Groundwater impact on environmental flow needs of streams in sandy catchments in the Netherlands

Abstract

During recent years, water managers and water users in the Netherlands experienced water shortages in numerous streams. Besides low rainfall amounts and high temperatures, anthropogenic alterations to the groundwater system are also responsible for the reduced baseflow in streams. These alterations may reduce resilience and increase a risk to streams as more droughts are expected in the Netherlands due to climate change. We propose a methodology to assess the impact of groundwater-related alterations and climate change on baseflow and environmental flow needs (EFN). Application of this methodology for two sandy catchments showed that, under average meteorological conditions, baseflow in the main streams still meets the EFN requirements. During dry years, baseflow is probably insufficient in the upper parts of the catchments. Anthropogenic alterations show a significant impact: drainage caused 25–40% baseflow reduction, groundwater abstractions caused 5–28% and climate change will potentially cause an additional reduction of 33–70% by 2050.

Editor D. Koutsoyiannis; Guest editor M. Acreman     

基于SOM的流域分类和无资料区径流模拟

无资料区的径流模拟问题是国内外水文研究的难点之一。基于相似流域的参数移植法是常用的解决方法之一,但如何判断相似流域是制约此类方法发展的难点。本文以滇池流域为例,采用自组织映射神经网络(SOM)和层次聚类分析(HCA)联合模式,选取16个流域物理特征为指标进行子流域分类,以确定相似流域。运用无分层的K-means分类的SOM法将整个滇池流域划分为7类具有水文属性的子流域组,分类情景与HCA基本一致,两者实现相互验证。采用HBV水文模型模拟子流域径流过程,并选择部分子流域进行组内参数移植交叉检验。结果显示,HBV模型可较好的模拟滇池流域径流过程;此外,子流域交叉检验结果优良,表明同组内参数可以相互移植。本文不仅为解决滇池流域无资料问题提供了可靠手段,而且由于SOM实现了高维流域特征可视化展示,有助于管理者全面、深入的把握滇池流域水文属性的空间分布特征,为进行水资源管理提供指导。     

Holocene alluviation and land-use change on Callaly Moor,Northumberland, England

Archaeological survey and palaeoenvironmental investigations in Coe Burn, Callaly Moor, Northumberland, have examined the chronology and nature of prehistoric and historic land-use change in a small upland river catchment. These studies have revealed an intensively used landscape with evidence of late Neolithic to Bronze Age, medieval and post-medieval land use and settlement. Two episodes of valley floor alluviation are identified and dated to the middle-late Bronze Age and post-medieval periods. Magnetic mineral and geochemical analyses of fine sediments from the older alluvial fill show it to contain a high proportion of topsoil, which is believed to have been produced by soil erosion resulting from Bronze Age tree clearance and cultivation. Deposition of the younger alluvial fill (post AD 1500) was associated with the inwash of relatively unweathered bedrock generated by coal mining in the catchment. It is demonstrated that by integrating archaeological field survey with off-site palaeoenvironmental investigations a much fuller picture of human activity and land-use change has emerged than otherwise would have been the case. Studies of this type may be particularly useful in other upland river catchments where archaeological survival is limited or poor.     

The potential role of freshwater luminescence measurements in exploring runoff pathways in upland catchments

The forested Coalburn catchment (1·5 km²) in northern England experiences episodic stream acidification. To plan for sustainable management of the plantation forest cycle, an understanding is required of the flow pathways and hydrochemical routing signatures of the organic and mineral soils that make up the source areas for runoff. A tentative mixing model, based on simple water chemistry exists for the major (terrestrial) sources and buffers of acidification; it is being expanded and consolidated by a detailed approach to the organic components of runoff, via sampling and analysis of the luminescence of surface waters at the catchment outlet and in two distinctive feeder streams. Luminescence measurements are presented that permit a simple apportionment of source areas. However, the technique also appears to have potential for identifying differential flow sourcing between the acrotelm and catotelm of intact peat deposits and for clarifying the influence of forest root systems in altering the organic chemistry of infiltrating waters. Applications may include the monitoring and prediction of coloured water events for the water supply industry. Copyright © 2001 John Wiley & Sons, Ltd.     

Hydrometeorological relationships in a glacierized catchment in the Canadian high arctic

Runoff from a small glacierized catchment in the Canadian high Arctic was monitored throughout one melt season. The stream discharge record is one aspect of a larger project involving glacier mass balance, superimposed ice formation and local climate on a glacier in the Sawtooth Range, Ellesmere Island, Northwest Territories, Canada. To better understand the main factors influencing the production of runoff on the glacier during the period of main summer melt, regression analyses were performed relating daily air temperature, shortwave incoming and net radiation, absorptivity and wind speed to daily glacier discharge. Air temperature at the glacier meteorological station on rain-free days is the element with the greatest correlation with runoff (r² = 0.57; n = 34). A multiple regression of discharge with air temperature, shortwave incoming radiation, net radiation hours and wind speed achieved the best fit (r² = 0.84; n = 34). Rain events (> 10mmd^?1) can dominate daily discharge when they occur during the period of ice melt, creating more runoff per unit area than can be produced by melt alone, and significantly reduce the accuracy of runoff predictions.     

The response of flow duration curves to afforestation

The hydrologic effect of replacing pasture or other short crops with trees is reasonably well understood on a mean annual basis. The impact on flow regime, as described by the annual flow duration curve (FDC) is less certain. A method to assess the impact of plantation establishment on FDCs was developed. The starting point for the analyses was the assumption that rainfall and vegetation age are the principal drivers of evapotranspiration. A key objective was to remove the variability in the rainfall signal, leaving changes in streamflow solely attributable to the evapotranspiration of the plantation. A method was developed to (1) fit a model to the observed annual time series of FDC percentiles; i.e. 10th percentile for each year of record with annual rainfall and plantation age as parameters, (2) replace the annual rainfall variation with the long term mean to obtain climate adjusted FDCs, and (3) quantify changes in FDC percentiles as plantations age. Data from 10 catchments from Australia, South Africa and New Zealand were used. The model was able to represent flow variation for the majority of percentiles at eight of the 10 catchments, particularly for the 10–50th percentiles. The adjusted FDCs revealed variable patterns in flow reductions with two types of responses (groups) being identified. Group 1 catchments show a substantial increase in the number of zero flow days, with low flows being more affected than high flows. Group 2 catchments show a more uniform reduction in flows across all percentiles. The differences may be partly explained by storage characteristics. The modelled flow reductions were in accord with published results of paired catchment experiments. An additional analysis was performed to characterise the impact of afforestation on the number of zero flow days (N_zero) for the catchments in group 1. This model performed particularly well, and when adjusted for climate, indicated a significant increase in N_zero. The zero flow day method could be used to determine change in the occurrence of any given flow in response to afforestation. The methods used in this study proved satisfactory in removing the rainfall variability, and have added useful insight into the hydrologic impacts of plantation establishment. This approach provides a methodology for understanding catchment response to afforestation, where paired catchment data is not available.     

Threshold response of small streams to surface coal mining,bituminous coal fields,central Pennsylvania

Stream response to surface coal mining and reclamation was studied in 29 small (0·13 to 5·72 km²) watersheds located in the bituminous coal fields of Central Pennsylvania. These basins, up to 82 per cent mined, were selected from 176 first-order tributaries of Beech Creek with similar vegetation, soil, lithology, and basin characteristics. Measurements were made at 262 cross-sections (an average of nine cross-sections per stream) of channel cross-section area, bankfull width, mean bankfull depth, dimensions of the largest moving blocks, stream slope, valley-side slope, basin area, and mined area. Observed differences in channel morphology were related to differences in extent of mining by means of scatter plots, correlation, cluster analysis, and bivariate regression. Stream response to increased peak discharge and channel shear stress produced by increased surface runoff from regraded mine spoil takes the form of enlarged channels and increases in the size of moving blocks. Large basin areas appear to dampen the effect of mining, resulting in limited channel enlargement with greater extent of mining. In contrast, where peak discharges and associated shear stresses exceed the combined erosional resistance of floodplain vegetation, colluvial blocks, and channel banks, streams adjust extensively to higher levels of mining, causing an abrupt increase in the size of transported blocks and eroded channels. In the first-order basins studied, this stepped response occurs at approximately 0·45 km² mined area and 50 per cent of the total basin area mined. For streams that have exceeded both threshold levels, disequilibrium is demonstrated by a strong, positive correlation between local stream slope and basin area. Where both threshold levels of mining are exceeded, steep channel slopes reinforce the tendency of stream cross-sections to increase with greater disturbance by mining, necessitating that these streams rapidly adjust their morphology in order to attain a new equilibrium which is compatible with the conditions imposed by mining and reclamation.     

The effect of forestry drainage operations on upland sediment yields: A case study

Forestry is generally considered to be a land use giving low sediment yields. However, in Britain most forestry is on land that requires extensive drainage, and it was found that the drainage of a small upland catchment resulted in sediment yields over the following five years equivalent to nearly half a century's load at pre-drainage rates. Subsequent sediment yields did not decline to pre-drainage levels, but remained about four times higher, as a result of erosion of the drains.     

Flow variability and hillslope hydrology

Examination of spatial variability of streamflow in headwater areas can provide important insight about factors that influence hillslope hydrology. Detailed observations of variations in stream channel input, based on a tracer experiment, indicate that topography alone cannot explain flow variability. However, determination of changes in channel input on a small spatial scale can provide valuable clues to factors, such as structural geology that control subsurface flows.