Nicola Fohrer

Comparison of two different approaches of sensitivity analysis

Abstract Due to spatial variability, budget constraints or access difficulties model input parameters always are uncertain to some extent. Therefore the knowledge of sensitive input parameters is beneficial for model development and application. It can lead to a better understanding and to better estimated values and thus reduced uncertainty. In the present paper two simple approaches of sensitivity analysis are compared by the use of the physically based, continuous time hydrological model SWAT. In both approaches, one parameter is varied at a time while holding the others fixed, but the way of defining the range of variation is different. Similar results are obtained suggesting that parameter sensitivity may be determined without the results being influenced by the chosen method. Most sensitive parameters for hydrology and water quality are the physical soil properties such as bulk density, available water capacity or hydraulic conductivity. Plant specific parameters like maximum stomatal conductance or maximum leaf area index as well as slope length, slope steepness, and curve number also show a high sensitivity. Both approaches can be considered as equivalent, as they provide the same overall ranking into more and less sensitive parameters. An identification of the sensitive parameters is possible independently from the chosen variation range.

Hydrologic Response to land use changes on the catchment scale

Abstract Regional land use changes due to European market policy have far reaching consequences for various landscape functions. Among others land use change influences the local water balance. Simulation models are mostly used to analyse the effect of management practices on water quality but they can also be a useful tool to quantify the hydrologic response of a catchment to different land use options. In this study the physically based, continuous time step model SWAT mod has been applied within the joint research project SFB 299 at the Giessen University to support the development of sustainable land use concepts. In a first step the model has been calibrated and validated for four mesoscale watersheds with differing land use distributions. Then the model performance for changing land use has been tested in an artificial watershed with a single crop at one time and one underlying soil type to eliminate the complex interactions of natural watersheds. In relation to forest barley produced the strongest response of the water balance components. Finally a case study for the Dietzholze watershed with two land use scenarios derived with the ProLand model has been carried out. The impact of land use change on the annual water balance was relatively small due to compensating effects in a complex catchment. The decrease of forest due to a grassland bonus amplifies the peak flow rate and thus increases the risk of flooding.

Long-term land use changes in a mesoscale watershed due to socio-economic factors : effects on landscape structures and functions

To examine the effects of land use changes in peripheral regions on landscape structures and functions, we have developed or adapted three simulation models focused on agricultural economics, ecology and hydrology. These models were linked via GIS and tested in the Aar watershed located in central Germany. The agro-economic simulation model ProLand predicts land use changes resulting from a particular framework of specified natural, economic and political characteristics on a regional scale. This land use prognosis serves as input for the ecological and hydrological models. The ecological model ELLA analyzes and predicts the influence of land use systems on the biodiversity of a landscape indicated by the spread and dispersal of selected species. The present study focuses on modeling the spread of skylark (Alauda arvensis) breeding habitats. The physically based hydrological model SWAT calculates components of the landscapes water balance over a daily time-step. This paper addresses the most relevant problems of integrated modeling approaches, including the use of common databases, metainformation, and interface requirements for data exchange. Initial modeling results are presented to demonstrate the impact of a changing framework in peripheral regions on ecological and economic indicators. The actual land use was compared with a scenario with an incentive for grassland developed by ProLand. The percentage of forested areas declined significantly, while the area of grassland increased from 20 to 41%. The result was a decline of connected areas suitable for skylark habitats. Stream flow and surface runoff increased due to the change in land use.

The impact of agricultural Best Management Practices on water quality in a North German lowland catchment

Research on water quality degradation caused by point and diffuse source pollution plays an important role in protecting the environment sustainably. Implementation of Best Management Practices (BMPs) is a conventional approach for controlling and mitigating pollution from diffuse sources. The objectives of this study were to assess the long-term impact of point and diffuse source pollution on sediment and nutrient load in a lowland catchment using the ecohydrological model Soil and Water Assessment Tool (SWAT) and to evaluate the cost and effectiveness of BMPs for water quality improvement in the entire catchment. The study area, Kielstau catchment, is located in the North German lowlands. The water quality is not only influenced by the predominating agricultural land use in the catchment as cropland and pasture, but also by six municipal wastewater treatment plants. Diffuse entries as well as punctual entries from the wastewater treatment plants are implemented in the model set-up. Results from model simulations indicated that the SWAT model performed satisfactorily in simulating flow, sediment, and nutrient load in a daily time step. Two approaches to structural and nonstructural BMPs have been recommended in relation to cost and effectiveness of BMPs in this study. These BMPs include extensive land use management, grazing management practice, field buffer strip, and nutrient management plan. The results showed that BMPs would reduce fairly the average annual load for nitrate and total nitrogen by 8.6% to 20.5%. However, the implementation of BMPs does not have much impact on reduction in the average annual load of sediment and total phosphorus at the main catchment outlet. The results obtained by implementing those BMPs ranged from 0.8% to 4.9% and from 1.1% to 5.3% for sediment and total phosphorus load reduction, respectively. This study also reveals that reduction only in one type of BMP did not achieve the target value for water quality according to the European Water Framework Directive. The combination of BMPs improved considerably water quality in the Kielstau catchment, achieving a 53.9% and a 46.7% load reduction in nitrate and total nitrogen load, respectively, with annual implementation cost of 93,000 Euro.

SWAT-G, a version of SWAT99.2 modified for application to low mountain range catchments

Abstract The Soil and Water Assessment Tool (SWAT) is a well established distributed eco-hydrologic model. However, using the example of a mesoscale catchment in Germany it is shown that the version SWAT99.2 is not able to correctly reproduce the runoff generation in a low mountain region. The calculated contribution of the baseflow to the streamflow is far too high whereas the interflow is strongly underestimated. Alternatively, the modified version SWAT-G can be used which, as is demonstrated in this paper, yields far better results for catchments with predominantly steep slopes and shallow soils over hard rock aquifers. In the example, calibrating the model over three hydrologic years of daily streamflow, the model efficiency increases from −0.17 to +0.76. The modifications in SWAT-G allow hydrological processes to be modelled in low mountain ranges while not restraining the applicability of the model to catchments with other characteristics.

Assessing the Impacts of Four Land Use Types on the Water Quality of Wetlands in Japan

This study examined how changes in the composition of land use can affect wetland water quality. Twenty-four wetlands located in Hiroshima prefecture in the western part of Japan were selected for this purpose. The water quality parameters that were explored include: pH, electrical conductivity, turbidity, dissolved oxygen, total dissolved solid, temperature and different forms of nitrogen. These important indicators of the water quality in the study area were measured from December 2005 to December 2006. The composition of land uses was determined for the catchments of the wetlands. They were then categorized into three classes, including non-disturbed, moderately-disturbed and highly-disturbed wetlands, based on the extent of urban area (as the most disruptive land use type within the catchment of the wetlands). The relationship between land use types and water quality parameters for the wetlands was statistically examined. The findings indicated that there were significant positive relationships between the proportion (%) of urban areas within catchments of the wetlands and EC (r = 0.67, p < 0.01), TDS (r = 0.69, p < 0.01), TN (r = 0.92, p < 0.01), DON (r = 0.6, p < 0.01), NH4+(r = 0.47, p < 0.05), NO2− (r = 0.50, p < 0.05), while negative relationships were observed between the proportion (%) of forest area in these wetlands and EC (r = −0.62, p < 0.01), TDS (r = −0.68, p < 0.01), TN (r = −0.68, p < 0.01), DON (r = -0.43, p < 0.05), and NH4+ (r = −0.55, p < 0.01). Analysis of the variance also revealed significant differences within the wetland groups in terms of the annual mean of electrical conductivity, total dissolved solids, total nitrogen, nitrite, dissolved inorganic nitrogen and dissolved organic nitrogen in the study area. Moreover, the study also indicated that the forest area plays a significant role in withholding nutrient loads from the wetlands, and hence, it can act as a sink for surface/subsurface nutrient inputs flowing into such water bodies from the watersheds.

An interdisciplinary modelling approach to evaluate the effects of land use change

A set of three GIS-based models from the field of agricultural economy (ProLand), ecology (YELL) and hydrology (SWAT-G) was applied in the mountainous mesoscale watershed of the Aar, Germany. In a joint modelling exercise a sequence of land use change scenarios was analysed. It was assumed that the average field size for land use systems increases due to changes in the economic and administrative framework. ProLand calculated the spatial distribution of the new production systems on a grid basis. The resulting land use maps were analysed with YELL with regard to the habitat suitability for Emberiza citrinella and with the model SWAT-G in terms of the effects of land use change on hydrologic processes. Multi-functional trade-off relations between economic, ecological and hydrological landscape functions were compiled.

Modelling hydrological processes in mesoscale lowland river basins with SWAT—capabilities and challenges

Abstract Lowland areas are characterized by specific properties, such as flat topography, low hydraulic gradients, shallow groundwater, and high potential for water retention in peatland and lakes. These characteristics and their dominating hydrological processes have to be assessed and considered for the analysis and modelling of water balances in lowland catchments. The capabilities and challenges of modelling hydrological processes and water balances in mesoscale lowland river basins with the SWAT model are presented. The investigated catchments Stör, Treene and Kielstau are located in northern Germany within a lowland area. Covering areas from 50 to 517 km2, these rural meso-catchments have sandy, loamy and peaty soils. Drainage, in terms of tile drainage and open ditches, has changed the natural water balance. The set-up and modifications of the model as applied in these case studies can be transferred to other similar catchments in lowland areas. The dominating hydrological processes were found to be mainly controlled by groundwater dynamics and storage, drainage, wetlands and ponds. Some groundwater parameters were found to be highly sensitive and they turned out to be the most influential factors for improving simulated water discharge.

Simulation of Streamflow and Sediment with the Soil and Water Assessment Tool in a Data Scarce Catchment in the Three Gorges Region, China

The Three Gorges Region in China is currently subject to a large-scale land use change, which was induced by the construction of the Three Gorges Dam on the Yangtze River. The relocation of towns, villages, and agricultural areas is expected to affect the water balance and increase erosion rates and sediment yields in the affected catchments. Hydrologic and water quality models are frequently used to assess the impact of land use changes on water resources. In this study, the eco-hydrological Soil and Water Assessment Tool (SWAT) model is applied to the Xiangxi Catchment in the Three Gorges Region. This paper presents the calibration and validation of streamflow and sediment loads at Xingshan gauging station. The calibration of daily streamflow resulted in a satisfactory fit of simulated and observed data, which is indicated by Nash-Sutcliffe efficiency (NSE) values of 0.69 and 0.67 for the calibration (1981-1986) and validation (1988-1993) periods, respectively. In contrast, the model was not able to simulate the monthly average sediment loads correctly, as indicated by very low NSE values of 0.47 (calibration) and 0.08 (validation). This might be due to inadequate representation of spatial rainfall variability by the available climate stations, insufficient input data, uncertainties in the model structure, or uncertainties in the observed sediment loads. The discussion of these possible reasons for the incorrect prediction of sediment loads by SWAT reveals the need for further research in the field of hydrological and water quality modeling in China.

Eco-hydrologic model cascades: Simulating land use and climate change impacts on hydrology, hydraulics and habitats for fish and macroinvertebrates.

Climate and land use changes affect the hydro- and biosphere at different spatial scales. These changes alter hydrological processes at the catchment scale, which impact hydrodynamics and habitat conditions for biota at the river reach scale. In order to investigate the impact of large-scale changes on biota, a cascade of models at different scales is required. Using scenario simulations, the impact of climate and land use change can be compared along the model cascade. Such a cascade of consecutively coupled models was applied in this study. Discharge and water quality are predicted with a hydrological model at the catchment scale. The hydraulic flow conditions are predicted by hydrodynamic models. The habitat suitability under these hydraulic and water quality conditions is assessed based on habitat models for fish and macroinvertebrates. This modelling cascade was applied to predict and compare the impacts of climate- and land use changes at different scales to finally assess their effects on fish and macroinvertebrates. Model simulations revealed that magnitude and direction of change differed along the modelling cascade. Whilst the hydrological model predicted a relevant decrease of discharge due to climate change, the hydraulic conditions changed less. Generally, the habitat suitability for fish decreased but this was strongly species-specific and suitability even increased for some species. In contrast to climate change, the effect of land use change on discharge was negligible. However, land use change had a stronger impact on the modelled nitrate concentrations affecting the abundances of macroinvertebrates. The scenario simulations for the two organism groups illustrated that direction and intensity of changes in habitat suitability are highly species-dependent. Thus, a joined model analysis of different organism groups combined with the results of hydrological and hydrodynamic models is recommended to assess the impact of climate and land use changes on river ecosystems.