Showing papers in "Hydrological Processes in 2001"

Distribution of oxygen-18 and deuterium in river waters across the United States†

Abstract: Reconstruction of continental palaeoclimate and palaeohydrology is currently hampered by limited information about isotopic patterns in the modern hydrologic cycle. To remedy this situation and to provide baseline data for other isotope hydrology studies, more than 4800, depth- and width-integrated, stream samples from 391 selected sites within the USGS National Stream Quality Accounting Network (NASQAN) and Hydrologic Benchmark Network (HBN) were analysed for δ18O and δ2H (http://water.usgs.gov/pubs/ofr/ofr00-160/pdf/ofr00-160.pdf). Each site was sampled bimonthly or quarterly for 2·5 to 3 years between 1984 and 1987. The ability of this dataset to serve as a proxy for the isotopic composition of modern precipitation in the USA is supported by the excellent agreement between the river dataset and the isotopic compositions of adjacent precipitation monitoring sites, the strong spatial coherence of the distributions of δ18O and δ2H, the good correlations of the isotopic compositions with climatic parameters, and the good agreement between the ‘national’ meteoric water line (MWL) generated from unweighted analyses of samples from the 48 contiguous states of δ2H=8·11δ18O+8·99 (r2=0·98) and the unweighted global MWL of sites from the Global Network for Isotopes in Precipitation (GNIP) of the International Atomic Energy Agency and the World Meteorological Organization (WMO) of δ2H=8·17δ18O+10·35. The national MWL is composed of water samples that arise in diverse local conditions where the local meteoric water lines (LMWLs) usually have much lower slopes. Adjacent sites often have similar LMWLs, allowing the datasets to be combined into regional MWLs. The slopes of regional MWLs probably reflect the humidity of the local air mass, which imparts a distinctive evaporative isotopic signature to rainfall and hence to stream samples. Deuterium excess values range from 6 to 15‰ in the eastern half of the USA, along the northwest coast and on the Colorado Plateau. In the rest of the USA, these values range from −2 to 6‰, with strong spatial correlations with regional aridity. The river samples have successfully integrated the spatial variability in the meteorological cycle and provide the best available dataset on the spatial distributions of δ18O and δ2H values of meteoric waters in the USA. Published in 2001 by John Wiley & Sons, Ltd.

Effects of urbanization on streamflow in the Atlanta area (Georgia, USA): a comparative hydrological approach

Abstract: For the period from 1958 to 1996, streamflow characteristics of a highly urbanized watershed were compared with less-urbanized and non-urbanized watersheds within a 20000 km 2 region in the vicinity of Atlanta, Georgia: in the Piedmont and Blue Ridge physiographic provinces of the southeastern USA. Water levels in several wells completed in surficial and crystalline-rock aquifers were also evaluated. Data were analysed for seven US Geological Survey (USGS) stream gauges, 17 National Weather Service rain gauges, and five USGS monitoring wells. Annual runoff coefficients (RCs; runoff as a fractional percentage of precipitation) for the urban stream (Peachtree Creek) were not significantly greater than for the less-urbanized watersheds. The RCs for some streams were similar to others and the similar streams were grouped according to location. The RCs decreased from the higher elevation and higher relief watersheds to the lower elevation and lower relief watersheds: values were 0! 54 for the two Blue Ridge streams, 0! 37 for the four middle Piedmont streams (near Atlanta), and 0! 28 for a southern Piedmont stream. For the 25 largest stormflows, the peak flows for Peachtree Creek were 30% to 100% greater than peak flows for the other streams. The storm recession period for the urban stream was 1‐2 days less than that for the other streams and the recession was characterized by a 2-day storm recession constant that was, on average, 40 to 100% greater, i.e. streamflow decreased more rapidly than for the other streams. Baseflow recession constants ranged from 35 to 40% lower for Peachtree Creek than for the other streams; this is attributed to lower evapotranspiration losses, which result in a smaller change in groundwater storage than in the less-urbanized watersheds. Low flow of Peachtree Creek ranged from 25 to 35% less than the other streams, possibly the result of decreased infiltration caused by the more efficient routing of stormwater and the paving of groundwater recharge areas. The timing of daily or monthly groundwater-level fluctuations was similar annually in each well, reflecting the seasonal recharge. Although water-level monitoring only began in the 1980s for the two urban wells, water levels displayed a notable decline compared with non-urban wells since then; this is attributed to decreased groundwater recharge in the urban watersheds due to increased imperviousness and related rapid storm runoff. Copyright ! 2001 John Wiley & Sons, Ltd.

Carbon and nitrogen isotopic compositions of particulate organic matter in four large river systems across the United States

Abstract: Riverine particulate organic matter (POM) samples were collected bi-weekly to monthly from 40 sites in the Mississippi, Colorado, Rio Grande, and Columbia River Basins (USA) in 1996–97 and analysed for carbon and nitrogen stable isotopic compositions. These isotopic compositions and C : N ratios were used to identify four endmember sources of POM: plankton, fresh terrestrial plant material, aquatic plants, and soil organic material. This large-scale study also incorporated ancillary chemical and hydrologic data to refine and extend the interpretations of POM sources beyond the source characterizations that could be done solely with isotopic and elemental ratios. The ancillary data were especially useful for differentiating between seasonal changes in POM source materials and the effects of local nutrient sources and in-stream biogeochemical processes. Average values of δ13C and C : N for all four river systems suggested that plankton is the dominant source of POM in these rivers, with higher percentages of plankton downstream of reservoirs. Although the temporal patterns in some rivers are complex, the low δ13C and C : N values in spring and summer probably indicate plankton blooms, whereas relatively elevated values in fall and winter are consistent with greater proportions of decaying aquatic vegetation and/or terrestrial material. Seasonal shifts in the δ13C of POM when the C : N remains relatively constant probably indicate changes in the relative rates of photosynthesis and respiration. Periodic inputs of plant detritus are suggested by C : N ratios >15, principally on the Columbia and Ohio Rivers. The δ15N and δ13C also reflect the importance of internal and external sources of dissolved carbon and nitrogen, and the degree of in-stream processing. Elevated δ15N values at some sites probably reflect inputs from sewage and/or animal waste. This information on the spatial and temporal variation in sources of POM in four major river systems should prove useful in future food web and nutrient transport studies. Published in 2001 by John Wiley & Sons, Ltd.

Erodibility of cohesive streambeds in the loess area of the midwestern USA

Abstract: Excess stress parameters, critical shear stress (τ c ) and erodibility coefficient (k d ), for degrading channels in the loess areas of the midwestern USA are presented based on in situ jet-testing measurements. Critical shear stress and k d are used to define the erosion resistance of the streambed, The jet-testing apparatus applies hydraulic stresses to the bed and the resulting scour due to the impinging jet is related to the excess stress parameters. Streams tested were primarily silt-bedded in texture with low densities, which is typical of loess soils. Results indicate that there is a wide variation in the erosion resistance of streambeds, spanning six orders of magnitude for τ c and four orders of magnitude for k d . Erosion resistance was observed to vary within a streambed, from streambed to streambed, and from region to region. An example of the diversity of materials within a river system is the Yalobusha River Basin in Mississippi. The median value of τ c for the two primary bed materials, Naheola and Porters Creek Clay Formations, was 1.31 and 256 Pa, respectively. Streambeds composed of the Naheola Formation are readily eroded over the entire range of shear stresses, whereas only the deepest flows generate boundary stresses great enough to erode streambeds composed of the Porters Creek Clay Formation. Therefore, assessing material resistance and location is essential in classifying and modelling streambed erosion processes of these streams.

The distribution and strength of riparian tree roots in relation to riverbank reinforcement

Abstract: The main influences of plants on the mass stability of riverbanks are those that affect the strength of bank sediments. Plants enhance bank strength by reducing pore-water pressures and by directly reinforcing bank material with their roots. In this paper we do not consider bank hydrology but focus on quantifying increases in sediment strength due to root reinforcement. Root reinforcement is a function of root strength, interface friction between the roots and the soil, and the distribution of roots within the soil. Field and laboratory studies of Australian riparian trees, river red gum (Eucalyptus camaldulensis) and swamp paperbark (Melaleuca ericifolia), indicate that bank reinforcement, due to the roots of these species, decreases exponentially with depth below the soil surface and distance away from the trees. Differences in the spatial distribution of root reinforcement are illustrated by a comparison of the apparent cohesion due to roots (cr) with the effective cohesion of the saturated bank material (15 kPa). Directly below the river red gum, root reinforcement is equal to effective cohesion at 1·7 m depth. At the dripline (17 m from the trunk), root reinforcement is equal to material strength at 0·4 m depth. For the swamp paperbark, cr=15 kPa at 1·1 m depth beneath surface, at the trunks, and at 0·4 m depth at the dripline (2·5 m from the trunk). A description of the spatial distribution of root reinforcement is important in the riverbank context. We find that interspecies differences in the strength of living roots have less significance for bank reinforcement than interspecies differences in root distribution. Copyright © 2001 John Wiley & Sons, Ltd.

Rethinking what constitutes suspended sediment

Abstract: Although cohesive suspended sediment is now known to be transported primarily as flocculated material, there is still a misconception of what constitutes suspended sediment. Flocs represent a complex matrix of microbial communities, organic particles (e.g. detritus, extracellular polymers and cellular debris), inorganic particles (e.g. clays and silts) and substantial interfloc spaces (pores), which allow for the retention or flow through of water. Flocculation results in significant alteration of the hydrodynamics of the constituent particles (by modifying their effective size, shape, density and porosity), thereby affecting the transport of sediment and associated contaminants. The composition and structure of a floc is in a continuous state of change as the medium in which it is transported provides the floc with further building materials, energy, nutrients and chemicals for biological growth, chemical reactions and morphological development. As such, a floc's physical (e.g. transport), chemical (e.g. contaminant adsorption) and biological (community development and contaminant biotransformation) behaviour are also in a continuous state of change, with concomitant effects on their aquatic environment as a whole. Although it is recognized that floc form will influence floc behaviour, there is still a basic lack of knowledge of the complex links between the structural components of a floc and how their individual properties and behaviours in combination with others will influence a floc's physical, chemical and biological behaviour. This paper provides a comprehensive conceptual model that links the many interrelated structural components of typical flocs and their interrelated behavioural aspects, in order to enhance our understanding of what constitutes suspended sediment. Copyright © 2001 John Wiley & Sons, Ltd.

Quantifying contributions to storm runoff through end-member mixing analysis and hydrologic measurements at the Panola Mountain Research Watershed (Georgia, USA).

Abstract: The geographic sources and hydrologic flow paths of stormflow in small catchments are not well understood because of limitations in sampling methods and insufficient resolution of potential end members. To address these limitations, an extensive hydrologic dataset was collected at a 10 ha catchment at Panola Mountain Research Watershed near Atlanta, GA, to quantify the contribution of three geographic sources of stormflow. Samples of stream water, runoff from an outcrop, and hillslope subsurface stormflow were collected during two rainstorms in the winter of 1996, and an end-member mixing analysis model that included five solutes was developed. Runoff from the outcrop, which occupies about one-third of the catchment area, contributed 50–55% of the peak streamflow during the 2 February rainstorm, and 80–85% of the peak streamflow during the 6–7 March rainstorm; it also contributed about 50% to total streamflow during the dry winter conditions that preceded the 6–7 March storm. Riparian groundwater runoff was the largest component of stream runoff (80–100%) early during rising streamflow and throughout stream recession, and contributed about 50% to total stream runoff during the 2 February storm, which was preceded by wet winter conditions. Hillslope runoff contributed 25–30% to peak stream runoff and 15–18% to total stream runoff during both storms. The temporal response of the three runoff components showed general agreement with hydrologic measurements from the catchment during each storm. Estimates of recharge from the outcrop to the riparian aquifer that were independent of model calculations indicated that storage in the riparian aquifer could account for the volume of rain that fell on the outcrop but did not contribute to stream runoff. The results of this study generally indicate that improvements in the ability of mixing models to describe the hydrologic response accurately in forested catchments may depend on better identification, and detailed spatial and temporal characterization of the mobile waters from the principal hydrologic source areas that contribute to stream runoff. Copyright  2001 John Wiley & Sons, Ltd.

Evaluation and generalization of temperature‐based methods for calculating evaporation

Abstract: Seven temperature-based equations, each representing a typical form, were evaluated and compared for determining evaporation at two climatological stations (Rawson Lake and Atikokan) in north-western Ontario, Canada The comparison was first made using the original constant values involved in each equation, and then using the recalibrated constant values The results show that when the original constant values were used, larger biases existed for most of the equations for both stations When recalibrated constant values were substituted for the original constant values, six of the seven equations improved for both stations Using locally calibrated parameter values, all seven equations worked well for determining mean seasonal evaporation values For monthly evaporation values, the modified Blaney–Criddle method produced least error for all months for both stations, followed by the Hargreaves and Thornthwaite methods The Linacre, Kharrufa and Hamon methods showed a significant bias in September for both stations With properly determined constant values, the modified Blaney–Criddle, the Hargreaves and Thornthwaite methods can be recommended for estimating evaporation in the study region, as far as temperature-based methods are concerned Copyright © 2001 John Wiley & Sons, Ltd

A dynamic TOPMODEL

Abstract: A new version of the rainfall-runoff model TOPMODEL is described in which the assumption of a quasi-steady state saturated zone configuration is replaced by a kinematic wave routing of subsurface flow implemented in a way that allows the simulation of dynamically variable upslope contributing areas. The new version retains the idea of a distribution of hydrologically similar points in the catchment but allows more flexibility in the definition of hydrologically similarity. This allows the new version to retain much of the computational efficiency of the original model while still allowing results to be mapped back into the space of the catchment. A comparison is gives of the original and dynamic versions in an application to the Slapton Wood catchment, Devon, UK, within the GLUE methodology. The new version provides results that are a better fit to observed discharges, with improved prediction bounds, and patterns of predicted deficits storage that are qualitatively more consistent with understanding of the responses of this catchment. Copyright © 2001 John Wiley & Sons, Ltd.

Predicting floodplain inundation: raster-based modelling versus the finite-element approach

Abstract: We compare two approaches to modelling floodplain inundation: a raster-based approach, which uses a relatively simple process representation, with channel flows being resolved separately from the floodplain using either a kinematic or diffusive wave approximation, and a finite-element hydraulic model aiming to solve the full two-dimensional shallow-water equations. A flood event on a short (c. 4 km) reach of the upper River Thames in the UK is simulated, the models being validated against inundation extent as determined from satellite synthetic aperture radar (SAR) imagery. The unconstrained friction parameters are found through a calibration procedure, where a measure of fit between predicted and observed shorelines is maximized. The raster and finite-element models offer similar levels of performance, both classifying approximately 84% of the model domain correctly, compared with 65% for a simple planar prediction of water surface elevation. Further discrimination between models is not possible given the errors in the validation data. The simple raster-based model is shown to have considerable advantages in terms of producing a straightforward calibration process, and being robust with respect to channel specification. Copyright © 2001 John Wiley & Sons, Ltd.

Post‐fire runoff and erosion from simulated rainfall on small plots, Colorado Front Range

Abstract: Wildfires in the Colorado Front Range can trigger dramatic increases in runoff and erosion. A better understanding of the causes of these increases is needed to predict the effects of future wildfires, estimate runoff and erosion risks from prescribed fires, and design effective post-fire rehabilitation treatments. The objective of this project was to determine whether runoff and sediment yields were significantly related to the site variables of burn severity, percent cover, soil water repellency, soil moisture, time since burning, and slope. To eliminate the variability due to natural rainfall events, we applied an artificial storm of approximately 80 mm h -1 on 26 1 m 2 plots in the summer and fall of 2000. The plots were distributed among a June 2000 wildfire, a November 1999 prescribed fire, and a July 1994 wildfire. For 23 of the 26 plots the ratio of runoff to rainfall exceeded 50%. Nearly all sites exhibited strong natural or fire-induced water repellency, so the runoff ratios were only 15-30% larger for the high-severity plots in the two more recent fires than for the unburned or low-severity plots. The two high-severity plots in the 1994 wildfire had very low runoff ratios, and this probably was due to the high soil moisture conditions at the time of the simulated rainfall and the resulting reduction in the natural water repellency. Sediment yields from the high-severity sites in the two more recent fires were 10-26 times greater than the unburned and low-severity plots. The plots burned at high severity in 1994 yielded only slightly more sediment than the unburned plots. Percent ground cover explained 81% of the variability in sediment yields, and the sediment yields from the plots in the 1994 wildfire are consistent with the observed recovery in percent ground cover.

A conceptual model of preferential flow systems in forested hillslopes: evidence of self‐organization

Abstract: Preferential flow paths are known to be important conduits of subsurface stormflow in forest hillslopes. Earlier research on preferential flow paths focused on vertical transport; however, lateral transport is also evident in steep forested slopes underlain by bedrock or till. Macropores consisting of decayed and live roots, subsurface erosion, surface bedrock fractures, and animal burrows form the basis of a backbone' for lateral preferential flow in such sites. Evidence from field studies in Japan indicates that although individual macropore segments are generally <0.5 m in length, they have a tendency to self-organize into larger preferential flow systems as sites become wetter. Staining tests show clear evidence of interconnected macropore flow segments, including: flow within decayed root channels and subsurface erosion cavities; flow in small depressions of the bedrock substrate; fracture flow in weathered bedrock; exchange between macropores and mesopores; and flow at the organic horizon-mineral soil interface and in buried pockets of organic material and loose soil. Here we develop a three-dimensional model for preferential flow systems based on distributed attributes of macropores and potential connecting nodes (e.g. zones of loose soil and buried organic matter). We postulate that the spatially variable and non-linear preferential flow response observed at our Japan field site, as well as at other sites, is attributed to discrete segments of macropores connecting at various nodes within the regolith. Each node is activated by local soil water conditions and is influenced strongly by soil depth, permeability, pore size, organic matter distribution, surface and substrate topography, and possibly momentum dissipation. This study represents the first attempt to characterize the spatially distributed nature of preferential flow paths at the hillslope scale and presents strong evidence that these networks exhibit complex system behaviour.

Strength and persistence of fire‐induced soil hydrophobicity under ponderosa and lodgepole pine, Colorado Front Range

Abstract: Fire-induced soil hydrophobicity is presumed to be a primary cause of the observed post-fire increases in runoff and erosion from forested watersheds in the Colorado Front Range, but the presence and persistence of hydrophobic conditions has not been rigorously evaluated. Hence the goals of this study were to: (1) assess natural and fire-induced soil hydrophobicity in the Colorado Front Range, and (2) determine the effect of burn severity, soil texture, vegetation type, soil moisture, and time since burning on soil hydrophobicity. Five wild and prescribed fires ranging in age from 0 to 22 months were studied. Each fire had four study sites in ponderosa pine forests that had been burned at high, moderate, or low severity, and three sites were in unburned areas. Additional sites were established in lodgepole pine stands and an area with unusually coarse-textured soils. At each site the soil hydrophobicity was assessed in two pits using the water drop penetration time (WDPT) and the critical surface tension (CST). Measurements were made at the mineral soil surface and depths of 3, 6, 9, 12, 15, and 18 cm. In sites burned at moderate or high severity the soils were often strongly hydrophobic at 0, 3, and 6 cm. Unburned sites or sites burned at low severity were typically hydrophobic only at the surface. Although soil hydrophobicity generally strengthened with increasing burn severity, statistically significant differences in soil hydrophobicity were difficult to detect because of the high variability within and between sites. Hydrophobicity also increased with increasing percent sand and was not present when soil moistures exceeded 12–25%. There were no significant differences in soil hydrophobicity between ponderosa and lodgepole pine stands, regardless of burn severity. Repeat measurements on one fire suggest a weakening of fire-induced soil hydrophobicity after 3 months. Comparisons between fires suggest that fire-induced soil hydrophobicity persists for at least 22 months. Overall, CST values were more consistent and more highly correlated with the independent variables than the WDPT, and the CST is recommended for assessing soil hydrophobicity rather than the more commonly used WDPT. Copyright © 2001 John Wiley & Sons, Ltd.

Comparison of soil infiltration rates in burned and unburned mountainous watersheds

Abstract: Steady-state infiltration measurements were made at mountainous sites in New Mexico and Colorado, USA, with volcanic and granitic soils after wildfires and at comparable unburned sites. We measured infiltration in the New Mexico volcanic soils under two vegetation types, ponderosa pine and mixed conifer, and in the Colorado granitic soils under ponderosa pine vegetation. These measurements were made within high-severity burn areas using a portable infiltrometer with a 0Ð017 m 2 infiltration area and artificial rainfall rates ranging from 97 to 440 mm h � 1 . Steadystate infiltration rates were less at all burned sites relative to unburned sites. The volcanic soil with ponderosa pine vegetation showed the greatest difference in infiltration rates with a ratio of steady-state infiltration rate in burned sites to unburned soils equal to 0Ð15. Volcanic soils with mixed conifer vegetation had a ratio (burned to unburned soils) of at most 0Ð38, and granitic soils with ponderosa pine vegetation had a ratio of 0Ð38. Steady-state infiltration rates on unburned volcanic and granitic soils with ponderosa pine vegetation are not statistically different. We present data on the particle-size distribution at all the study sites and examples of wetting patterns produced during the infiltration experiments. Published in 2001 by John Wiley & Sons, Ltd.

Post‐fire runoff and erosion from rainfall simulation: contrasting forests with shrublands and grasslands

Abstract: Rainfall simulations allow for controlled comparisons of runoff and erosion among ecosystems and land cover conditions. Runoff and erosion can increase greatly following fire, yet there are few rainfall simulation studies for post-fire plots, particularly after severe fire in semiarid forest. We conducted rainfall simulations shortly after a severe fire (Cerro Grande) in ponderosa pine forest near Los Alamos, New Mexico, USA, which completely burned organic ground cover and exposed unprotected soil. Measurements on burned plots showed 74% of mineral soil was exposed compared with an estimated 3% exposed prior to the fire. Most of the remaining 26% surface area was covered by easily moveable ash. Rainfall was applied at 60 mm h � 1 in three repeated tests over 2 days. Runoff from burned plots was about 45% of the total 120 mm of applied precipitation, but only 23% on the unburned plots. The most striking difference between the response of burned and unburned plots was the amount of sediment production; burned plots generated 25 times more sediment than unburned plots (76 kg ha � 1 and 3 kg ha � 1 respectively per millimetre of rain). Sediment yields were well correlated with percentage bare soil (r D 0Ð84). These sediment yields were more than an order of magnitude greater than nearly all comparable rainfall simulation studies conducted on burned plots in the USA, most of which have been in grasslands or shrublands. A synthesis of comparable studies suggests that an erosion threshold is reached as the amount of soil exposed by fire increases to 60–70%. Our results provide sediment yield and runoff data from severely burned surfaces, a condition for which little rainfall simulation data exist. Further, our results contrast post-fire hydrologic responses in forests with those in grasslands and shrublands. These results can be applied to problems concerning post-fire erosion, flooding, contaminant transport, and development of associated remediation strategies. Copyright  2001 John Wiley & Sons, Ltd.

Post-fire, rainfall intensity-peak discharge relations for three mountainous watersheds in the western USA

Abstract: Wildfire alters the hydrologic response of watersheds, including the peak discharges resulting from subsequent rainfall. Improving predictions of the magnitude of flooding that follows wildfire is needed because of the increase in human population at risk in the wildland–urban interface. Because this wildland–urban interface is typically in mountainous terrain, we investigated rainfall-runoff relations by measuring the maximum 30 min rainfall intensity and the unit-area peak discharge (peak discharge divided by the area burned) in three mountainous watersheds (17–26Ð 8k m 2 )a fter a wildfire. We found rainfall-runoff relations that relate the unit-area peak discharges to the maximum 30 min rainfall intensities by a power law. These rainfall-runoff relations appear to have a threshold value for the maximum 30 min rainfall intensity (around 10 mm h � 1 ) such that, above this threshold, the magnitude of the flood peaks increases more rapidly with increases in intensity. This rainfall intensity could be used to set threshold limits in rain gauges that are part of an early-warning flood system after wildfire. The maximum unit-area peak discharges from these three burned watersheds ranged from 3Ð 2t o 50 m 3 s � 1 km � 2 . These values could provide initial estimates of the upper limits of runoff that can be used to predict floods after wildfires in mountainous terrain. Published in 2001 by John Wiley & Sons, Ltd.

Effects of pipeflow on hydrological process and its relation to landslide: a review of pipeflow studies in forested headwater catchments

Abstract: Since the 1980s, several field studies of pipeflow hydrology have been conducted in forested, steep headwater catchments. However, adequate information is lacking with regard to questions as to how representative these previous studies are and how widespread the phenomena might be. Thus, the aim of this study is to review some studies of pipeflow hydrology on forested steep hillslopes. Several points were clarified. (1) The maximum discharge of pipeflow was mainly determined by the soil pipe diameter. When the condition of both the soil pipe diameter and the slope gradient in forest soil were similar to those in peaty podzol, the maximum discharge of pipeflow on forest slopes was slighter than that in peaty podzol. (2) Pipeflow was delivered from a variety of sources, and the contributing area of pipeflow extended as the soil layer became wetter. Therefore, the dominant contributor (new water and old water) was varied and the contribution of pipeflow to streamflow increased with the increase of rainfall magnitude. (3) The roles of pipeflow in the storm runoff generation processes demonstrated two roles: the concentration of water and the rapid drainage to downslopes. Thus, soil pipes can increase the peak discharge from the hillslope and decrease the peak lag time of the storm hydrograph, when compared to the unpiped hillslope. (4) The roles of pipeflow on hydrological process suggested that the soil pipes contribute to the slope stability by increasing the rate of soil drainage and limiting the development of perched groundwater conditions. However, if the rate of water concentration to the soil pipe network is in excess of the pipeflow transmission capacity, the cavity of the soil pipe could readily be filled with water during a rain event, increasing pore water pressure in the surrounding matrix. In this case, the soil pipe induced slope instability. (5) Moreover, pipe erosion has significant impact on the runoff characteristics of pipeflow, since pipe erosion contributed to a change in the limited drainage capacity of soil pipe. Thus, pipe erosion plays an important role in shallow landslide initiation. Copyright © 2001 John Wiley & Sons, Ltd.

Intelligent control for modelling of real‐time reservoir operation

Abstract: This paper presents a new approach to improving real-time reservoir operation. The approach combines two major procedures: the genetic algorithm (GA) and the adaptive network-based fuzzy inference system (ANFIS). The GA is used to search the optimal reservoir operating histogram based on a given inflow series, which can be recognized as the base of input–output training patterns in the next step. The ANFIS is then built to create the fuzzy inference system, to construct the suitable structure and parameters, and to estimate the optimal water release according to the reservoir depth and inflow situation. The practicability and effectiveness of the approach proposed is tested on the operation of the Shihmen reservoir in Taiwan. The current M-5 operating rule curves of the Shihmen reservoir are also evaluated. The simulation results demonstrate that this new approach, in comparison with the M-5 rule curves, has superior performance with regard to the prediction of total water deficit and generalized shortage index (GSI). Copyright © 2001 John Wiley & Sons, Ltd.

Long‐term changes in concentrations and flux of nitrogen in the Mississippi River Basin, USA

Abstract: Current and historical data show that nitrogen concentrations and flux in the Mississippi River Basin have increased significantly during the past 100 years. Most of the increase observed in the lower Mississippi River has occurred since the early 1970s and is due almost entirely to an increase in nitrate. The current (1980–99) average annual nitrogen (N) flux from the Mississippi Basin to the Gulf of Mexico is about 1 555 500 t year−1, of which about 62% is nitrate-N. The remaining 38% is organic nitrogen and a small amount of ammonium. The current (1980–99) average nitrate flux to the Gulf is almost three times larger than it was during 1955–70. This increased supply of nitrogen to the Gulf is believed to be partly responsible for the increasing size of a large hypoxic zone that develops along the Louisiana–Texas shelf each summer. This zone of oxygen-depleted water has doubled in areal extent since it was first measured in 1985. The increase in annual nitrate flux to the Gulf can be largely explained by three factors: increased fertilizer use, annual variability in precipitation and increased streamflow, and the year-to-year variability in the amount of nitrogen available in the soil-ground water system for leaching to streams. The predominant source areas for the nitrogen transported to the Gulf of Mexico are basins draining southern Minnesota, Iowa, Illinois, Indiana, and Ohio. Basins in this region yield 1801 to 3050 kg N km−2 year−1 to streams, several times the N yield of basins outside this region. Published in 2001 by John Wiley & Sons, Ltd.

Causes and consequences of fire‐induced soil water repellency

Abstract: A wettable surface layer overlying a water-repellent layer is commonly observed following a fire on a watershed. High surface temperatures burn' off organic materials and create vapours that move downward in response to a temperature gradient and then condense on soil particles causing them to become water repellent. Water-repellent soils have a positive water entry pressure hp that must be exceeded or all the water will runoff. Water ponding depths h o that exceeds hp will cause infiltration, but the profile is not completely wetted. Infiltration rate and soil wetting increase as the value of h o /h p increases. The consequence is very high runoff, which also contributes to high erosion on fire-induced water-repellent soils during rain storms. Grass establishment is impaired by seeds being eroded and lack of soil water for seeds that do remain and germinate. Extrapolation of these general findings to catchment or watershed scales is difficult because of the very high temporal and spatial variabilities that occur in the field.

Estimating areal snowmelt infiltration into frozen soils

Abstract: An algorithm for estimating areal snowmelt infiltration into frozen soils is developed. Frozen soils are grouped into classes according to surface entry condition as: (a) Restricted-water entry is impeded by surface conditions, (b) Limited-capillary flow predominates and water entry is influenced primarily by soil physical properties, and (c) Unlimited-gravity flow predominates and most of the meltwater infiltrates. For Limited soils cumulative infiltration over time is estimated by a parametric equation from surface saturation, initial soil moisture content (water + ice), initial soil temperature and infiltration opportunity time. Total infiltration into Unlimited and Limited soils is constrained by the available water storage capacity. This constraint is also used to determine when Limited soils have thawed. The minimum spatial scale of the infiltration model is established for Limited soils by the variabilities in surface saturation, snow water equivalent, soil infiltrability, soil moisture (water + ice) and depth of soil freezing. Since snowmelt infiltration is influenced by other processes and factors that affect snow ablation, it is assumed that the infiltrability spatial scale should be consistent with the scales used to describe these variables. For open, northern, cold regions the following order in spatial scales is hypothesized: frozen ground > snowmelt > snow water equivalent ≥ frozen soil infiltrability > soil moisture (water + ice) and snow water. For mesoscale application of the infiltration model it is recommended that the infiltrability scale be taken equal to the scale used to describe the areal extent and distribution of the water equivalent of the snowcover that covers frozen ground. Scaling the infiltrability of frozen soils in this manner allows one to exploit established landscape-stratification methodology used to derive snow accumulation means and distribution. Scaling of soil infiltrability at small scales (microscale) is complicated and requires information on the association(s) between the spatial distributions of soil moisture (water + ice) and snow water. A flow chart of the algorithm is presented.

Scaling in hydrology

Abstract: Correspondence to: G Bloschl, Institut fur Hydraulik, Gewasserkunde und Wasserwirtschaft, Technische Universitat Wien, Karrsplatz 13/223, A-1040 Wien, Austria E-mail: bloeschl@hydrotuwienacat The term ‘scaling’, to many, is veiled in a nimbus of exciting mystery At a basic level, part of the mystery simply comes from confusion of two connotations of the word—meaning either scale invariance (ie processes behaving similarly at small and large scales) or upscaling/downscaling (ie aggregating/disaggregating data) However, once this hurdle is surmounted, from whence does this excitement come and in what direction does it lead? If we follow the scale invariance track of enquiry, some hidden signature of hydrologic systems that can be encapsulated in beautifully simple equations is promised The idea of self-similarity, first conceived by L F Richardson and expertly marketed by B Mandelbrot, is compelling given so much visual evidence of variability at small and large scales And, indeed, if you believe there exists a single universal relationship underlying hydrologic processes at many scales it is hard not to fly off to cloud-cuckoo land with this idea The upscaling/downscaling track of enquiry is more practical In hydrology, much of the recent interest began in the 1970s with the early work of A Freeze and L Gelhar aggregating the groundwater flow equation based on a stochastic approach, and gained additional momentum in the 1980s when it was realized that spatial heterogeneity of the land surface matters for atmospheric models Those subdisciplines of hydrology in which the basic equations are known with some degree of confidence (eg groundwater flow) have a head start, but for catchment hydrology and hillslope hydrology there is still a long way to go before the derivation of an aggregate large-scale equation from first principles will be possible It is likely that ad hoc relationships with little theoretical justification will be with us for another few years Field hydrologists may wonder what role field observations and onsite experience have in all this, and I wonder too Is it coincidence that most of the celebrated (and rightly so) pioneers of the scaling community never were personally involved in fieldwork, or is there a causal relationship? I believe it is the latter Fieldwork and scaling theory, apparently, are too widely divergent for a single individual to excel in both Or perhaps it is something else I continue to be intrigued by the complexity of hydrological processes when in the field The rich diversity in the spatial arrangements of flow paths and mechanisms that, to the observer, quite obviously change with scale make it difficult for me, when back in the office, to write down simple formulations that neglect most of what I know is out there Many of the betterknown scaling relationships do neglect the important bits For example, stochastically averaged groundwater flow equations usually assume that

Landscape evolution models: a software review

Abstract: Coulthard, T. J. (2001). Landscape evolution models: a software review. Hydrological Processes, 15(1), 165-173.

Fire, storms, and erosional events in the Idaho batholith

Abstract: In late December 1996, the South Fork Payette River basin in west-central Idaho experienced a prolonged storm that culminated on January 1, 1997, with intense rain on melting snow that triggered slide failures, producing debris flows and sediment-charged floods. Failures occurred in saturated, cohesionless, grussy colluvium derived from weathered Idaho batholith granitic rocks. Many failures along the South Fork Payette River originated in ponderosa pine forests burned in the 1989 stand-replacing Lowman fire. An example is the 0Ð49 km 2 ‘Jughead’ Creek basin, where a single large colluvial failure produced almost 40% of the total volume eroded from the basin and generated a massive and rapid debris flow. Failures also occurred in steep, unburned, and unforested drainages such as Hopkins Creek. In this south-facing 0Ð58 km 2 basin, 15 colluvial hollows failed, but no single failure produced more than 10% of the total eroded volume. Sediment transport in Hopkins Creek occurred by prolonged sediment-charged sheetflooding. Despite vegetation differences, sediment yields from the geomorphically similar Hopkins Creek (¾42 000 Mg km � 2 ) and Jughead Creek (¾44 000 Mg km � 2 ) basins were quite similar. These 1997 erosion events are equivalent to several thousand years of sediment yield at low rates (2Ð7–30 Mg km 2 year � 1 ) measured by short-term sediment trapping and gauging in Idaho batholith watersheds. If similar large events were solely responsible for sediment export, recurrence intervals (RIs) of several hundred years would account for higher sediment yields averaged over ¾10 4 year from Idaho batholith watersheds. Dating of small fire-induced sheetflooding events in an early Holocene tributary junction fan of Jughead Creek indicates that frequent small sedimentation events (RI ³ 33–80 year) occurred between 7400 and 6600 cal year BP, with an average yield not greatly exceeding 16 Mg km � 2 year � 1 . Compared with the Holocene average, erosion rates during that 800 year period were unusually low, suggesting that sediment yields have not been constant over time, and that climatic variations and related fire regime changes may exert a strong influence on the probability of major erosional events. Copyright  2001 John Wiley & Sons, Ltd.

Hydrologic flowpaths in tropical rainforest soilscapes—a review

Abstract: A conceptual framework of hillslope hydrological response patterns and their environmental controls is used to evaluate past and present research sites in tropical rainforests with regard to their predominant controls and hydrological functioning. This evaluation highlights the importance of near-surface flowpaths for runoff generation in Acrisol landscapes, and lack thereof in Ferralsol landscapes. Both soil types tentatively define the width of a spectrum of possible hillslope hydrologic flowpaths, from predominantly vertical to predominantly lateral flowpaths. This result allows the testing of hypotheses concerning the soil geographical occurrence of particular processes of runoff generation even where soil hydraulic data are not available.

Seasonal variation in the energy and water exchanges above and below a larch forest in eastern Siberia

Abstract: The water and energy exchanges in forests form one of the most important hydro-meteorological systems. There have been far fewer investigations of the water and heat exchange in high latitude forests than of those in warm, humid regions. There have been few observations of this system in Siberia for an entire growing season, including the snowmelt and leaf-fall seasons. In this study, the characteristics of the energy and water budgets in an eastern Siberian larch forest were investigated from the snowmelt season to the leaf-fall season. The latent heat flux was strongly affected by the transpiration activity of the larch trees and increased quickly as the larch stand began to foliate. The sensible heat dropped at that time, although the net all-wave radiation increased. Consequently, the seasonal variation in the Bowen ratio was clearly ‘U’-shaped, and the minimum value (1·0) occurred in June and July. The Bowen ratio was very high (10–25) in early spring, just before leaf opening. The canopy resistance for a big leaf model far exceeded the aerodynamic resistance and fluctuated over a much wider range. The canopy resistance was strongly restricted by the saturation deficit, and its minimum value was 100 s m−1 (10 mm s−1 in conductance). This minimum canopy resistance is higher than values obtained for forests in warm, humid regions, but is similar to those measured in other boreal conifer forests. It has been suggested that the senescence of leaves also affects the canopy resistance, which was higher in the leaf-fall season than in the foliated season. The mean evapotranspiration rate from 21 April 1998 to 7 September 1998 was 1·16 mm day−1, and the maximum rate, 2·9 mm day−1, occurred at the beginning of July. For the growing season from 1 June to 31 August, this rate was 1·5 mm day−1. The total evapotranspiration from the forest (151 mm) exceeded the amount of precipitation (106 mm) and was equal to 73% of the total water input (211 mm), including the snow water equivalent. The understory evapotranspiration reached 35% of the total evapotranspiration, and the interception evaporation was 15% of the gross precipitation. The understory evapotranspiration was high and the interception evaporation was low because the canopy was sparse and the leaf area index was low. Copyright © 2001 John Wiley & Sons, Ltd.

A bivariate gamma distribution for use in multivariate flood frequency analysis

Abstract: A gamma distribution is one of the most frequently selected distribution types for hydrological frequency analysis. The bivariate gamma distribution with gamma marginals may be useful for analysing multivariate hydrological events. This study investigates the applicability of a bivariate gamma model with five parameters for describing the joint probability behavior of multivariate flood events. The parameters are proposed to be estimated from the marginal distributions by the method of moments. The joint distribution, the conditional distribution, and the associated return periods are derived from marginals. The usefulness of the model is demonstrated by representing the joint probabilistic behaviour between correlated flood peak and flood volume and between correlated flood volume and flood duration in the Madawask River basin in the province of Quebec, Canada. Copyright © 2001 John Wiley & Sons, Ltd.

A process for fire‐related debris flow initiation, Cerro Grande fire, New Mexico

Abstract: In this study we examine factors that pertain to the generation of debris flows from a basin recently burned by wildfire. Throughout the summer 2000 thunderstorm season, we monitored rain gauges, channel cross-sections, hillslope transects, and nine sediment-runoff traps deployed in a steep, 0.15 km 2 basin burned by the May 2000 Cerro Grande fire in New Mexico. Debris flows were triggered in the monitored basin during a rainstorm on July 16, 2000, in response to a maximum 30 min rainfall intensity of 31 mm h -1 (return period of approximately 2 years). Eleven other storms occurred before and after the July storm; these storms resulted in significant runoff, but did not generate debris flows. The debris flows generated by the July 16 storm initiated on a broad, open hillslope as levee-lined rills. The levees were composed of gravel- and cobble-sized material supported by an abundant fine-grained matrix. Debris-flow deposits were observed only on the hillslopes and in the first and second-order drainages of the monitored basin. No significant amounts of channel incision were measured following the passage of the debris flows, indicating that most of the material in the flows originated from the hillslopes. Sediment-runoff concentrations of between 0.23 and 0.81 kg l -1 (with a mean of 0.42 kg l -1 ) were measured from the hillslope traps following the debris-flow-producing storm. These concentrations, however, were not unique to the July 16 storm. The materials entrained by the July 16 storm contained a higher proportion of silt- plus clay-sized materials in the <2 mm fraction than the materials collected from storms that produced comparable sediment-runoff concentrations but not debris flows. The difference in materials demonstrates the critical role of the availability of fine-grained wood ash mantling the hillslopes in the runoff-dominated generation of post-wildfire debris flows. The highest sediment-runoff concentrations, again not unique to debris-flow production, were produced from maximum 30 min rainfall intensities greater than 20 mm h -1 .

Hydrology and dissolved organic carbon biogeochemistry in an ombrotrophic bog

Abstract: At the Mer Bleue bog, Ontario, Canada, DOC export measured at the basin outflow was � 8Ð3 s 3Ð 7gCm � 2 yr � 1 , and DOC loading via precipitation was estimated to be 1Ð5 s 0Ð 7gCm � 2 yr � 1 . Discharge and DOC export calculated using a Dupuit–Forchheimer approximation compared well (within 1 g C m � 2 yr � 1 ) to outflow estimates of DOC export, and confirmed that outflow measurements were a suitable proxy for DOC seepage at the peatland margins. DOC export was 12% of the magnitude of the residual carbon sink measured at the peatland. The [DOC] across groundwater transects decreased with depth, and [DOC] sampled below 0Ð75 m depths remained fairly constant over the study period. However, [DOC] exported through the acrotelm (0 to 0Ð45 m peat depth) was variable, ranging from 40 mg l � 1 after snowmelt to 70 mg l � 1 during the growing season. Fluorescence analysis revealed that exported DOC was ‘allochthonous-like’, whereas DOC in the catotelm (deeper layers of peat) became more ‘autochthonous-like’ with depth. A conceptual model is developed to summarize the hydrological processes and controls which affect DOC biogeochemistry at the Mer Bleue. Copyright  2001 John Wiley & Sons, Ltd.

Soil frost effects on soil water and runoff dynamics along a boreal forest transect: 1. Field investigations

Abstract: Soil frost effects on soil water and runoff dynamics along a boreal forest transect. 1. Field investigation