Land use/land cover changes (LULCC) directly impact the surface temperature by modifying the radiative, physiological, and aerodynamic properties controlling the surface energy and water balances. In ...this study, we propose a new method to attribute changes in the surface temperature induced by LULCC to changes in radiative and turbulent heat fluxes, with the partition of turbulent fluxes controlled by aerodynamic and surface resistances. We demonstrate that previous attribution studies have overestimated the contribution of aerodynamic resistance by assuming independence between the aerodynamic resistance and the Bowen ratio. Our results further demonstrate that acceptable agreement between modeled and observed temperature anomalies does not guarantee correct attribution by the model. When performing an attribution analysis, the covariance among attributing variables needs to be taken into consideration in order to accurately interpret the results.
Key Points
A widely used method for attributing surface temperature changes assumes that the Bowen ratio is independent of aerodynamic resistance
The independence assumption leads to an overestimation of the impact of aerodynamic resistance
A new method that does not invoke the assumption of independence between the Bowen ratio and aerodynamic resistance is proposed
More than half of the world's population now live in cities, which are known to be heat islands. While daytime urban heat islands (UHIs) are traditionally thought to be the consequence of less ...evaporative cooling in cities, recent work sparks new debate, showing that geographic variations of daytime UHI intensity were largely explained by variations in the efficiency with which urban and rural areas convect heat from the land surface to the lower atmosphere. Here, we reconcile this debate by demonstrating that the difference between the recent finding and the traditional paradigm can be explained by the difference in the attribution methods. Using a new attribution method, we find that spatial variations of daytime UHI intensity are more controlled by variations in the capacity of urban and rural areas to evaporate water, suggesting that strategies enhancing the evaporation capability such as green infrastructure are effective ways to mitigate urban heat.
Evapotranspiration, defined as the total flux of water from the land surface to the atmosphere, is a major component of the hydrologic cycle and surface energy balance. Although evapotranspiration is ...expected to intensify with increasing temperatures, long‐term, regional trends in evapotranspiration remain uncertain due to spatially and temporally limited direct measurements. In this study, we utilize an emergent relation between the land surface and atmospheric boundary layer to infer daily evapotranspiration from historical meteorological data collected at 236 weather stations across the United States. Our results suggest a statistically significant (α = 0.05) decrease in evapotranspiration of approximately 6% from 1961 to 2014, with a significant (α = 0.05) sharp decline of 13% from 1998 to 2014. We attribute the decrease in evapotranspiration mostly to declines in surface conductance, but also to offsetting changes in longwave radiation, wind speed, and incoming solar radiation. Using an established stomatal conductance model, we explain the changes in inferred surface conductance as a response to increases in carbon dioxide and, more recently, to an abrupt decrease in atmospheric humidity.
Evapotranspiration (ET) is challenging to model because it depends on heterogeneous land surface features—such as soil moisture, land cover type, and plant physiology—resulting in rising model ...complexity and substantial disagreement between models. We show that the evaporative fraction (ET as a proportion of available energy at the surface) can be estimated accurately across a broad range of conditions using a simple equation with no free parameters and no land surface information; only near‐surface air temperature and specific humidity observations are required. The equation performs well when compared to eddy covariance measurements at 76 inland continental sites, with prediction errors comparable to errors in the eddy covariance measurements themselves, despite substantial variability in surface conditions across sites. This reveals an emergent simplicity to continental ET that has not been previously recognized, in which land‐atmosphere coupling efficiently embeds land surface information in the near‐surface atmospheric state on daily to monthly time scales.
Key Points
Land‐atmosphere coupling embeds surface controls on ET in the atmospheric state
We evaluate a simple equation for actual ET, with no parameters or surface inputs
Across 76 sites, errors in the equation's predictions are comparable to those in eddy covariance data
Malagasy subsistence farmers, who comprise 70% of the nearly 26 million people in Madagascar, often face food insecurity because of unreliable food production systems and adverse crop conditions. The ...2020–2021 drought in Madagascar, in particular, is associated with an exceptional food crisis, yet we are unaware of peer-reviewed studies that quantitatively link variations in weather and climate to agricultural outcomes for staple crops in Madagascar. In this study, we use historical data to empirically assess the relationship between soil moisture and food production. Specifically, we focus on major staple crops that form the foundation of Malagasy food systems and nutrition, including rice, which accounts for 46% of the average Malagasy caloric intake, as well as cassava, maize, and sweet potato. Available data associated with survey-based crop statistics constrain our analysis to 2010–2017 across four clusters of Malagasy districts. Strong correlations are observed between remotely sensed soil moisture and rice production, ranging between 0.67 to 0.95 depending on the cluster and choice of crop calendar. Predictions are shown to be statistically significant at the 90% confidence level using bootstrapping techniques, as well as through an out-of-sample prediction framework. Soil moisture also shows skill in predicting cassava, maize, and sweet potato production, but only when the months most vulnerable to water stress are isolated. Additional analyses using more survey data, as well as potentially more-refined crop maps and calendars, will be useful for validating and improving soil-moisture-based predictions of yield.
•A physically-based method is used to study springtime energy balance changes.•Interacting biophysical and meteorological properties regulate the Bowen ratio.•Increasing specific humidity imposes a ...negative feedback on evapotranspiration.•Synoptic scale decreases in wind speed lead to increases in aerodynamic resistance.
In ecosystems characterized by strong seasonality in leaf area, the emergence of leaves during springtime modifies land surface energy balance by altering surface biophysical properties during a period when atmospheric conditions are also changing. However, the relative importance and interactions among surface biophysical and atmospheric variables in modifying the surface energy balance are not well understood. In this study, we use a physically-based attribution method to quantify the relative importance of covarying surface biophysical and atmospheric variables in modifying the surface energy balance during springtime. Results show that the widely observed decrease in the Bowen ratio that occurs with leaf emergence is not solely attributable to the sharp decrease in surface resistance caused by increasing leaf area. Rather, decreases in the Bowen ratio reflect the combined effects of changes in surface properties and atmospheric conditions. Specifically, decreasing surface resistance and increasing air temperature both act to reduce the Bowen ratio, while concurrent increases in specific humidity provide a negative feedback that constrains evaporative fluxes. In parallel, aerodynamic resistance tends to increase after leaf emergence largely because wind speed tends to decrease during springtime. These findings provide a refined characterization of surface energy balance dynamics during springtime when both surface and atmospheric conditions are changing rapidly and reveal previously understudied properties of the near-surface atmosphere that influence surface Bowen ratio and aerodynamic resistance.
Evaporation (E) and transpiration (T) respond differently
to ongoing changes in climate, atmospheric composition, and land use. It is
difficult to partition ecosystem-scale evapotranspiration (ET) ...measurements
into E and T, which makes it difficult to validate satellite data and land
surface models. Here, we review current progress in partitioning E and T and
provide a prospectus for how to improve theory and observations going
forward. Recent advancements in analytical techniques create new
opportunities for partitioning E and T at the ecosystem scale, but their
assumptions have yet to be fully tested. For example, many approaches to
partition E and T rely on the notion that plant canopy conductance and
ecosystem water use efficiency exhibit optimal responses to atmospheric
vapor pressure deficit (D). We use observations from 240 eddy covariance flux
towers to demonstrate that optimal ecosystem response to D is a reasonable
assumption, in agreement with recent studies, but more analysis is necessary
to determine the conditions for which this assumption holds. Another
critical assumption for many partitioning approaches is that ET can be
approximated as T during ideal transpiring conditions, which has been
challenged by observational studies. We demonstrate that T can exceed 95 %
of ET from certain ecosystems, but other ecosystems do not appear to reach
this value, which suggests that this assumption is ecosystem-dependent with
implications for partitioning. It is important to further improve approaches
for partitioning E and T, yet few multi-method comparisons have been
undertaken to date. Advances in our understanding of carbon–water coupling
at the stomatal, leaf, and canopy level open new perspectives on how to
quantify T via its strong coupling with photosynthesis. Photosynthesis can be
constrained at the ecosystem and global scales with emerging data sources
including solar-induced fluorescence, carbonyl sulfide flux measurements,
thermography, and more. Such comparisons would improve our mechanistic
understanding of ecosystem water fluxes and provide the observations
necessary to validate remote sensing algorithms and land surface models to
understand the changing global water cycle.
Land use and land cover change such as deforestation can directly induce changes in land surface temperature (LST). Using observational data from four paired eddy covariance sites, we attribute ...changes in LST induced by deforestation to changes in radiation, aerodynamic resistance, the Bowen ratio or surface resistance, and heat storage using two different methods: the intrinsic biophysical mechanism (IBM) method and the two‐resistance mechanism method. The two models are first optimized to reduce the root‐mean‐square error of the simulated daily LST change by using daily‐averaged inputs and a weighted average approach for computing the sensitivities. Both methods indicate that the daytime warming effect of deforestation is mostly induced by changes in aerodynamic resistance as the surface becomes smoother after deforestation, and the nighttime cooling effect of deforestation is controlled by changes in aerodynamic resistance, surface resistance, radiation, and heat storage. Both methods also indicate that changes in atmospheric temperature have a large impact on LST and need to be included in the LST attribution. However, there are significant differences between the two methods. The IBM method tends to overestimate the contribution of aerodynamic resistance due to the assumption that aerodynamic resistance and the Bowen ratio are independent. Additionally, the IBM method underestimates the contributions of radiation and heat storage during the daytime but overestimates them at night. By highlighting the similarity and dissimilarity between the two methods, this study suggests that acceptable agreement between observed and modeled LST change is the prerequisite for attribution but does not guarantee correct attribution.
Key Points
Land surface temperature response to deforestation is attributed to different biophysical changes using two attribution methods
Similarity and dissimilarity between the attribution results from the two methods are identified
Acceptable agreement between observed and modeled land surface temperature change is the prerequisite for attribution
Many individual studies have shown that the timing of leaf senescence in boreal and temperate deciduous forests in the northern hemisphere is influenced by rising temperatures, but there is limited ...consensus on the magnitude, direction and spatial extent of this relationship.
A meta-analysis was conducted of published studies from the peer-reviewed literature that reported autumn senescence dates for deciduous trees in the northern hemisphere, encompassing 64 publications with observations ranging from 1931 to 2010.
Among the meteorological measurements examined, October temperatures were the strongest predictors of date of senescence, followed by cooling degree-days, latitude, photoperiod and, lastly, total monthly precipitation, although the strength of the relationships differed between high- and low-latitude sites. Autumn leaf senescence has been significantly more delayed at low (25° to 49°N) than high (50° to 70°N) latitudes across the northern hemisphere, with senescence across high-latitude sites more sensitive to the effects of photoperiod and low-latitude sites more sensitive to the effects of temperature. Delays in leaf senescence over time were stronger in North America compared with Europe and Asia.
The results indicate that leaf senescence has been delayed over time and in response to temperature, although low-latitude sites show significantly stronger delays in senescence over time than high-latitude sites. While temperature alone may be a reasonable predictor of the date of leaf senescence when examining a broad suite of sites, it is important to consider that temperature-induced changes in senescence at high-latitude sites are likely to be constrained by the influence of photoperiod. Ecosystem-level differences in the mechanisms that control the timing of leaf senescence may affect both plant community interactions and ecosystem carbon storage as global temperatures increase over the next century.
US crop yield losses from hydroclimatic hazards Choi, Eunkyoung; Rigden, Angela J; Tangdamrongsub, Natthachet ...
Environmental research letters,
01/2024, Letnik:
19, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Abstract
Hydroclimatic stresses can negatively impact crop production via water deficits (low soil water supply and high atmospheric demand) or surpluses (high soil water supply and low atmospheric ...demand). However, the impact of both stresses on crop yields at regional scales is not well understood. Here we quantified yield sensitivities and corresponding spatio-temporal yield losses of US rainfed maize, soybeans, sorghum, and spring wheat to hydroclimatic stresses by considering the joint impacts of root-zone soil moisture and atmospheric evaporative demand from 1981 to 2020. We show that crop yields can be reduced similarly by two major hydroclimatic hazards, which are defined as the most yield damaging conditions over time: ‘Low Supply + High Demand’ and ‘High Supply + Low Demand’. However, more exposure to ‘Low Supply + High Demand’ hazard led to the largest annual yield losses (7%–17%) across all four crops over time. Modeled yield losses due to these hazards were significantly associated with crop insurance lost costs. The extent of yield losses varies considerably by crop and location, highlighting the need for crop-specific and regionally tailored adaptation strategies.
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