Showing papers in "Hydrological Processes in 2008"

Recent advances in stream and river temperature research

Abstract: Research on stream and river temperatures is reviewed with particular attention being given to advances in understanding gained since 1990 and on investigations of fundamental controls on thermal behaviour, thermal heterogeneity at different spatial scales, the influence of human impacts and the nature of past and future trends. Copyright  2008 John Wiley & Sons, Ltd.

Reconciling theory with observations: elements of a diagnostic approach to model evaluation

Abstract: This paper discusses the need for a well-considered approach to reconciling environmental theory with observations that has clear and compelling diagnostic power. This need is well recognized by the scientific community in the context of the ‘Predictions in Ungaged Basins’ initiative and the National Science Foundation sponsored ‘Environmental Observatories’ initiative, among others. It is suggested that many current strategies for confronting environmental process models with observational data are inadequate in the face of the highly complex and high order models becoming central to modern environmental science, and steps are proposed towards the development of a robust and powerful ‘Theory of Evaluation’. This paper presents the concept of a diagnostic evaluation approach rooted in information theory and employing the notion of signature indices that measure theoretically relevant system process behaviours. The signature-based approach addresses the issue of degree of system complexity resolvable by a model. Further, it can be placed in the context of Bayesian inference to facilitate uncertainty analysis, and can be readily applied to the problem of process evaluation leading to improved predictions in ungaged basins. Copyright  2008 John Wiley & Sons, Ltd.

Modelling runoff from highly glacierized alpine drainage basins in a changing climate

Abstract: The future runoff from three highly glacierized alpine catchments is assessed for the period 2007-2100 using a glacio-hydrological model including the change in glacier coverage. We apply scenarios for the seasonal change in temperature and precipitation derived from regional climate models. Glacier surface mass balance and runoff are calculated in daily time-steps using a distributed temperature-index melt and accumulation model. Model components account for changes in glacier extent and surface elevation, evaporation and runoff routing. The model is calibrated and validated using decadal ice volume changes derived from four digital elevation models (DEMs) between 1962 and 2006, and monthly runoff measured at a gauging station (1979-2006). Annual runoff from the drainage basins shows an initial increase which is due to the release of water from glacial storage. After some decades, depending on catchment characteristics and the applied climate change scenario, runoff stabilizes and then drops below the current level. In all climate projections, the glacier area shrinks dramatically. There is an increase in runoff during spring and early summer, whereas the runoff in July and August decreases significantly. This study highlights the impact of glaciers and their future changes on runoff from high alpine drainage basins.

Representation of agricultural conservation practices with SWAT

Abstract: Results of modelling studies for the evaluation of water quality impacts of agricultural conservation practices depend heavily on the numerical procedure used to represent the practices. Herein, a method for the representation of several agricultural conservation practices with the Soil and Water Assessment Tool (SWAT) is developed and evaluated. The representation procedure entails identifying hydrologic and water quality processes that are affected by practice implementation, selecting SWAT parameters that represent the affected processes, performing a sensitivity analysis to ascertain the sensitivity of model outputs to selected parameters, adjusting the selected parameters based on the function of conservation practices, and verifying the reasonableness of the SWAT results. This representation procedure is demonstrated for a case study of a small agricultural watershed in Indiana in the Midwestern USA. The methods developed in the present work can be applied with other watershed models that employ similar underlying equations to represent hydrologic and water quality processes.

Advancing process‐based watershed hydrological research using near‐surface geophysics: A vision for, and review of, electrical and magnetic geophysical methods

Abstract: We want to develop a dialogue between geophysicists and hydrologists interested in synergistically advancing process based watershed research. We identify recent advances in geophysical instrumentation, and provide a vision for the use of electrical and magnetic geophysical instrumentation in watershed scale hydrology. The focus of the paper is to identify instrumentation that could significantly advance this vision for geophysics and hydrology during the next 3–5 years. We acknowledge that this is one of a number of possible ways forward and seek only to offer a relatively narrow and achievable vision. The vision focuses on the measurement of geological structure and identification of flow paths using electrical and magnetic methods. The paper identifies instruments, provides examples of their use, and describes how synergy between measurement and modelling could be achieved. Of specific interest are the airborne systems that can cover large areas and are appropriate for watershed studies. Although airborne geophysics has been around for some time, only in the last few years have systems designed exclusively for hydrological applications begun to emerge. These systems, such as airborne electromagnetic (EM) and transient electromagnetic (TEM), could revolutionize hydrogeological interpretations. Our vision centers on developing nested and cross scale electrical and magnetic measurements that can be used to construct a three-dimensional (3D) electrical or magnetic model of the subsurface in watersheds. The methodological framework assumes a ‘top down’ approach using airborne methods to identify the large scale, dominant architecture of the subsurface. We recognize that the integration of geophysical measurement methods, and data, into watershed process characterization and modelling can only be achieved through dialogue. Especially, through the development of partnerships between geophysicists and hydrologists, partnerships that explore how the application of geophysics can answer critical hydrological science questions, and conversely provide an understanding of the limitations of geophysical measurements and interpretation. Copyright © 2008 John Wiley & Sons, Ltd.

Decadal trend of climate in the Tibetan Plateau—regional temperature and precipitation

Abstract: The Tibetan Plateau has one of the most complex climates in the world. Analysis of the climate in this region is important for understanding the climate change worldwide. In this study, climate patterns and trends in the Tibetan Plateau were analysed for the period from 1961 to 2001. Air temperature and precipitation were analysed on monthly and annual time scales using data collected from the National Meteorological Centre, China Meteorological Administration. Nonlinear slopes were estimated and analysed to investigate the spatial and temporal trends of air temperature and precipitation in the Tibetan Plateau using a Mann-Kendall method. Spatial analysis of air temperature and precipitation variability across the Tibetan Plateau was undertaken. While most trends are local in nature, there are general basinwide patterns. Temperature during the last several decades showed a long-term warmer trend, especially the areas around Dingri and Zogong stations, which formed two increasing centres. Only one of the stations investigated exhibited decreasing trend, and this was not significant. Precipitation in the Tibetan Plateau has increased in most regions of the study area over the past several decades, especially in the eastern and central part, while the western Tibetan Region exhibited a decreased trend over the same period.

Aqua Incognita: the unknown headwaters

Abstract: Running water comprises just over one millionth of the world’s water.The importance of those streams and rivers as a resource for humanwelfare and biodiversity, however, is far out of proportion to thatminuscule fraction. This explains why protecting running waters (theflow regimes, water quality and biota) is such a vital concern for society.Yet for all the focus and concern, how much do we actually know aboutthese running waters, and the lotic habitat they comprise?Consider what would happen if one asked any national environmentalauthority to assess the basic chemical and ecological status of runningwaters. At the river mouths, there would be enough information tomake a reasonable assessment of the status. But somewhere on theway upstream, available data would run dry, long before most streamchannels did (in non-arid regions).In Sweden, with an ambitious programme for monitoring and assess-ing surface waters, it came as a surprise several years ago to realizethat the length of all perennial streams on the country’s maps was notknown. When that was modelled in the form of a ‘virtual network’ froma50m

``Amount effect'' of water isotopes and quantitative analysis of post-condensation processes

Abstract: A numerical model is proposed that describes the interaction between raindrops and water vapour near the planetary boundary layer to explain the “amount effect”. This model relates the intensity to the isotopic composition of precipitation. The model resolves raindrop sizes, and explicitly includes: (1) the isotopic equilibration time of raindrops that is drop-size dependent; (2) raindrop transit times through the atmosphere; and (3) the evolution of the isotopic composition of vapour at various rain rates. At high rain rate, the precipitation through a layer is less equilibrated with the vapour because the isotopic equilibration time is long compared to the fast transit time, and there is a preponderance of large drops, which take longer to equilibrate. The δ18O of vapour in the lower atmosphere becomes lower as a result of the interaction with these raindrops of low δ18O, and the degree of depletion of 18O is higher when precipitation rates are high. The model reproduces time-series observations of isotopic composition of precipitation in Japan, and a vapour replenishment rate is inferred by either advection or evaporation of about 5% of the precipitation rate. The results could be the basis for a new parameterization of the isotopic equilibration for different precipitation types and rates in General Circulation Models (GCMs). When the model is applied to a GCM, this parameterization is important for places where precipitation occurs at cold temperatures (<15 °C). Copyright © 2007 John Wiley & Sons, Ltd.

Carbon isotope fractionation of dissolved inorganic carbon (DIC) due to outgassing of carbon dioxide from a headwater stream

Abstract: The stable isotopic composition of dissolved inorganic carbon (δ13C-DIC) was investigated as a potential tracer of streamflow generation processes at the Sleepers River Research Watershed, Vermont, USA. Downstream sampling showed δ13C-DIC increased between 3-5% from the stream source to the outlet weir approximately 0.5 km downstream, concomitant with increasing pH and decreasing PCO2. An increase in δ13C-DIC of 2.4 ± 0.1% per log unit decrease of excess PCO2 (stream PCO2 normalized to atmospheric PCO2) was observed from downstream transect data collected during snowmelt. Isotopic fractionation of DIC due to CO2 outgassing rather than exchange with atmospheric CO2 may be the primary cause of increased δ13C-DIC values downstream when PCO2 of surface freshwater exceeds twice the atmospheric CO2 concentration. Although CO2 outgassing caused a general increase in stream δ13C-DIC values, points of localized groundwater seepage into the stream were identified by decreases in δ13C-DIC and increases in DIC concentration of the stream water superimposed upon the general downstream trend. In addition, comparison between snowmelt, early spring and summer seasons showed that DIC is flushed from shallow groundwater flowpaths during snowmelt and is replaced by a greater proportion of DIC derived from soil CO2 during the early spring growing season. Thus, in spite of effects from CO2 outgassing, δ13C of DIC can be a useful indicator of groundwater additions to headwater streams and a tracer of carbon dynamics in catchments.

Do Nash values have value? Discussion and alternate proposals

Abstract: where Qobs is the observed streamflow, Qcalc is the streamflow predicted by some models, and Qobs is the mean observed flow over the interval of interest. Schaefli and Gupta (2007) point out that few modellers in other environmental fields know what an NSE value is. However, they follow Legates and McCabe (1999), Seibert (2001) and several others who propose to modify and improve this metric by the substitution of a suitable objective benchmark Qbench for the mean flow Qobs in Equation (1). Schaefli and Gupta (2007) do not mention the one major point made by Legates and McCabe (1999) and Krause et al. (2005), which is that the NSE overemphasizes large flows relative to other measurements because the various deviations are squared. This problem is shared by the familiar correlation coefficients, R and R2, and while this weighting scheme is statistically questionable, it could be argued that large flows have the greatest importance to humans. However, this defect is serious because the very flows that are overrepresented in such ‘goodness-of-fit’ calculations are for many reasons the least accurately measured. To offset this problem, Legates and McCabe (1999) propose that Equation (1) be replaced with a more general formulation of efficiency ‘E ’ based on absolute values:

A spatial assessment of hydrologic alteration caused by dam construction in the middle and lower Yellow River, China

Abstract: The ‘range of variability approach’ (RVA) and mapping technique are used to investigate the spatial variability of hydrologic alterations (HA) due to dam construction along the middle and lower Yellow River, China, over the past five decades. The impacts of climate variability on hydrological process have been removed during wet and dry periods and the focus is on the impacts of human activities, such as dam construction, on hydrological processes. Results indicate the following: (1) The impacts of the Sanmenxia reservoir on the hydrologic alteration are relatively slight with a mean HA value of 0·48, ranking in the last place among the four large reservoirs. (2) Xiaolangdi reservoir has significantly changed the natural flow regime downstream with mean HA value of 0·56, ranking it in first place among the large reservoirs. (3) The results of ranked median degrees of 33 hydrologic alteration indicators for 10 stations in the Yellow River show that the hydrologic alteration of Huayuankou ranks the highest among 10 stream gauges. (4) Impacts of reservoirs on hydrological processes downstream of the dams are closely associated with the regulating activities of the reservoirs. At the same time, alterations of streamflow regimes resulting from climatic changes (e.g. precipitation variability) make the situation more complicated and more hydrological observations will be necessary for further analysis. The results of the current study will be greatly beneficial to the regional water resources management and restoration of eco-environmental systems in the middle and lower Yellow River characterized by intensified dam construction under a changing environment. Copyright © 2008 John Wiley & Sons, Ltd.

Climate change and fluvial flood risk in the UK: more of the same?

Abstract: The potential impact of climate change on fluvial flooding is receiving considerable scientific and political interest thanks to evidence from climate model projections and a widely held belief that flood risk may be increasing at European levels. This review compares published work on historical trends in UK rainfall and river flow records with high-resolution regional climate change projections, and attempts to reconcile apparent differences between the two. Attention is focused on the techniques used for climate change detection and attribution, as well as the potential confounding effects of land-use change. International and domestic efforts to build adaptive capacity rest on improved quantification of uncertainty in flood risk at very local, catchment and regional scales. This will involve further research to better integrate climate and land-management interactions, to understand changes in the dependence between different flood generating mechanisms, and to improve the characterization and communication of uncertainty at all stages of analysis. Resources are also needed to ensure that latest, but still uncertain, science is presented in an appropriate form to underpin policy development and is translated into sensible guidance for practitioners.

Rainfall partitioning into throughfall, stemflow, and interception within a single beech (Fagus sylvatica L.) canopy: influence of foliation, rain event characteristics, and meteorology

Abstract: While the hydrological balance of forest ecosystems has often been studied at the annual level, quantitative studies on the factors determining rainfall partitioning of individual rain events are less frequently reported. Therefore, the effect of the seasonal variation in canopy cover on rainfall partitioning was studied for a mature deciduous beech (Fagus sylvatica L.) tree over a 2-year period. At the annual level, throughfall amounted to 71% of precipitation, stemflow 8%, and interception 21%. Rainfall partitioning at the event level depended strongly on the amount of rainfall and differed significantly (p 0·85, n = 205) with foliation, rainfall characteristics and meteorological variables as predictor variables. For a given amount of rainfall, foliation significantly increased interception and decreased throughfall and stemflow amounts. In addition, rainfall duration, maximum rainfall rate, vapour pressure deficit, and wind speed significantly affected rainfall partitioning at the event level. Increasing maximum hourly rainfall rate increased throughfall and decreased stemflow generation, while higher hourly vapour pressure deficit decreased event throughfall and stemflow amounts. Wind speed decreased throughfall in the growing period only. Since foliation and the event rainfall amount largely determined interception loss, the observed net water input under the deciduous canopy was sensitive to the temporal distribution of rainfall. Copyright © 2007 John Wiley & Sons, Ltd.

On the estimation of antecedent wetness conditions in rainfall–runoff modelling

Abstract: The analysis of the physical processes involved in a conceptual model of soil water content balance is addressed with the objective of its application as a component of rainfall–runoff modelling. The model uses routinely measured meteorological variables (rainfall and air temperature) and incorporates a limited number of significant parameters. Its performance in estimating the soil moisture temporal pattern was tested through local measurements of volumetric water content carried out continuously on an experimental plot located in central Italy. The analysis was carried out for different periods in order to test both the representation of infiltration at the short time-scale and drainage and evapotranspiration processes at the long time-scale. A robust conceptual model was identified that incorporated the Green–Ampt approach for infiltration and a gravity-driven approximation for drainage. A sensitivity analysis was performed for the selected model to assess the model robustness and to identify the more significant parameters involved in the principal processes that control the soil moisture temporal pattern. The usefulness of the selected model was tested for the estimation of the initial wetness conditions for rainfall–runoff modelling at the catchment scale. Specifically, the runoff characteristics (runoff depth and peak discharge) were found to be dependent on the pre-event surface soil moisture. Both observed values and those estimated by the model gave good results. On the contrary, with the antecedent wetness conditions furnished by two versions of the antecedent precipitation index (API), large errors were obtained. Copyright © 2007 John Wiley & Sons, Ltd.

Evaluating the effect of scale in flood inundation modelling in urban environments

Abstract: Cellular-based approaches for flood inundation modelling have been extensively calibrated and evaluated for the prediction of flood flows on rural river reaches. However, there has only been limited application of these approaches to urban environments, where the need for flood management is greatest. Practical application of two-dimensional (2D) flood inundation models is often limited by computation time and processing power on standard desktop PCs when attempting to resolve flows on the high-resolution grids necessary to replicate urban features. Consequently, it is necessary to evaluate the effectiveness of coarse grids to represent flood flows through urban environments. To examine these effects, LISFLOOD-FP, a 2D storage cell model, is applied to hypothetical flooding scenarios in Greenfields, Glasgow. Grid resampling techniques in GIS software packages are evaluated and a bilinear gridding technique appears to provide the most accurate and physically intuitive results. A gridding method maintaining sharp elevation changes at building interfaces and neighbouring land is presented and estimates of the discretization noise associated with the coarse resolution grids suggest little improvement over current gridding methods. The variation in model results from the friction sensitivity analysis suggests a non-stationary response to Manning's n with changing model resolution. Model results suggests that a coarse resolution model for urban applications is limited by the representation of urban media in coarse model grids. Furthermore, critical length scales related to building dimensions and building separation distances exist in urban areas that determine maximum possible grid resolutions for hydraulic models of urban flooding. Copyright ©, 2008 John Wiley & Sons, Ltd.

The impact of land use change and check-dams on catchment sediment yield

Abstract: This work was financially supported by a research project of the Fundacion Instituto Euromediterraneo del Agua (CE) and by the Spanish Ministry of Science and Education through the projects CANOA CGL2004- 04 949-C02-01; PROBASE CGL2006-11 619 and ERCO CGL2007-62 590/BTE. The first author was financially supported by an I3P postdoc contract CSIC-CE and a “Ramon y Cajal” contract from the Spanish Ministry of Science and Education. Gonzalo G. Barbera also was supported by I3P postdoc contract. Joris de Vente was supported by a Seneca (Region of Murcia Government) foreign researchers grant and through a contract in the integrated project DESIRE of the EU FP6 037 046 (GOCE).

Modelling the probability of occurrence of shallow landslides and channelized debris flows using GEOtop‐FS

Abstract: This paper describes a coupled, distributed, hydrological-geotechnical model, GEOtop-FS, which simulates the probability of occurrence of shallow landslides and debris flows. We use a hydrological distributed model, GEOtop, which, models latent and sensible heat fluxes and surface runoff, and computes soil moisture in 3-D by solving Richards'equation numerically, together with an infinite-slope geotechnical model, GEOtop-FS. The combined model allows both the hydraulic and geotechnical properties of soil to be considered and realistically modelled. In particular, the model has been conceived to make direct use of field surveys, geotechnical characteristics and soil moisture measurements. In the model the depth of available sediments is also used to characterize the hydraulic properties of the area examined. To account for the uncertainty related to the natural variability in the factors influencing the stability of natural slopes, the safety factor is computed with a probabilistic approach. In order to determine the likelihood of slope failures, soil parameters are assigned distributions instead of single deterministic values. The analysis presented was carried out for an alpine watershed, located in the Friuli region, Italy, for which some geological and geotechnical data were available. In the past, this watershed experienced landslides and debris flows during intense storms following long and moderate intensity rainfall events. The distributed coupled GEOtop-FS model was calibrated by reproducing some of these events and validated in order to map future failure probabilities.

Identifying individual rain events from pluviograph records: a review with analysis of data from an Australian dryland site

Abstract: Rainfall is routinely reported as falling in ‘events’ or ‘storms’ whose beginning and end are defined by rainless intervals of a nominated duration (minimum inter-event time, MIT). Rain events commonly exhibit fluctuations in rain rate as well as periods when rain ceases altogether. Event characteristics such as depth, mean rain rate, and the surface runoff volume generated, are defined in relation to the length of the rain event. These derived properties are dependent upon the value of MIT adopted to define the event, and the literature reveals a wide range of MIT criteria. Surprisingly little attention has been paid to this dependency, which limits the inter-comparison of results in published work. The diversity in criteria also diminishes the usefulness of historical data on event durations, rain rates, etc., in attempts to document changes in the rainfall climate. This paper reviews the range of approaches used in the recognition of rain events, and a 5 year pluviograph record from an arid location is analysed. Changing MIT from 15 min to 24 h (lying within the range of published criteria) alters the number of rain events from 550 to 118. The mean rain rate declines from 2·04 mm h−1 to 0·94 mm h−1, and the geometric mean event duration rises from 0·66 h to 3·98 h. This wide variation in the properties of rain events indicates that more attention needs to be paid to the selection and reporting of event criteria in studies that adopt event-based data analysis. The selection of a MIT criterion is shown to involve a compromise between the independence of widely-spaced events and their increasingly variable intra-event characteristics. Copyright © 2008 John Wiley & Sons, Ltd.

Surveying flash floods: gauging the ungauged extremes

Abstract: The monitoring of flash-flood events gives us the unique opportunity toobserve how catchments respond when most of the surface and subsur-face hydrologic flow paths are active. These events often reveal aspectsof hydrological behaviour that either were unexpected on the basis ofweaker responses or highlight anticipated but previously unobservedbehaviour (Delrieu

An ANN‐based model for spatiotemporal groundwater level forecasting

Abstract: This paper evaluates the feasibility of using an artificial neural network (ANN) methodology for estimating the groundwater levels in some piezometers placed in an aquifer in north-western Iran. This aquifer is multilayer and has a high groundwater level in urban areas. Spatiotemporal groundwater level simulation in a multilayer aquifer is regarded as difficult in hydrogeology due to the complexity of the different aquifer materials. In the present research the performance of different neural networks for groundwater level forecasting is examined in order to identify an optimal ANN architecture that can simulate the piezometers water levels. Six different types of network architectures and training algorithms are investigated and compared in terms of model prediction efficiency and accuracy. The results of different experiments show that accurate predictions can be achieved with a standard feedforward neural network trained usung the Levenberg-Marquardt algorithm. The structure and spatial regressions of the ANN parameters (weights and biases) are then used for spatiotemporal model presentation. The efficiency of the spatio-temporal ANN (STANN) model is compared with two hybrid neural-geostatistics (NG) and multivariate time series-geostatistics (TSG) models. It is found in this study that the ANNs provide the most accurate predictions in comparison with the other models. Based on the nonlinear intrinsic ANN approach, the developed STANN model gives acceptable results for the Tabriz multilayer aquifer.

Rain event properties in nature and in rainfall simulation experiments: a comparative review with recommendations for increasingly systematic study and reporting

Abstract: In hydrology and geomorphology, less attention has been paid to rain event properties such as duration, mean and peak rain rate than to rain properties such as drop size or kinetic energy. A literature review shows a lack of correspondence between natural and simulated rain events. For example, 26 studies that report event statistics from substantial records of natural rain reveal a mean rain rate of just 3·47 mm h−1 (s.d. 2·38 mm h−1). In 17 comparable studies dealing with extreme rain rates including events in cyclonic, tropical convective, and typhoon conditions, a mean maximum rain rate (either hourly or mean event rain rate) of 86·3 mm h−1 (s.d. 57·7 mm h−1) is demonstrated. However, 49 studies using rainfall simulation involve a mean maximum rain rate of 103·1 mm h−1 (s.d. 81·3 mm h−1), often sustained for > 1 h, exceeding even than of extreme rain events, and nearly 30 times the mean rain rate in ordinary, non-exceptional, rain events. Thus rainfall simulation is often biased toward high rain rates, and many of the rates employed (in several instances exceeding 150 mm h−1) appear to have limited relevance to ordinary field conditions. Generally, simulations should resemble natural rain events in each study region. Attention is also drawn to the raindrop arrival rate at the surface. In natural rain, this is known to vary from 5000 m−2 s−1. Arrival rate may need to be added to the list of parameters that must be reproduced realistically in rainfall simulation studies. Copyright © 2008 John Wiley & Sons, Ltd.

Phosphorus in groundwater – an overlooked contributor to eutrophication?

Abstract: This paper presents the first international assessment of phosphorus concentrations in groundwater, using data from the Republic of Ireland, Northern Ireland, Scotland, England and Wales. Phosphorus is considered to be the main limiting nutrient in most freshwater ecosystems. Controlling phosphorus inputs is thus considered the key to reducing eutrophication and managing ecological quality. Very little attention has been paid to evaluating transfers via groundwater due to the long-held belief that adsorption and metal complex formation retain the majority of potentially mobile phosphorus. In each country, ecologically-important phosphorus thresholds are exceeded in a significant number of groundwater samples. The relative contributions of potential sources for these elevated concentrations are currently unclear but there is evidence to suggest that they are at least partly anthropogenic. The results suggest that groundwater P concentrations are such that they may be a more important contributor to surface water phosphorus than previously thought. Copyright © 2008 John Wiley & Sons, Ltd.

Effects of subsurface drainage tiles on streamflow in Iowa agricultural watersheds: Exploratory hydrograph analysis

Abstract: Flow from artificial subsurface (tile) drainage systems may be contributing to increasing baseflow in Midwestern rivers and increased losses of nitrate-nitrogen. Standard hydrograph analysis techniques were applied to model simulation output and field monitoring from tile-drained landscapes to explore how flow from drainage tiles affects stream baseflow and streamflow recession characteristics. DRAINMOD was used to simulate hydrologic response from drained (24 m tile spacing) and undrained agricultural systems. Hydrograph analysis was conducted using programs PART and RECESS. Field monitoring data were obtained from several monitoring sites in Iowa typical of heavily drained and less-drained regions. Results indicate that flow from tile drainage primarily affects the baseflow portion of a hydrograph, increasing annual baseflow in streams with seasonal increases primarily occurring in the late spring and early summer months. Master recession curves from tile-drained watersheds appear to be more linear than less-tiled watersheds although comparative results of the recession index k were inconsistent. Considering the magnitude of non-point source pollutant loads coming from tile-drained landscapes, it is critical that more in-depth research and analysis be done to assess the effects of tile drainage on watershed hydrology if water quality solutions are to be properly evaluated. Copyright © 2008 John Wiley & Sons, Ltd.

Effects of vegetation on snow accumulation and ablation in a mid‐latitude sub‐alpine forest

Abstract: We quantified the effects of forest vegetation on snow accumulation and ablation in a mid-latitude montane environment in Northern New Mexico. Detailed observations of snow depth along transects extending radially from trees showed that snow depth was 25% greater on the northern versus southern side of trees. At maximum accumulation, canopy interception resulted in a 47% reduction of snow water equivalent (SWE) under canopy. An array of ultrasonic snow depth sensors showed that snow ablation rates were 54% greater in open locations compared to locations under canopy. Maximum accumulation of SWE occurred 21 days later on the north versus south side of the trees. Binary regression tree models indicated strong correlation (R2 = 0·68) between micro-scale (i.e. 10-cm resolution) canopy structure indices and snow depth. The regression tree model adequately resolved general tree-well structure, suggesting that future remotely sensed vegetation data may improve snow distribution models. Copyright © 2008 John Wiley & Sons, Ltd.

Modelling hydrological response to different land‐use and climate change scenarios in the Zamu River basin of northwest China

Abstract: Changes in climate and land use can significantly influence the hydrological cycle and hence affect water resources. Understanding the impacts of climate and land-use changes on streamflow can facilitate development of sustainable water resources strategies. This study investigates the flow variation of the Zamu River, an inland river in the arid area of northwest China, using the Soil and Water Assessment Tool distributed hydrological model. Three different land-use and climate-change scenarios were considered on the basis of measured climate data and land-use cover, and then these data were input into the hydrological model. Based on the sensitivity analysis, model calibration and verification, the hydrological response to different land-use and climate-change scenarios was simulated. The results indicate that the runoff varied with different land-use type, and the runoff of the mountain reaches of the catchment increased when grassland area increased and forestland decreased. The simulated runoff increased with increased precipitation, but the mean temperature increase decreased the runoff under the same precipitation condition. Application of grey correlation analysis showed that precipitation and temperature play a critical role in the runoff of the Zamu River basin. Sensitivity analysis of runoff to precipitation and temperature by considering the 1990s land use and climate conditions was also undertaken. Copyright © 2007 John Wiley & Sons, Ltd.

Peat hydraulic conductivity in cold regions and its relation to pore size and geometry

Abstract: Subsurface flow through peat plays a critical role in the hydrology of organic-covered, permafrost terrains, which occupy a large part the continental arctic, sub-arctic, and boreal regions. Hillslope drainage in these terrains occurs predominantly through the active flow zone between the relatively impermeable frost table and the water table above it. The hydraulic conductivity profile within this zone controls the subsurface drainage of snowmelt and storm water. Peat hydraulic conductivity profiles were examined at three sites in north-western Canada, each representing a widely occurring organic-covered, permafrost terrain type. Three independent measures of saturated hydraulic conductivity were used—tracer tests, constant-head well-permeameter tests, and laboratory measurements of undisturbed samples. At all three sites, the conductivity profiles contained very high values (10–1000 m d−1) within the top ca 0·1 m where the peat is only lightly decomposed, a large reduction with increasing depth below the ground surface in the transition zone, and relatively low values in a narrow range (0·5–5 m d−1) below ca 0·2 m depth, where the peat is in an advanced state of decomposition. Digital image analysis of resin-impregnated peat samples showed that hydraulic conductivity is essentially controlled by pore hydraulic radius. The strong dependence of hydraulic conductivity on hydraulic radius implies that peat soils subjected to similar degrees of decomposition and compaction have a similar hydraulic conductivity regardless of the location. This explains the similarity of the depth-conductivity profiles among all three terrain types. Copyright © 2008 John Wiley & Sons, Ltd.

A comparison of forest and moorland stream microclimate, heat exchanges and thermal dynamics

Abstract: Although the importance of riparian forest in moderating stream temperature variability is recognized, most previous research focuses on conifer harvesting effects and summer maximum temperature with highly variable findings. This article compares stream temperature, microclimate and heat exchange dynamics between semi-natural forest and moorland (no trees) reaches in the Scottish Cairngorms over two calendar years to provide a longer-term perspective. Mean daily water column temperature is warmer for moorland than forest in late winter–early spring, but cooler in summer. Daily water column temperature range is greater for moorland than forest. Streambed temperature dynamics are markedly different between reaches, reflecting contrasting groundwater–surface water (GW–SW) interactions. Mean, minimum and maximum daily air temperature is cooler, humidity is lower, and wind speed is much higher for moorland than forest on average. Net radiation is the dominant heat sink in autumn–winter and major heat source in spring–summer for moorland and summer for forest. Net radiation is greater in summer and lower in winter for moorland than forest. Sensible heat is an energy source in autumn–winter and sink in spring–summer, with loss (gain) greater in summer (winter) for moorland than forest. Latent heat is predominantly a sink for both reaches, with magnitude and variability higher for moorland than forest. Streambed heat flux is much smaller than fluxes at the air–water interface, with moorland and forest illustrating seasonal and between-reach differences attributable to different GW–SW interactions. Seasonal patterns in stream energy budget partitioning are illustrated schematically. To our knowledge, this is the first such study of mixed woodland, which generates notably different results to work on coniferous forest. This research provides a process basis to model stream thermal impact of changes in forest practice, and so inform decision making by land and water resource managers. Copyright © 2008 John Wiley & Sons, Ltd.

Effect of water table drawdown on peatland dissolved organic carbon export and dynamics

Abstract: Peatlands play an important role in the global carbon cycle, and loss of dissolved organic carbon (DOC) has been shown to be important for peatland carbon budgets. The objective of this study was to determine how net production and export of DOC from a northern peatland may be affected by disturbance such as drainage and climate change. The study was conducted at a poor fen containing several pool–ridge complexes: (1) control site–no water table manipulation; (2) experimental site–monitored for one season in a natural state and then subjected to a water table drawdown for 3 years; (3) drained site–subjected to a water table drawdown 9 years prior to monitoring. The DOC concentration was measured in pore water along a microtopographic gradient at each site (hummock, lawn and hollow), in standing water in pools, and in discharge from the experimental and drained sites. The initial water table drawdown released ∼3 g of carbon per square metre in the form of DOC, providing a large pulse of DOC to downstream ecosystems. This value, however, represents only 1–9% of ecosystem respiration at this site. Seasonal losses of DOC following drainage were 8–11 g of carbon per square metre, representing ∼17% of the total carbon exchange at the experimental study site. Immediately following water table drawdown, DOC concentrations were elevated in pore water and open water pools. In subsequent seasons, DOC concentration in the pool declined, but remained higher than the control site even 11 years after water-table drawdown. This suggests continued elevated net DOC production under lower water table conditions likely related to an increase in vegetation biomass and larger water table fluctuations at the experimental and drained sites. However, the increase in concentration was limited to initially wet microforms (lawns and hollows) reflecting differences in vegetation community changes, water table and soil subsidence along the microtopographic gradient. Copyright © 2008 John Wiley & Sons, Ltd and Her Majesty the Queen in right of Canada.

A new temperature based method to separate rain and snow

Abstract: This paper presents the development and testing of a new method to estimate daily snowfall from precipitation and associated temperature records. The new method requires two variables; the threshold mean daily air temperature at which 50% of precipitation is considered snow, and the temperature range within which mixed precipitation can occur. Sensitivity analyses using 15 climate stations across south-western Alberta, Canada, and ranging from prairie to alpine regions investigates the sensitivity of those two variables on mean annual snowfall (MAS), the coefficient of determination, and the MAS-weighted coefficient of determination. Existing methods, including the static threshold method, one linear transition method used by Quick and Pipes, and the Leavesley method employed in the PRMS hydrological modelling system are compared with the new method, using a total of 963 years of daily data from the 15 climate stations used for the sensitivity analyses. Four different approaches to using the two input variables (threshold temperature and range) were tested and statistically compared: mean annual variables based on the 15 stations, mean annual variables for each station, mean monthly variables for each station, and a sine curve representing seasonal variation of the variables. In almost all cases the proposed new method resulted in higher MAS-weighted coefficients of determination, and, on average, they were significantly different from those of other methods. The paper concludes with a decision tree to help decide which method and approach to apply under a variety of data availabilities.

Long-term trend of precipitation and runoff in Korean river basins

Abstract: The spatial and temporal variations of precipitation and runoff for 139 basins in South Korea were investigated for 34 years (1968-2001). The Precipitation-Runoff Modelling System (PRMS) was selected for the assessment of basin hydrologic response to varying climates and physiology. A non-parametric Mann-Kendall's test and regression analysis are used to detect trends in annual, seasonal, and monthly precipitation and runoff, while Moran's I is adapted to determine the degree of spatial dependence in runoff trend among the basins. The results indicated that the long-term trends in annual precipitation and runoff were increased in northern regions and decreased in south-western regions of the study area during the study period. The non-parametric Mann-Kendall test showed that spring streamflow was decreasing, while summer streamflow was increasing. April precipitation decreased between 15% and 74% for basins located in south-western part of the Korean peninsula. June precipitation increased between 18% and 180% for the majority of the basins. Trends in seasonal and monthly streamflow show similar patterns compared to trends in precipitation. Decreases in spring runoff are associated with decreases in spring precipitation which, accompanied by rising temperatures, are responsible for reducing soil moisture. The regional patterns of precipitation and runoff changes show a strong to moderate positive spatial autocorrelation, suggesting that there is a high potential for severe spring drought and summer flooding in some parts of Korea if these trends continue in the future.