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Showing papers in "Hydrological Processes in 2011"


Journal ArticleDOI
TL;DR: In this paper, the authors identify the causal mechanisms that underpin the observed negative response exhibited by fish populations to enhanced fine sediment loads, and the variability across different fish species across different families.
Abstract: Elevated fine sediment input from terrestrial and aquatic sources as a result of anthropogenic activity is widely recognized to impact negatively on aquatic ecosystems. In rivers, freshwater fish are exposed to a range of impacts resulting from fine sediment pressures. To date, research on the effects of fine sediments on fish has been concentrated within relatively few families, notably the salmonidae. This paper reviews the literature describing indirect and direct impacts of fine sediment on freshwater fish as a contribution towards enhancing the understanding of the impacts of anthropogenic activities on freshwater ecosystems. We identify the causal mechanisms that underpin the observed negative response exhibited by fish populations to enhanced fine sediment loads, and the variability across different fish species.

457 citations


Journal ArticleDOI
TL;DR: A review of recent advances in flash flood forecasting can be found in this paper, where the authors focus on the use of quantitative precipitation estimates and forecasts, use of remotely sensed data in hydrological modelling, developments in forecasting models and techniques, and uncertainty estimates.
Abstract: Flash flooding is one of the most hazardous natural events, and it is frequently responsible for loss of life and severe damage to infrastructure and the environment. Research into the use of new modelling techniques and data types in flash flood forecasting has increased over the past decade, and this paper presents a review of recent advances that have emerged from this research. In particular, we focus on the use of quantitative precipitation estimates and forecasts, the use of remotely sensed data in hydrological modelling, developments in forecasting models and techniques, and uncertainty estimates. Over the past decade flash flood forecast lead-time has expanded up to six hours due to improved rainfall forecasts. However the largest source of uncertainty of flash flood forecasts remains unknown future precipitation. An increased number of physically based hydrological models have been developed and used for flash flood forecasting and they have been found to give more plausible results when compared with the results of conceptual, statistical, and neural network models. Among the three methods for deciding flash flood occurrence discussed in this review, the rainfall comparison method (flash flood guidance) is most commonly used for flash flood forecasting as it is easily understood by the general public. Unfortunately, no existing model is capable of making reliable flash flood forecasts in urban watersheds even though the incidence of urban flash flooding is increasing due to increasing urbanisation. Copyright © 2011 John Wiley & Sons, Ltd.

328 citations


Journal ArticleDOI
TL;DR: The surface energy balance algorithm for land (SEBAL) and mapping evapotranspiration at high resolution with Internalized Calibration (METRIC) as mentioned in this paper are two popular methods for estimating the evaporation rate.
Abstract: Surface Energy Balance Algorithms for Land (SEBAL) and Mapping EvapoTranspiration at high Resolution with Internalized Calibration (METRIC) are satellite-based image-processing models that calculate evapotranspiration (ET) as a residual of a surface energy balance. Both models are calibrated using inverse modelling at extreme conditions approach to develop image-specific estimations of the sensible heat flux (H) component of the surface energy balance and to effectively remove systematic biases in net radiation, soil heat flux, radiometric temperature and aerodynamic estimates. SEBAL and METRIC express the near-surface temperature gradient as an indexed function of radiometric surface temperature, eliminating the need for absolutely accurate surface temperature and the need for air temperature measurements. Slope and aspect functions and temperature lapsing are used in METRIC applications in mountainous terrains. SEBAL and METRIC algorithms are designed for relatively routine application by trained professionals familiar with energy balance, aerodynamics and basic radiation physics. The primary inputs for the models are short-wave and long-wave (thermal) images from satellite (e.g. Landsat and MODIS), a digital elevation model and ground-based weather data measured within or near the area of interest. ET ‘maps’ (i.e. images) developed using Landsat images provide means to quantify ET on a field basis in terms of both rate and spatial distribution. METRIC takes advantage of calibration using weather-based reference ET so that both calibration and extrapolation of instantaneous ET to 24-h and longer periods compensate for regional advection effects where ET can exceed daily net radiation. SEBAL and METRIC have advantages over conventional methods of estimating ET using crop coefficient curves or vegetation indices in that specific crop or vegetation type does not need to be known and the energy balance can detect reduced ET caused by water shortage, salinity or frost as well as evaporation from bare soil. Copyright © 2011 John Wiley & Sons, Ltd.

312 citations


Journal ArticleDOI
TL;DR: In this article, the authors show that large-scale river flow datasets are crucial to advance hydrological science and propose ways forward to consolidate historical data and secure future river flow data.
Abstract: Time-series for river gauging stations are core blue-skies and applied research resources for understanding impacts of climate and anthropogenic change on basin hydrology. River flow archives hold vital information for evidence-based assessment of past hydrological variability, and support hydrological modelling of future changes. River discharge is an integration of basin input, storage and transfer processes to the gauging point. It is important to set basin outlet data in regional to global and long-term contexts: to better understand nested scales of variability; to pinpoint locations and time periods most sensitive to climate and human impacts; to make predictions for ungauged basins; and to inform decision makers on water security issues, and where and when to take measures to mitigate water hazards and stress, including floods and droughts (Dai et al., 2009; Bonnell et al., 2006; Feyen & Dankers, 2009; Haddeland et al., 2006; Hannah et al., 2005). Thus, there is clear rationale for supporting large-scale (i.e. regional to continental to global) river flow archives. Notable examples of such databases include that held by the WMO Global Runoff Data Centre (GRDC) and the UNESCO Flow Regimes from International Experimental and Network Data (FRIEND) European Water Archive (EWA). For large-scale river flow archives to be valuable research resources, they must be fit for purpose. However, these databases are at risk due to a possible decline in network coverage, associated time-series truncation, growing human impact on (near-) natural flows, and increasingly restricted access to national-scale data. This commentary aims: (1) to demonstrate largescale river flow datasets are crucial to advance hydrological science and solve operational issues; (2) to assess the current status of large-scale river flow datasets; and (3) to propose ways forward to consolidate historical data and secure future river flow data.

298 citations


Journal ArticleDOI
TL;DR: In this paper, the authors presented a regional map of modelled amounts of fog interception across the tropical montane cloud forests (TMCF) and showed major spatial variability, showing that field-measured annual ‘cloud-water interception (CWI) totals determined with the wet-canopy water budget method (WCWB) vary widely between locations and range between 22 and 1990 mm.
Abstract: Tropical montane cloud forests (TMCF) typically experience conditions of frequent to persistent fog. On the basis of the altitudinal limits between which TMCF generally occur (800–3500 m.a.s.l. depending on mountain size and distance to coast) their current areal extent is estimated at ∼215 000 km2 or 6·6% of all montane tropical forests. Alternatively, on the basis of remotely sensed frequencies of cloud occurrence, fog-affected forest may occupy as much as 2·21 Mkm2. Four hydrologically distinct montane forest types may be distinguished, viz. lower montane rain forest below the cloud belt (LMRF), tall lower montane cloud forest (LMCF), upper montane cloud forest (UMCF) of intermediate stature and a group that combines stunted sub-alpine cloud forest (SACF) and ‘elfin’ cloud forest (ECF). Average throughfall to precipitation ratios increase from 0·72 ± 0·07 in LMRF (n = 15) to 0·81 ± 0·11 in LMCF (n = 23), to 1·0 ± 0·27 (n = 18) and 1·04 ± 0·25 (n = 8) in UMCF and SACF–ECF, respectively. Average stemflow fractions increase from LMRF to UMCF and ECF, whereas leaf area index (LAI) and annual evapotranspiration (ET) decrease along the same sequence. Although the data sets for UMCF (n = 3) and ECF (n = 2) are very limited, the ET from UMCF (783 ± 112 mm) and ECF (547 ± 25 mm) is distinctly lower than that from LMCF (1188 ± 239 mm, n = 9) and LMRF (1280 ± 72 mm; n = 7). Field-measured annual ‘cloud-water’ interception (CWI) totals determined with the wet-canopy water budget method (WCWB) vary widely between locations and range between 22 and 1990 mm (n = 15). Field measured values also tend to be much larger than modelled amounts of fog interception, particularly at exposed sites. This is thought to reflect a combination of potential model limitations, a mismatch between the scale at which the model was applied (1 × 1 km) and the scale of the measurements (small plots), as well as the inclusion of near-horizontal wind-driven precipitation in the WCWB-based estimate of CWI. Regional maps of modelled amounts of fog interception across the tropics are presented, showing major spatial variability. Modelled contributions by CWI make up less than 5% of total precipitation in wet areas to more than 75% in low-rainfall areas. Catchment water yields typically increase from LMRF to UMCF and SACF–ECF reflecting concurrent increases in incident precipitation and decreases in evaporative losses. The conversion of LMCF (or LMRF) to pasture likely results in substantial increases in water yield. Changes in water yield after UMCF conversion are probably modest due to trade-offs between concurrent changes in ET and CWI. General circulation model (GCM)-projected rates of climatic drying under SRES greenhouse gas scenarios to the year 2050 are considered to have a profound effect on TMCF hydrological functioning and ecology, although different GCMs produce different and sometimes opposing results. Whilst there have been substantial increases in our understanding of the hydrological processes operating in TMCF, additional research is needed to improve the quantification of occult precipitation inputs (CWI and wind-driven precipitation), and to better understand the hydrological impacts of climate- and land-use change. Copyright © 2010 John Wiley & Sons, Ltd.

295 citations


Journal ArticleDOI
TL;DR: In this paper, the uncertainties associated with atmosphere-ocean General Circulation Models (GCMs) and hydrologic models are assessed by means of multi-modeling and using the statistically downscaled outputs from eight GCM simulations and two emission scenarios.
Abstract: The uncertainties associated with atmosphere-ocean General Circulation Models (GCMs) and hydrologic models are assessed by means of multi-modelling and using the statistically downscaled outputs from eight GCM simulations and two emission scenarios. The statistically downscaled atmospheric forcing is used to drive four hydrologic models, three lumped and one distributed, of differing complexity: the Sacramento Soil Moisture Accounting (SAC-SMA) model, Conceptual HYdrologic MODel (HYMOD), Thornthwaite-Mather model (TM) and the Precipitation Runoff Modelling System (PRMS). The models are calibrated based on three objective functions to create more plausible models for the study. The hydrologic model simulations are then combined using the Bayesian Model Averaging (BMA) method according to the performance of each models in the observed period, and the total variance of the models. The study is conducted over the rainfall-dominated Tualatin River Basin (TRB) in Oregon, USA. This study shows that the hydrologic model uncertainty is considerably smaller than GCM uncertainty, except during the dry season, suggesting that the hydrologic model selection-combination is critical when assessing the hydrologic climate change impact. The implementation of the BMA in analysing the ensemble results is found to be useful in integrating the projected runoff estimations from different models, while enabling to assess the model structural uncertainty. Copyright © 2011 John Wiley & Sons, Ltd.

232 citations


Journal ArticleDOI
TL;DR: In this article, the authors developed a method to determine crop water needs based on the evapotranspiration (ET) of a reference crop under a given set of meteorological conditions, which can help reduce agricultural water use by matching irrigation rates to the actual water needs of a crop as it grows.
Abstract: Crop coefficients were developed to determine crop water needs based on the evapotranspiration (ET) of a reference crop under a given set of meteorological conditions. Starting in the 1980s, crop coefficients developed through lysimeter studies or set by expert opinion began to be supplemented by remotely sensed vegetation indices (VI) that measured the actual status of the crop on a field-by-field basis. VIs measure the density of green foliage based on the reflectance of visible and near infrared (NIR) light from the canopy, and are highly correlated with plant physiological processes that depend on light absorption by a canopy such as ET and photosynthesis. Reflectance-based crop coefficients have now been developed for numerous individual crops, including corn, wheat, alfalfa, cotton, potato, sugar beet, vegetables, grapes and orchard crops. Other research has shown that VIs can be used to predict ET over fields of mixed crops, allowing them to be used to monitor ET over entire irrigation districts. VI-based crop coefficients can help reduce agricultural water use by matching irrigation rates to the actual water needs of a crop as it grows instead of to a modeled crop growing under optimal conditions. Recently, the concept has been applied to natural ecosystems at the local, regional and continental scales of measurement, using time-series satellite data from the MODIS sensors on the Terra satellite. VIs or other visible-NIR band algorithms are combined with meteorological data to predict ET in numerous biome types, from deserts, to arctic tundra, to tropical rainforests. These methods often closely match ET measured on the ground at the global FluxNet array of eddy covariance moisture and carbon flux towers. The primary advantage of VI methods for estimating ET is that transpiration is closely related to radiation absorbed by the plant canopy, which is closely related to VIs. The primary disadvantage is that they cannot capture stress effects or soil evaporation. Copyright © 2011 John Wiley & Sons, Ltd.

206 citations


Journal ArticleDOI
TL;DR: On red herrings and real herrings: disinformation and information in hydrological inference as discussed by the authors, where the authors focus on two types of herrings, namely, false information and real information.
Abstract: On red herrings and real herrings: disinformation and information in hydrological inference

196 citations


Journal ArticleDOI
TL;DR: In this paper, the authors examined the spatial coherence of drought in Europe using regional indicators of precipitation and streamflow deficit, and developed a methodology for forecasting the termination of drought conditions.
Abstract: Droughts tend to evolve slowly and affect large areas simultaneously, which suggests that improved understanding of spatial coherence of drought would enable better mitigation of drought impacts through enhanced monitoring and forecasting strategies. This study employs an up-to-date dataset of over 500 river flow time series from eleven European countries, along with a gridded precipitation dataset, to examine the spatial coherence of drought in Europe using regional indicators of precipitation and streamflow deficit. The drought indicators were generated for 23 homogeneous regions, and for selected regions historical drought characteristics were corroborated with previous work. The spatial coherence of drought characteristics was then examined at a European scale. Historical droughts generally have distinctive signatures in their spatio-temporal development, so there was limited scope for using the evolution of historical events to inform forecasting. Rather, relationships were explored in time series of drought indicators between regions. Correlations were generally low, but multivariate analyses revealed broad continental-scale patterns which appear to be related to large-scale atmospheric circulation indices (in particular, the North Atlantic Oscillation and the East Atlantic-West Russia pattern). A novel methodology for forecasting was developed (and demonstrated with reference to the United Kingdom) which predicts ‘drought from drought’ – i.e. uses spatial coherence of drought to facilitate early warning of drought in a target region, from drought which is developing elsewhere in Europe. Whilst the skill of the methodology is relatively modest at present, this approach presents a potential new avenue for forecasting, which offers significant advantages in that it allows prediction for all seasons , and also shows some potential for forecasting the termination of drought conditions.

189 citations


Journal ArticleDOI
TL;DR: In this paper, the authors demonstrate the importance of horizontal heat flows in thawing discontinuous permafrost, and show that such thaw produces dramatic land-cover changes that can alter basin runoff production in this region.
Abstract: Climate warming and human disturbance in north-western Canada have been accompanied by degradation of permafrost, which introduces considerable uncertainty to the future availability of northern freshwater resources. This study demonstrates the rate and spatial pattern of permafrost loss in a region that typifies the southern boundary of permafrost. Remote-sensing analysis of a 1·0 km2 area indicates that permafrost occupied 0·70 km2 in 1947 and decreased with time to 0·43 km2 by 2008. Ground-based measurements demonstrate the importance of horizontal heat flows in thawing discontinuous permafrost, and show that such thaw produces dramatic land-cover changes that can alter basin runoff production in this region. A major challenge to northern water resources management in the twenty-first century therefore lies in predicting stream flows dynamically in the context of widely occurring permafrost thaw. The need for appropriate water resource planning, mitigation, and adaptation strategies is explained. Copyright © 2010 John Wiley & Sons, Ltd.

183 citations


Journal ArticleDOI
TL;DR: In this article, the authors present a catchment comparison exercise to evaluate the value of viewing catchments from the storage perspective, based on existing data from five watersheds, no common experimental design, and no integrated modelling efforts.
Abstract: The volume of water stored within a catchment, and its partitioning among groundwater, soil moisture, snowpack, vegetation, and surface water are the variables that ultimately characterize the state of the hydrologic system. Accordingly, storage may provide useful metrics for catchment comparison. Unfortunately, measuring and predicting the amount of water present in a catchment is seldom done; tracking the dynamics of these stores is even rarer. Storage moderates fluxes and exerts critical controls on a wide range of hydrologic and biologic functions of a catchment. While understanding runoff generation and other processes by which catchments release water will always be central to hydrologic science, it is equally essential to understand how catchments retain water. We have initiated a catchment comparison exercise to begin assessing the value of viewing catchments from the storage perspective. The exercise is based on existing data from five watersheds, no common experimental design, and no integrated modelling efforts. Rather, storage was estimated independently for each site. This briefing presents some initial results of the exercise, poses questions about the definitions and importance of storage and the storage perspective, and suggests future directions for ongoing activities. Copyright. (C) 2011 John Wiley & Sons, Ltd.

Journal ArticleDOI
TL;DR: In this paper, the authors compare quantitative measures of soil water retention capacity for two opposing slopes in a semi-arid catchment in southwest Idaho, USA, and find that soils on the north aspect retain as much as 25% more water at any given soil water pressure than samples from the south aspect slope.
Abstract: : Many catchment hydrologic and ecologic processes are impacted by the storage capacity of soil water, which is dictated by the profile thickness and water retention properties of soil. Soil water retention properties are primarily controlled by soil texture, which in turn varies spatially in response to microclimate-induced differences in insolation, wetness, and temperature. All of these variables can be strongly differentiated by slope aspect. In this study, we compare quantitative measures of soil water retention capacity for two opposing slopes in a semi-arid catchment in southwest Idaho, USA. Undisturbed soil cores from north and south aspects were subjected to a progressive drainage experiment to estimate the soil water retention curve for each sample location. The relatively large sample size (35) supported statistical analysis of slope scale differences in soil water retention between opposing aspects. Soils on the north aspect retain as much as 25% more water at any given soil water pressure than samples from the south aspect slope. Soil porosity, soil organic matter, and silt content were all greater on the north aspect, and each contributed to greater soil water retention. These results, along with the observation that soils on north aspect slopes tend to be deeper indicate that north aspect slopes can store more water from the wet winter months into the dry summer in this region, an observation with potential implications for ecological function and landscape evolution.

Journal ArticleDOI
TL;DR: In this article, the authors used a simple water balance analysis to estimate watershed storage changes during a rainy season (dV) and found that the watersheds store significant amounts of rainfall with little corresponding runoff in the beginning of the wet season and then release considerably more water to the streams after they reach and exceed their storage capacities.
Abstract: Subsurface runoff dominates the hydrology of many steep humid regions, and yet the basic elements of water collection, storage, and discharge are still poorly understood at the watershed scale. Here, we use exceptionally dense rainfall and runoff records from two Northern California watersheds (~100 km2) with distinct wet and dry seasons to ask the simple question: how much water can a watershed store? Stream hydrographs from 17 sub-watersheds through the wet season are used to answer this question where we use a simple water balance analysis to estimate watershed storage changes during a rainy season (dV). Our findings suggest a pronounced storage limit and then ‘storage excess’ pattern; i.e. the watersheds store significant amounts of rainfall with little corresponding runoff in the beginning of the wet season and then release considerably more water to the streams after they reach and exceed their storage capacities. The amount of rainfall required to fill the storages at our study watersheds is the order of a few hundred millimeters (200–500 mm). For each sub-watershed, we calculated a variety of topographic indices and regressed these against maximum dV. Among various indices, median gradient showed the strongest control on dV where watershed median slope angle was positively related to the maximum volume of storage change. We explain this using a hydrologically active bedrock hypothesis whereby the amount of water a watershed can store is influenced by filling of unrequited storage in bedrock. The amount of water required to activate rapid rainfall–runoff response is larger for steeper watersheds where the more restricted expansion of seepage from bedrock to the soil limits the connectivity between stored water and stream runoff. Copyright © 2011 John Wiley & Sons, Ltd.

Journal ArticleDOI
TL;DR: In this paper, the Macro-scale Probability-Distributed Moisture model (Mac-PDM.09) has been used to model the land surface hydrologic dynamics of continental-scale river basins.
Abstract: Global hydrological models (GHMs) model the land surface hydrologic dynamics of continental-scale river basins. Here we describe one such GHM, the Macro-scale - Probability-Distributed Moisture model.09 (Mac-PDM.09). The model has undergone a number of revisions since it was last applied in the hydrological literature. This paper serves to provide a detailed description of the latest version of the model. The main revisions include the following: (1) the ability for the model to be run for n repetitions, which provides more robust estimates of extreme hydrological behaviour, (2) the ability of the model to use a gridded field of coefficient of variation (CV) of daily rainfall for the stochastic disaggregation of monthly precipitation to daily precipitation, and (3) the model can now be forced with daily input climate data as well as monthly input climate data. We demonstrate the effects that each of these three revisions has on simulated runoff relative to before the revisions were applied. Importantly, we show that when Mac-PDM.09 is forced with monthly input data, it results in a negative runoff bias relative to when daily forcings are applied, for regions of the globe where the day-to-day variability in relative humidity is high. The runoff bias can be up to - 80% for a small selection of catchments but the absolute magnitude of the bias may be small. As such, we recommend future applications of Mac-PDM.09 that use monthly climate forcings acknowledge the bias as a limitation of the model. The performance of Mac-PDM.09 is evaluated by validating simulated runoff against observed runoff for 50 catchments. We also present a sensitivity analysis that demonstrates that simulated runoff is considerably more sensitive to method of PE calculation than to perturbations in soil moisture and field capacity parameters.

Journal ArticleDOI
TL;DR: In this paper, the uncertainties in global land evapotranspiration (ET) estimates using three process-based ET models and a set of remote sensing and observational based radiation and meteorological forcing datasets are explored.
Abstract: Estimating evapotranspiration (ET) at continental to global scales is central to understanding the partitioning of energy and water at the earth's surface and the feedbacks with the atmosphere and biosphere, especially in the context of climate change. Recent evaluations of global estimates from remote sensing, upscaled observations, land surface models and atmospheric reanalyses indicate large uncertainty across the datasets of the order of 50% of the global annual mean value. In this paper, we explore the uncertainties in global land ET estimates using three process-based ET models and a set of remote sensing and observational based radiation and meteorological forcing datasets. Input forcings were obtained from International Satellite Cloud Climatology Project (ISCCP) and Surface Radiation Budget (SRB). The three process-based ET models are: a surface energy balance method (SEBS), a revised Penman–Monteith (PM) model, and a modified Priestley–Taylor model. Evaluations of the radiation products from ISCCP and SRB show large differences in the components of surface radiation, and temporal inconsistencies that relate to changes in satellite sensors and retrieval algorithms. In particular, step changes in the ISCCP surface temperature and humidity data lead to spurious increases in downward and upward longwave radiation that contributes to a step change in net radiation, and the ISCCP data are not used further. An ensemble of global estimates of land surface ET are generated at daily time scale and 0.5 degree spatial resolution for 1984–2007 using two SRB radiation products (SRB and SRBqc) and the three models. Uncertainty in ET from the models is much larger than the uncertainty from the radiation data. The largest uncertainties relative to the mean annual ET are in transition zones between dry and humid regions and monsoon regions. Comparisons with previous studies and an inferred estimate of ET from long-term inferred ET indicate that the ensemble mean value is reasonable, but generally biased high globally. Long-term changes over 1984–2007 indicate a slight increase over 1984–1998 and decline thereafter, although uncertainties in the forcing radiation data and lack of direct linkage with soil moisture limitations in the models prevents attribution of these changes. Copyright © 2011 John Wiley & Sons, Ltd.

Journal ArticleDOI
TL;DR: In this article, the effects of climate variability and human activities on runoff were quantified using three methods, namely, multi-regression, hydrologic sensitivity analysis, and hydrological model simulation.
Abstract: Much attention has recently been focused on the effects that climate variability and human activities have had on runoff. In this study, these effects are quantified using three methods, namely, multi-regression, hydrologic sensitivity analysis, and hydrologic model simulation. A conceptual framework is defined to separate the effects. As an example, the change in annual runoff from the semiarid Laohahe basin (18 112 km2) in northern China was investigated. Non-parametric Mann-Kendall test, Pettitt test, and precipitation-runoff double cumulative curve method were adopted to identify the trends and change-points in the annual runoff from 1964 to 2008 by first dividing the long-term runoff series into a natural period (1964–1979) and a human-induced period (1980–2008). Then the three quantifying methods were calibrated and calculated, and they provided consistent estimates of the percentage change in mean annual runoff for the human-induced period. In 1980–2008, human activities were the main factors that reduced runoff with contributions of 89–93%, while the reduction percentages due to changes in precipitation and potential evapotranspiration only ranged from 7 to 11%. For the various effects at different durations, human activities were the main reasons runoff decreased during the two drier periods of 1980–1989 and 2000–2008. Increased runoff during the wetter period of 1990–1999 is mainly attributed to climate variability. This study quantitatively separates the effects of climate variability and human activities on runoff, which can serve as a reference for regional water resources assessment and management. Copyright © 2011 John Wiley & Sons, Ltd.

Journal ArticleDOI
TL;DR: In this article, uncertainty in discharge data must be critically assessed before data can be used in, e. g. water resources estimation or hydrological modelling. In the alluvial Choluteca River in Honduras, the ri...
Abstract: Uncertainty in discharge data must be critically assessed before data can be used in, e. g. water resources estimation or hydrological modelling. In the alluvial Choluteca River in Honduras, the ri ...

Journal ArticleDOI
TL;DR: In this paper, the effects of the hyporheic zone on stream ecosystems were examined using data from existing groundwater modelling studies of hyporheeic exchange flow at five sites in a fifth-order, mountainous stream network.
Abstract: Many hyporheic papers state that the hyporheic zone is a critical component of stream ecosystems, and many of these papers focus on the biogeochemical effects of the hyporheic zone on stream solute loads. However, efforts to show such relationships have proven elusive, prompting several questions: Are the effects of the hyporheic zone on stream ecosystems so highly variable in place and time (or among streams) that a consistent relationship should not be expected? Or, is the hyporheic zone less important in stream ecosystems than is commonly expected? These questions were examined using data from existing groundwater modelling studies of hyporheic exchange flow at five sites in a fifth-order, mountainous stream network. The size of exchange flows, relative to stream discharge (QHEF :Q), was large only in very small streams at low discharge (area ³100 ha; Q 0·7) and in all larger streams, QHEF :Q was small. These data show that biogeochemical processes in the hyporheic zone of small streams can substantially influence the stream’s solute load, but these processes become hydrologically constrained at high discharge or in larger streams and rivers. The hyporheic zone may influence stream ecosystems in many ways, however, not just through biogeochemical processes that alter stream solute loads. For example, the hyporheic zone represents a unique habitat for some organisms, with patterns and amounts of upwelling and downwelling water determining the underlying physiochemical environment of the hyporheic zone. Similarly, hyporheic exchange creates distinct patches of downwelling and upwelling. Upwelling environments are of special interest, because upwelling water has the potential to be thermally or chemically distinct from stream water. Consequently, micro-environmental patches created by hyporheic exchange flows are likely to be important to biological and ecosystem processes, even if their impact on stream solute loads is small. Published in 2011 by John Wiley & Sons, Ltd.

Journal ArticleDOI
TL;DR: A review of methods in estimating basin scale evapotranspiration and its applications in understanding basin water balance dynamics is presented in this article, where the authors identify the need to compare and evaluate the different ET approaches using standard data sets in basins covering different hydro-climatic regions of the world.
Abstract: Evapotranspiration (ET) is an important hydrological process that can be studied and estimated at multiple spatial scales ranging from a leaf to a river basin. We present a review of methods in estimating basin scale ET and its applications in understanding basin water balance dynamics. The review focuses on two aspects of ET: (i) how the basin scale water balance approach is used to estimate ET; and (ii) how ‘direct’ measurement and modelling approaches are used to estimate basin scale ET. Obviously, the basin water balance-based ET requires the availability of good precipitation and discharge data to calculate ET as a residual on longer time scales (annual) where net storage changes are assumed to be negligible. ET estimated from such a basin water balance principle is generally used for validating the performance of ET models. On the other hand, many of the direct estimation methods involve the use of remotely sensed data to estimate spatially explicit ET and use basin-wide averaging to estimate basin scale ET. The direct methods can be grouped into soil moisture balance modelling, satellite-based vegetation index methods, and methods based on satellite land surface temperature measurements that convert potential ET into actual ET using a proportionality relationship. The review also includes the use of complementary ET estimation principles for large area applications. The review identifies the need to compare and evaluate the different ET approaches using standard data sets in basins covering different hydro-climatic regions of the world. Copyright © 2011 John Wiley & Sons, Ltd.

Journal ArticleDOI
TL;DR: In this article, a new in situ monthly rainfall dataset has been collected (1998-2007) and compared with Tropical Rainfall Measuring Mission (TRMM) 3B43 monthly precipitation data for regions located above 3000 m. In the Peruvian mountain ranges, these correction models better approximate TRMM rainfall monthly values, as already verified for annual values.
Abstract: In an attempt to estimate the spatial and temporal behaviour of rainfall over the mountainous areas of the Peruvian Andes, a new in situ monthly rainfall dataset has been collected (1998–2007) and compared with Tropical Rainfall Measuring Mission (TRMM) 3B43 monthly precipitation data for regions located above 3000 m. The reliability of the TRMM 3B43 data varies depending on the root mean squared error ratio (%RMSE) and correlation coefficient. Because of the discrepancy between the two datasets, the use of additive and multiplicative correction models is proposed for the TRMM 3B43 data. In the Peruvian mountain ranges, these correction models better approximate TRMM rainfall monthly values, as already verified for annual values. Copyright © 2011 John Wiley & Sons, Ltd.

Journal ArticleDOI
TL;DR: In this article, a three-component mixing model has been employed using the values of δ18O and electrical conductivity (EC) of the river water, and its constituents, to estimate the time-varying relative fraction of each component.
Abstract: The observed retreat of several Himalayan glaciers and snow packs is a cause of concern for the huge population in southern Asia that is dependent on the glacial-fed rivers emanating from Himalayas There is considerable uncertainty about how cryospheric recession in the Himalayan region will respond to climate change, and how the water resource availability will be affected As a first step towards quantifying the contribution of glacier-melt water, hydrograph separation of River Ganga at Rishikesh into its constituent components, namely (i) surface runoff, (ii) glacial ice-melt and (iii) groundwater discharge has been done in this paper A three-component mixing model has been employed using the values of δ18O and electrical conductivity (EC) of the river water, and its constituents, to estimate the time-varying relative fraction of each component The relative fraction of the surface runoff peaks (70–90%) during winter, due to the near-zero contribution of glacial ice-melt, essentially represents the melting of surface snow from the catchment The contribution of glacial ice-melt to the stream discharge peaks during summer and monsoon reaches a maximum value of ∼40% with an average of 32% The fraction of groundwater discharge varies within a narrow range (15 ± 5%) throughout the year On the basis of the variation in the d-excess values of river water, it is also suggested that the snow-melt and ice-melt component has a significant fraction derived from winter precipitation with moisture source from mid-latitude westerlies (also known as western disturbances) Copyright © 2010 John Wiley & Sons, Ltd

Journal ArticleDOI
TL;DR: In this article, the authors suggest an alternative approach where different sources of field data are used to build a synthesis of dominant hydrological processes and hence provide recommendations for representing those processes in a time-stepping simulation model.
Abstract: Hydrological scientists develop perceptual models of the catchments they study, using field measurements and observations to build an understanding of the dominant processes controlling the hydrological response. However, conceptual and numerical models used to simulate catchment behaviour often fail to take advantage of this knowledge. It is common instead to use a pre-defined model structure which can only be fitted to the catchment via parameter calibration. In this article, we suggest an alternative approach where different sources of field data are used to build a synthesis of dominant hydrological processes and hence provide recommendations for representing those processes in a time-stepping simulation model. Using analysis of precipitation, flow and soil moisture data, recommendations are made for a comprehensive set of modelling decisions, including Evapotranspiration (ET) parameterization, vertical drainage threshold and behaviour, depth and water holding capacity of the active soil zone, unsaturated and saturated zone model architecture and deep groundwater flow behaviour. The second article in this two-part series implements those recommendations and tests the capability of different model sub-components to represent the observed hydrological processes. Copyright © 2010 John Wiley & Sons, Ltd.

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TL;DR: In this paper, the storage-discharge relationship of two nested (3·6 and 30·4 km2) upland catchments using rainfall-runoff models was investigated using a nonlinear discharge sensitivity function and a tracer-constrained process-based conceptual model.
Abstract: We investigate the storage–discharge relationships of two nested (3·6 and 30·4 km2) upland catchments using rainfall–runoff models: (1) a nonlinear discharge sensitivity function and (2) a tracer-constrained process-based conceptual model. Both approaches explicitly acknowledge that water storage is neither time nor space invariant and this provided insight into the respective scaling relationships. Both modelling approaches consistently demonstrated small seasonal storage fluctuations consistent with the wet, cool Scottish climate: the smaller catchment exhibited a greater average dynamic storage (ca 55 mm) compared to the larger scale (ca 40 mm). However, there are differences in simulated storage quantities and ranges inferred from both models, which can largely be explained by model uncertainties and model assumptions. In contrast to the dynamic (active) storage indicated by the rainfall-runoff models, input-output relationships of δ18O in both catchments were used to estimate the passive storage available for mixing and tracer damping in streams. This showed that catchment storage is an order of magnitude greater (ranging from 500 to 900 mm) than the dynamic storage estimated by both models, though again, storage estimates were greater for the smaller catchment. The passive storage inferred for mixing indicates that discussion of dynamic storage revealed by water balance considerations masks a much larger catchment storage which may in turn determine sensitivity to environmental change. Copyright © 2011 John Wiley & Sons, Ltd.

Journal ArticleDOI
TL;DR: In this article, an assessment of EFAS results based on European Centre for Medium Range Weather Forecasts (ECMWF) weather forecasts over a period of 10 years is presented, where the data are analysed with regard to skill, bias and quality of river discharge forecast.
Abstract: The European Flood Alert System (EFAS) provides early flood alerts on a pre-operational basis to National hydrological services. EFAS river discharge forecasts are based on probabilistic techniques, using ensemble system and deterministic numerical weather prediction data. The performance of EFAS is regularly analysed with regard to individual flood events and case studies. Although this analysis provides important insight into the strengths and weaknesses of the forecast system, it lacks statistical and independent measures of its long-term performance. In this paper, an assessment of EFAS results based on European Centre for Medium Range Weather Forecasts (ECMWF) weather forecasts over a period of 10 years is presented. EFAS river discharge forecasts have been rerun every week for a period of 10 years using the weather forecast available at the time. These are evaluated for a total of 1025 river gauging stations distributed across Europe. The selected stations are sufficiently separated in space to avoid autocorrelation of station time series. Also, analysis is performed with a gap of 3 days between each forecast which reduces the temporal correlation of the time series of the same station. The data are analysed with regard to skill, bias and quality of river discharge forecast. The 10-year simulations clearly show that the skill of the river discharge forecasts has undergone an evolution linked to the quality of the operational meteorological forecast. Overall, over a period of 10 years, the skill of the EFAS forecasts has steadily increased. Important hydrological extreme events cannot be clearly identified with the skill score analysis, highlighting the necessity for event-based analysis in addition to statistical long-term assessments for a better understanding of the EFAS system and large scale river discharge predictions in general. The predictability is shown to depend on catchment size and geographical location. It can be shown that performance tends to increase with catchment size and is influenced by the underlying topography in Europe. Copyright © 2010 John Wiley & Sons, Ltd.

Journal ArticleDOI
TL;DR: In this paper, the authors explored links between severe hydrological droughts and weather types (WTs) to improve the understanding of hydroclimatological processes involved in the development of regional hydrologogical drought in north-western Europe.
Abstract: Links between severe hydrological droughts and weather types (WTs) were explored to improve the understanding of hydroclimatological processes involved in the development of regional hydrological drought in north-western Europe. A new Regional Drought Area Index (RDAI) was developed, using daily streamflow, to represent the drought-affected area. Daily RDAI series (1964–2001) were created for two regions with homogeneous drought behaviour in Denmark and four regions in Great Britain. An objective version of the Hess–Brezowsky Grosswetterlagen yielding 29 WTs was used. Regional drought characteristics, including duration and frequency, were found to vary considerably between regions. However, in 1976 and 1996, all regions experienced severe events, and these years were found to be the most severe drought years across the study region as a whole. The hydrological response time (i.e. the time over which WTs influence drought development) was found to vary markedly (45–210 days) between regions according to basin storage properties. WT-frequency anomalies (FAs) before and during the onset of the five most severe droughts were identified for each region. The dominant drought-yielding WTs changed between regions and between events within each region. High-pressure systems centred over the respective region were most frequently associated with droughts as well as WTs with a northern (N, NE or NW) or a southern (S, SE or SW) airflow over the Danish and British regions. Five of the six WTs associated with drought for all regions represented a northern high-pressure system (i.e. over Great Britain, Fennoscandia or the Norwegian Sea). This article demonstrates (1) hydrological response time to be fundamental in moderating drought response to mesoscale climatic drivers and (2) severe hydrological droughts may be caused by a complex set of hydroclimatological processes that vary between regions and events. Copyright © 2010 John Wiley & Sons, Ltd.

Journal ArticleDOI
TL;DR: In this article, a physically based water balance was coded in the SWAT model to replace the traditional curve number (CN) method of runoff generation, which is less than ideal in many situations (e.g. monsoonal climates and areas dominated by variable source area hydrology).
Abstract: Watershed scale hydrological and biogeochemical models rely on the correct spatial-temporal prediction of processes governing water and contaminant movement. The Soil and Water Assessment Tool (SWAT) model, one of the most commonly used watershed scale models, uses the popular curve number (CN) method to determine the respective amounts of infiltration and surface runoff. Although appropriate for flood forecasting in temperate climates, the CN method has been shown to be less than ideal in many situations (e.g. monsoonal climates and areas dominated by variable source area hydrology). The CN model is based on the assumption that there is a unique relationship between the average moisture content and the CN for all hydrologic response units (HRUs), and that the moisture content distribution is similar for each runoff event, which is not the case in many regions. Presented here is a physically based water balance that was coded in the SWAT model to replace the CN method of runoff generation. To compare this new water balance SWAT (SWAT-WB) to the original CN-based SWAT (SWAT-CN), two watersheds were initialized; one in the headwaters of the Blue Nile in Ethiopia and one in the Catskill Mountains of New York. In the Ethiopian watershed, streamflow predictions were better using SWAT-WB than SWAT-CN [Nash–Sutcliffe efficiencies (NSE) of 0·79 and 0·67, respectively]. In the temperate Catskills, SWAT-WB and SWAT-CN predictions were approximately equivalent (NSE > 0·70). The spatial distribution of runoff-generating areas differed greatly between the two models, with SWAT-WB reflecting the topographical controls imposed on the model. Results show that a water balance provides results equal to or better than the CN, but with a more physically based approach. Copyright © 2010 John Wiley & Sons, Ltd.

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TL;DR: In this paper, the authors proposed that more emphasis should be placed on developing generic modelling toolkits with the functionality for coupling current or future projected sediment regimes with biological response for a range of biota, which should permit the identification of river catchment specific targets within a national context, based on biological effect and incorporate sufficient flexibility for utilizing updated physical, chemical, biological and catchment attribute data.
Abstract: Sediment plays a pivotal role in determining the physical, chemical and biological integrity of aquatic ecosystems. A range of factors influences the impacts of sediment pressures on aquatic biota, including concentration, duration of exposure, composition and particle size. In recognition of the need to assess environmental status for sediment and mitigate excessive sediment pressures on aquatic habitats, both water column and river substrate metrics have been proposed as river sediment targets. Water column metrics include light penetration, turbidity, sediment concentration summary statistics and sediment regimes. Substrate metrics include embeddedness, the fredle index and riffle stability. Identification of meaningful numeric targets along these lines has, however, been undermined by various issues including the uncertainty associated with toxicological dose-response profiles and the impracticalities of deploying statistically robust sampling strategies capable of supporting catchment-scale targets. Many of the thresholds reported are based on correlative relationships that fail to capture the specific mechanisms controlling sediment impacts on aquatic habitats and are stationary in nature. Temporal windows represented by the key life stages of specific species must be given greater emphasis. Given such issues and the need to support the revision of sediment targets for river catchment management, it is proposed that greater emphasis should be placed on developing generic modelling toolkits with the functionality for coupling current or future projected sediment regimes with biological response for a range of biota. Such tools should permit the identification of river catchment-specific targets within a national context, based on biological effect and incorporate sufficient flexibility for utilizing updated physical, chemical, biological and catchment attribute data. Confidence will continue to be required in compliance screening to ensure cost-effective management programmes for avoiding disproportionate investment in impacted river catchments.

Journal ArticleDOI
TL;DR: In this paper, a 150 km2 subarctic Precambrian Shield catchment where the poorly drained heterogeneous mosaic of lakes, exposed bedrock, and soil filled areas creates variable contributing areas, hydrological connectivity was measured in 11 sub-basins with a particular focus on three representative subbasins.
Abstract: A drainage basin's runoff response can be determined by the connectivity of generated runoff to the stream network and the connectivity of the downstream drainage network The connectivity of a drainage basin modulates its ability to produce streamflow and respond to precipitation events and is a function of the complex and variable storage capacities throughout the drainage basin and along the drainage network An improved means to measure and account for the dynamics of stream network connectivity at the catchment scale is needed to predict basin scale streamflow At a 150 km2 subarctic Precambrian Shield catchment where the poorly drained heterogeneous mosaic of lakes, exposed bedrock, and soil filled areas creates variable contributing areas, hydrological connectivity was measured in 11 sub-basins with a particular focus on three representative sub-basins The three sub-basins, although of similar relative size, vary considerably in the dominant typology and topology of their constituent elements At a 10-m spatial resolution, saturated areas were mapped using both multispectral satellite imagery and onsite measurements of storage according to land cover To measure basin-scale hydrological connectivity, the drainage network was represented using graph theory where stream reaches are ‘edges’ connecting sub-basin ‘nodes’ The overall hydrological connectivity of the stream network was described as the ratio of actively flowing relative to potentially flowing stream reaches The hydrological connectivity of the stream network to the outlet was described as the ratio of actively flowing stream reaches that were connected to the outlet to the potentially flowing stream reaches Hydrological connectivity was then related to daily average streamflow and basin runoff ratio Improved understanding of causal factors for the variable streamflow response to runoff generation in this environment will serve as a first step towards improved streamflow prediction in formerly glaciated landscapes, especially in small ungauged basins Copyright © 2011 John Wiley & Sons, Ltd

Journal ArticleDOI
TL;DR: Competing model representations of hydrological processes with respect to their capability to simulate observed processes in the Mahurangi River basin in Northland, New Zealand are evaluated.
Abstract: The current generation of hydrological models has been widely criticized for their inability to adequately simulate hydrological processes. In this study, we evaluate competing model representations of hydrological processes with respect to their capability to simulate observed processes in the Mahurangi River basin in Northland, New Zealand. In the first part of this two-part series, the precipitation, soil moisture, and flow data in the Mahurangi were used to estimate the dominant hydrological processes and explore several options for their suitable mathematical representation. In this paper, diagnostic tests are applied to gain several insights for model selection. The analysis highlights dominant hydrological processes (e.g. the importance of vertical drainage and baseflow compared to sub-surface stormflow), provides guidance for the choice of modelling approaches (e.g. implicitly representing sub-grid heterogeneity in soils), and helps infer appropriate values for model parameters. The approach used in this paper demonstrates the benefits of flexible model structures in the context of hypothesis testing, in particular, supporting a more systematic exploration of current ambiguities in hydrological process representation. The challenge for the hydrological community is to make better use of the available data, not only to estimate parameter values but also to diagnostically identify more scientifically defensible model structures. Copyright © 2010 John Wiley & Sons, Ltd.

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TL;DR: In this article, the microwave radiation from freeze-thaw soil was examined by carrying out experimental measurements at 18·7 and 36·5 GHz using a Truck-mounted Multi-frequency Microwave Radiometer (TMMR) in the Heihe River of China.
Abstract: The soil freeze–thaw controls the hydrological and carbon cycling and thus affects water and energy exchanges at land surface. This article reported a newly developed algorithm for distinguishing the freeze/thaw status of surface soil. The algorithm was based on information from Advanced Microwave Scanning Radiometer Enhanced (AMSR-E) which records brightness temperature (Tb) in the afternoon and after midnight. The criteria and discriminant functions were obtained from both radiometer observations and model simulations. First of all, the microwave radiation from freeze–thaw soil was examined by carrying out experimental measurements at 18·7 and 36·5 GHz using a Truck-mounted Multi-frequency Microwave Radiometer (TMMR) in the Heihe River of China. The experimental results showed that the soil moisture is a key component that differentiates the microwave radiation behaviours during the freeze–thaw process, and the differences in soil temperature and emissivity between frozen and thawed soils were found to be the most important criteria. Secondly, a combined model was developed to consider the impacts of complex ground surface conditions on the discrimination. The model simulations quite followed the trend of in situ observations with an overall relation coefficient (R) of approximately 0·88. Finally, the ratio of Tb18·7H (horizontally polarized Tb at 18·7 GHz) to Tb36·5V was considered primarily as the quasi-emissivity, which is more reasonable and explicit in measuring the microwave radiation changes in soil freezing and thawing than the spectral gradient. By combining Tb36·5V to indicate the soil temperature variety, a Fisher linear discrimination analysis was used to establish the discriminant functions. After being corrected by TMMR measurements, the new discriminant algorithm had an overall accuracy of 86% when validated by 4-cm soil temperature. The multi-year discriminant results also provided a good agreement with the classification map of frozen ground in China. Copyright © 2011 John Wiley & Sons, Ltd.