Assessment of a land surface model over a mediterranean crop site for a long period of time : impact of uncertainties in soil parameters

International audienc

Evaluation of land surface model simulations of evapotranspiration over a 12 year crop succession: impact of the soil hydraulic properties

Evapotranspiration has been recognized as one of the most uncertain term in the surface water balance simulated by land surface models. In this study, the SURFEX/ISBAA- gs simulations of evapotranspiration are assessed at local scale over a 12 year 5 Mediterranean crop succession. The model is evaluated in its standard implementation which relies on the use of the ISBA pedotransfer estimates of the soil properties. The originality of this work consists in explicitly representing the succession of crop cycles and inter-crop bare soil periods in the simulations and assessing its impact on the dynamic of simulated and measured evapotranspiration over a long period of time. 10 The analysis focuses on key soil parameters which drive the simulation of evapotranspiration, namely the rooting depth, the soil moisture at saturation, the soil moisture at field capacity and the soil moisture at wilting point. The simulations achieved with the standard values of these parameters are compared to those achieved with the in situ values. The portability of the ISBA pedotransfer functions is evaluated over a typical 15 Mediterranean crop site. Various in situ estimates of the soil parameters are considered and distinct parametrization strategies are tested to represent the evapotranspiration dynamic over the crop succession. This work shows that evapotranspiration mainly results from the soil evaporation when it is continuously simulated over a Mediterranean crop succession. The evapo20 transpiration simulated with the standard surface and soil parameters of the model is largely underestimated. The deficit in cumulative evapotranspiration amounts to 24% over 12 years. The bias in daily daytime evapotranspiration is −0.24mmday−1. The ISBA pedotransfer estimates of the soil moisture at saturation and at wilting point are overestimated which explains most of the evapotranspiration underestimation. The 25 overestimation of the soil moisture at wilting point causes the underestimation of transpiration at the end of the crop cycles. The overestimation of the soil moisture at saturation triggers the underestimation of the soil evaporation during the wet soil periods. The use of field capacity values derived from laboratory retention measurements leads to inaccurate simulation of soil evaporation due to the lack of representativeness of the soil structure variability at the field scale. The most accurate simulation is achieved with the values of the soil hydraulic properties derived from field measured soil moisture. Their temporal analysis over each crop cycle provides meaningful estimates of the 5 wilting point, the field capacity and the rooting depth to represent the crop water needs and accurately simulate the evapotranspiration over the crop succession. We showed that the uncertainties in the eddy-covariance measurements are significant and can explain a large part of the unresolved random differences between the simulations and the measurements of evapotranspiration. Other possible model shortcomings include 10 the lack of representation of soil vertical heterogeneity and root profile along with inaccurate energy balance partitioning between the soil and the vegetation at low LAI

Multi-year assessment of a land surface model: Impact of the uncertainty on soil parameters on the evapotranpiration of a mediterranean crop site

International audienc

Rfx6 directs islet formation and insulin production in mice and humans.

Insulin from the beta-cells of the pancreatic islets of Langerhans controls energy homeostasis in vertebrates, and its deficiency causes diabetes mellitus. During embryonic development, the transcription factor neurogenin 3 (Neurog3) initiates the differentiation of the beta-cells and other islet cell types from pancreatic endoderm, but the genetic program that subsequently completes this differentiation remains incompletely understood. Here we show that the transcription factor Rfx6 directs islet cell differentiation downstream of Neurog3. Mice lacking Rfx6 failed to generate any of the normal islet cell types except for pancreatic-polypeptide-producing cells. In human infants with a similar autosomal recessive syndrome of neonatal diabetes, genetic mapping and subsequent sequencing identified mutations in the human RFX6 gene. These studies demonstrate a unique position for Rfx6 in the hierarchy of factors that coordinate pancreatic islet development in both mice and humans. Rfx6 could prove useful in efforts to generate beta-cells for patients with diabetes

Rfx6 directs islet formation and insulin production in mice and humans.

Insulin from the beta-cells of the pancreatic islets of Langerhans controls energy homeostasis in vertebrates, and its deficiency causes diabetes mellitus. During embryonic development, the transcription factor neurogenin 3 (Neurog3) initiates the differentiation of the beta-cells and other islet cell types from pancreatic endoderm, but the genetic program that subsequently completes this differentiation remains incompletely understood. Here we show that the transcription factor Rfx6 directs islet cell differentiation downstream of Neurog3. Mice lacking Rfx6 failed to generate any of the normal islet cell types except for pancreatic-polypeptide-producing cells. In human infants with a similar autosomal recessive syndrome of neonatal diabetes, genetic mapping and subsequent sequencing identified mutations in the human RFX6 gene. These studies demonstrate a unique position for Rfx6 in the hierarchy of factors that coordinate pancreatic islet development in both mice and humans. Rfx6 could prove useful in efforts to generate beta-cells for patients with diabetes.Journal ArticleResearch Support, N.I.H. ExtramuralResearch Support, Non-U.S. Gov'tinfo:eu-repo/semantics/publishe

La répétition des feux et des sécheresses menace la biodiversité fonctionnelle des écosystèmes méditerranéens

International audienceBetween 2005 and 2008, a group of 10 research teams studied vegetation, litter, earthworms, microfauna, bacteria, soil physics, nutriments, organic matter (quality and quantity) and toxic compounds in a multidisciplinary research program dealing with the impact of repeated forest fires on biodiversity and the soil in mediterranean forests. Plots were laid in old growth forests and in sites burned between 1 and 4 times in the last 50 years. For each fire regime, fires dating 1, 3, 5, 15 and 25 years were chosen. The goals were: (1) to look for biodiversity indicators for the assessment of the ecosystem status and functioning, (2) to understand interactions between biotic and abiotic factors in ecosystem resistance and resilience, (3) to look for critical fire frequency thresholds. Taking advantage of the exceptional drought which occurred during the experiment, we studied the interaction between fire and drought. Two thresholds appeared in fire frequency (1 and 4 fires in 50 years) leading to deep changes in the ecosystem biodiversity and functioning. Few differences were observed 3 and 15 years after fire between plots burned 1, 2 or 3 times, the ecosystem being used for millenniums to such fire frequencies. Earthworms and bacteria diversity and activity appeared critical for ecosystem functioning and recovery after fire. Due to drought, this activity was severely reduced and strong interactions were found between fire and drought, each of these disturbances amplifying the impact of the other. High disturbance frequency by fire or drought reduced biological diversity, increasing the resistance but reducing the resilience of communities. Some obligate seeders tree species disappeared when fire occurred at less than 15 to 20 years interval, changing the ecosystem structure. Trees mortality due to drought was far higher in recently burned areas, degrading vegetation from forest to matorals. The quantitative resilience of several physical and chemical parameters is achieved within 15 to 25 years after fire, but 50 years are necessary to fully recover the qualitative values and balance for these parameters as well as for most of the biological factors. Most of the critical biological, physical and chemical processes were concentrated in the topsoil (0-5 cm), deeper horizons being impoverished by a long fire history, except in old growth forests. These ecosystems are fragile and their dynamics may shift to desertification with increasing drought due to climate change

Sécheresses et incendies répétés accroissent mutuellement leur impact sur l'écosystème

International audienceIn a recent research project performed in South-Eastern France on the impact of repeated fires on many components of forest environment (vegetation, fauna, microbiology, soil physics, chemical properties, organic matter, nutrients) we showed a strong interaction between repeated droughts and repeated fires. Each disturbance significantly increases the impact of the other one. Repeated droughts can stop and even reverse the recovery process after fire, and delay this recovery when they occurred before fire. Repeated fires lessen the resistance and resilience of the ecosystem to drought. Forest regeneration processes are threatened even in usually fire-prone environments usually resilient. Soil biological activity is severely affected, and particularly some key groups as earthworms and bacteria contributing to nitrogen cycle. Soil physics and chemical properties appear to be degraded as a habitat, and indirectly the reduction of biological activity limits their recovery, including a negative carbon balance. Four successive years appear to be the critical threshold for drought and four times in 50 years the critical threshold for fires. As climate change may lead to higher drought frequency and fire occurrence is tied to drought, the drought/fire interaction may degrade forest ecosystems more rapidly than expected from separate assessment of drought and fire impacts