USE OF HISTORIC FLOOD INFORMATION IN ESTIMATING FLOOD PEAKS ON UNGAGED WATERSHEDS1

Regional procedures to estimate flood magnitudes for ungaged watersheds typically ignore available site-specific historic flood information such as high water marks and the corresponding flow estimates, otherwise referred to as limited site-specific historic (LSSH) flood data. A procedure to construct flood frequency curves on the basis of LSSH flood observations is presented. Simple inverse variance weighting is employed to systematically combine flood estimates obtained from the LSSH data base with those from a regional procedure to obtain improved estimtes of flood peaks on the ungaged watershed. For the region studied, the variance weighted estimates of flow had a lower logarithmic standard error than either the regional or the LSSH flow estimates, when compared to the estimates determined by three standard distributions for gaged watersheds investigated in the development of the methodology. Use of the simple inverse variance weighting procedure is recommended when “reliable” estimates of LSSH floods for the ungaged site are available.     

SCALE PROBLEMS IN INTERDISCIPLINARY WATER RESOURCES INVESTIGATION1

ABSTRACT The problems encountered in estimating scale parameters in interdisciplinary watershed research are discussed. Meteorology, hydrology, geology and water quality are discussed with respect to their individual strengths and weaknesses when applied to different scale problems. Areas where incompatibilities exist are pointed out and suggestions are made for effecting compromise. A watershed scale of roughly a hundred acres may be the scale at which all disciplines can effect optimum interaction.     

INSTITUTIONAL INTERACTION IN METROPOLITAN WATER RESOURCE PLANNING1

ABSTRACT Methods of institutional coordination derived from the applied behavioral sciences have been useful in determining the policy planning, and implementation responsibilities that must be shared between local governments, watershed districts, and a regional planning body in the Minneapolis-St. Paul Metropolitan Area. The confines of the traditional behavioral science models of organizations and institutional change processes, and the realities of administrative systems imbedded in political processes at both the local and state levels, have created conflicts between regional planners, watershed district staff and consultants, and municipal administrators. A conceptual framework based on work by Selznick on institutions was applied to two watershed districts, and the results evaluated for other research purposes as well as policy development for the 1973 Minnesota legislative session.     

Management of Sedimentation in Tropical Watersheds

/ The sedimentation of reservoirs is a serious problem throughout the tropics, yet most attempts to control sedimentation in large river basins have not been very successful. Reliable information on erosion rates and sources of sediments has been lacking. In regions where geologically unstable terrain combines with high rainfall, natural erosion rates might be so high that the effects of human activity are limited. Estimates of natural erosion in these situations often have been poor because of the episodic nature of most erosion during large storms and because mass-wasting may supply much of the sediment. The predominance of mass-wasting in some watersheds can result in an unexpectedly high ratio of bedload to suspended load, shifting sedimentation to "live" rather than "dead" storage within reservoirs. Furthermore, the inappropriate use of the Universal Soil Loss Equation to assess the effectiveness of erosion control measures has led to inaccurate estimates of the sediment reduction benefits that could accrue to watershed treatment efforts. Although reducing erosion from cultivated areas is desirable for other reasons, efforts aimed at reducing reservoir sedimentation by controlling agricultural sources of erosion may have limited benefits if the principal sources are of natural origin or are associated with construction of the dams and reservoirs and with rural roads and trails. Finally, the most appropriate locations for watershed rehabilitation depend on the magnitude of temporary storage of colluvium and alluvium within the river basin: Where storage volume is large and residence time of sediment very long, reducing agricultural erosion may have limited impacts on sedimentation within the expected life of a reservoir. Systematic development and analysis of sediment budgets for representative watersheds is needed to address these limitations and thereby improve both the planning of river basin development schemes and the allocation of resources towards reducing sedimentation. When sedimentation of reservoirs is the key issue, sediment budgets must focus especially on channel transport rates and sediment delivery from hillsides. Sediment budgets are especially critical for tropical areas where project funds and technical help are limited. Once sediment budgets are available, watershed managers will be able to direct erosion control programs towards locations where they will be most effective. KEY WORDS: Tropical watersheds; Sedimentation; Reservoirs; Erosion control     

A Review of Water Quality Responses to Air Temperature and Precipitation Changes 2: Nutrients,Algal Blooms,Sediment, Pathogens

In this paper we review the published, scientific literature addressing the response of nutrients, sediment, pathogens, and cyanobacterial blooms to historical and potential future changes in air temperature and precipitation. The goal is to document how different attributes of water quality are sensitive to these drivers, to characterize future risk, to inform management responses, and to identify research needs to fill gaps in our understanding. Results suggest that anticipated future changes present a risk of water quality and ecosystem degradation in many United States locations. Understanding responses is, however, complicated by inherent high spatial and temporal variability, interactions with land use and water management, and dependence on uncertain changes in hydrology in response to future climate. Effects on pollutant loading in different watershed settings generally correlate with projected changes in precipitation and runoff. In all regions, increased heavy precipitation events are likely to drive more episodic pollutant loading to water bodies. The risk of algal blooms could increase due to an expanded seasonal window of warm water temperatures and the potential for episodic increases in nutrient loading. Increased air and water temperatures are also likely to affect the survival of waterborne pathogens. Responding to these challenges requires understanding of vulnerabilities, and management strategies to reduce risk.     

农田非点源污染研究中的降雨径流关系——SCS法的修正

SCS法是当前被广泛应用于非点源污染研究中降雨径流关系的一种方法，但是由于地理环境的差异，直接将该方法应用在上海郊区时误差较大，因此，笔者根据实测资料对模式的2个系数（前期损失量和径流曲线数）予以修正，并提出相应的模型。     

Dissolved Organic Carbon in Upland Forested Watersheds Underlain by Continuous Permafrost in Central Siberia

Hydroclimatic variability and plant species ecology cause mosaics in forested watersheds in permafrost zones. Measurements of organic matter accumulation, stock of dissolved organic carbon (DOC), DOC concentrations in litter leachates, subsurface flow, stream and seasonal and annual export were made in two contrasting slopes and valleys in the northern taiga of Central Siberia. Increased organic carbon accumulation in litter was found in poor hydroclimatic conditions of the north-facing slope and bottom valleys. In contrast, DOC contents and its export to soil were almost two-fold higher in warmer well-drained sites of the south-facing slope. The overall DOC flux to mineral soil from the beginning of June to mid-September was 17 g C m⁻² in the south-facing slope sites and only 9 g C m⁻² in the north-facing slope sites. DOC export was positively correlated with precipitation stimulating leaching of mobilizable organic matter. Accordingly intra-seasonal and inter-annual variability of the DOC fluxes was tightly coupled with water input. Meanwhile DOC export in Sphagnum and feathermoss sites showed different behavior in dry and wet years. The presence of permafrost preventing deep seepage of organic solutes results in higher stream DOC fluxes compared to permafrost-free or island permafrost regions. However, thawing of seasonally frozen soil layer during the growing season led to the decreasing concentrations of DOC in the stream from June to September. For two seasons of continuous stream water sampling (June and August–September), the riverine DOC flux constituted about 14% of DOC entering mineral soil on both slopes. The ratio of hydrologic DOC loss to NPP of larch forests of the region was estimated to be 1.1%.     

IBIS改进及其多尺度流域分布式生态水文模拟

针对集成生物圈模拟器IBIS未考虑高分辨率模拟情况下地表起伏对太阳辐射等参数影响的特点,改进了IBIS的地表太阳辐射计算模块、日气象数据读取模块,并增加了坡度、坡向数据读取模块。在土地利用覆被数据、DEM、坡度、坡向和降水、气温、云量、湿度、风速等气象数据的支持下,利用改进的IBIS分别对泾河及其子流域在0.041 5°和0.001°空间尺度上的生态水文过程进行了模拟,并用二源遥感估算的蒸散发对模拟结果进行了空间比较分析。结果表明,改进的IBIS在日、月、年不同时间尺度和不同空间尺度上均具有很好的模拟适应性与模拟能力,模拟蒸散发与二源遥感估算蒸散发结果比较一致,总体空间分布形态和趋势具有相似特征。     

Septic tank and agricultural non‐point source pollution within a rural watershed

This investigation addresses the problem of Non‐Point Source (NPS) pollution in the rural Lake Weatherford watershed in Parker County, Texas. This reservoir is the primary municipal water supply for the City of Weatherford, Texas. The principal method of wastewater disposal is the on‐site system or septic tanks for the small residential areas surrounding the reservoir.

Sources of NPS pollution of interest in this watershed include agricultural operations as well as the residential areas. These sites were identified with the aid of aerial photography and field investigation. Suspected NPS problems were substantiated through a sampling program involving chemical and biological testing of the reservoir. Results indicate that there is significant NPS pollution contamination of Lake Weatherford from agricultural sources and seepage from on‐site wastewater disposal systems. Excessive fecal coliform and fecal streptococcus counts (>500 bacteria/100 ml) were generally associated with rainfall events and several samples showed values > 100000 bacteria/100 ml. The fecal coliform/fecal streptococcus ratios indicated contamination from human sources, animal sources, and a combination of both. Nutrient concentrations fluctuated from quite low to high with ammonia as the most consistent problem. High ammonia values were also associated with rainfall events.     

Assessment of Tree Rings as a Hydrologic Record in a Humid Subtropical Environment1

Crockett, Kris, Jonathan B. Martin, Henri D. Grissino-Mayer, Evan R. Larson, and Thomas Mirti, 2010. Assessment of Tree Rings as a Hydrologic Record in a Humid Subtropical Environment. Journal of the American Water Resources Association (JAWRA) 1-13. DOI: 10.1111/j.1752-1688.2010.00464.x Abstract: Information about long-term variability of streamflow is important to allocate water resources, but few historical records extend more than 75 years into the past, requiring proxy records to evaluate flow prior to that time. Flow proxies have been found in tree-ring widths in temperate regions, but have rarely been used in humid subtropical environments because the relationship between tree growth and climate was believed to be weakened by limited seasonality and stress on tree growth from drought conditions. Tree-ring residual chronologies from two forests sampled from northern Florida correlate well with historical annual discharge (r² values as high as 0.47) from 3 of 15 river-gauging stations that were used to compare with the tree-ring chronologies. The best correlations occur where streamflow has little contribution from spring discharge or continuous flow from lakes and wetlands. Streams lack correlations with the tree-ring residual chronologies (r² values as low as 0.0002) where springs and continuous discharge from lakes mute variations in their flow. Tree-ring chronologies appear to be useful for reconstruction of prehistorical variations of some streamflow in humid subtropical regions, but interpretations of the reconstructions must consider the local hydrologic conditions.