Georg Hörmann

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.

Modeling the effects of environmental variables on short-term spatial changes in phytoplankton biomass in a large shallow lake, Lake Taihu

Understanding the short-term response of phytoplankton biomass on environmental variables is needed for issuing early warnings of harmful algal blooms in aquatic ecosystems. Predicting harmful algal blooms are particularly challenging in large shallow lakes due to their complex mixing patterns. This study used a two-dimensional hydrodynamic–phytoplankton model to evaluate the effects of environmental variables on short-term changes in the horizontal distribution of phytoplankton biomass in a large shallow lake, Lake Taihu, China. Two simulations were performed using daily and hourly average wind condition and water temperature data collected in 2009. Other model inputs were identical for these two simulations. The response of phytoplankton to wind conditions, light intensity, water temperature, and total dissolved phosphorus and nitrogen concentrations were examined based on a sensitivity analysis using the hourly data. Hourly simulation achieved a more realistic distribution of phytoplankton biomass than the daily simulation. This finding implies that data with a higher temporal resolution are more useful for short-term prediction of phytoplankton biomass in this lake. Sensitivity analysis indicated that water temperature and light intensity dominate short-term changes in phytoplankton biomass in this lake. Wind conditions also affect phytoplankton biomass distribution by causing advective water movement.

Structural uncertainty assessment in a discharge simulation model

Abstract A major goal in hydrological modelling is to identify and quantify different sources of uncertainty in the modelling process. This paper analyses the structural uncertainty in a streamflow modelling system by investigating a set of models with increasing model structure complexity. The models are applied to two basins: Kielstau in Germany and XitaoXi in China. The results show that the model structure is an important factor affecting model performance. For the Kielstau basin, influences from drainage and wetland are critical for the local runoff generation, while for the XitaoXi basin accurate distributions of precipitation and evapotranspiration are two of the determining factors for the success of the river flow simulations. The derived model uncertainty bounds exhibit appropriate coverage of observations. Both case studies indicate that simulation uncertainty for the low-flow period contributes more to the overall uncertainty than that for the peak-flow period, although the main hydrological features in these two basins differ greatly. Citation Zhang, X. Y., Hörmann, G., Gao, J. F. & Fohrer, N. (2011) Structural uncertainty assessment in a discharge simulation model. Hydrol. Sci. J. 56(5), 854–869.

Accuracy, reproducibility and sensitivity of acoustic Doppler technology for velocity and discharge measurements in medium-sized rivers

Abstract With increased interest and requirements in surface water quality and hydrodynamics, additional information is needed about water flow in streams. The mobile OTT Qliner with acoustic Doppler technology (ADQ) provides a highly efficient and accurate way of collecting this information. For this study we completed 366 measurements of flow velocity, water depth and discharge with ADQ from September 2010 to June 2011 at 174 cross-sections in eight catchments of different sizes located in northern Germany, central Germany and southeastern China. The measurements were used to study the accuracy, reproducibility and sensitivity of the device, and to improve the hydrodynamic sampling for medium-sized rivers and channels by investigating its internal settings. The observations reported clearly show that the results of flow average, profile, layer and point values obtained with the ADQ compare very well with those of electromagnetic or ultrasonic devices. In general, the average flow velocity gives the highest agreement. Vertical velocity has a better quality than the layer velocity, which indicates a greater precision in the horizontal than in the perpendicular direction. Point velocity, the composite of vertical velocity and layer velocity, has intermediate precision. Tests on internal settings revealed that measurement is more sensitive to cell size than to time interval setting. A cell size to depth ratio of between 0.1 and 0.2 m produced the highest reliability. A measurement period of 30 s is needed for velocities faster than 0.3 m/s; for shallow and slow-flowing rivers, an interval of 50 s or even greater is recommended. The closer the measured points were to the river bank or bed, the greater the measurement error. The river bed can also influence the measurement more distinctly than the river bank. Editor D. Koutsoyiannis; Associate editor A. Montanari Citation Song, S., Schmalz, B., Hörmann, G., and Fohrer, N., 2012. Accuracy, reproducibility and sensitivity of acoustic Doppler technology for velocity and discharge measurements in medium-sized rivers. Hydrological Sciences Journal, 57 (8), 1626–1641.

Predicting effects of temperature increase on the water balance of beech forest : An application of the 'KAUSHA' model

The water balance model ‘KAUSHA’ (Halldin, 1989) was applied to a 100-year-old beech (Fagus sylvatica L.) forest in northern Germany. Overall, a satisfying agreement between modelled evapotranspiration values and independent micrometeorological measurements (Bowen ratio energy balance method) could be observed, although for rainy days KAUSHA showed a tendency to overestimate evapotranspiration. The model was used to predict the effects of a climate warming on the water budgets of the forest. It is shown that a temperature increase of 2°C due to a rising CO2 content of the atmosphere will not change the yearly totals of evapotranspiration significantly, but could have serious effects on the soil water balance during the vegetation period. Because under climate change conditions a higher amount of the available soil water has already been evaporated in winter and spring, soil water content will limit the transpiration of the trees from July to September much more strongly. Therefore, the yield of beech forest might also suffer from drought effects. It can be concluded that a better knowledge of the seasonal distribution of rainfall under climate change conditions is indispensable for predicting effects of rising temperatures and CO2 concentrations on ecosystems.

Natural and Anthropogenic Causes of Vegetation Changes in Riparian Wetlands Along the Lower Reaches of the Yellow River, China

There are many natural and anthropogenic impacts on the vegetation in riparian wetlands. The Yellow River in the North of China is one of the biggest rivers of the world, but the riparian wetland area is very small. The monitoring of these wetlands has increased in the last years because of the high ecological importance and increasing pressure due to economic development and increasing use of water resources. We use remote sensing and hydrologic data to analyze the change of wetland vegetation in the Zhengzhou natural reserve, which is affected by a rapid population growth and change of water flow. The main findings are: (1) the vegetation cover of the study area decreased from 1984 to 2011, (2) the hydrologic boundary conditions changed dramatically after the construction of a dam upstream and floods have almost disappeared under the new water resource management, (3) there is a conflict between the protection of the wetland vegetation and its use for recreation and tourism. Actually, anthropogenic factors are the main reasons for wetland degradation, and the government and the water management department should pay more attention to the protection of the remaining wetlands and the environmental balance.

Detailed spatial analysis of SWAT-simulated surface runoff and sediment yield in a mountainous watershed in China

Abstract In this study, surface runoff and sediment yields, simulated by the Soil and Water Assessment Tool (SWAT) for the Xiangxi catchment, China, are analysed at the spatial level of hydrologic response unit (HRU). This allows for a more precise targeting of best management practices than analysis at the coarser sub-basin level. It also provides the opportunity to validate simulated surface runoff and sediment yield by evaluating the plausibility of their spatial variation within the watershed. The results indicate that satisfactory model performance at a gauge does not guarantee plausible results at HRU level. Both surface runoff and sediment yields were found to vary reasonably with land use and soil types, but not with slope. The credibility of simulated surface runoff was increased by modifying selected SWAT algorithms and parameters. However, these modifications had only a marginal effect on sediment yield, which can be explained by the nonlinear relationship between sediment yield and HRU area in the modified Universal Soil Loss Equation. This study demonstrates that evaluation of model output at HRU level is highly recommended. Editor D. Koutsoyiannis; Guest editor V. Krysanova

Water Relations at Different Scales

The comprehensive data set of meteorological, hydrological, soil physical and plant physiological measurements permits a detailed inquiry into how ecosystems use water and how the water cycle is coupled with other cycles in agricultural and forest ecosystems. Since characteristic features of such cycling processes are distinctly scale-dependent and comprehensive water budgets are subject to detailed discussion in Chapter 9 of the present book, in the following the spatial scale is restricted to the range from leaves to stands of the ecosystems investigated in the core area of research (cf. Chapter 3). Thus, the first part presents water relations at the leaf scale, in the second the focus is on the ecophysiology of water exchange at the stand level, and finally there is a comparison of the empirical results of this enquiry with results of modelling approaches. The methodology underlying both measuring and modelling procedures is summarized in Section 1.3.

Lentic small water bodies: Variability of pesticide transport and transformation patterns

Lentic small water bodies have a high ecological potential as they fulfill several ecosystem services such as the retention of water and pollutants. They serve as a hot spot of biodiversity. Due to their location in or adjacent to agricultural fields, they can be influenced by inputs of pesticides and their transformation products. Since small water bodies have rarely been part of monitorings/campaigns up to now, their current exposure and processes guiding the pesticide input are not understood, yet. This study presents results of a sampling campaign of 10 lentic small water bodies from 2015 to 2016. They were sampled once after the spring application for a pesticide target screening, before autumn application and three times after rainfall events following the application. The autumn sampling focused on the herbicides metazachlor, flufenacet and their transformation products - oxalic acid and - sulfonic acid as representatives for common pesticides in the study region. The concentrations were associated with rainfall before and after application, characteristics of the site and the water bodies, physicochemical parameters and the applied amount of pesticides. The key results of the pesticide screening in spring indicate positive detections of pesticides which have not been applied for years to the single fields. The autumn sampling showed frequent occurrences of the transformation products, which are formed in soil, from 39% to 94% of all samples (n=71). Discharge patterns were observed for metazachlor with highest concentrations in the first sample after application and then decreasing, but not for flufenacet. The concentrations of the transformation products increased over time and revealed highest values mainly in the last sample. Besides rainfall patterns right after application, the spatial and temporal dissemination of the pesticides to the water bodies seems to play a major role to understand the exposure of lentic small water bodies.