

2021 ◽

Author(s):

Thibault Hallouin ◽

Richard J. Ellis ◽

Douglas B. Clark ◽

Simon J. Dadson ◽

Andrew G. Hughes ◽

...

Keyword(s):

Land Surface ◽

Hydrological Cycle ◽

Water Cycle ◽

System Model ◽

Unified Framework ◽

Future Developments ◽

Land System ◽

Road Map ◽

Terrestrial Water ◽

Floods And Droughts

Abstract. Land surface, hydrological, and groundwater modelling communities all have expertise in simulating the hydrological processes at play in the land system, but these communities have largely remained distinct with limited collaboration between disciplines. In order to address key societal questions regarding the future availability of water resources and the intensity of extreme events such as floods and droughts in a changing climate, these communities must build on the strengths of one another. The development of a common modelling infrastructure, a framework, can contribute to stimulating cross-fertilisation between them. By allowing (parts of) their existing models to be coupled together, improved land system models can be built to better understand and simulate the terrestrial hydrological cycle. This paper presents a Python implementation of such a framework named the Unified Framework for Hydrology (unifhy). The framework aims to provide the technical infrastructure required to couple models, taking into account the specific needs of a land system model. Its conceptual design and technical capabilities are outlined first, before its usage and useful characteristics are demonstrated through case studies. The limitations of the current framework and necessary future developments are finally presented as a road map for later versions and/or other implementations of the framework.

Download Full-text

Surface heterogeneity impacts on boundary layer dynamics via energy balance partitioning

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-10-17815-2010 ◽

2010 ◽

Vol 10 (7) ◽

pp. 17815-17851 ◽

Cited By ~ 1

Author(s):

N. A. Brunsell ◽

D. B. Mechem ◽

M. C. Anderson

Keyword(s):

Boundary Layer ◽

Heat Flux ◽

Latent Heat ◽

Latent Heat Flux ◽

Land Surface ◽

Surface Heterogeneity ◽

Length Scales ◽

Surface Conditions ◽

Boundary Layer Dynamics

Abstract. The role of land-atmosphere interactions under heterogeneous surface conditions is investigated in order to identify mechanisms responsible for altering surface heat and moisture fluxes. Twelve coupled land surface – large eddy simulation scenarios with four different length scales of surface variability under three different horizontal wind speeds are used in the analysis. The base case uses Landsat ETM imagery over the Cloud Land Surface Interaction Campaign (CLASIC) field site for 3 June 2007. Using wavelets, the surface fields are band-pass filtered in order to maintain the spatial mean and variances to length scales of 200 m, 1600 m, and 12.8 km as lower boundary conditions to the model. The simulations exhibit little variation in net radiation. Rather, a change in the partitioning of the surface energy between sensible and latent heat flux is responsible for differences in boundary layer dynamics. The sensible heat flux is dominant for intermediate surface length scales. For smaller and larger scales of surface heterogeneity, which can be viewed as being more homogeneous, the latent heat flux becomes increasingly important. The results reflect a general decrease of the Bowen ratio as the surface conditions transition from heterogeneous to homogeneous. Air temperature is less sensitive to surface heterogeneity than water vapor, which implies that the role of surface heterogeneity in modifying the local temperature gradients in order to maximize convective heat fluxes. More homogeneous surface conditions, on the other hand, tend to maximize latent heat flux. Scalar vertical profiles respond predictably to the partitioning of surface energy. Fourier spectra of the vertical wind speed, air temperature and specific humidity (w, T and q) and associated cospectra (w'T', w'q' and T'q'), however, are insensitive to the length scale of surface heterogeneity, but the near surface spectra are sensitive to the mean wind speed.

Download Full-text

Water Cycle Extremes in the GRACE and GRACE-FO Data Record

10.5194/gstm2020-35 ◽

2020 ◽

Author(s):

Matthew Rodell ◽

Bailing Li

Keyword(s):

Land Surface ◽

Water Mass ◽

Water Cycle ◽

Land Surface Model ◽

Surface Model ◽

Drought Event ◽

Terrestrial Water Storage ◽

Data Record ◽

Terrestrial Water ◽

Hydrological Droughts

A unique aspect of satellite gravimetry is its ability to quantify changes in all water stored at all depths on and beneath the land surface.&#160; Hence, GRACE and GRACE-FO are well suited for quantifying both hydrological droughts, when terrestrial water storage (TWS) is low, and pluvial events, when TWS is high.&#160; In this study we use GRACE and GRACE-FO data assimilation within a land surface model to fill the 1-year gap between the two missions and to replace other missing data.&#160; We apply a cluster analysis approach to identify the locations and extents of TWS extreme events in resulting data record.&#160; We then rank these events based on their intensity, i.e., the integral of the non-seasonal water mass anomaly over the period of the event. &#160;In this presentation we report on the largest wet and dry events over each continent.&#160; During the period of study, Africa, North America, and Australia each had a wet event with an intensity that exceeded 10,000 km3 * month, although the 2010-2012 event in Australia can largely be attributed to a depressed baseline TWS during the period caused by the millennial drought.&#160; With 30 more years of data it is probable that the intensity of that drought would have been greater than the recovery and wet event during 2010-2012.&#160; As it stands, the biggest drought event was determined to be one occurred in South America during 2015-2016, with an intensity of over 10,000 km3 * month.

Download Full-text

Evaluating Surface Water Cycle Simulated by the Australian Community Land Surface Model (CABLE) across Different Spatial and Temporal Domains

Journal of Hydrometeorology ◽

10.1175/jhm-d-12-0123.1 ◽

2013 ◽

Vol 14 (4) ◽

pp. 1119-1138 ◽

Cited By ~ 20

Author(s):

Huqiang Zhang ◽

Bernard Pak ◽

Ying Ping Wang ◽

Xinyao Zhou ◽

Yongqiang Zhang ◽

...

Keyword(s):

Land Surface ◽

Water Cycle ◽

Land Surface Model ◽

Surface Model ◽

Model Parameters ◽

Global Soil ◽

Australian Community ◽

Forest Regions ◽

Terrestrial Water ◽

Multimodel Ensembles

Abstract The terrestrial water cycle in the Australian Community Atmosphere Biosphere Land Exchange (CABLE) model has been evaluated across a range of temporal and spatial domains. A series of offline experiments were conducted using the forcing data from the second Global Soil Wetness Project (GSWP-2) for the period of 1986–95, but with its default parameter settings. Results were compared against GSWP-2 multimodel ensembles and a range of observationally driven datasets. CABLE-simulated global mean evapotranspiration (ET) and runoff agreed well with the GSWP-2 multimodel climatology and observations, and the spatial variations of ET and runoff across 150 large catchments were well captured. Nevertheless, at regional scales it underestimated ET in the tropics and had some significant runoff errors. The model sensitivity to a number of selected parameters is further examined. Results showed some significant model uncertainty caused by its sensitivity to soil wilting point as well as to the root water uptaking efficiency and canopy water storage parameters. The sensitivity was large in tropical rain forest and midlatitude forest regions, where the uncertainty caused by the model parameters was comparable to a large part of its difference against the GSWP-2 multimodel mean. Furthermore, the discrepancy among the CABLE perturbation experiments caused by its sensitivity to model parameters was equivalent to about 20%–40% of the intermodel difference among the GSWP-2 models, which was primarily caused by different model structure/processes. Although such results are model dependent, they suggest that soil/vegetation parameters could be another source of uncertainty in estimating global surface energy and water budgets.

Download Full-text