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551.
Dilip Mathur Eric S. McClellan Susan A. Haney 《Journal of the American Water Resources Association》1988,24(1):159-167
ABSTRACT: The effects of variable discharges during the summer on the dissolved oxygen (DO) content and water temperature upstream and downstream of the Conowingo Hydroelectric Power Station were investigated. The DO dynamics are controlled primarily by meteorological factors that are independent of the mode of hydrostation operation. DO stratification occurred during the summer in Conowingo Pond, but thermal stratification was not observed. The magnitude and duration of off-peak discharges including a run-of-the-river operation did not affect DO stratification in Conowingo Pond; little vertical mixing occurred. However, strong winds and/or high river flows temporarily destroyed DO stratification. The run-of-the-river operation or off-peak continuous discharge schemes did not provide better DO conditions downstream of the hydrostation than the peaking operation with intermittent off-peak releases. A statistical model predicted that a DO of 5 ppm occurs 0.6 miles downstream of the powerhouse when the natural river flow is consistently greater than 15,000 cfs and water temperature is less than 80°F. A mean daily DO of at least 4 ppm was predicted to occur over 80 percent of the time during the 92-day summer period. Farther downstream (1.3 miles from the powerhouse) a mean daily DO of at least 4 ppm was predicted to occur 90 percent of the time in summer. 相似文献
552.
Hongjie Xie Xiaobing Zhou Jan M.H. Hendrickx Enrique R. Vivoni Huade Guan Yong Q. Tian Eric E. Small 《Journal of the American Water Resources Association》2006,42(1):237-256
This study examines NEXRAD Stage III product (hourly, cell size 4 km by 4 km) for its ability in estimating precipitation in central New Mexico, a semiarid area. A comparison between Stage III and a network of gauge precipitation estimates during 1995 to 2001 indicates that Stage III (1) overestimates the hourly conditional mean (CM) precipitation by 33 percent in the monsoon season and 55 percent in the nonmonsoon season; (2) overestimates the hourly CM precipitation for concurrent radar‐gauge pairs (nonzero value) by 13 percent in the monsoon season and 6 percent in the nonmonsoon season; (3) overestimates the seasonal precipitation accumulation by 11 to 88 percent in monsoon season and underestimates by 18 to 89 percent in the nonmonsoon season; and (4) either overestimates annual precipitation accumulation up to 28.2 percent or underestimates it up to 11.9 percent. A truncation of 57 to 72 percent of the total rainfall hours is observed in the Stage III data in the nonmonsoon season, which may be the main cause for both the underestimation of the radar rainfall accumulation and the lower conditional probability of radar rainfall detection in the nonmonsoon season. The study results indicate that the truncation caused loss of small rainfall amounts (events) is not effectively corrected by the real‐time rain gauge calibration that can adjust the rainfall rates but cannot recover the truncated small rainfall events. However, the truncation error in the monsoon season may be suppressed due to the larger rainfall rate and/or combined effect of overestimates by bright band and hail contaminations, virga, advection, etc. In general, improvement in NEXRAD performance since the monsoon season in 1998 is observed, which is consistent with the systematic improvement in the NEXRAD network. 相似文献
553.
Eric W. Larsen Alexander K. Premier Steven E. Greco 《Journal of the American Water Resources Association》2006,42(4):1077-1097
ABSTRACT: Bank erosion along a river channel determines the pattern of channel migration. Lateral channel migration in large alluvial rivers creates new floodplain land that is essential for riparian vegetation to get established. Migration also erodes existing riparian, agricultural, and urban lands, sometimes damaging human infrastructure (e.g., scouring bridge foundations and endangering pumping facilities) in the process. Understanding what controls the rate of bank erosion and associated point bar deposition is necessary to manage large alluvial rivers effectively. In this study, bank erosion was proportionally related to the magnitude of stream power. Linear regressions were used to correlate the cumulative stream power, above a lower flow threshold, with rates of bank erosion at 13 sites on the middle Sacramento River in California. Two forms of data were used: aerial photography and field data. Each analysis showed that bank erosion and cumulative effective stream power were significantly correlated and that a lower flow threshold improves the statistical relationship in this system. These correlations demonstrate that land managers and others can relate rates of bank erosion to the daily flow rates of a river. Such relationships can provide information concerning ecological restoration of floodplains related to channel migration rates as well as planning that requires knowledge of the relationship between flow rates and bank erosion rates. 相似文献
554.
Juliana Jaramillo Eric G. Chapman Fernando E. Vega James D. Harwood 《Die Naturwissenschaften》2010,97(3):291-298
The coffee berry borer, Hypothenemus hampei, is the most important pest of coffee throughout the world, causing losses estimated at US $500 million/year. The thrips Karnyothrips flavipes was observed for the first time feeding on immature stages of H. hampei in April 2008 from samples collected in the Kisii area of Western Kenya. Since the trophic interactions between H. hampei and K. flavipes are carried out entirely within the coffee berry, and because thrips feed by liquid ingestion, we used molecular gut-content
analysis to confirm the potential role of K. flavipes as a predator of H. hampei in an organic coffee production system. Species-specific COI primers designed for H. hampei were shown to have a high degree of specificity for H. hampei DNA and did not produce any PCR product from DNA templates of the other insects associated with the coffee agroecosystems.
In total, 3,327 K. flavipes emerged from 17,792 H. hampei-infested berries collected from the field between April and September 2008. Throughout the season, 8.3% of K. flavipes tested positive for H. hampei DNA, although at times this figure approached 50%. Prey availability was significantly correlated with prey consumption,
thus indicating the potential impact on H. hampei populations. 相似文献
555.
Hu Li Jianjun Qiu Ligang Wang Huajun Tang Changsheng Li Eric Van Ranst 《Agriculture, ecosystems & environment》2010,135(1-2):24-33
Agricultural production plays an important role in affecting atmospheric greenhouse gas concentrations. Field measurements were conducted in Quzhou County, Hebei Province in the North China Plains to quantify carbon dioxide (CO2) and nitrous oxide (N2O) emissions from a winter wheat–maize rotation field, a common cropping system across the Chinese agricultural regions. The observed flux data in conjunction with the local climate, soil and management information were utilized to test a process-based model, Denitrification–Decomposition or DNDC, for its applicability for the cropping system. The validated DNDC was then used for predicting impacts of three management alternatives (i.e., no-till, increased crop residue incorporation and reduced fertilizer application rate) on CO2 and N2O emissions from the target field. Results from the simulations indicated that (1) CO2 emissions were significantly affected by temperature, initial SOC, tillage method, and quantity and quality of the organic matter added in the soils; (2) increases in temperature, initial SOC, total fertilizer N input, and manure amendment substantially increased N2O emissions; and (3) temperature, initial SOC, tillage, and quantity and quality of the organic matter added in the soil all had significant effects on global warming. Finally, five 50-year scenarios were simulated with DNDC to predict their long-term impacts on crop yield, soil C dynamics, nitrate leaching losses, and N2O emissions. The modelled results suggested that implementation of manure amendment or crop residue incorporation instead of increased fertilizer application rates would more efficiently mitigate GHG emissions from the tested agro-ecosystem. The multi-impacts provided a sound basis for comprehensive assessments on the management alternatives. 相似文献
556.
Eric S. Marland Kirk Stellar Gregg H. Marland 《Mitigation and Adaptation Strategies for Global Change》2010,15(1):71-91
With an evolving political environment of commitments to limit emissions of greenhouse gases, and of markets to trade in emissions
permits, there is growing scientific, political, and economic need to accurately evaluate carbon (C) stocks and flows—especially
those related to human activities. One component of the global carbon cycle that has been contentious is the stock of carbon
that is physically held in harvested wood products. The carbon stored in wood products has been sometimes overlooked, but
the amount of carbon contained in wood products is not trivial, it is increasing with time, and it is significant to some
Parties. This paper is concerned with accurate treatment of harvested wood products in inventories of CO2 emissions to the atmosphere. The methodologies outlined demonstrate a flexible way to expand current methods beyond the assumption
of a simple, first-order decay to include the use of more accurate and detailed data while retaining the simplicity of simple
formulas. The paper demonstrates that a more accurate representation of decay time can have significant economic implications
in a system where emissions are taxed or emissions permits are traded. The method can be easily applied using only data on
annual production of wood products and two parameters to characterize their expected lifetime. These methods are not specific
to wood products but can be applied to long-lived, carbon-containing products from sources other than wood, e.g. long-lived
petrochemical products. A single unifying approach that is both simple and flexible has the potential to be both more accurate
in its results, more efficient in its implementation, and economically important to some Parties. 相似文献
557.
558.
Ann Verdoodt Stephen M. Mureithi Liming Ye Eric Van Ranst 《Agriculture, ecosystems & environment》2009,129(1-3):332-339
The establishment of enclosures has become an important measure to combat land degradation in many of the world's semi-arid rangelands. In view of the increased pressure exerted by this land reclamation strategy on the neighbouring agricultural lands, knowledge of the time required for restoring vegetation cover and soil health, and of the potential positive impact of an adapted management strategy, is highly required. This paper assesses the vegetation and soil rehabilitation in a 23-year chronosequence of two different enclosure management types. In the severely degraded, semi-arid Njemps Flats plain of the Lake Baringo Basin in Kenya communal enclosures characterised by high quality inputs and strict control, and private enclosures managed by individual farmers, were installed since the 1980s. Six communal enclosures (3–17 years since establishment) and six private enclosures (13–23 years since establishment) were selected. Vegetation cover was estimated along three 50 m transects set within each enclosure and in the adjacent open grazing area using the point-to-line transect method. Five 0.5 m2 quadrats systematically placed alongside each transect were sampled for herbaceous standing biomass and topsoil physical, chemical and biological analyses. Grass cover and herbaceous biomass production proved to be the most responsive biotic parameters under both management types, whereas the recovery of the forbs was unsuccessful. Under communal management, the biomass production fully recovered up to its optimal level as recorded in the neighbouring nature reserves. Within private enclosures however, the adopted management strategies seriously restricted biomass production to a significantly lower level. Soil quality generally recovered more slowly with time. Significant improvements compared to the open rangeland were recorded in topsoil bulk density, organic C and total N stocks, and microbial biomass C and N stocks of the communal enclosures. Unlike the communal enclosures, only topsoil bulk density and the microbial biomass C stock showed a significant difference in the private enclosures. With respect to C and total N stocks, and the microbial biomass N a non-significant improving trend was recorded. The level of chemical and biological soil quality obtained under both management types is still low and draws the attention to the importance of careful monitoring of grazing and grass cutting activities under both enclosure management strategies. The chronosequences further highlight the potential of some well-managed private enclosures, whereas intrinsic soil properties such as high alkalinity, as well as changes in management, limit the rehabilitation of some other private as well as communal enclosures. 相似文献
559.
On flows in simulated urban canopies 总被引:1,自引:0,他引:1
560.