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991.
Vicente L. Lopes H. Evan Canfield 《Journal of the American Water Resources Association》2004,40(2):311-319
ABSTRACT: This paper evaluates the effects of watershed geometric representation (i.e., plane and channel representation) on runoff and sediment yield simulations in a semiarid rangeland watershed. A process based, spatially distributed runoff erosion model (KINEROS2) was used to explore four spatial representations of a 4.4 ha experimental watershed. The most complex representation included all 96 channel elements identifiable in the field. The least complex representation contained only five channel elements. It was concluded that oversimplified watershed representations greatly influence runoff and sediment yield simulations by inducing excessive infiltration on hillslopes and distorting runoff patterns and sediment fluxes. Runoff and sediment yield decrease systematically with decreasing complexity in watershed representation. However, less complex representations had less impact on runoff and sediment‐yield simulations for small rainfall events. This study concludes that the selection of the appropriate level of watershed representation can have important theoretical and practical implications on runoff and sediment yield modeling in semiarid environments. 相似文献
992.
Brent J. Dalzell Prasanna H. Gowda David J. Mulla 《Journal of the American Water Resources Association》2004,40(2):533-543
ABSTRACT: This paper studies the effectiveness of alternative farm management strategies at improving water quality to meet Total Maximum Daily Loads (TMDLs) in agricultural watersheds. A spatial process model was calibrated using monthly flow, sediment, and phosphorus (P) losses (1994 to 1996) from Sand Creek watershed in south‐central Minnesota. Statistical evaluation of predicted and observed data gave r2 coefficients of 0.75, 0.69, and 0.49 for flow (average 4.1 m3/s), sediment load (average 0.44 ton/ha), and phosphorus load (average 0.97 kg/ha), respectively. The calibrated model was used to evaluate the effects of conservation tillage, conversion of crop land to pasture, and changes in phosphorus fertilizer application rate on pollutant loads. TMDLs were developed for sediment and P losses based on existing water quality standards and guidelines. Observed annual sediment and P losses exceeded these TMDLs by 59 percent and 83 percent, respectively. A combination of increased conservation tillage, reduced application rates of phosphorus fertilizer, and conversion of crop land to pasture could reduce sediment and phosphorus loads by 23 percent and 20 percent of existing loads, respectively. These reductions are much less than needed to meet TMDLs, suggesting that control of sediment using buffer strips and control of point sources of phosphorus are needed for the remaining reductions. 相似文献
993.
E. Conrad Lamon Song S. Qian Daniel D. Richter 《Journal of the American Water Resources Association》2004,40(5):1219-1229
ABSTRACT: Dynamic linear models (DLM) and seasonal trend decomposition (STL) using local regression, or LOESS, were used to analyze the 50‐year time series of suspended sediment concentrations for the Yadkin River, measured at the U.S. Geological Survey station at Yadkin College, North Carolina. A DLM with constant trend, seasonality, and a log10 streamflow regressor provided the best model to predict monthly mean log10 suspended sediment concentrations, based on the forecast log likelihood. Using DLM, there was evidence (odds approximately 69:1) that the log10 streamflow versus log10 suspended sediment concentration relationship has changed, with an approximate 20 percent increase in the log10 streamflow coefficient over the period 1981 to 1996. However, sediment concentrations in the Yadkin River have decreased during the decade of the 1990s, which has been accompanied by a concomitant increase in streamflow variability. Although STL has been shown to be a versatile trend analysis technique, DLM is shown to be more suitable for discovery and inference of structural changes (trends) in the model coefficient describing the relationship between flow and sediment concentration. 相似文献
994.
Marji J. Patz Katta J. Ready Quentin D. Skinner 《Journal of the American Water Resources Association》2004,40(5):1247-1255
ABSTRACT: The objective of this study was to examine the chemistry of Coalbed Methane (CBM) discharge water reacting with semi‐arid ephemeral stream channels in the Powder River Basin, Wyoming. The study area consisted of two ephemeral streams: Burger Draw and Sue Draw. These streams are tributaries to the perennial Powder River. Samples were collected bimonthly from three CBM discharge points and seven channel locations in Burger Draw and Sue Draw. Samples were also collected bimonthly from the Powder River above and below the confluence of Burger Draw. Before sample collection, the pH and electrical conductivity (EC) were measured in the field. Samples were transported to the laboratory and analyzed for alkalinity, major cations, and anions. From the measurement of sodium (Na), calcium (Ca), and magnesium (Mg), practical sodium adsorption ratio (SARp) and true sodium adsorption ratio (SARt) were calculated. Results suggest pH and EC of CBM discharge water was 7.1 and 4.3 dS/m, respectively. The CBM discharge water consisted of higher concentrations of sodium and alkalinity compared to other components. The pH of CBM discharge water increased significantly (p = 0.000) in the downstream channel of Burger Draw from 7.1 to 8.84 before it joined with the Powder River. Dissolved calcium concentration of CBM discharge water decreased significantly (p = 0.000) in the downstream channel water. Subsequently, SARp increased approximately from 24 to 29. The SARt also increased significantly (p = 0.001) in the downstream channel water. For instance, SARt of CBM discharge water increased from 32.93 to 45.5 downstream channels after the confluence of Sue Draw with the Burger Draw. The only significant difference in water chemistry above and below the confluence of Burger Draw with the Powder River was pH, which increased from 8.36 to 8.52. The significant increase in SAR values of CBM discharge water in Burger Draw and Sue Draw tributaries suggest a careful monitoring of salinity and sodicity is needed if CBM discharge water is used for irrigation in semi‐arid environments. Results discussed in this study will be useful to downstream water users who depend on water for irrigation. 相似文献
995.
Ryan C. Schwartz Peter J. Deadman Daniel J. Scott Linda D. Mortsch 《Journal of the American Water Resources Association》2004,40(3):647-662
ABSTRACT: Recent research that couples climate change scenarios based on general circulation models (GCM) with Great Lakes hydrologic models has indicated that average water levels are projected to decline in the future. This paper outlines a methodology to assess the potential impact of declining water levels on Great Lakes waterfront communities, using the Lake Huron shoreline at Goderich, Ontario, as an example. The methodology utilizes a geographic information system (GIS) to combine topographic and bathymetric datasets. A digital elevation surface is used to model projected shoreline change for 2050 using water level scenarios. An arbitrary scenario, based on a 1 m decline from February 2001 lake levels, is also modeled. By creating a series of shoreline scenarios, a range of impact and cost scenarios are generated for the Goderich Harbor and adjacent marinas. Additional harbor and marina dredging could cost as much as CDN $7.6 million. Lake freighters may experience a 30 percent loss in vessel capacity. The methodology is used to provide initial estimates of the potential impacts of climate change that can be readily updated as more robust climate change scenarios become available and is adaptable for use in other Great Lakes coastal communities. 相似文献
996.
997.
We draw on the economic, corporate governance, and family business literatures to explain why the effects of family on family firms makes this governance form theoretically distinct from those of public and private non‐family firms. Our thesis is that parental altruism, when combined with private ownership and owner‐management, influences the ability of the firm's owner‐manager to exercise self‐control, which, in turn, can expose some family firms to conflicts rooted in the agency threats of moral hazard, hold‐up, and adverse selection. We then discuss why some other family firms are able to minimize these dark side threats and thereby attain altruism's brighter side. Finally, we discuss how altruism's influence changes over time as ownership becomes dispersed among family members and across generations. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
998.
B. Y. Aminuddin M. H. Ghulam W. Y. Wan Abdullah M. Zulkefli R. B. Salama 《Water, Air, & Soil Pollution: Focus》2005,5(1-2):89-101
Cameron Highlands is a mountainous region with steep slopes. Gradients exceeding 20∘ are common. The climate is favourable to the cultivation of tea, sub-tropical vegetables and flowers (under rain-shelter).
Crop production is sustained by high fertiliser and manure applications. However, agriculture in this environment is characterised
by high levels of soil erosion and environmental pollution. A study on the sustainability of these agro-ecosystems was conducted.
Results indicated that soil loss was in the range of 24–42 ton/ha/yr under vegetables and 1.3 ton under rain-shelter. Sediment
load in the vegetable sub-catchment reached 3.5 g/L, 50 times higher than that associated with flowers under rain-shelter
and tea. The sediments contained high nutrient loads of up to 470 kg N/ha/yr. The N, P and K lost in runoff from cabbage farms
was 154 kg/season/ha, whereas in chrysanthemum farms it was 5 kg. In cabbage farms, the N, P, and K lost through leaching
was 193 kg/season/ha. The NO3–N concentration in the runoff from the cabbage farms reached 25 ppm but less than 10 ppm in runoff from rain-shelters. Inorganic
pollution in the rivers was within the acceptable limit of 10 ppm. The sustainability of the agro-ecosystems is in the order
of tea { > } rain–shelter ≫ vegetables. 相似文献
999.
P. Hill C. Marshall H. Harmens D. L. Jones J. Farrar 《Water, Air, & Soil Pollution: Focus》2005,4(6):177-186
Soil respiration is a large C flux which is of primary importance in determining C sequestration. Here we ask how it is altered by atmospheric CO2 concentration and N additions. Swards of Lolium perenne L. were grown in a Eutric cambisol under controlled conditions with and without the addition of 200 kg NO? 3 ?N ha?1, at either 350 ppm or 700 ppm CO2, for 3 months. Soil respiration and net canopy photosynthesis were both increased by added N and elevated CO2, but soil respiration increased proportionately less than fixation by photosynthesis. Thus, both elevated CO2 and N appeared to increase potential C sequestration, although adding N at elevated CO2 reduced the C sequestered as a proportion of that fixed relative to elevated CO2 alone. Across all treatments below-ground respiratory C losses were predicted by root biomass, but not by soil solution C and N concentrations. Specific root-dependent respiration was increased by elevated CO2, such that below-ground respiration per unit biomass and per unit plant N was increased. 相似文献
1000.
L. J. Sheppard A. Crossley I. D. Leith K. J. Hargreaves J. A. Carfrae N. van Dijk J. N. Cape D. Sleep D. Fowler J. A. Raven 《Water, Air, & Soil Pollution: Focus》2005,4(6):197-205
Critical N loads for ombrotrophic bogs, which often contain rare and N-sensitive plants (especially those in lower plant groups: lichens, mosses and liverworts), are based on very few experimental data from measured, low background N deposition areas. Additionally the relative effects of reduced versus oxidised N are largely unknown. This paper describes an automated field exposure system (30 km S. of Edinburgh, Scotland) for treating ombrotrophic bog vegetation with fine droplets of oxidised N (NaNO3) and reduced N (NH4Cl). Whim Moss exists in an area of low ambient N deposition (ca. 8 kg N ha?1 y?1), the sources and quantification of which are described. The wet N treatment system is run continuously, and is controlled/activated by wind speed and rainfall to provide a unique simulation of “real worl” treatment patterns (no rain=no treatment). Simulated precipitation is supplied at ionic concentrations below 4 mM in rainwater collected on site. Treatments provide a replicated dose response to 16, 32 and 64 kg N ha?1 y?1 adjusted for ambient deposition (8 kg N ha?1 y?1). The 16 and 64 kg N ha?1 y?1 are duplicated with a P+K supplement. Baseline soil chemistry and foliar nutrient status was established for all 44 plots for Calluna vulgaris, Sphagnum capillifolium, Hypnum jutlandicum and Cladonia portentosa. 相似文献