Performance of Gradient and Gradient-Free Optimizers in Transient Hydraulic Tomography

Sub-surface characterization in fractured aquifers is challenging due to the co-existence of contrasting materials namely matrix and fractures. Transient hydraulic tomography (THT) is proved to be an efficient and robust technique to estimate hydraulic (K_m, K_f) and storage (S_m, S_f) properties in such complex hydrogeologic settings. However, performance of THT is governed by data quality and optimization technique used in inversion. We assessed the performance of gradient and gradient-free optimizers with THT inversion. Laboratory experiments were performed on a two-dimensional, granite rock (80 cm × 45 cm × 5 cm) with known fracture pattern. Cross-hole pumping experiments were conducted at 10 ports (located on fractures), and time-drawdown responses were monitored at 25 ports (located on matrix and fractures). Pumping ports were ranked based on weighted signal-to-noise ratio (SNR) computed at each observation port. Noise-free, good quality (SNR > 100) datasets were inverted using Levenberg–Marquardt: LM (gradient) and Nelder–Mead: NM (gradient-free) methods. All simulations were performed using a coupled simulation-optimization model. Performance of the two optimizers is evaluated by comparing model predictions with observations made at two validation ports that were not used in simulation. Both LM and NM algorithms have broadly captured the preferential flow paths (fracture network) via K and S tomograms, however LM has outperformed NM during validation ( $$). Our results conclude that, while method of optimization has a trivial effect on model predictions, exclusion of low quality (SNR ≤ 100) datasets can significantly improve the model performance.     

Relationships of nitrogen loadings,residential development,and physical characteristics with plant structure in New England salt marshes

We examined the vascular plant species richness and the extent, density, and height ofSpartina species of ten Narragansett Bay, Rhode Island (United States) fringe salt marshes which had a wide range of residential land development and N-loadings associated with their watersheds. Significant inverse relationships of tallS. alterniflora with species richness and with the extent and density ofS. patens and shortS. alterniflora were observed. Extent and density ofS. patens and extent of shortS. alterniflora were positively and significantly related with plant species richness. Marsh elevation and area did not significantly correlate with plant structure. Flood tide height significantly and inversely correlated withS. patens, but did not significantly relate toS. alterniflora or plant species richness. Marsh width significantly and positively correlated with plant species richness andS. patens and inversely correlated with tallS. alterniflora. Significant inverse relationships were observed for N-load, % residential development, and slope withS. patens, shortS. alterniflora, and species richness, and significant positive relationships with tallS. alterniflora. The marsh slope and width were significantly correlated with N-load and residential development that made it difficult to determine to what extent anthropogenic stressors were contributing to the variation in the plant structure among the marshes. At five marhes with similar slopes, there were significant inverse relationships of N-load withS. patens (density and extent) and a positive relationship with tallS. alterniflora (extent). Although there were no significant relationships of slope with the plant metrics among the five sites, other physical factors, such as the flood tide height and marsh width, significantly correlated with the extent and density ofSpartina species. Significant relationships of N-load with plant structure (albeit confounded by the effect of the physical characteristics) support the hypothesis of competitive displacement of dominant marsh plants under elevated nitrogen. It is likely that the varying plant structure in New England marshes is a response to a combination of natural factors and multiple anthropogenic stressors (e.g., eutrophication and sea level rise).     

Nekton habitat quality at shallow water sites in two Rhode Island coastal systems

We evaluated nekton habitat quality at 5 shallow-water sites in 2 Rhode Island systems by comparing nekton densities and biomass, number of species, prey availability and feeding, and abundance of winter flounderPseudopleuronectes americanus. Nekton density and biomass were compared with a 1.75-m² drop ring at 3 sites (marsh, intertidal, and subtidal) in Coggeshall Cove in Narragansett Bay and two subtidal sites (eelgrass and macroalgae) in Ninigret Pond, a coastal lagoon. We collected benthic core samples and examined nekton stomach contents in Coggeshall Cove. We identified 16 species of fish, 16 species of crabs, and 3 species of shrimp in our drop ring samples. A multivariate analysis of variance indicated differences in total nekton, invertebrates, fish, and winter flounder across the five sites. Relative abundance of benthic invertebrate taxa did not match relative abundance of prey taxa identified in the stomachs. Nonmetric multidimensional scaling plots showed groupings in nekton and benthic invertebrate prey assemblages among subtidal, intertidal, and marsh sites in Coggeshall Cove. Stepwise multiple regression indicated that biomass of macroalgae was the most important variable predicting abundance of nekton in Coggeshall Cove, followed by elevation and depth. In Rhode Island systems that do not experience chronic hypoxia, macroalgae adds structure to unvegetated areas and provides refuge for small nekton. All sites sampled were characterized by high abundance and diversity of nekton pointing to the importance of shallow inshore areas for production of fishes and decapods. Measurements of habitat quality should include assessment of the functional significance of a habitat (this can be done by comparing nekton numbers and biomass), some measure of habitat diversity, and a consideration of how habitat quality varies in time and space.