Analysis of catchment evapotranspiration at different scales using bottom-up and top-down approaches

Physically-based hydrological models are used to predict catchment water balance through detailed simulation of hydrological processes at small temporal and spatial scales. However, annual catchment water balance can also be easily and simply predicted using lumped conceptual model. Comparison between physically-based hydrological models and lumped conceptual models can help us understand the dominant factors on catchment water balance at different scales. In this paper, a distributed physically-based hydrological model (i.e., bottom-up approach) and a simple water-energy balance model (i.e., top-down approach) are used to predict actual evapotranspiration in nine sub-catchments, and the whole basin of the Luan River in northern China. Both simulations give very close values of annual evapotranspiration and show the same complementary relationship between actual and potential evapotranspiration at annual time scale. From the analysis at different time scales through comparison of the top-down and the bottom-up methods, it is shown that the annual catchment evapotranspiration is controlled mainly by annual precipitation and potential evapotranspiration, and the variability of soil water and vegetation becomes more important at a smaller time scale in the study areas. It is also known that the relationship between potential and actual evapotranspiration shows a highly nonlinear relationship at the annual and catchment scale but can be simplified to a linear relationship at hourly temporal and hillslope scales, which is commonly used in the physically-based hydrological models.     

Sankey Brook Catchment Study

The Sankey Brook catchment lies in the heart of the Mersey basin. As part of a major study into development issues in the Sankey Brook catchment, a hydraulic model was constructed and successfully used to simulate the river system and to assist in the derivation of discharge control policies.
The background to the study and the subsequent hydrological and hydraulic modelling are described, particularly in relation to the more complex systems in the lower reaches of Sankey Brook.     

Karstentwässerung im Kaisergebirge (Tirol, Österreich) – Abgrenzung hydrographischer Einzugsgebiete durch Kombination hydrogeologischer Untersuchungen mit Isotopenmethoden und hydrologischer Modellierung

Water resources investigations in karstic mountainous regions face the difficulty that the hydrological catchment boundaries are usually not identical with the orographic catchment boundaries. In this study an integrative method is presented that combines different sources of hydrogeological information and results of distributed water balance modelling to identify hydrological catchment boundaries and subsurface water ways in a karstic mountainous region. The different information comprises, in addition to meteorological and hydrographic data, region-wide point measurements of discharge, field surveys, isotope data, tracer tests, spring discharge data, soil and geological surveys and mapping, etc. The study was set-up in the Kaisergebirge mountain range in Tyrol/Austria, using the well known model MIKE SHE. Based on the additional information, the original model structure was adjusted to the specific situation in the karst, e.g. by introducing a bypass-flow within the unsaturated zone. The modelling results based on the orographic catchments were evaluated in terms of closure and—equally important—non-closure of the water balance. Spatial patterns of simulated hydrologic quantities are interpreted in light of the different sources of information. As a result, a regional map of surplus and deficit between observed and simulated runoff was drawn and combined with the additional information to form a general conceptual model of karst water flow directions and the location and altitude of the contributing recharge area in the high-alpine region. The results of the study provide a valuable basis for the assessment and protection of karst water resources for water supply.     

Analysis of catchment evapotranspiration at different scales using bottom-up and top-down approaches

Physically-based hydrological models are used to predict catchment water balance through detailed simulation of hydrological processes at small temporal and spatial scales. However, annual catchment water balance can also be easily and simply predicted using lumped conceptual model. Comparison between physically-based hydrological models and lumped conceptual models can help us understand the dominant factors on catchment water balance at different scales. In this paper, a distributed physically-based hydrological model (i.e., bottom-up approach) and a simple water-energy balance model (i.e., top-down approach) are used to predict actual evapotranspiration in nine sub-catchments, and the whole basin of the Luan River in northern China. Both simulations give very close values of annual evapotranspiration and show the same complementary relationship between actual and potential evapotranspiration at annual time scale. From the analysis at different time scales through comparison of the top-down and the bottom-up methods, it is shown that the annual catchment evapotranspiration is controlled mainly by annual precipitation and potential evapotranspiration, and the variability of soil water and vegetation becomes more important at a smaller time scale in the study areas. It is also known that the relationship between potential and actual evapotranspiration shows a highly nonlinear relationship at the annual and catchment scale but can be simplified to a linear relationship at hourly temporal and hillslope scales, which is commonly used in the physically-based hydrological models.     

Agriculture and groundwater nitrate contamination in the Seine basin. The STICS-MODCOU modelling chain

A software package is presented here to predict the fate of nitrogen fertilizers and the transport of nitrate from the rooting zone of agricultural areas to surface water and groundwater in the Seine basin, taking into account the long residence times of water and nitrate in the unsaturated and aquifer systems. Information on pedological characteristics, land use and farming practices is used to determine the spatial units to be considered. These data are converted into input data for the crop model STICS which simulates the water and nitrogen balances in the soil-plant system with a daily time-step. A spatial application of STICS has been derived at the catchment scale which computes the water and nitrate fluxes at the bottom of the rooting zone. These fluxes are integrated into a surface and groundwater coupled model MODCOU which calculates the daily water balance in the hydrological system, the flow in the rivers and the piezometric variations in the aquifers, using standard climatic data (rainfall, PET). The transport of nitrate and the evolution of nitrate contamination in groundwater and to rivers is computed by the model NEWSAM. This modelling chain is a valuable tool to predict the evolution of crop productivity and nitrate contamination according to various scenarios modifying farming practices and/or climatic changes. Data for the period 1970-2000 are used to simulate the past evolution of nitrogen contamination. The method has been validated using available data bases of nitrate concentrations in the three main aquifers of the Paris basin (Oligocene, Eocene and chalk). The approach has then been used to predict the future evolution of nitrogen contamination up to 2015. A statistical approach allowed estimating the probability of transgression of different concentration thresholds in various areas in the basin. The model is also used to evaluate the cost of the damage resulting of the treatment of drinking water at the scale of a groundwater management unit in the Seine river basin.     

Long-term changes in pollutant load outflows and purification function in a paddy field watershed using a circular irrigation system

A long-term investigation on the water quality and hydrology was carried out for 8 years and 7 months (from October 1991 to April 2000) in a paddy field watershed using a circular irrigation system. The annual amount of rainfall ranged from 1270 to 2226 mm and it was found that the amount of irrigation water tended to decrease as rainfall increased. Phosphorus and chemical oxygen demand (COD) concentrations tended to decrease with the river flowing down, whereas nitrogen concentrations showed no significant difference. The annual outflow loads (sum of the net-outflow load during irrigation periods and the outflow load during non-irrigation periods) of total nitrogen (T-N), total phosphorus (T-P), and COD ranged from 13.6 to 75.3 kg ha(-1)yr(-1), -3.55 to 2.21 kg ha(-1)yr(-1), and -24.7 to 48.5 kg ha(-1)yr(-1), respectively. The negative values for T-P and COD loads indicated that the study watershed performed a purification function. The change in annual pollutant loads was primarily attributed to the amount of hydrological water volumes (the annual amount of rainfall or that of rainfall plus irrigation water) for T-N and COD loads and partially for T-P load. In addition, the purification function was related to the hydraulic retention time, and the study watershed allows sufficient retention for pollutant purification for phosphorus and COD contents and partially for nitrogen content.     

Towards quantification of uncertainty in predicting water quality failures in integrated catchment model studies

This paper describes the development and application of a method for estimating uncertainty in the prediction of sewer flow quantity and quality and how this may impact on the prediction of water quality failures in integrated catchment modelling (ICM) studies. The method is generic and readily adaptable for use with different flow quality prediction models that are used in ICM studies. Use is made of the elicitation concept, whereby expert knowledge combined with a limited amount of data are translated into probability distributions describing the level of uncertainty of various input and model variables. This type of approach can be used even if little or no site specific data is available. Integrated catchment modelling studies often use complex deterministic models. To apply the results of elicitation in a case study, a computational reduction method has been developed in order to determine levels of uncertainty in model outputs with a reasonably practical level of computational effort. This approach was applied to determine the level of uncertainty in the number of water quality failures predicted by an ICM study, due to uncertainty associated with input and model parameters of the urban drainage model component of the ICM. For a small case study catchment in the UK, it was shown that the predicted number of water quality failures in the receiving water could vary by around 45% of the number predicted without consideration of model uncertainty for dissolved oxygen and around 32% for unionised ammonia. It was concluded that the potential overall levels of uncertainty in the ICM outputs could be significant. Any solutions designed using modelling approaches that do not consider uncertainty associated with model input and model parameters may be significantly over-dimensioned or under-dimensioned. With changing external inputs, such as rainfall and river flows due to climate change, better accounting for uncertainty is required.     

Modelling the impacts of climate change on surface runoff in small Mediterranean catchments: empirical evidence from Greece

During the last decades, a major factor that analysts and policy makers take into account in the assessment of the environment is climate change. This global physical process is expected to cause problems in natural and human environment. Thus, modelling climate change impacts may lead to prevention policies to minimize the degradation of life quality because of lack of water resources in the future. This study implicates Geographical Information System (GIS) and hydrological modelling tools to various scenarios of climate change such as the increase of temperature the decrease of rainfall, or even both of them, to estimate the potential impact of climate change on surface runoff in a typical catchment in Andros Island, Greece. Primary results indicate a proportional runoff decrease in the next 50 years because of global climate changes.     

An integrated multi-level watershed-reservoir modeling system for examining hydrological and biogeochemical processes in small prairie watersheds

Eutrophication of small prairie reservoirs presents a major challenge in water quality management and has led to a need for predictive water quality modeling. Studies are lacking in effectively integrating watershed models and reservoir models to explore nutrient dynamics and eutrophication pattern. A water quality model specific to small prairie water bodies is also desired in order to highlight key biogeochemical processes with an acceptable degree of parameterization. This study presents a Multi-level Watershed-Reservoir Modeling System (MWRMS) to simulate hydrological and biogeochemical processes in small prairie watersheds. It integrated a watershed model, a hydrodynamic model and an eutrophication model into a flexible modeling framework. It can comprehensively describe hydrological and biogeochemical processes across different spatial scales and effectively deal with the special drainage structure of small prairie watersheds. As a key component of MWRMS, a three-dimensional Willows Reservoir Eutrophication Model (WREM) is developed to addresses essential biogeochemical processes in prairie reservoirs and to generate 3D distributions of various water quality constituents; with a modest degree of parameterization, WREM is able to meet the limit of data availability that often confronts the modeling practices in small watersheds. MWRMS was applied to the Assiniboia Watershed in southern Saskatchewan, Canada. Extensive efforts of field work and lab analysis were undertaken to support model calibration and validation. MWRMS demonstrated its ability to reproduce the observed watershed water yield, reservoir water levels and temperatures, and concentrations of several water constituents. Results showed that the aquatic systems in the Assiniboia Watershed were nitrogen-limited and sediment flux played a crucial role in reservoir nutrient budget and dynamics. MWRMS can provide a broad context of decision support for water resources management and water quality protection in the prairie region.     

A hybrid neural-genetic algorithm for reservoir water quality management

A combined neural network and genetic algorithm (GA) was developed for water quality management of Feitsui Reservoir in Taiwan. First, an artificial neural network (ANN) model was employed to simulate the behavior of nutrient loads into the reservoir. The data from watershed loads, precipitation in the watershed, and outflow were used in the ANN model to forecast the total phosphorus concentration in the reservoir. A 6-year (1992-97) record of water quality data was used for network training, and additional data collected in 1998-2000 were used for model verification. Further, a GA was used with this ANN model to optimize the control of nutrient loads from the watershed. The GA was used as a search strategy to determine the proper reduction rates of nutrient loads from the watershed so that the objective function could be as close to the optimal value as possible. The study results indicate that the ANN model can effectively simulate the dynamics of reservoir water quality. The GA is able to identify control schemes that reduce the in-reservoir total phosphorus concentration by as much as 60%, and water quality in the reservoir can be expected to achieve an oligotrophic (most of the time) or mesotrophic level if the watershed nutrient loads are reduced by 10-80%.     

Modelling of mercury transport and transformation processes in the Idrijca and Soca river system

In the town of Idrija, Slovenia, the world's second largest mercury mine was active for 500 years and about 37,000 tons of mercury has been lost in the environment. Mercury is still drained from soil, riverbed and floodplains and transported with the Idrijca and Soca Rivers to the Gulf of Trieste. A part of inorganic mercury is methylated either in the river system, or later in the coastal area, and, due to its bioaccumulation and biomagnification represents potential danger to human health. A 1-D aquatic model MeRiMod was used to simulate hydrodynamics and sediment transport in the river system from Idrija to the Soca River mouth. Transport of particle bound and dissolved mercury as well as potential net methylation of mercury in the river system was simulated. The simulation of an observed flood wave with 20-year recurrence period was performed in order to validate the model. Methylation was simulated at lower discharges, as higher methylation rates occur in such conditions. The measurement data and the MeRiMod model were also used to establish a historical mercury mass balance of the Idrijca and Soca Rivers catchment. Sediment core data from the Gulf of Trieste and the measured concentrations from floodplains were used to verify and calibrate the model. Simulations of different high discharges were performed as most of the transport of particulate mercury occurs within flood wave conditions. Compared to the measurements, the results of the model showed an agreement within an order of magnitude, for the transport of total mercury mostly within a factor of 4, and for the methylation within a factor of 5. However, proper trends of the phenomena were obtained by simulations. The combination of modelling and measurements has resulted in some interesting conclusions about the phenomenon of the transport and transformations of mercury in the observed river system.     

Impacts of Climatic Change on River Flow Regimes in the UK

Much has been written in recent years about the potential threats posed by increasing greenhouse gas concentrations. This paper summarizes the implications of global warming for hydrological processes in general and river flow characteristics in the UK in particular, emphasizing the present high degree of uncertainty. Current climate change scenarios for the UK imply that rainfall between autumn and spring will increase, and this may have beneficial implications for UK water resources. However, the effect of this increase may be outweighed by higher evapotranspiration. Average annual runoff in a catchment in southern UK may be reduced by around 5% by the middle of the next century, but this estimate is very uncertain: runoff may reduce by 30% or increase by 30%. Runoff in northern and western UK is likely to show a slight increase (but with similarly large confidence intervals). It is probable that river flows in the UK will be much more concentrated in winter than at present. The effect of a given climate change scenario on monthly flow regimes depends on the current summer water balance and on catchment geological conditions.     

The role of climate on inter-annual variation in stream nitrate fluxes and concentrations

In recent decades, temporal variations in nitrate fluxes and concentrations in temperate rivers have resulted from the interaction of anthropogenic and climatic factors. The effect of climatic drivers remains unclear, while the relative importance of the drivers seems to be highly site dependent. This paper focuses on 2-6 year variations called meso-scale variations, and analyses the climatic drivers of these variations in a study site characterized by high N inputs from intensive animal farming systems and shallow aquifers with impervious bedrock in a temperate climate. Three approaches are developed: 1) an analysis of long-term records of nitrate fluxes and nitrate concentrations in 30 coastal rivers of Western France, which were well-marked by meso-scale cycles in the fluxes and concentration with a slight hysteresis; 2) a test of the climatic control using a lumped two-box model, which demonstrates that hydrological assumptions are sufficient to explain these meso-scale cycles; and 3) a model of nitrate fluxes and concentrations in two contrasted catchments subjected to recent mitigation measures, which analyses nitrate fluxes and concentrations in relation to N stored in groundwater. In coastal rivers, hydrological drivers (i.e., effective rainfall), and particularly the dynamics of the water table and rather stable nitrate concentration, explain the meso-scale cyclic patterns. In the headwater catchment, agricultural and hydrological drivers can interact according to their settings. The requirements to better distinguish the effect of climate and human changes in integrated water management are addressed: long-term monitoring, coupling the analysis and the modelling of large sets of catchments incorporating different sizes, land uses and environmental factors.     

A hydrological investigation using a lumped water balance model: the Aison River Basin case (Greece)

The purpose of the present paper is to investigate the hydrological components of the Aison River Basin in northern Greece. The orography of the area and the increasing irrigation needs require a specifically adapted hydrological model in order to address water management issues. With this aim in view, a parsimonious lumped simulation–optimization model with a snowmelt routine is elaborated in a monthly time step. The Nelder–Mead algorithm is applied for automatic optimization of the model parameters using the Nash criterion as an objective function. The model results are also evaluated by additional statistical criteria. In order to further reduce data uncertainty influence, annual actual evapotranspiration values are compared with those derived using three empirical methods (Turk, Coutagne and Schreiber methods). Model outputs were shown to be a good estimation of the hydrological cycle components, indicating that water losses represent almost 62% of the total precipitation volume.     

May land use change reduce the water deficiency problem caused by reduced brown coal mining in the state of Brandenburg?

Surface mining alters the water regime not only locally, but also regionally. The reduced brown coal mining in the south-east of the state Brandenburg (Germany) leads to decreasing river discharge and consequently to a shortage in the water supply. Land use change is one possible option to counteract this development. In this simulation study, we explored the impact of temporary and permanent set-aside of arable land on Brandenburg’s regional water balance. Temporary and permanent set-aside were considered as major measures towards deintensification of agriculture. Simulations were performed using the regional ecohydrological model SWIM, which integrates hydrological processes, vegetation growth, erosion and nutrient dynamics. The model was used to simulate the consequences of different land use change scenarios on main components of the regional water balance. Changes in the use of arable land altered clearly its water balance. The impact of these changes on the regional water balance for Brandenburg did not exceed ±10% for its single components. Opposite tendencies were established in the simulations by contrasting effects of temporary and permanent set-aside of arable cropland. While temporary set-aside increased runoff from the whole area up to 6.7% due to lower evapotranspiration and higher soil moisture in arable land, the conversion of agricultural land within river corridors to meadows had an opposite effect on regional runoff (6.9% decrease) due to higher water retention coefficients and higher evapotranspiration losses. Therefore, only temporary set-aside may compensate to some extent for the anticipated decrease in river discharge.     

RCP8.5情景下流域景观水文与泥沙 动态连通变化研究

在气候变化背景下,水文连通是风景园林学学科中 景观水文研究的热点和重点之一。目前的景观水文连通研究 多基于静态地表,而忽略地表演化过程,尤其是河底泥沙动态 变化。在总结现有水文连通研究的基础上,结合气候预测模 型、水动力地表景观演变模型及景观连通指数研究方法,以英 国赛文河凯尔苏斯流域为研究区,采用UKCP18气候模型预 测RCP8.5情景下未来10年间(2021—2030年)日降雨量, 运用凯撒二维水动力地表景观演变模型(CAESAR-Lisflood) 模拟10年连续河流演化及淹没模式变化,基于景观连通指数 评价流域景观水文的动态连通变化。结果表明：泥沙变化对淹 没模式和水文连通性产生影响,未来10年间整体连接度(IIC)与 可能连通性(PC)变低,雨季水文IIC与PC变化幅度较大,旱季 平稳。凯尔苏斯流域洪泛区域比河道的沉积作用更明显,流域 总沉积量是侵蚀量的4.6倍。泥沙输移作用使得淹没面积减少 0.25km2 、流域总水量减少40%,水文IIC与PC降低     

Integrated river flow modelling: A case study

The paper presents a holistic approach to river flow modelling in which catchment rainfall-runoff, sewer hydrodynamic and river hydrodynamic models are linked together and applied to the Bradford catchment (UK). The catchment features steep gradients, flashy supercritical flow in combination with dry bed condition and is highly urbanised. Given this complexity and the paucity of data, a holistic approach was found essential, although its application proved challenging.

Most important uncertainties inherent in each sub-model were assessed individually as well as propagation effects through the integrated model. The relative contribution of uncertainty sources to the total uncertainty in river flow predictions was quantified using the concept of variance decomposition. The results show that uncertainties in model structure and rainfall input in the rainfall-runoff model were significant. However, within the river hydrodynamic model, uncertainties propagated from the rainfall-runoff model were less apparent due to additional uncertainty arising from the urban drainage system.     

Modelling of hydrologic processes and potential response to climate change through the use of a multisite SWAT

Hydrologic models that use components for integrated modelling of surface water and groundwater systems help conveniently simulate the dynamically linked hydrologic and hydraulic processes that govern flow conditions in watersheds. The Soil and Water Assessment Tool (SWAT) is one such model that allows continuous simulations over long time periods in the land phase of the hydrologic cycle by incorporating surface water and groundwater interactions. This study provides a verified structure for the SWAT to evaluate existing flow regimes in a small-sized catchment in Denmark and examines a simple simulation to help quantify the effects of climate change on regional water quantities. SWAT can be regarded among the alternative hydrologic simulation tools applicable for catchments with similar characteristics and of similar sizes in Denmark. However, the modellers would be required to determine a proper set of effective model parameters and agree on a proper balancing among different calibration sites during all parameter optimization trials.     

CONTROL-UNIT-BASED ANALYSIS OF THE ECOLOGICAL RESTORATION MEASURE SYSTEM AND ARRANGEMENT OF THE XIAONANHAI LAKE WATERSHED IN HUBEI PROVINCE,CHINA

To deal with the water environmental degradation and ecological damage of the Xiaonanhai Lake watershed in Songzi City of Hubei Province, China, this study first divided the watershed into 32 control units according to the administrative division and catchment zones, then analyzed the pollution source and load and calculated the water environmental capacity of the watershed with the water environment system model coupled by the Soil and Water Assessment Tool model and the MIKE 21 model. To better deal with different pollutants and divide the responsibility more efficiently, the study proposed a control-unit-based system of five ecological restoration measures including the three-stage constructed wetland, the natural wetland, the clean water corridor, the lakeshore buffer zone, and the emerged and floating plant belt. Finally, the performance evaluation of these measures was conducted under the “Dual Control” system of concentration control and total load control of pollutants. The result proved that the five measures could effectively reduce the total amount of COD, TN, TP, and NH₃-N to improve the water quality, meeting the Surface Water Class Ⅲ Standard.     

River flow and associated transport of sediments and solutes through a highly urbanised catchment, Bradford, West Yorkshire

The hydrological characteristics of catchments become drastically modified in response to urbanisation. The total contributions and dynamics of runoff, suspended sediment and solutes may change significantly and have important implications downstream where they may affect flooding, instream ecological habitat, water quality and siltation of river channels and lakes. Although an appreciation of the likely hydrological changes is crucial for effective catchment management they are still poorly understood. In this paper we present data from a network of river monitoring stations throughout the heavily urbanised Bradford catchment, West Yorkshire. Sites are upstream, within and downstream of the highly urbanised central part of the catchment. Flow, turbidity (calibrated to suspended sediment concentration) and specific conductance (surrogate for solute concentration), logged at 15-min intervals, are presented for a 12-month period (June 2000 to June 2001). The total amounts and dynamics of flow, solute and suspended sediment transport were investigated. Estimated total flow and suspended sediment transport for the monitoring period were found to be high in response to the high total rainfall. Flow and sediment transport regimes were extremely 'flashy' throughout the catchment and became increasingly flashy in a downstream direction. Suspended sediment discharged from the Bradford subcatchment makes an important contribution to downstream sediment transport on the river Aire at Beal. Data suggest that the urbanised part of the Bradford catchment is extremely important in contributing solutes to the Beck (river). Although flow and sediment are also contributed to the Bradford Beck in the urbanised part of the catchment the data suggest that significant amounts may enter the combined sewer system and bypass the river. Understanding the spatial and temporal variations of flow and the transport of suspended sediment and solutes in rivers in urbanized subcatchments is crucial to their effective management and monitoring. Furthermore, this knowledge may be extremely important to the management and monitoring of downstream rivers in large scale mixed catchments.