• Energy Research

Abstract. The responses of crop functioning to changing climate and atmospheric CO2 concentration ([CO2]) could have large effects on food production, and impact carbon, water and energy fluxes, causing feedbacks to climate. To simulate the responses of temperate crops to changing climate and [CO2], accounting for the specific phenology of crops mediated by management practice, we present here the development of a process-oriented terrestrial biogeochemical model named ORCHIDEE-CROP (v0), which integrates a generic crop phenology and harvest module and a very simple parameterization of nitrogen fertilization, into the land surface model (LSM) ORCHIDEEv196, in order to simulate biophysical and biochemical interactions in croplands, as well as plant productivity and harvested yield. The model is applicable for a range of temperate crops, but it is tested here for maize and winter wheat, with the phenological parameterizations of two European varieties originating from the STICS agronomical model. We evaluate the ORCHIDEE-CROP (v0) model against eddy covariance and biometric measurements at 7 winter wheat and maize sites in Europe. The specific ecosystem variables used in the evaluation are CO2 fluxes (NEE), latent heat and sensible heat fluxes. Additional measurements of leaf area index (LAI), aboveground biomass and yield are used as well. Evaluation results reveal that ORCHIDEE-CROP (v0) reproduces the observed timing of crop development stages and the amplitude of pertaining LAI changes in contrast to ORCHIDEEv196 in which by default crops have the same phenology than grass. A near-halving of the root mean square error of LAI from 2.38 ± 0.77 to 1.08 ± 0.34 m2 m−2 is obtained between ORCHIDEEv196 and ORCHIDEE-CROP (v0) across the 7 study sites. Improved crop phenology and carbon allocation lead to a general good match between modelled and observed aboveground biomass (with a normalized root mean squared error (NRMSE) of 11.0–54.2 %), crop yield, as well as of the daily carbon and energy fluxes with NRMSE of ~9.0–20.1 and ~9.4–22.3 % for NEE, and sensible and latent heat fluxes, respectively. The model data mistfit for energy fluxes are within uncertainties of the measurements, which themselves show an incomplete energy balance closure within the range 80.6–86.3 %. The remaining discrepancies between modelled and observed LAI and other variables at specific sites are partly attributable to unrealistic representation of management events. In addition, ORCHIDEE-CROP (v0) is shown to have the ability to capture the spatial gradients of carbon and energy-related variables, such as gross primary productivity, NEE, sensible heat fluxes and latent heat fluxes, across the sites in Europe, an important requirement for future spatially explicit simulations. Further improvement of the model with an explicit parameterization of nutrition dynamics and of management, is expected to improve its predictive ability to simulate croplands in an Earth System Model.

Abstract. Accurate quantification of the amount and spatial variation of evapotranspiration is important in a wide range of disciplines. Remote sensing based surface energy balance models have been developed to estimate turbulent surface energy fluxes at different scales. The objective of this study is to evaluate the Surface Energy Balance System (SEBS) model on a landscape scale, using tower-based flux measurements at different land cover units during an overpass of the ASTER sensor over the SPARC 2004 experimental site in Barrax (Spain). A sensitivity analysis has been performed in order to investigate to which variable the sensible heat flux is most sensitive. Taking into account their estimation errors, the aerodynamic parameters (hc, z0M and d0) can cause large deviations in the modelling of sensible heat flux. The effect of replacement of empirical derivation of these aerodynamic parameters in the model by field estimates or literature values is investigated by testing two scenarios: the Empirical Scenario in which empirical equations are used to derive aerodynamic parameters and the Field Scenario in which values from field measurements or literature are used to replace the empirical calculations of the Empirical Scenario. In the case of a homogeneous land cover in the footprints of the measurements, the Field Scenario only resulted in a small improvement, compared to the Empirical Scenario. The Field Scenario can even worsen the result in the case of heterogeneous footprints, by creating sharp borders related to the land cover map. In both scenarios modelled fluxes correspond better with flux measurements over uniform land cover compared to cases where different land covers are mixed in the measurement footprint. Furthermore SEBS underestimates sensible heat flux especially over dry and sparsely vegetated areas, which is common in single-source models.

Water scarcity is one of the problems in water management that hinders European rivers in reaching a good ecological status as defined in the European Water Framework Directive. Water scarcity often coincides with high water temperature and low water quality. High water temperatures decrease the oxygen supply and often coincide with low flows that tend to increase the load of various compounds that affect the equilibrium in the ecosystem. The river Meuse regularly encounters situations of low water flow. In these situations, the river Rur, an important tributary of the river Meuse in Germany, contributes significantly to the Meuse discharge. Climate change can lead to more frequent periods of water scarcity. Moreover, plans exist to divert water from the Rur to former brown coal quarries in Germany. This exploratory study examines the relationships between discharge, water temperature and water quality under future climate change and water diversion scenarios in low-flow situations for the rivers Meuse and Rur. The results of the study confirm that rising air temperatures as a result of climate change will lead to higher water temperatures which will negatively impact the water quality of the Meuse in particular. Despite the fact that the contribution of the Rur has a positive impact on the water quality of the Meuse, the results suggest that effects of plans to divert water from the Rur may be small on average. However, the impact of the diversion may be stronger on individual hot summer days with low water levels when the Rur contributes significantly to the discharge of the Meuse.

Comprendre les relations entre le climat et l'échange de carbone par les écosystèmes terrestres est essentiel pour prédire les niveaux futurs de dioxyde de carbone atmosphérique en raison des effets d'accélération potentiels des rétroactions positives du cycle climat-carbone. Cependant, les relations directement observées entre le climat et l'échange de CO2 terrestre avec l'atmosphère à travers les biomes et les continents font défaut. Nous présentons ici des données décrivant les relations entre l'échange net de carbone par les écosystèmes (NEE) et les facteurs climatiques tels que mesurés à l'aide de la méthode de covariance de Foucault sur 125 sites uniques dans divers écosystèmes sur six continents avec un total de 559 années de site. Nous trouvons que le NEE observé aux sites de covariance tourbillonnaire est (1) une fonction forte de la température annuelle moyenne aux latitudes moyennes et élevées, (2) une fonction forte de la sécheresse aux latitudes moyennes et basses, et (3) une fonction à la fois de la température et de la sécheresse autour de la ceinture moyenne-latitudinale (45°N). La sensibilité du NEE à la température annuelle moyenne se décompose à ~ 16 °C (une valeur seuil de la température annuelle moyenne), au-delà de laquelle aucune augmentation supplémentaire de l'absorption de CO2 avec la température n'a été observée et la sécheresse influence les règles de dépassement de l'influence de la température. Comprender las relaciones entre el clima y el intercambio de carbono por parte de los ecosistemas terrestres es fundamental para predecir los niveles futuros de dióxido de carbono en la atmósfera debido a los posibles efectos aceleradores de las retroalimentaciones positivas del ciclo clima-carbono. Sin embargo, faltan relaciones directamente observadas entre el clima y el intercambio terrestre de CO2 con la atmósfera a través de biomas y continentes. Aquí presentamos datos que describen las relaciones entre el intercambio neto de carbono (NEE) del ecosistema y los factores climáticos medidos utilizando el método de covarianza de remolinos en 125 sitios únicos en varios ecosistemas de seis continentes con un total de 559 años-sitio. Encontramos que la NEE observada en los sitios de covarianza de remolinos es (1) una fuerte función de la temperatura media anual en latitudes medias y altas, (2) una fuerte función de sequedad en latitudes medias y bajas, y (3) una función tanto de la temperatura como de la sequedad alrededor del cinturón latitudinal medio (45°N). La sensibilidad de NEE a la temperatura media anual se rompe a ~ 16 °C (un valor umbral de la temperatura media anual), por encima del cual no se observó ningún aumento adicional de la absorción de CO2 con la temperatura y la influencia de la sequedad anula la influencia de la temperatura. Understanding the relationships between climate and carbon exchange by terrestrial ecosystems is critical to predict future levels of atmospheric carbon dioxide because of the potential accelerating effects of positive climate–carbon cycle feedbacks. However, directly observed relationships between climate and terrestrial CO2 exchange with the atmosphere across biomes and continents are lacking. Here we present data describing the relationships between net ecosystem exchange of carbon (NEE) and climate factors as measured using the eddy covariance method at 125 unique sites in various ecosystems over six continents with a total of 559 site-years. We find that NEE observed at eddy covariance sites is (1) a strong function of mean annual temperature at mid- and high-latitudes, (2) a strong function of dryness at mid- and low-latitudes, and (3) a function of both temperature and dryness around the mid-latitudinal belt (45°N). The sensitivity of NEE to mean annual temperature breaks down at ~ 16 °C (a threshold value of mean annual temperature), above which no further increase of CO2 uptake with temperature was observed and dryness influence overrules temperature influence. يعد فهم العلاقات بين المناخ وتبادل الكربون بواسطة النظم الإيكولوجية الأرضية أمرًا بالغ الأهمية للتنبؤ بالمستويات المستقبلية لثاني أكسيد الكربون في الغلاف الجوي بسبب التأثيرات المتسارعة المحتملة للتغذية المرتدة الإيجابية لدورة المناخ والكربون. ومع ذلك، لا توجد علاقات ملحوظة مباشرة بين المناخ والتبادل الأرضي لثاني أكسيد الكربون مع الغلاف الجوي عبر المناطق الحيوية والقارات. نقدم هنا بيانات تصف العلاقات بين صافي تبادل النظام البيئي للكربون (NEE) والعوامل المناخية كما تم قياسها باستخدام طريقة التباين الدوامي في 125 موقعًا فريدًا في أنظمة بيئية مختلفة عبر ست قارات بإجمالي 559 سنة موقع. نجد أن NEE التي لوحظت في مواقع التباين الدوامي هي (1) وظيفة قوية لمتوسط درجة الحرارة السنوية عند خطوط العرض المتوسطة والعالية، (2) وظيفة قوية للجفاف عند خطوط العرض المتوسطة والمنخفضة، و (3) وظيفة لكل من درجة الحرارة والجفاف حول حزام العرض المتوسط (45درجةشمالاً). تنهار حساسية NEE لمتوسط درجة الحرارة السنوية عند حوالي 16 درجة مئوية (قيمة عتبة لمتوسط درجة الحرارة السنوية)، والتي لم يلاحظ فوقها أي زيادة أخرى في امتصاص ثاني أكسيد الكربون مع درجة الحرارة ويتجاوز تأثير الجفاف تأثير درجة الحرارة.