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《制图学和地理信息科学》2013,40(2):161-164
Fragmentary river segments have to be reconnected before addressing various routing and tracking problems. Elevation determines drainage directions, so the partial heights available through LiDAR may provide useful hints on how the segments should be joined. However, it is not trivial how this information can be applied. This paper bridges this gap by proposing the induced structure approach, which first approximates a terrain compatible with those observations, and then derives a river network from that induced terrain. Since the network is derived from an induced terrain that honors the partial observations, we expect that the derived river network will enforce most restrictions imposed by the partial observations. This paper also provides specifics on the implementation. In the first step regarding terrain reconstruction, we find that the optimal scheme depends on the height sample distribution. If the samples are sparsely yet evenly distributed, natural neighbor interpolation with stream burning (NN-SB) is the most cost-effective. If the samples are offered only at the given river locations, the hydrology-aware version of Over determined Palladian Partial Differential Equation (HA-ODETLAP) should be used instead. In the second step concerning river derivation, we find it necessary to favor those given river locations. Otherwise they will be missed out. We set their respective initial water amounts to the critical accumulation level to ensure a river flows across them. In the subsequent branch thinning process, those locations are protected from being trimmed. We foresee applications of our solution framework in a few 2D and 3D network tracing problems with similar observation distribution, like dendrite network reconstruction. 相似文献
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《African Journal of Marine Science》2013,35(1):241-264
Bayesian methods are useful in fisheries stock assessment because they provide a conceptually elegant and statistically rigorous approach to making decisions under uncertainty. The application of Bayesian stock assessment methods in the management of Namibian orange roughy Hoplosthethus atlanticus within the 200 mile EEZ of Namibia is reviewed. Time-series of relative abundance are short and their reliability in indicating abundance trends is uncertain. The development of informative prior probability density functions (pdfs) for the constants of proportionality (q) for hydro-acoustic, commercial trawl swept area, and research trawl swept area indices produced statistically consistent prior estimates of absolute abundance for each of the three grounds where more than one index of abundance was available. The posterior pdfs for stock assessment model parameters were used to account for uncertainty in evaluations of the potential consequences of alternative harvesting policies under a stock reduction model in which catch removals were assumed to account for any declines. It appears that all orange roughy stocks off Namibia have been depleted below the limit reference point (50% of long-term unfished biomass). However, the stock reduction model could not easily account for the large declines in indices on the four fishing grounds over the period from 1995 until 1999 when the informative priors for q were applied. In the 2000 stock assessment, the Bayesian procedure was updated to account formally for uncertainty in model structures that could explain the decline in abundance. The possibility of very low stock abundance could still not be discounted when these uncertainties were accounted for. Although this most recent methodology applies more statistical rigour, its complexity has hindered its acceptance in Namibia. However, if it is worth quantifying risks and uncertainties in future stock assessments for the provision of precautionary management advice, it is proposed that the assessment protocols adopted be probabilistic to account for uncertainty in model parameters, that careful attention be given to subjective judgements about their inputs and the representation of uncertainty within them, and that, where appropriate, alternative hypotheses about stock abundance and mechanisms for catchability and stock decline be taken into account. 相似文献
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全球气候变化对贵州省径流模数的潜在影响 总被引：1，自引：0，他引：1
揭示全球气候变化对贵州省径流模数潜在影响可为该区优化配置水资源、确定水土保持治理的重点区域以及减少由降雨季节性分布不均引发旱涝灾害提供依据。该文以贵州省22个主要气象和水文站的降雨和径流等资料为基础,建立降雨和径流之间的统计关系。采用DELTA方法,根据HadCM3模型对A2情景(人口快速增长、经济发展缓慢)和B2情景(强调社会技术创新)下输出的不同时期的降雨量以及实测的贵州省降雨量和径流模数资料,推算出贵州省2006-2035年、2036-2065年和2066-2095年前后两个时期之间径流模数的增减量;研究结果为:A2和B2情景下未来3个时期径流模数逐渐增大,相对于基准期1961-1990年最大增幅分别达17.52%和10.58%。不同情景和不同时期径流模数变化的空间分布差异较大,A2情景下径流模数的变化比B2情景下更为剧烈。在贵州省石漠化严重的地区,当径流增加较多时,不仅需要考虑水资源的充分合理利用,还需兼顾防止水土流失的加剧。 相似文献
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《African Journal of Marine Science》2013,35(1):181-203
Orange roughy Hoplostethus atlanticus are unusual fish. They form dense aggregations that have fuelled lucrative fisheries at great depths (600–1 400 m), especially off Namibia, New Zealand and Australia. They are thought to be very long-lived (>100 years, maturity at 22 – 40 years), and to have exceptionally low natural mortality (M = 0.045–0.064 year?1) and slow growth rates (K = 0.055–0.070 year?1). In addition, they spawn large eggs and have low fecundity. These factors combine to make orange roughy highly susceptible to overfishing; most stocks are below 30% of pristine levels. Assessments are obtained from indices of catch rate and trawl, acoustic and egg surveys. Acoustic estimates are the most direct, but are confounded by the species' low target strength (?50 to ?53 dB)–attributable to the wax-filled swim bladder. Extracellular wax esters are stored in abundance and comprise mostly mono-unsaturated fatty acids, with low concentrations of the ω-3 fatty acid family. This unusual composition (resultant from the species' diet) ensures neutral buoyancy. Stock separation has been inferred mainly from biological studies, but genetic studies have also found differences among stocks within New Zealand and Australia. Deep-water habitat may be damaged by trawling operations and may take many years to recover, so in some quarters there is a call for a portion of suitable habitat to be set aside for preservation. Although Namibian orange roughy are shallower, smaller and younger than those in other stocks, the Namibian fishery sustained high catches for only a few years before quotas were reduced, from 12 000 to 1 875 tons. Three management lessons are suggested for developing orange roughy fisheries based on the Namibian experience: (1) imposition of catch limits during exploratory fishing; (2) starting the acoustic surveys earlier in the fishery, if possible; (3) greater reliance on trends in catch rate until a survey series has been established. 相似文献
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《African Journal of Marine Science》2013,35(1):223-240
Orange roughy form dense spawning aggregations in specific small areas in deep water on the Namibian shelf between late June and early August each year. The biomass in three such areas, where most commercial fishing occurs (the Johnies, Frankies and Rix Quota Management Areas, or QMAs) has been assessed acoustically each year since 1997. Acoustic estimates of the aggregated portion of the biomass (the only component that can be assessed reliably using acoustics) were obtained for all three QMAs in 1997 and 1998, but only for Frankies in 1999 owing to increased problems with target identification as the biomass declined. The methodology developed for these surveys, including the equipment used, survey design, target identification, data processing and error analysis are described. Some important biases that should be corrected for when estimating absolute abundance of orange roughy acoustically are addressed. Individual sources of error were quantified as well as possible, and input to an error model that simulated the error process and produced probability density functions of absolute biomass, from which the mean absolute biomass and its standard error could be computed for each survey, effectively correcting for identified sources of bias and quantifying the overall uncertainty. The correction factors ranged from 1.58 to 1.71 and the CVs increased by factors of 1.2–2.1. Target strength uncertainty and negative bias attributable to the dead zone close to the bottom were considered to be the most serious errors. The acoustic estimates indicate a substantial decline in orange roughy biomass in all three QMAs since 1997, in accord with indices from contemporaneous swept-area surveys and the catch rate of the commercial fleet. Acoustic estimates have already been used extensively to manage the resource and are likely to remain important in the future. 相似文献
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《African Journal of Marine Science》2013,35(1):449-464
The results of the first seasonal survey of the fish of the Kavango River flood-plain along the Angola/Namibia border are reported. The river experiences peak flooding from February through June, with the 375-km long floodplain extending up to 5 km across. The floodplain was sampled five times in 1992 by seine, fish traps and rotenone. The data indicated a pronounced structural and functional response of the fish community in relation to the alternating flood and drought conditions in the river. Catch per unit effort and diversity were highest during months of peak flooding (May and June), and lowest during the month of least flow (November). The reproductive strategies of K-selected piscivorous cichlids and tigerfish were in advance of flooding. Many r-selected invertivores, especially cyprinids, were in relative synchrony with flooding and the stimulation of littoral zone plant growth, while other invertivores lagged the cyprinids. Herbivores had lowest relative abundance during peak flooding; this seemingly inverse relationship with the invertivores should not be interpreted as replacement, but rather the swamping of the system with young-of-the-year r-selected invertivores. The data support the Flood Pulse Concept, which hypothesizes that flooding is the major "driver" of productivity in lowland or floodplain rivers. 相似文献
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《African Journal of Marine Science》2013,35(1):205-221
Exploration for orange roughy Hoplostethus atlanticus in Namibia started in 1994 and within 12 months several aggregations had been discovered, suggesting the existence of a biomass sufficient to support a viable fishery. At that early stage it was realized that few, if any, recognized management procedures existed for newly developing fisheries, especially with the paucity of data such as existed on Namibian orange roughy. The development of the Namibian orange roughy fishery is reviewed to document the management strategies implemented and how the management of the fishery evolved. The first six years of the fishery are described, including the three-year exploration phase, several years of profitable exploitation, and the severe decline in catch rates. Whether the decline is attributable to fishing mortality or to change in the aggregating behaviour of orange roughy, or both, is not clear. Although many aspects of the precautionary approach were followed, a risk analysis applied and a number of innovative management methods implemented (e.g. incentives to promote exploratory fishing, use of Bayesian statistical methods, implementation of a management plan for long-term total allowable catches), the aggregating biomass declined to between 10 and 50% of virgin levels within the six years. The management methods applied are evaluated in the light of the severe decline in catch rate experienced in 1998 and 1999, so that others may learn from the experience. 相似文献
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《African Journal of Marine Science》2013,35(3-4):511-524
The textural and compositional characteristics of the surficial shelf sediments north and south of the Orange River Delta are reviewed and compared. Sediments are fractionated and dispersed both north- and southwards of the Orange River mouth by wave action, longshore drift and subsurface currents. The mean grain size decreases both offshore and southwards in response to decreased wave influence at the seabed and the competence of the weak poleward undercurrent respectively. The increasing dominance of marine biogenic components in sediments south of the prodelta indicates a greater marine influence, modifying previous inferences that the Namaqualand mudbelt is primarily derived from the southward transport of Orange River sediments. A sharp distinction can be drawn between sediments of the Orange Shelf to the south and the Walvis Shelf to the north. Foraminiferarich deposits that dominate the Orange middle shelf and slope indicate that upwelling is an inner-shelf phenomenon. On the Walvis Shelf, foraminiferal sediments are confined to the slope and outer shelf. Fish debris is more common in Walvis Shelf sediments. Although phosphorite and glauconite sands often occur together in the same deposits on the Orange Shelf, the two minerals are concentrated in separate deposits on the Walvis Shelf. 相似文献
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