Modélisation statistique des modules annuels et des étiages du bassin roumain du Timis-Bega, similitudes régionales avec la Moselle française

Le bassin hydrographique du Timis-Bega, d’une superficie d’environ 13 000 km2, est situé à l’ouest de la Roumanie. Le contexte orographique, l’influence climatique méditerranéenne, la nature et l’occupation du sol font que cette zone géographique relativement restreinte présente une assez grande diversité des écoulements. Cette dernière est considérée à travers le choix de 19 sous-bassins, de quelque km2 à plusieurs centaines de km2. La régionalisation des modules et des débits d’étiage du Timis-Bega a été entreprise à la suite de travaux analogues réalisés sur le bassin hydrographique de la Moselle française (GALÉA et CANALI, 2005). Ces travaux avaient montré l’adéquation de la loi de Weibull à deux paramètres pour décrire la variabilité temporelle et spatiale des modules annuels et des étiages des sous-bassins observés. Deux principaux résultats avaient été soulignés, l’un concernait la faible différenciation des lois régionales établies pour les modules et les étiages des années moyennes à « sèches »  (F ≤ 0,5), l’autre mettait l’accent sur l’usage de jaugeages épisodiques d’étiage pour améliorer la performance des modèles régionaux. Notre propos est de vérifier ces considérations, établies pour le bassin de la Moselle, dans le contexte hydro-climatique du Timis-Bega. Les résultats obtenus permettent de confirmer tant l’aspect peu différencié des lois régionales que l’aspect amélioration de la performance des modèles régionaux. Plus précisément, cette amélioration résulte d’une meilleure précision, que celle obtenue par régression multiple classique, de l’estimation du descripteur de régime local, le module annuel médian ou le débit journalier médian d’étiage qui représente une source d’incertitude importante du modèle régional respectif. L’usage d’une information épisodique sur les débits d’étiage, concomitante à un site étudié et à un sous-bassin de référence, permet l’estimation du débit journalier médian et, par extension, celle du module annuel médian. Une explication à cette extension pourrait être trouvée dans la faible différenciation des lois régionales pour les fréquences moyennes à « sèches ». L’intérêt des jaugeages épisodiques d’étiage nous semble assez porteur pour être pris en considération lors d’une démarche ultérieure de régionalisation à bases géostatistiques des descripteurs de régime. Enfin, un résultat inattendu concerne la quasi similitude des lois régionales de la Moselle et du Timis-Bega tant en ce qui concerne les modules annuels que les étiages. Ce résultat, à considérer avec prudence, abonderait dans le sens de la théorie des régions hydrologiques homogènes.The catchment area of Timis-Bega (13,000 km2) is located in the western part of Romania. The orographical context, the Mediterranean climatic influence, the nature of the ground and the vegetation contribute to this relatively restricted geographical area’s great diversity of flows. This latter characteristic is based on a study of 19 sub-basins, ranging in size from a few km2 to several hundred km2. The regionalization of annual mean discharge and low flow in the Timis-Bega catchment was undertaken following similar work realized on the French Moselle catchment. This earlier work showed that the two-parameter Weibull distribution adequately described the spatial and temporal variability of annual mean discharge and low flow for the studied sub-catchments. The two principal results of the preceding study were the weak differentiation among the regional models established for annual mean discharge and low flow, and the recommended use of episodic gauging of low flows for the improvement of the regional models. Our intention was to verify these preceding considerations, established for the French Moselle basin, in the hydro-climatic context of Timis-Bega. The results obtained allowed us to confirm the weak differentiation among the regional models as well as the improvement of the regional models. More specifically, this improvement resulted from the better precision, compared to that obtained by traditional multiple regression methods, of the estimation of the local discharge descriptor, the median annual mean discharge or the median minimal daily low flow discharge. The use of low flow episodic discharge measurements, that are concomitant between the studied and reference sub-catchments, allows the estimation of the median minimal daily low flow discharge and the median annual mean discharge. This fortuitous result is explained by the weak differentiation observed between the regional annual mean discharge model and the regional average low flow discharge model for average to dry years. We believe episodic measurements of low flow should be considered when collecting data that will be used for the generation of regionalization flow descriptors by geostatistical methods. Lastly, the similarities between the regional distributions of annual mean discharge and low flow for both the Moselle and Timis-Bega regions were unexpected. This result should, however, be considered with caution, as it implies the existence of homogeneous hydrological regions

The use of airborne laser scanning to develop a pixel-based stratification for a verified carbon offset project

Background The voluntary carbon market is a new and growing market that is increasingly important to consider in managing forestland. Monitoring, reporting, and verifying carbon stocks and fluxes at a project level is the single largest direct cost of a forest carbon offset project. There are now many methods for estimating forest stocks with high accuracy that use both Airborne Laser Scanning (ALS) and high-resolution optical remote sensing data. However, many of these methods are not appropriate for use under existing carbon offset standards and most have not been field tested. Results This paper presents a pixel-based forest stratification method that uses both ALS and optical remote sensing data to optimally partition the variability across an ~10,000 ha forest ownership in Mendocino County, CA, USA. This new stratification approach improved the accuracy of the forest inventory, reduced the cost of field-based inventory, and provides a powerful tool for future management planning. This approach also details a method of determining the optimum pixel size to best partition a forest. Conclusions The use of ALS and optical remote sensing data can help reduce the cost of field inventory and can help to locate areas that need the most intensive inventory effort. This pixel-based stratification method may provide a cost-effective approach to reducing inventory costs over larger areas when the remote sensing data acquisition costs can be kept low on a per acre basis

Partitioning vegetation response to anthropogenic stress to develop multi-taxa wetland indicators

Emergent plants can be suitable indicators of anthropogenic stress in coastal wetlands if their responses to natural environmental variation can be parsed from their responses to human activities in and around wetlands. We used hierarchical partitioning to evaluate the independent influence of geomorphology, geography, and anthropogenic stress on common wetland plants of the U.S. Great Lakes coast and developed multi-taxa models indicating wetland condition. A seven-taxon model predicted condition relative to watershed-derived anthropogenic stress, and a four-taxon model predicted condition relative to within-wetland anthropogenic stressors that modified hydrology. The Great Lake on which the wetlands occurred explained an average of about half the variation in species cover, and subdividing the data by lake allowed us to remove that source of variation. We developed lake-specific multi-taxa models for all of the Great Lakes except Lake Ontario, which had no plant species with significant independent effects of anthropogenic stress. Plant responses were both positive (increasing cover with stress) and negative (decreasing cover with stress), and plant taxa incorporated into the lake-specific models differed by Great Lake. The resulting models require information on only a few taxa, rather than all plant species within a wetland, making them easier to implement than existing indicators

THE MORDELL–LANG QUESTION FOR ENDOMORPHISMS OF SEMIABELIAN VARIETIES

Abstract. The Mordell–Lang conjecture describes the intersection of a finitely generated subgroup with a closed subvariety of a semiabelian variety. Equivalently, this conjecture describes the intersection of closed subvarieties with the set of images of the origin under a finitely generated semigroup of translations. We study the analogous question in which the translations are replaced by algebraic group endomorphisms (and the origin is replaced by another point). We show that the conclusion of the Mordell–Lang conjecture remains true in this setting if either (1) the semiabelian variety is simple, (2) the semiabelian variety is A 2, where A is a one-dimensional semiabelian variety, (3) the subvariety is a connected one-dimensional algebraic subgroup, or (4) each endomorphism has diagonalizable Jacobian at the origin. We also give examples showing that the conclusion fails if we make slight modifications to any of these hypotheses. 1

Inventory and Ventilation Efficiency of Nonnative and Native Phragmites australis (Common Reed) in Tidal Wetlands of the Chesapeake Bay

Nonnative Phragmites is among the most invasive plants in the U.S. Atlantic coast tidal wetlands, whereas the native Phragmites has declined. Native and nonnative patches growing side by side provided an ideal setting for studying mechanisms that enable nonnative Phragmites to be a successful invader. We conducted an inventory followed by genetic analysis and compared differences in growth patterns and ventilation efficiency between adjacent native and nonnative Phragmites stands. Genetic analysis of 212 patches revealed that only 14 were native suggesting that very few native Phragmites populations existed in the study area. Shoot density decreased towards the periphery of native patches, but not in nonnative patches. Ventilation efficiency was 300 % higher per unit area for nonnative than native Phragmites, likely resulting in increased oxidation of the rhizosphere and invasive behavior of nonnative Phragmites. Management of nonnative Phragmites stands should include mechanisms that inhibit pressurized ventilation of shoots