ALBERT

Description: Droughts are expected to become more frequent and more intense under climate change. Plant mortality rates and biomass declines in response to drought depend on stomatal and xylem flow regulation. Plants operate on a continuum of xylem and stomatal regulation strategies from very isohydric (strict regulation) to very anisohydric. Co-existing species may display a variety of isohydricity behaviors. As such, it can be difficult to predict how to model the degree of isohydricity at the ecosystem scale by aggregating studies of individual species. This is nonetheless essential for accurate prediction of ecosystem drought resilience. In this study, we define a metric for the degree of isohydricity at the ecosystem scale in analogy with a recent metric introduced at the species-level. Using data from the AMSR-E satellite, this metric is evaluated globally based on diurnal variations in microwave vegetation optical depth (VOD), which is directly related to leaf water potential. Areas with low annual-mean radiation are found to be more anisohydric. Except for evergreen broadleaf forests in the tropics, which are very isohydric, and croplands, which are very anisohydric, land cover type is a poor predictor of ecosystem isohydricity, in accordance with previous species-scale observations. It is therefore also a poor basis for parameterizing water stress response in land-surface models. For taller ecosystems, canopy height is correlated with higher isohydricity (so that rainforests are mostly isohydric). Highly anisohydric areas show either high or low underlying water use efficiency. In seasonally dry locations, most ecosystems display a more isohydric response (increased stomatal regulation) during the dry season. In several seasonally dry tropical forests, this trend is reversed, as dry-season leaf-out appears to coincide with a shift towards more anisohydric strategies. The metric developed in this study allows for detailed investigations of spatial and temporal variations in plant water behavior. This article is protected by copyright. All rights reserved.

Description: The diurnal hydrologic cycle, a sequence of evapotranspiration, boundary layer growth, moist convection, and precipitation, is described in a thermodynamic framework, assuming an atmosphere composed solely of water. This idealized cycle is shown to be equivalent to an abbreviated version of the classical Rankine cycle where not all the water vapor is condensed. Energy and entropy fluxes of the processes involved in the cycle are quantified using the reversible approximation as a function of the quality of the liquid-vapor mixture (the ratio of the residual background vapor and the total mass of water) and the different temperatures at which evaporation and condensation take place. The proposed framework allows quantitative estimates of the net work (which is used by the cycle to drive the atmospheric circulation and dissipated by various frictional forces and nonidealities) as well as of the thermodynamic efficiency of the cycle. Possible extensions of the idealized framework relating to the role of dry air and the inclusion of various irreversible processes are also discussed.

Description: Vegetation pattern morphology is suggested as one indicator of system closeness to desertification. Using pattern morphology as an indicator requires understanding the timescales at which patterned vegetation systems respond to drought. Modeling these timescales requires accounting for rainfall intermittency and all the pathways controlling vegetation-precipitation feedbacks. In this paper, a model of rainfall initiation and intensity based on the dynamics of a single-column atmospheric boundary layer is coupled to a patterned vegetation model. The coupled climate-vegetation model, parameterized to represent a typical vegetation morphology in southwestern Niger, is used to investigate the timescales of desertification due to shifts in the total annual rainfall regime, and the effect of precipitation feedbacks on these timescales. Depending on the exact rainfall history, biomass and spatial morphology may not respond monotonically to a decrease in rainfall. The model results suggest changes in pattern morphology responding to shifts in annual rainfall require at least 4–5 years. Feedbacks acting through vegetation's influence on surface albedo and, to a lesser extent, surface evapotranspiration act to speed up the vegetation response to drought. Although the overall local-scale vegetation-precipitation feedback is positive, individual storm events may exhibit negative feedbacks, in which rainfall only occurs for low vegetation cover, depending on the free atmospheric conditions. Vegetation-precipitation feedbacks are sufficiently important to speed up changes in vegetation patterns, even in marginal drylands with low biomass levels.

Description: Watersheds can be characterized as complex space-time filters that transform incoming fluxes of energy, water, and nutrients into variable output signals. The behavior of these filters is driven by climate, geomorphology, and ecology and, accordingly, varies from site to site. We investigated this variation by exploring the behavior of evapotranspiration signals from 14 different AmeriFlux sites. Evapotranspiration is driven by water and energetic forcing and is mediated by ecology and internal redistribution of water and energy. As such, it integrates biological and physical controls, making it an ideal signature to target when investigating watershed filtering. We adopted a paradigmatic approach (referred to as the null model) that couples the Penman-Monteith equation to a soil moisture model and explored the deviations between the predictions of the null model and the observed AmeriFlux data across the sites in order to identify the controls on these deviations and their commonalities and differences across the sites. The null model reproduced evapotranspiration fluxes reasonably well for arid, shallow-rooted systems but overestimated the effects of water limitation and could not reproduce seasonal variation in evapotranspiration at other sites. Accounting for plant access to groundwater (or deep soil moisture) reserves and for the effects of soil temperature on limiting evapotranspiration resolved these discrepancies and greatly improved prediction of evapotranspiration at multiple time scales. The results indicate that site-specific hydrology and climatic factors pose important controls on biosphere-hydrosphere interactions and suggest that plant–water table interactions and early season phenological controls need to be incorporated into even simple models to reproduce the seasonality in evapotranspiration.

Description: Author(s): Gabriel G. Katul, Alexandra G. Konings, and Amilcare Porporato [Phys. Rev. Lett. 107, 268502] Published Thu Dec 22, 2011

Description: The bulk velocity Ub in streams is conventionally estimated from Manning's equation, but difficulties remain in parameterizing the roughness coefficient n when the streambed is covered with vegetation. A two-layer velocity model is proposed to determine n and Ub for the submerged vegetation case. The modeled n is derived as a function of flow and vegetation properties that can be inferred from remote sensing platforms, such as canopy height, leaf area density, and flow depth. The main novelty in the proposed formulation is that the shear stress is related to the mean velocity profile by considering both ejective and sweeping motions by dominant eddies. The proposed model is tested against a large data set from the literature and is shown to perform well, particularly for rigid vegetation. Poorer model performance for flexible vegetation can be partially attributed to the shape of the assumed mean velocity profile. The roughness coefficient n is found to be robust to variations in the average spacing between canopy elements, allowing the model to be applied to heterogeneous canopies.

Description: Calibration and validation of geophysical measurement systems typically requires knowledge of the “true” value of the target variable. However, the data considered to represent the “true” values often include their own measurement errors, biasing calibration and validation results. Triple collocation (TC) can be used to estimate the root-mean-square-error (RMSE), using observations from three mutually-independent, error-prone measurement systems. Here, we introduce Extended Triple Collocation (ETC): using exactly the same assumptions as TC, we derive an additional performance metric, the correlation coefficient of the measurement system with respect to the unknown target, . We demonstrate that is the scaled, unbiased signal-to-noise ratio, and provides a complementary perspective compared to the RMSE. We apply it to three collocated wind datasets. Since ETC is as easy to implement as TC, requires no additional assumptions, and provides an extra performance metric, it may be of interest in a wide range of geophysical disciplines.

Description: Abstract Over the past decade, the concept of isohydry or anisohydry, which describes the link between soil water potential (ΨS), leaf water potential (ΨL), and stomatal conductance (gs), has soared in popularity. However, its utility has recently been questioned, and a surprising lack of coordination between the dynamics of ΨL and gs across biomes has been reported. Here, we offer a more expanded view of the isohydricity concept that considers effects of vapour pressure deficit (VPD) and leaf area index (AL) on the apparent sensitivities of ΨL and gs to drought. After validating the model with tree‐ and ecosystem‐scale data, we find that within a site, isohydricity is a strong predictor of limitations to stomatal function, though variation in VPD and leaf area, among other factors, can challenge its diagnosis. Across sites, the theory predicts that the degree of isohydricity is a good predictor of the sensitivity of gs to declining soil water in the absence of confounding effects from other drivers. However, if VPD effects are significant, they alone are sufficient to decouple the dynamics of ΨL and gs entirely. We conclude with a set of practical recommendations for future applications of the isohydricity framework within and across sites.

Description: Nature Geoscience 10, 410 (2017). doi:10.1038/ngeo2957 Authors: Julia K. Green, Alexandra G. Konings, Seyed Hamed Alemohammad, Joseph Berry, Dara Entekhabi, Jana Kolassa, Jung-Eun Lee & Pierre Gentine