Discontinuous headwater stream networks with stable flowheads,Salmon River basin,Idaho

Headwater streams expand, contract, and disconnect in response to seasonal moisture conditions or those related to individual precipitation events. The fluctuation of the surface flow extent, or active drainage network, reflects catchment storage characteristics and has important impacts on stream ecology; however, the hydrological mechanisms that drive this phenomenon are still uncertain. Here, we present field surveys of the active drainage networks of four headwater streams in Central Idaho's Frank Church‐River of No Return Wilderness (7–21 km²) spanning the spring and summer months of 2014. We report the total length of the active drainage networks, which varied as a power law function with stream discharge with an average exponent of 0.11 ± 0.03 (range of 0.05–0.20). Generally, these active drainage networks were less responsive to changes in discharge than many streams in past studies. We observed that the locations where surface flow originates, or flowheads, were often stable, and an average of 64% of the change in active drainage network length was explained by downstream discontinuities. Analysis of geologic and geomorphic characteristics of individual watersheds and flowheads suggests that most flowheads below approximately 2200 m are supported by stable flowpaths controlled by bedrock structure. At higher elevations, small accumulation areas and saturation of shallow and conductive soil and colluvium after snowmelt result in more mobile flowhead locations. The dynamics of active drainage networks can help illuminate the spatiotemporal structure of flowpaths supporting surface flow. Copyright © 2016 John Wiley & Sons, Ltd.     

The assumption of uniform specific discharge: unsafe at any time?

Nearby catchments in the same landscape are often assumed to have similar specific discharge (runoff per unit catchment area). Five years of streamflow from 14 nested catchments in a 68 km² landscape was used to test this assumption, with the hypothesis that the spatial variability in specific discharge is smaller than the uncertainties in the measurement. The median spatial variability of specific discharge, defined as subcatchment deviation from the catchment outlet, was 33% at the daily scale. This declined to 24% at a monthly scale and 19% at an annual scale. These specific discharge differences are on the same order of magnitude as predicted for major land‐use conversions or a century of climate change. Spatial variability remained when considering uncertainties in specific discharge, and systematic seasonal patterns in specific discharge variation further provide confidence that these differences are more than just errors in the analysis of catchment area, rainfall variability or gauging. Assuming similar specific discharge in nearby catchments can thus lead to spurious conclusions about the effects of disturbance on hydrological and biogeochemical processes. Copyright © 2016 John Wiley & Sons, Ltd.     

RESPONSE OF SEDIMENT DISCHARGE TO ECOHYDROLOGICAL FACTORS IN LUERGOU CATCHMENT BASED ON WAVELET ANALYSIS

The response of sediment discharge rate to the following four ecohydrological factors: temperature, rainfall, evapotranspiration (ET), and stream flow was evaluated by conducting wavelet analysis on Luergou small catchment data ranging from 1982 to 2000. For sediment discharge rate, there was an overall trend of reduction that included a periodic oscillation of 6 to 7 years per cycle. Rainfall also had an overall trend of reduction that included two periodic oscillations of 7 years per cycle and 2 years per cycle, respectively. Stream flow had the same trend as rainfall but had one periodic oscillation of 6 to 7 years per cycle. In contrast with rainfall and stream flow, the trends for temperature and ET each showed an overall increasing tendency, and both had the same two periodic oscillations of 6 to 7 years per cycle and 4 years per cycle, respectively. The sediment discharge rate had significant relationships with the four ecohydrological factors, with stream flow and rainfall having positive correlations, while ET and temperature had negative correlations. The correlation between ET and sediment discharge rate became stronger when ET was compared to the sediment discharge rate of the following year. The relationship between sediment discharge rate and the four ecohydrological factors was further expressed by the multi-linear regression model that was constructed, which makes sediment discharge rate a function of stream flow, rainfall, ET, and temperature.     

Combining a rainfall–runoff model and a regionalization approach for flood and water resource assessment in the western Po Valley,Italy

ABSTRACT

Selecting the best structure and parameterization of rainfall–runoff models is not straightforward and depends on a broad number of factors. In this study, the “Modello Idrologico Semi-Distribuito in continuo” (MISDc) was tested on 63 mountainous catchments in the western Po Valley (Italy) and the optimal model parameters were regionalized using different strategies. The model performance was evaluated through several indexes analysing hydrological regime, high-flow condition and flow–duration curve (FDC). In general, MISDc provides a good fit behaviour with a Kling-Gupta Efficiency index greater than 0.5 for 100% and 84% of cases for calibration and validation, respectively. Concerning the regionalization, spatial proximity approach is the most accurate solution obtaining satisfactory performance. Lastly, the predicted FDCs showed an excellent similarity with the observed ones. Results encourage to apply MISDc over the study area for flood forecasting and for assessing water resources availability thanks to the modest computational efforts and data requirements.     

一种通过地形指数提取流域自由水蓄水容量空间分布的方法

对于分布式水文模型而言,如何获得参数的空间分布是模型应用的重点和难点问题.本文将分水源参数中的敏感参数——自由水蓄水容量为研究对象.建立地形指数与自由水蓄水容量的函数关系,以此提取流域内的自由水蓄水容量空间分布.最后利用本方法提取了陕西省陈河流域的自由水蓄水容量空间分布,并将之作为栅格型新安江模型的参数进行洪水模拟演算.应用结果表明本文提出的方法得到了理想的模拟结果.该方法以物理规律为基础能较为准确地计算出流域内自由水蓄水容量的空间分布,为分布式模型的发展奠定了坚实的基础.     

Influences of forest canopy on snowpack accumulation and isotope ratios

The stable water isotopes, ²H and ¹⁸O, can be useful environmental tracers for quantifying snow contributions to streams and aquifers, but characterizing the isotopic signatures of bulk snowpacks is challenging because they can be highly variable across the catchment landscape. In this study, we investigate one major source of isotopic heterogeneity in snowpacks: the influence of canopy cover. We measured amounts and isotopic compositions of bulk snowpack, throughfall, and open precipitation during seven campaigns in mid-winter 2018 along forest-grassland transects at three different elevations (1196, 1297, and 1434 m above sea level) in a pre-Alpine catchment in Switzerland. Snowpack storages under forest canopies were 67 to 93% less than in adjacent open grasslands. On average, the water isotope ratios were higher in the snowpacks under forest canopy than in open grasslands (by 13.4 ‰ in δ²H and 2.3 ‰ in δ¹⁸O). This isotopic difference mirrored the higher isotope values in throughfall compared with open snowfall (by 13.5 ‰ in δ²H and 2.2 ‰ in δ¹⁸O). Although this may suggest that most of the isotopic differences in snowpacks under forests versus in open grasslands were attributable to canopy interception effects, the temporal evolution of snowpack isotope ratios indicated preferential effluxes of lighter isotopes as energy inputs increased and the snowpack ripened and melted. Understanding these effects of forest canopy on bulk snowpack snow water equivalent and isotopic composition are useful when using isotopes to infer snowmelt processes in landscapes with varying forest cover.     

Constraining water age dynamics in a south-eastern Australian catchment using an age-ranked storage and stable isotope approach

Improving our knowledge of the travel times of water through catchments is critical for the management and protection of water resources and to improve our understanding of fundamental catchment behaviour. In this study we use the age-ranked storage framework StorAge Selection (SAS) to investigate travel times in the Corin catchment, a headwater catchment in the south-east of Australia covered by native Eucalyptus species. Few studies have applied the SAS framework globally and in energy-intensive areas where catchment losses are heavily in favour of evapotranspiration relative to streamflow. A combination of observed and modelled values of oxygen-18 (δ¹⁸O ), the stable isotope in water, are used to constrain storage selection preferences of streamflow and evapotranspiration and the size of the catchment active storage. The highest performing parameter combinations that could reproduce δ¹⁸O in streamflow were dependent on a strong preference for young water in evapotranspiration, and a mixture of weak young and old water preference in streamflow. The mean travel time of streamflow over the study period 2007–2019, weighted by the flow rate, is limited to within a probable range of 2.81–9.77 years. The size of the active storage, a key parameter in the SAS framework, was poorly identified, and in combination with the isotopic inputs into the model, contributed to the uncertainty of the results. We discuss the implications of the results with respect to the study area, as well as within the context of SAS research globally and identify ways to improve the modelling process.     

Seasonal snow cover decreases young water fractions in high Alpine catchments

Estimation of young water fractions (F_yw), defined as the fraction of water in a stream younger than approximately 2–3 months, provides key information for water resource management in catchments where runoff is dominated by snowmelt. Knowing the average dependence of summer flow on winter precipitation is an essential context for comparing regional drought severity and provides the hydrological template for downstream water users and ecosystems. However, F_yw estimation based on seasonal signals of stable isotopes of oxygen and hydrogen has not yet explicitly addressed how to parsimoniously include the seasonal shift of water input from snow. Using experimental data from three high-elevation, Alpine catchments (one dominated by glacier and two by snow), we propose a framework to explicitly include the delays induced by snow storage into estimates of F_yw. Scrutinizing the key methodological choices when estimating F_yw from isotope data, we find that the methods used to construct precipitation input signals from sparse isotope samples can significantly impact F_yw. Given this sensitivity, our revised procedure estimates a distribution of F_yw values that incorporates a wide range of possible methodological choices and their uncertainties; it furthermore compares the commonly used amplitude ratio approach to a direct convolution approach, which circumvents the assumption that the isotopic signals have a sine curve shape, an assumption that is generally violated in snow-dominated environments. Our new estimates confirm that high-elevation Alpine catchments have low F_yw values, spanning from 8 to 11%. Such low values have previously been interpreted as the impact of seasonal snow storage alone, but our comparison of different F_yw estimation methods suggests that these low F_yw values result from a combination of both snow cover effects and longer storage in the subsurface. In contrast, in the highest elevation, glacier dominated catchment, F_yw is 3–4 times greater compared to the other two catchments, due to the lower storage and faster drainage processes. A future challenge, capturing spatio-temporal snowmelt isotope signals during winter baseflow and the snowmelt period, remains to improve constraints on the F_yw estimation technique.     

自贡市西山公园地形对地震动的影响

不规则地形和土层对地震动的影响较大,建(构)筑物选址及其抗震设防必须考虑地形和土层场地的放大效应,以避免或减轻其震害.利用自贡地形台阵记录的汶川地震(Ms8.0)的主震加速度时程,基于传统谱比法分别研究了地形和土层场地对地震动的放大效应.结果表明:(1)地形场地在低频的放大效应不明显,最大仅为1.24;在高频的放大效应较显著,在1~10 Hz频带,山顶的放大效应最大,EW、NS和UD地震动的最大放大效应分别为4.15、3.61和2.41,对应频率分别为5.72 Hz、6.46 Hz和7.44 Hz;在10~20 Hz频带,靠近山顶的山脊上某个地震动分量的放大效应最大,7#台站EW、5#台站NS和7#台站UD地震动的最大放大效应分别为9.10、5.56和2.52,对应频率分别为16.97 Hz、16.91 Hz和17.91 Hz.(2)地形场地的最大放大效应随高度有增加的趋势,且在0.1~10 Hz频带随高度增加的趋势更加明显.(3)土层场地水平向地震动在2 Hz以上开始明显放大,竖向地震动在4 Hz以上开始明显放大;EW、NS和UD地震动的最大放大效应分别为13.4、12.168和6.0,对应频率分别为6.94 Hz、7.55 Hz和10.8 Hz.(4)土层场地与地形场地的最大放大效应相比较,前者显著大于后者,对于水平向地震动,前者至少是后者的3倍以上;对于竖向地震动,前者至少是后者的2.5倍以上.(5)无论是地形场地还是土层场地,地震动的最大放大效应均有水平向大于竖向的特征.     

无人机影像和机载激光雷达技术在南方线状工程带状地形图中的应用

将通过控制点精度检验和碎部控制点精度检验分析,基于无人机影像数据和无人机激光雷达技术制作的1:500大比例尺地形图精度,结果显示,无论控制点还是碎部点,其平面位置精度完全满足1:500比例尺地形图的要求;其高程精度方面,控制点和平地碎部点的高程精度也满足1:500比例尺地形图的要求,高山地区的碎部控制点精度基本满足1:...