HESSD - Latest Articles

Trends in rainfall erosivity in NE Spain at annual, seasonal and daily scales, 1955–2006

Wed, 16 May 2012 00:00:00 +0200

Trends in rainfall erosivity in NE Spain at annual, seasonal and daily scales, 1955–2006

Hydrology and Earth System Sciences Discussions, 9, 6285-6309, 2012

Author(s): M. Angulo-Martínez and S. Beguería

Rainsplash – the detachment and transport of soil particles by the impact of raindrops on a bare soil – is a major mechanism of soil degradation and erosion on semiarid areas and agricultural lands. Rainfall erosivity refers to the ability of precipitation to erode soil, and depends on the characteristics of the raindrops – size and velocity – and on the rainfall intensity and duration. Despite the relevance of rainfall erosivity for soil degradation prevention very few studies addressed its spatial and temporal variability. On this study the time variation of rainfall erosivity in the Ebro valley (NE Spain) is assessed for the period 1955–2006. The results show a general decrease in annual and seasonal rainfall erosivity, which is explained by a decrease of very intense rainfall events whilst the frequency of moderate and low events increased. This trend is related to prevailing positive conditions of the main atmospheric teleconnection indices affecting the West Mediterranean, i.e. the North Atlantic Oscillation (NAO), the Mediterranean Oscillation (MO) and the Western Mediterranean Oscillation (WeMO).

Integrating MODIS images in a water budget model for dynamic functioning and drought simulation of a Mediterranean forest in Tunisia

Wed, 16 May 2012 00:00:00 +0200

Integrating MODIS images in a water budget model for dynamic functioning and drought simulation of a Mediterranean forest in Tunisia

Hydrology and Earth System Sciences Discussions, 9, 6251-6284, 2012

Author(s): H. Chakroun, F. Mouillot, M. Nouri, and Z. Nasr

The use of remote sensing at different spatio-temporal resolutions is being common during the last decades since sensors offer many inputs to water budget estimation. Various water balance models use the LAI as a parameter for accounting water interception, evapotranspiration, runoff and available ground water. The objective of the present work is to improve vegetation stress monitoring at regional scale for a natural forested ecosystem. LAI-MODIS and spatialized vegetation, soil and climatic data have been integrated in a water budget model that simulates evapotranspiration and soil water content at daily step. We first explore LAI-MODIS in the specific context of Mediterranean natural ecosystem. Results showed that despite coarse resolution of LAI-MODIS product (1 km), it was possible to discriminate evergreen and coniferous vegetation and that LAI values are influenced by underlying soil capacity of water holding. The dynamic of vegetation has been integrated into the water budget model by weekly varying LAI-MODIS. Results of simulations were analysed in terms of actual evapotranspoiration, deficit of soil water to field capacity and vegetation stress index based on actual and potential evapotranspiration. Comparing dynamic LAI variation, afforded by MODIS, to a hypothetic constant LAI all over the year correspond to 30% of fAPAR increase. A sensitivity analysis of simulation outputs to this fAPAR variation reveals that increase of both deficit of soil water to field capacity and stress index are respectively 18% and 27%, (in terms of RMSE, these variations are respectively 1258 mm yr⁻¹ and 11 days yr⁻¹). These results are consistent with previous studies led at local scale showing that LAI increase is accompanied by stress conditions increase in Mediterranean natural ecosystems. In this study, we also showed that spatial modelisation of drought conditions based on water budget simulations is an adequate tool for quantifying expositions of different species to stress and for analysing most influent factors on ecosystem vulnerability to drought.

Behavior analysis of convective and stratiform rain using Markovian approach over Mediterranean region from meteorological radar data

Wed, 16 May 2012 00:00:00 +0200

Behavior analysis of convective and stratiform rain using Markovian approach over Mediterranean region from meteorological radar data

Hydrology and Earth System Sciences Discussions, 9, 6225-6250, 2012

Author(s): M. Lazri, S. Hameg, S. Ameur, J. M. Brucker, F. Ouallouche, and Y. Mohia

The aim of this study is to analyze the chronological behavior of precipitation in the north of Algeria using a Markovian approach. The probabilistic approach presented here proposes to study the evolution of the rainfall phenomenon in two distinct study areas, one located in sea and other located in ground. The data that we have used are provided by the National Office of Meteorology in Algiers (ONM). They are a series of images collected by the meterological radar of Setif during the rainy season 2001/2002. A decision criterion is established and based on radar reflectivity in order to classify the precipitation events located in both areas. At each radar observation, a state of precipitations is classified, either convective (heavy precipitation) or stratiform (average precipitation) both for the "sea" and for the "ground". On the whole, a time series of precipitations composed of three states; S₀ (no raining), S₁ (stratiform precipitation) and S₂ (convective precipitation), is obtained for each of the two areas. Thereby, we studied and characterized the behavior of precipitation in time by a Markov chain of order one with three states. Transition probabilities P_ij of state S_i to state S_j are calculated. The results show that rainfall is well described by a Markov chain of order one with three states. Indeed, the stationary probabilities, which are calculated by using the Markovian model, and the actual probabilities are almost identical.

A framework for upscaling short-term process-level understanding to longer time scales

Tue, 15 May 2012 00:00:00 +0200

A framework for upscaling short-term process-level understanding to longer time scales

Hydrology and Earth System Sciences Discussions, 9, 6203-6224, 2012

Author(s): W. H. Lim and M. L. Roderick

General experience in hydrologic modelling suggests that the parameterisation of a model could change over different time scales. As a result, hydrologists often re-parameterise their models whenever different temporal resolutions are required. Here, we investigate theoretical aspects of this issue in a search for the cause(s) of the need for re-parameterisations. Based on Taylor series expansion, we present a mathematical framework for temporal upscaling and evaluate it using a simple experimental system. For that, we use a unique database of half-hourly pan evaporation measurements (comprising 237 days) and examine how the model parameters change for daily and monthly integration periods. We show that the model parameters change over different integration periods with changes in the covariance between the model variables. The theory presented here is general and can be used as a basis for temporal upscaling.

Technical Note: Downscaling RCM precipitation to the station scale using quantile mapping – a comparison of methods

Tue, 15 May 2012 00:00:00 +0200

Technical Note: Downscaling RCM precipitation to the station scale using quantile mapping – a comparison of methods

Hydrology and Earth System Sciences Discussions, 9, 6185-6201, 2012

Author(s): L. Gudmundsson, J. B. Bremnes, J. E. Haugen, and T. Engen Skaugen

The impact of climate change on water resources is usually assessed at the local scale. However, regional climate models (RCM) are known to exhibit systematic biases in precipitation. Hence, RCM simulations need to be post-processed in order to produce reliable estimators of local scale climate. A popular post-processing approach is quantile mapping (QM), which is designed to adjust the distribution of modeled data, such that it matches observed climatologies. However, the diversity of suggested QM methods renders the selection of optimal techniques difficult and hence there is a need for clarification. In this paper, QM methods are reviewed and classified into: (1) distribution derived transformations, (2) parametric transformations and (3) nonparametric transformations; each differing with respect to their underlying assumptions. A real world application, using observations of 82 precipitation stations in Norway, showed that nonparametric transformations have the highest skill in systematically reducing biases in RCM precipitation.

Modelling climate change effects on a Dutch coastal groundwater system using airborne Electro Magnetic measurements

Mon, 14 May 2012 00:00:00 +0200

Modelling climate change effects on a Dutch coastal groundwater system using airborne Electro Magnetic measurements

Hydrology and Earth System Sciences Discussions, 9, 6135-6184, 2012

Author(s): M. Faneca Sànchez, J. L. Gunnink, E. S. van Baaren, G. H. P. Oude Essink, B. Siemon, E. Auken, W. Elderhorst, and P. G. B. de Louw

The forecast of climate change effects on the groundwater system in coastal areas is of key importance for policy makers. The Dutch water system has been deeply studied because of its complex system of low-lying areas, dunes, land won to the sea and dikes, but nowadays large efforts are still being done to find out the best techniques to describe complex fresh-brackish-saline groundwater dynamic systems. In this article, we describe a methodology consisting of high-resolution airborne Electro Magnetic (EM) measurements used in a 3-D variable-density transient groundwater model for a coastal area in the Netherlands. We used the Airborne EM measurements in combination with borehole-logging data, Electrical Conductivity Cone Penetration Tests and groundwater samples to create a 3-D fresh-brackish-saline groundwater distribution of the study area. The EM measurements proved to be an improvement compared to older techniques and provided quality input for the model. With the help of the built 3-D variable-density groundwater model, we removed the remaining inaccuracies of the 3-D chloride field and predicted the effects of three climate scenarios on the groundwater and surface water system. Results showed significant changes in the groundwater system, and gave direction for future water policy. Future research should provide more insight in the improvement of data collection for fresh-brackish-saline groundwater systems as it is of high importance to further improve the quality of the model.

Reservoir computing as an alternative to traditional artificial neural networks in rainfall-runoff modelling

Fri, 11 May 2012 00:00:00 +0200

Reservoir computing as an alternative to traditional artificial neural networks in rainfall-runoff modelling

Hydrology and Earth System Sciences Discussions, 9, 6101-6134, 2012

Author(s): N. J. de Vos

Despite theoretical benefits of recurrent artificial neural networks over their feedforward counterparts, it is still unclear whether the former offer practical advantages as rainfall-runoff models. The main drawback of recurrent networks is the increased complexity of the training procedure due to their architecture. This work uses recently introduced, conceptually simple reservoir computing models for one-day-ahead forecasts on twelve river basins in the Eastern United States, and compares them to a variety of traditional feedforward and recurrent models. Two modifications on the reservoir computing models are made to increase the hydrologically relevant information content of their internal state. The results show that the reservoir computing networks outperform feedforward networks and are competitive with state-of-the-art recurrent networks, across a range of performance measures. This, along with their simplicity and ease of training, suggests that reservoir computing models can be considered promising alternatives to traditional artificial neural networks in rainfall-runoff modelling.

Modelling the effects of climate and land cover change on groundwater recharge in south-west Western Australia

Thu, 10 May 2012 00:00:00 +0200

Modelling the effects of climate and land cover change on groundwater recharge in south-west Western Australia

Hydrology and Earth System Sciences Discussions, 9, 6063-6099, 2012

Author(s): W. Dawes, R. Ali, S. Varma, I. Emelyanova, G. Hodgson, and D. McFarlane

The groundwater resource contained within the sandy aquifers of the Swan Coastal Plain, south west Western Australia, provides approximately 60% of the drinking water for the metropolitan population of Perth. Rainfall decline over the past three decades coupled with increasing water demand from a growing population has resulted in falling dam storage and groundwater levels. Projected future changes in climate across south-west Western Australia consistently show a decline in annual rainfall of between 5 and 15%. There is expected to be a continuing reduction of diffuse recharge across the Swan Coastal Plain. This study aims to quantify the change in groundwater recharge in response to a range of future climate and land cover patterns across south-west Western Australia.

Modelling the impact on the groundwater resource of potential climate change was achieved with a dynamically linked unsaturated/saturated groundwater model. A Vertical Flux Manager was used in the unsaturated zone to estimate groundwater recharge using a variety of simple and complex models based on land cover type (e.g. native trees, plantation, cropping, urban, wetland), soil type, and taking into account the groundwater depth. These recharge estimates were accumulated on a daily basis for both observed and projected climate scenarios and used in a MODFLOW simulation with monthly stress periods.

In the area centred on the city of Perth, Western Australia, the patterns of recharge change and groundwater level change are not consistent spatially, or consistently downward. In the Dandaragan Plateau to the north-east of Perth there has been groundwater level rise since the 1970s associated with land clearing, and with rainfall projected to reduce the least in this area the groundwater levels are estimated to continue to rise. Along the coastal zone north of Perth there is an interaction between projected rainfall decline and legislated removal to pine forests. This results in areas of increasing recharge and rising water levels into the future despite a drying climate signal. To the south of Perth city there are large areas where groundwater levels are close to the land surface and not expected to change more than 1m upward or downward over the next two decades; it is beyond the accuracy of the model to conclude any definite trend.

In the south western part of the study area, the patterns of groundwater recharge are dictated primarily by soil, geology and land cover. In the sandy Swan (northern boundary) and Scott Coastal Plains (southern boundary) there is little response to future climates, because groundwater levels are shallow and much rainfall is rejected recharge. The profile dries out more in summer but this allows more rainfall to infiltrate in winter. Until winter recharge is insufficient to refill the aquifers these areas will not experience significant falls in groundwater levels. On the Blackwood Plateau however, the combination of native vegetation and clayey surface soils that restrict possible infiltration and recharge mean the area is very sensitive to climate change. With low capacity for recharge and low storage in the aquifers, small reductions in recharge can lead to large reductions in groundwater levels.

In the northern part of the study area both climate and land cover strongly influence recharge rates. Recharge under native vegetation is minimal and is relatively higher where grazing and pasture systems have been introduced after clearing of native vegetation. In some areas the low recharge values can be reduced to almost zero, even under dryland agriculture, if the future climate becomes very dry. In the Albany Area the groundwater resource is already over allocated, and the combination of existing permanent native vegetation with decreasing annual rainfall indicate reduced recharge. The area requires a reduction in groundwater abstraction to maintain the sustainability of the existing resource.

Climatic controls on diffuse groundwater recharge across Australia

Wed, 09 May 2012 00:00:00 +0200

Climatic controls on diffuse groundwater recharge across Australia

Hydrology and Earth System Sciences Discussions, 9, 6023-6062, 2012

Author(s): O. V. Barron, R. S. Crosbie, D. Pollock, W. R. Dawes, S. P. Charles, T. Pickett, and M. Donn

Reviews of field studies of groundwater recharge have attempted to investigate how climate characteristics control recharge, but due to a lack of data have not been able to draw any strong conclusions beyond that rainfall is the major determinant. This study has used numerical modeling for a range of Köppen-Geiger climate types (tropical, arid and temperate) to investigate the effect of climate variables on recharge for different soil and vegetation types. For the majority of climate types the total annual rainfall had a weaker correlation with recharge than the rainfall parameters reflecting rainfall intensity. In regions with winter-dominated rainfall, annual recharge under the same annual rainfall, soils and vegetation conditions is greater than in regions with summer-dominated rainfall. The relative importance of climate parameters other than rainfall is higher for recharge under annual vegetation, but overall is highest in the tropical climate type. Solar radiation and vapour pressure deficit show a greater relative importance than mean annual daily mean temperature. Climate parameters have lowest relative importance in the arid climate type (with cold winters) and the temperate climate type. For 75% of all considered cases of soil, vegetation and climate types recharge elasticity varies between 2 and 4, indicating a 20% to 40% change in recharge for a 10% change in annual rainfall Understanding how climate controls recharge under the observed historical climate allows more informed choices of analogue sites if they are to be used for climate change impact assessments.

The importance of glacier and forest change in hydrological climate-impact studies

Wed, 09 May 2012 00:00:00 +0200

The importance of glacier and forest change in hydrological climate-impact studies

Hydrology and Earth System Sciences Discussions, 9, 5983-6021, 2012

Author(s): N. Köplin, B. Schädler, D. Viviroli, and R. Weingartner

Changes in land cover alter the water balance components of a catchment, due to strong interactions between soils, vegetation and the atmosphere. Therefore, hydrological climate impact studies should also integrate scenarios of associated land cover change. To reflect two severe climate-induced changes in land cover, we applied scenarios of glacier retreat and forest cover increase that were derived from the temperature signals of the climate scenarios used in this study. The climate scenarios consist of ten regional climate models from the ENSEMBLES project; their respective temperature and precipitation deltas are used to run a hydrological model. The relative importance of each of the three types of scenarios (climate, glacier, forest) is assessed through an analysis of variance (ANOVA). Altogether, 15 mountainous catchments in Switzerland are analysed, exhibiting different degrees of glaciation during the control period (0–51%) and different degrees of forest cover increase under scenarios of change (12–55% of the catchment area). The results show that even an extreme change in forest cover is negligible with respect to changes in runoff, but it is crucial as soon as evaporation or soil moisture is concerned. For the latter two variables, the relative impact of forest change is proportional to the magnitude of its change. For changes that concern 35% of the catchment area or more, the effect of forest change on summer evapotranspiration is equally or even more important than the climate signal. For catchment with a glaciation of 10% or more in the control period, the glacier retreat significantly determines summer and annual runoff. The most important source of uncertainty in hydrological climate impact studies is the climate scenario, though, and it is highly recommended to apply an ensemble of climate scenarios in impact studies. The results presented here are valid for the climatic region they were tested for, i.e. a humid, mid-latitude mountainous environment. They might be different for regions where the evaporation is a major component of the water balance, for example. Nevertheless, a hydrological climate-impact study that assesses the additional impacts of forest and glacier change is new so far and provides insight into the question whether or not it is necessary to account for land cover changes as part of climate change impacts on hydrological systems.

Characterizing interactions between surface water and groundwater in the Jialu River basin using major ion chemistry and stable isotopes

Wed, 09 May 2012 00:00:00 +0200

Characterizing interactions between surface water and groundwater in the Jialu River basin using major ion chemistry and stable isotopes

Hydrology and Earth System Sciences Discussions, 9, 5955-5981, 2012

Author(s): L. Yang, X. Song, Y. Zhang, D. Han, B. Zhang, and D. Long

The Jialu River, a secondary tributary of the Huaihe River, has been severely contaminated for the major contaminant sources, such as a number of untreated or lightly treated sewage wastes in some cities. Groundwater along the river is not an isolated component of the hydrologic system, but instead connected with the surface water. This study aims to characterize the relationships between surface water (e.g. reservoirs, lakes and rivers) and groundwater near the river in the shallow Quaternary aquifer. The concentration of Cl⁻ in North Zhengzhou City increased prominently due to the discharge of a large amount of domestic water. Nitrate and potassium show maximum concentrations in groundwater in Fugou County. These high levels can be attributed to the use of a large quantity of fertilizer over this region. The regional well had water with a constant stable isotopic signature, which illustrates that the groundwater never or rarely receive recharge from surface water. However, the groundwater of transitional well (location SY3) seemed to be recharged by river water via bank infiltration in September 2010. Fractional contributions of river water to the groundwater were calculated based on isotopic and chemical data using a mass-balance approach. Results show that the groundwater was approximately composed of 60–70% river water. These findings would be useful for a better understanding of hydrogeological processes at the river-aquifer interface and ultimately benefit water management in the future.

Downscaling ERA-Interim temperature data in complex terrain

Wed, 09 May 2012 00:00:00 +0200

Downscaling ERA-Interim temperature data in complex terrain

Hydrology and Earth System Sciences Discussions, 9, 5931-5953, 2012

Author(s): L. Gao, M. Bernhardt, and K. Schulz

Air temperature controls a large variety of environmental processes, and is an essential input parameter for land surface models e.g. in hydrology, ecology and climatology. However, meteorological networks, which can provide the necessary information, are commonly sparse in complex terrains, especially in high mountainous regions. In order to provide temperature data in an adequate temporal and spatial resolution for local scale applications, we have developed a new downscaling method able to scale 3-hourly ERA-Interim temperature data. The scheme is based on model internal vertical lapse rates derived from different ERA-Interim pressure levels. The results are validated for three meteorological stations, located within the same ERA-Interim grid element: Zugspitze, Garmisch-Partenkirchen and Zugspitzplatt, in the German Alps; they are also compared with two other statistical, lapse rate based downscaling approaches. The results indicate that the use of model internal ERA-Interim lapse rates can significantly improve the downscaling performance when compared to the standard procedure of using fixed lapse rates.

Teaching hydrological modeling with a user-friendly catchment-runoff-model software package

Wed, 09 May 2012 00:00:00 +0200

Teaching hydrological modeling with a user-friendly catchment-runoff-model software package

Hydrology and Earth System Sciences Discussions, 9, 5905-5930, 2012

Author(s): J. Seibert and M. J. P. Vis

Computer models, and especially conceptual models, are frequently used for catchment hydrology studies. Teaching hydrological modeling, however, is challenging as students, when learning to apply computer models, have both to understand general model concepts and to be able to use particular computer programs. Here we present a new version of the HBV model. This software provides a user-friendly version which is especially useful for education. Different functionalities like an automatic calibration using a genetic algorithm or a Monte Carlo approach as well as the possibility to perform batch runs with predefined model parameters make the software also interesting for teaching in more advanced classes and research projects. Different teaching goals related to hydrological modeling are discussed and a series of exercises is suggested to reach these goals.

Uncertainty in computations of the spread of warm water in a river – lessons from Environmental Impact Assessment

Tue, 08 May 2012 00:00:00 +0200

Uncertainty in computations of the spread of warm water in a river – lessons from Environmental Impact Assessment

Hydrology and Earth System Sciences Discussions, 9, 5871-5904, 2012

Author(s): M. B. Kalinowska and P. M. Rowiński

The present study aims at evaluation of sources of uncertainty in modelling of heat transport in a river caused by the discharge coming from a cooling system of a designed gas-stem power plant. This study was a part of Environmental Impact Assessment and was based on two-dimensional modelling of temperature distribution in an actual river. The problems with proper description of the computational domain, velocity field and hydraulic characteristics were considered in the paper. An in-depth discussion on the methods of evaluation of dispersion coefficients in the model comprising all four components of the dispersion tensor was carried out. Numerical methods and their influence on final results of computations were also discussed. All computations were based upon a real case study performed in Vistula River in Poland.

Estimation of overland flow metrics at semiarid condition: Patagonian Monte

Mon, 07 May 2012 00:00:00 +0200

Estimation of overland flow metrics at semiarid condition: Patagonian Monte

Hydrology and Earth System Sciences Discussions, 9, 5837-5869, 2012

Author(s): M. J. Rossi and J. O. Ares

Water infiltration and overland flow (WIOF) processes are relevant in considering water partition among plant life forms, the sustainability of vegetation and the design of sustainable hydrological management. WIOF processes in arid and semiarid regions present regional characteristic trends imposed by the prevailing physical conditions of the upper soil as evolved under water-limited climate. A set of plot-scale field experiments at the semi-arid Patagonian Monte (Argentina) was performed in order to estimate infiltration-overland descriptive flow parameters. The micro-relief of undisturbed field plots at z-scale <1 mm was characterized through close-range stereo-photogrammetry and geo-statistical modelling. The overland flow areas produced by experimental runoff events were video-recorded and the runoff speed was measured with ortho-image processing software. Antecedent and post-inflow moisture were measured, and texture, bulk density and physical properties of the soil at the upper vadose zone were estimated. Field data were used to calibrate a physically-based, time explicit model of water balance in the upper soil and overland flows with a modified Green-Ampt (infiltration) and Chezy's (overland flow) algorithms. Modelling results satisfy validation criteria based on the observed overland flow areas, runoff-speed, water mass balance of the upper vadose zone, infiltration depth, slope along runoff-plume direction, and depression storage intensity. The experimental procedure presented supplies plot-scale estimates of overland flow and infiltration intensities at various intensities of water input which can be incorporated in larger-scale hydrological grid-models of arid regions. Findings were: (1) Overland flow velocities as well as infiltration-overland flow mass balances are consistently modelled by considering variable infiltration rates corresponding to depression storage and/or non-ponded areas. (2) The statistical relations presented allow the estimation of theoretical hydrodynamic parameters (Chezy's frictional C, average overland flow depth d^*) through measurable characteristics of the surface soil and overland flow kinetics. (3) A protocol of field experiments and coupled time-distributed modelling to 1–2 above is described. The methodology and results obtained in this study are probably relevant to similar arid-semiarid areas of the world.

The hydrological responses of different land cover types in a re-vegetation catchment area of the Loess Plateau, China

Fri, 04 May 2012 00:00:00 +0200

The hydrological responses of different land cover types in a re-vegetation catchment area of the Loess Plateau, China

Hydrology and Earth System Sciences Discussions, 9, 5809-5835, 2012

Author(s): S. Wang, B. J. Fu, G. Y. Gao, and J. Zhou

The impact of re-vegetation on soil moisture dynamics was investigated by comparing five land cover types. Soil moisture and temperature variations under grass (Andropogon), subshrub (Artemisia scoparia), shrub (Spiraea pubescens), tree (Robinia pseudoacacia), and crop (Zea mays) vegetation were monitored in an experiment performed during the growing season of 2011. There were more than 10 soil moisture pulses during the period of data collection, and the surface soil moisture of all of the land cover types showed an increasing trend. Corn cover was associated with consistently higher soil moisture readings than the other surfaces. Grass and subshrubs showed an intermediate moisture level, with that of grass being slightly higher than that of subshrub most of the time. Shrubs and trees were characterized by lower soil moisture readings, with the shrub levels consistently being slightly higher than those of the trees. With the exception of the corn land cover type, the average soil temperature showed the same regime as the average moisture content, but exhibiting a downward trend throughout the observation period. Three typical decreasing periods were chosen to compare the differences in water losses. In periods of both relatively lower and higher water soil moisture contents, subshrubs lost the largest amount of water. The daily water loss associated with corn was most variable. The tree and shrub sites presented an intermediate level, with that of tree being slightly higher compared to shrub; the daily water loss trends of these two land cover types were similar and were more stable than those of the other types. The amount of water loss related to the grass land cover type is determined by the initial moisture content. Soil under subshrubs acquired and retained soil moisture resources more efficiently than the other cover types, representing an adaptive vegetation type in this area.

Assessing water footprint at river basin level: a case study for the Heihe River Basin in northwest China

Thu, 03 May 2012 00:00:00 +0200

Assessing water footprint at river basin level: a case study for the Heihe River Basin in northwest China

Hydrology and Earth System Sciences Discussions, 9, 5779-5808, 2012

Author(s): Z. Zeng, J. Liu, P. H. Koeneman, E. Zarate, and A. Y. Hoekstra

Increasing water scarcity places considerable importance on the quantification of water footprint (WF) at different levels. Despite progress made previously, there are still very few WF studies focusing on specific river basins, especially for those in arid and semi-arid regions. The aim of this study is to quantify WF within the Heihe River Basin (HRB), a basin located in the arid and semi-arid northwest of China. The findings show that the WF was 1768 million m³ yr⁻¹ in the HRB over 2004–2006. Agricultural production was the largest water consumer, accounting for 96% of the WF (92% for crop production and 4% for livestock production). The remaining 4% was for the industrial and domestic sectors. The "blue" component of WF was 811 million m³ yr⁻¹. This indicates a blue water proportion of 46%, which is much higher than the world average and China's average, which is mainly due to the aridness of the HRB and a high dependence on irrigation for crop production. However, even in such a river basin, blue WF was still smaller than green WF, indicating the importance of green water. We find that blue WF exceeded blue water availability during eight months per year and also on an annual basis. This indicates that WF of human activities was achieved at a cost of violating environmental flows of natural freshwater ecosystems, and such a WF pattern is not sustainable. Considering the large WF of crop production, optimizing the crop planting pattern is often a key to achieving more sustainable water use in arid and semi-arid regions.

How extreme is extreme? An assessment of daily rainfall distribution tails

Wed, 02 May 2012 00:00:00 +0200

How extreme is extreme? An assessment of daily rainfall distribution tails

Hydrology and Earth System Sciences Discussions, 9, 5757-5778, 2012

Author(s): S. M. Papalexiou, D. Koutsoyiannis, and C. Makropoulos

The upper part of a probability distribution, usually known as the tail, governs both the magnitude and the frequency of extreme events. The tail behaviour of all probability distributions may be, loosely speaking, categorized in two families: heavy-tailed and light-tailed distributions, with the latter generating more "mild" and infrequent extremes compared to the former. This emphasizes how important for hydrological design is to assess correctly the tail behaviour. Traditionally, the wet-day daily rainfall has been described by light-tailed distributions like the Gamma, although heavier-tailed distributions have also been proposed and used, e.g. the Lognormal, the Pareto, the Kappa, and others. Here, we investigate the issue of tails for daily rainfall by comparing the upper part of empirical distributions of thousands of records with four common theoretical tails: those of the Pareto, Lognormal, Weibull and Gamma distributions. Specifically, we use 15 029 daily rainfall records from around the world with record lengths from 50 to 163 yr. The analysis shows that heavier-tailed distributions are in better agreement with the observed rainfall extremes than the more often used lighter tailed distributios, with clear implications on extreme event modelling and engineering design.

The problems of overexploitation of aquifers in semi-arid areas: the Murcia Region and the Segura Basin (South-east Spain) case

Wed, 02 May 2012 00:00:00 +0200

The problems of overexploitation of aquifers in semi-arid areas: the Murcia Region and the Segura Basin (South-east Spain) case

Hydrology and Earth System Sciences Discussions, 9, 5729-5756, 2012

Author(s): T. Rodríguez-Estrella

A general analysis of the problems arising from aquifer exploitation in semi-arid areas such as the Autonomous Region of Murcia, which belongs to the Segura Basin is presented, with particular reference to the Ascoy-Sopalmo aquifer, which is the most overexploited aquifer in Spain. It has suffered intense overabstraction over the last forty years, given renewable water resources of 2 Mm³ yr⁻¹ and abstractions amounting to as much as 55 Mm³ yr⁻¹. This has resulted in the drying of springs, continuous drawdown of water levels (5 m yr⁻¹); piezometric drops (over 30 m in one year, as a consequence of it being a karstic aquifer); increase in pumping costs (elevating water from more than 320 m depth); abandoning of wells (45 reduced to 20), diminishing groundwater reserves, and deteriorating water quality (progressing from a mixed sodium bicarbonate-chloride facies to a sodium chloride one). This is a prime example of poor management with disastrous consequences. In this sense, a series of internal measures is proposed to alleviate the overexploitation of this aquifer and of the Segura Basin, with the aim of contributing to a sustainable future.

Distributed hydrological modeling in a large-scale watershed of Northern China: multi-site model calibration, validation, and sensitivity analysis

Wed, 02 May 2012 00:00:00 +0200

Distributed hydrological modeling in a large-scale watershed of Northern China: multi-site model calibration, validation, and sensitivity analysis

Hydrology and Earth System Sciences Discussions, 9, 5697-5727, 2012

Author(s): S. Wang, Z. Zhang, G. Sun, P. Strauss, J. Guo, and Y. Tang

Model calibration is a complex task for large watersheds, especially for those in a heterogeneous mountain environment where multi-objective calibration strategy is essential. That may improve a model's capability to capture the spatial variations of the internal hydrologic variables. This study used the physically-based distributed hydrologic model, MIKESHE, to contrast a lumped calibration protocol that uses data measured at one single outlet to a multi-site calibration method which employed streamflow measurements at three separate stations within the large Chaohe River basin in Northern China. The results showed that, the single-site calibrated model was able to sufficiently simulate the hydrographs for two of the three stations (Nash-sutchliffe coefficient of 0.65–0.75, and correlation coefficient 0.81–0.87 during the testing period), but model performance was poor at the third station (EF only 0.44). By using the multi-site measurements model calibration reached a compromise between the different stations, the model reasonably representing the hydrographs of all three stations with EF ranging from 0.59–0.72. The modeling calibration results suggested that the dominant hydrological processes varied across the large watershed with upstream area dominated by slow groundwater and middle- and down-stream areas dominated by relatively quick interflow. We conclude that to account for the different hydrological process of watershed with large heterogeneity, it is necessary to employ a multi-site calibration protocol to reduce prediction errors.