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81.
Karen Schleeweis Samuel N. Goward Chengquan Huang John L. Dwyer Jennifer L. Dungan Mary A. Lindsey 《International Journal of Digital Earth》2016,9(10):963-980
Using the NASA Earth Exchange platform, the North American Forest Dynamics (NAFD) project mapped forest history wall-to-wall, annually for the contiguous US (1986–2010) using the Vegetation Change Tracker algorithm. As with any effort to identify real changes in remotely sensed time-series, data gaps, shifts in seasonality, misregistration, inconsistent radiometry and cloud contamination can be sources of error. We discuss the NAFD image selection and processing stream (NISPS) that was designed to minimize these sources of error. The NISPS image quality assessments highlighted issues with the Landsat archive and metadata including inadequate georegistration, unreliability of the pre-2009 L5 cloud cover assessments algorithm, missing growing-season imagery and paucity of clear views. Assessment maps of Landsat 5–7 image quantities and qualities are presented that offer novel perspectives on the growing-season archive considered for this study. Over 150,000+ Landsat images were considered for the NAFD project. Optimally, one high quality cloud-free image in each year or a total of 12,152 images would be used. However, to accommodate data gaps and cloud/shadow contamination 23,338 images were needed. In 220 specific path-row image years no acceptable images were found resulting in data gaps in the annual national map products. 相似文献
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Fluid exchange across the sediment–water interface in a sandy open continental shelf setting was studied using heat as a tracer. Summertime tidal oscillation of cross-shelf thermal fronts on the South Atlantic Bight provided a sufficient signal at the sediment–water interface to trace the advective and conductive transport of heat into and out of the seabed, indicating rapid flushing of ocean water through the upper 10–40 cm of the sandy seafloor. A newly developed transport model was applied to the in situ temperature data set to estimate the extent to which heat was transported by advection rather than conduction. Heat transported by shallow 3-D porewater flow processes was accounted for in the model by using a dispersion term, the depth and intensity of which reflected the depth and intensity of shallow flushing. Similar to the results of past studies in shallower and more energetic nearshore settings, transport of heat was greater when higher near-bed velocities and shear stresses occurred over a rippled bed. However, boundary layer processes by themselves were insufficient to promote non-conductive heat transport. Advective heat transport only occurred when both larger boundary layer stresses and thermal instabilities within the porespace were present. The latter process is dependent on shelf-scale heating and cooling of bottom water associated with upwelling events that are not coupled to local-scale boundary layer processes. 相似文献
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Karen L. Knee Elizabeth D. Crook James L. Hench James J. Leichter Adina Paytan 《Estuaries and Coasts》2016,39(6):1651-1668
Previous work has documented large fluxes of freshwater and nutrients from submarine groundwater discharge (SGD) into the coastal waters of a few volcanic oceanic islands. However, on the majority of such islands, including Moorea (French Polynesia), SGD has not been studied. In this study, we used radium (Ra) isotopes and salinity to investigate SGD and associated nutrient inputs at five coastal sites and Paopao Bay on the north shore of Moorea. Ra activities were highest in coastal groundwater, intermediate in coastal ocean surface water, and lowest in offshore surface water, indicating that high-Ra groundwater was discharging into the coastal ocean. On average, groundwater nitrate and nitrite (N + N), phosphate, ammonium, and silica concentrations were 12, 21, 29, and 33 times greater, respectively, than those in coastal ocean surface water, suggesting that groundwater discharge could be an important source of nutrients to the coastal ocean. Ra and salinity mass balances indicated that most or all SGD at these sites was saline and likely originated from a deeper, unsampled layer of Ra-enriched recirculated seawater. This high-salinity SGD may be less affected by terrestrial nutrient sources, such as fertilizer, sewage, and animal waste, compared to meteoric groundwater; however, nutrient-salinity trends indicate it may still have much higher concentrations of nitrate and phosphate than coastal receiving waters. Coastal ocean nutrient concentrations were virtually identical to those measured offshore, suggesting that nutrient subsidies from SGD are efficiently utilized. 相似文献
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Julian Damashek Karen L. Casciotti Christopher A. Francis 《Estuaries and Coasts》2016,39(4):1050-1071
Understanding rates of nitrogen cycling in estuaries is crucial for understanding their productivity and resilience to eutrophication. Nitrification, the microbial oxidation of ammonia to nitrite and nitrate, links reduced and oxidized forms of inorganic nitrogen and is therefore an important step of the nitrogen cycle. However, rates of nitrification in estuary waters are poorly characterized. In fall and winter of 2011–2012, we measured nitrification rates throughout the water column of all major regions of San Francisco Bay, a large, turbid, nutrient-rich estuary on the west coast of North America. Nitrification rates were highest in regions furthest from the ocean, including many samples with rates higher than those typically measured in the sea. In bottom waters, nitrification rates were commonly at least twice the magnitude of surface rates. Strong positive correlations were found between nitrification and both suspended particulate matter and ammonium concentration. Our results are consistent with previous studies documenting high nitrification rates in brackish, turbid regions of other estuaries, many of which also showed correlations with suspended sediment and ammonium concentrations. Overall, nitrification in estuary waters appears to play a significant role in the estuarine nitrogen cycle, though the maximum rate of nitrification can differ dramatically between estuaries. 相似文献
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Karen Culcasi 《Geographical review》2016,106(2):258-263
The goal of this forum is to garner attention to the issues surrounding the Boycott, Divestment, and Sanctions (BDS) campaign against Israel, to help a broad audience better understand the complexity of BDS, and to engage geographers in the BDS debate. Including the editorial introduction, seven different experts on the issues of Palestine/ Israel have written six essays on BDS for this forum. The six essays represent a wide range of opinions while also echoing some similar themes. 相似文献
90.
Tom R. Robinson Tim R. H. Davies Thomas M. Wilson Caroline Orchiston Nicolas Barth 《Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards》2016,10(2):146-163
Coseismic landsliding presents a major hazard to infrastructure in mountains during large earthquakes. This is particularly true for road networks, as historically coseismic landsliding has resulted in road losses larger than those due to ground shaking. Assessing the exposure of current and planned highway links to coseismic landsliding for future earthquake scenarios is therefore vital for disaster risk reduction. This study presents a method to evaluate the exposure of critical infrastructure to landsliding from scenario earthquakes from an underlying quantitative landslide hazard assessment. The method is applied to a proposed new highway link in South Island, New Zealand, for a scenario Alpine Fault earthquake and compared to the current network. Exposure (the likelihood of a network being affected by one or more landslides) is evaluated from a regional-scale coseismic landslide hazard model and assessed on a relative basis from 0 to 1. The results show that the proposed Haast-Hollyford Highway (HHH) would be highly exposed to coseismic landsliding with at least 30–40?km likely to be badly affected (the Simonin Pass route being the worse affected of the two routes). In the current South Island State Highway network, the HHH would be the link most exposed to landsliding and would increase the total network exposure by 50–70% despite increasing the total road length by just 3%. The present work is intended to provide an effective method to assess coseismic landslide hazard of infrastructure in mountains with seismic hazard, and potentially identify mitigation options and critical network segments. 相似文献