Author(s): Fisher, RA; Koven, CD | Abstract: Land surface models (LSMs) are a vital tool for understanding, projecting, and predicting the dynamics of the land surface and its role within the Earth system, under global change. Driven by the need to address a set of key questions, LSMs have grown in complexity from simplified representations of land surface biophysics to encompass a broad set of interrelated processes spanning the disciplines of biophysics, biogeochemistry, hydrology, ecosystem ecology, community ecology, human management, and societal impacts. This vast scope and complexity, while warranted by the problems LSMs are designed to solve, has led to enormous challenges in understanding and attributing differences between LSM predictions. Meanwhile, the wide range of spatial scales that govern land surface heterogeneity, and the broad spectrum of timescales in land surface dynamics, create challenges in tractably representing processes in LSMs. We identify three “grand challenges” in the development and use of LSMs, based around these issues: managing process complexity, representing land surface heterogeneity, and understanding parametric dynamics across the broad set of problems asked of LSMs in a changing world. In this review, we discuss progress that has been made, as well as promising directions forward, for each of these challenges.

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Perspectives on the Future of Land Surface Models and the Challenges of Representing Complex Terrestrial Systems

This contribution aims at presenting the principles that underlined the creation of the OZCAR research infras-
tructure, gathering various Critical Zone Observatories in France, and the scientific questions that drives the
observation settings.
The Critical Zone includes the fine zone between the lower atmosphere at the top of the canopy down to the
bedrock-soil interface. This lithosphere-atmosphere boundary is critical for the availability of life-sustaining
resources and critical for humanity because this is the zone where we live, where we build our cities, from which
we extract our food and our water and where we release most of our wastes. This is the fragile zone on which the
natural ecosystem relies because this is where nutrients are being released from the rocks.
OZCAR is a distributed research infrastructure gathering instrumented sites and catchments on continental
surfaces all dedicated to the observation and monitoring of the different compartments of the Critical Zone at
the national scale. All these observatories (more that 40) were all built up on specific questions (acid deposition,
flood prediction, urban hydrology. . . ), some of them more than 50 years ago, but they have all in common to
be highly instrumented, permanently funded as infrastructures. They all share the same overarching goal of
understanding and predicting the Critical Zone in a changing world. OZCAR gathers instrumented catchments,
hydrogeological sites, peatlands, glacier and permafrost regions and a spatial observatory under the common
umbrella of understanding water and biogeochemical cycles and the associated fluxes of energy by using natural
gradients and experimentation. Based on the collaboration with Southern Countries, OZCAR’s sites have a global
coverage including tropical areas and high mountainous regions in the Andes and the Himalaya.
OZCAR benefits from a French investments project called CRITEX (Innovative equipment for the critical zone,
https://www.critex.fr/critex-3/observatories/ ) that is centered on the development and deployment of innovative
instrumentation in the sites.
OZCAR was launched in 2016 under the leadership of the French Ministry in charge of Higher Education and
Research, assembling all French Research Institutions involved in environmental studies and with the ambition
of facilitating interdisciplinary research in terrestrial surfaces, stimulating instrumental development and being
visible at the international level.
The paper will presents the main common scientific questions, challenges in terms of instrumentation and
experimentation deployment, in particular in terms of co-location of sites, data base and modelling activities that
the OZCAR network plan to address in the next years.

OZCAR: the French network of Critical Zone Observatories: principlesand scientific objectives

Climate change can reduce surface-water supply by enhancing evapotranspiration in forested mountains, especially during heatwaves. We investigate this ‘drought paradox’ for the European Alps using a 1,212-station database and hyper-resolution ecohydrological simulations to quantify blue (runoff) and green (evapotranspiration) water fluxes. During the 2003 heatwave, evapotranspiration in large areas over the Alps was above average despite low precipitation, amplifying the runoff deficit by 32% in the most runoff-productive areas (1,300–3,000 m above sea level). A 3 °C air temperature increase could enhance annual evapotranspiration by up to 100 mm (45 mm on average), which would reduce annual runoff at a rate similar to a 3% precipitation decrease. This suggests that green-water feedbacks—which are often poorly represented in large-scale model simulations—pose an additional threat to water resources, especially in dry summers. Despite uncertainty in the validation of the hyper-resolution ecohydrological modelling with observations, this approach permits more realistic predictions of mountain region water availability. Mountain forest drought can paradoxically increase evapotranspiration (green water), helping vegetation at the expense of runoff (blue water). This is quantified for the 2003 event in the European Alps, highlighting underappreciated vulnerability of blue-water resources to future warmer summers.

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More green and less blue water in the Alps during warmer summers

Topographic Wetness Index calculation guidelines based on measured soil moisture and plant species composition.

. Plant functional traits determine vegetation responses to
environmental variation, but variation in trait values is large, even within
a single site. Likewise, uncertainty in how these traits map to Earth system
feedbacks is large. We use a vegetation demographic model (VDM), the
Functionally Assembled Terrestrial Ecosystem Simulator (FATES), to explore
parameter sensitivity of model predictions, and comparison to observations,
at a tropical forest site: Barro Colorado Island in Panama. We define a
single 12-dimensional distribution of plant trait variation, derived
primarily from observations in Panama, and define plant functional types
(PFTs) as random draws from this distribution. We compare several model
ensembles, where individual ensemble members vary only in the plant traits
that define PFTs, and separate ensembles differ from each other based on
either model structural assumptions or non-trait, ecosystem-level
parameters, which include (a) the number of competing PFTs present in any
simulation and (b) parameters that govern disturbance and height-based
light competition. While single-PFT simulations are roughly consistent with
observations of productivity at Barro Colorado Island, increasing the number
of competing PFTs strongly shifts model predictions towards higher
productivity and biomass forests. Different ecosystem variables show greater
sensitivity than others to the number of competing PFTs, with the
predictions that are most dominated by large trees, such as biomass, being
the most sensitive. Changing disturbance and height-sorting parameters, i.e., the rules of competitive trait filtering, shifts regimes of dominance or
coexistence between early- and late-successional PFTs in the model. Increases
to the extent or severity of disturbance, or to the degree of determinism in
height-based light competition, all act to shift the community towards
early-successional PFTs. In turn, these shifts in competitive outcomes alter
predictions of ecosystem states and fluxes, with more early-successional-dominated forests having lower biomass. It is thus crucial to differentiate
between plant traits, which are under competitive pressure in VDMs, from
those model parameters that are not and to better understand the
relationships between these two types of model parameters to quantify
sources of uncertainty in VDMs.

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Benchmarking and parameter sensitivity of physiological and vegetation dynamics using the Functionally Assembled Terrestrial Ecosystem Simulator (FATES) at Barro Colorado Island, Panama

Earth System Models (ESMs) are essential tools for understanding and predicting global change, but they cannot explicitly resolve hillslope‐scale terrain structures that fundamentally organize water, energy, and biogeochemical stores and fluxes at subgrid scales. Here we bring together hydrologists, Critical Zone scientists, and ESM developers, to explore how hillslope structures may modulate ESM grid‐level water, energy, and biogeochemical fluxes. In contrast to the one‐dimensional (1‐D), 2‐ to 3‐m deep, and free‐draining soil hydrology in most ESM land models, we hypothesize that 3‐D, lateral ridge‐to‐valley flow through shallow and deep paths and insolation contrasts between sunny and shady slopes are the top two globally quantifiable organizers of water and energy (and vegetation) within an ESM grid cell. We hypothesize that these two processes are likely to impact ESM predictions where (and when) water and/or energy are limiting. We further hypothesize that, if implemented in ESM land models, these processes will increase simulated continental water storage and residence time, buffering terrestrial ecosystems against seasonal and interannual droughts. We explore efficient ways to capture these mechanisms in ESMs and identify critical knowledge gaps preventing us from scaling up hillslope to global processes. One such gap is our extremely limited knowledge of the subsurface, where water is stored (supporting vegetation) and released to stream baseflow (supporting aquatic ecosystems). We conclude with a set of organizing hypotheses and a call for global syntheses activities and model experiments to assess the impact of hillslope hydrology on global change predictions.

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G. W. Grant

Papers

Hillslope Hydrology in Global Change Research and Earth System Modeling