2. Thesis and Dissertations
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Item Short-term Offshore Wind Speed Forecasting using Buoy Observations and Regional scale Wind Resource Assessment based on Scatterometer Data(National Institute of Technology Karnataka, Surathkal, 2016) Gadad, Sanjeev; Deka, Paresh ChandraOffshore winds are valuable source of renewable energy. To recognize the potential of area it is essential to assess the available resource and understand the sporadic nature of winds. Wind Resource Assessment (WRA) coupled with short-term forecast of winds will aid in establishing the confidence for undertaking offshore wind farm development. Wind speed forecasting is important for estimating power generation capacity of turbines. The knowledge of availability of the winds in future time steps will be pivotal in planning and improving the efficiency of energy production. Buoys are the fundamental source of in situ atmospheric parameter observations. One of the primary objectives of the present research is to determine suitable technique for short-term forecasting of offshore winds. So, the present study focuses on assessing accuracy of the ANFIS hybrid model for short-term wind speed forecasting. In addition, the Arabian Sea belongs to tropical humid climate zone and therefore the influence of Relative Humidity (RH) on the ANFIS model to estimate offshore wind speed was investigated. In the study, two buoys with id– AD07 and CB02 apart approximately by 500 km were selected. Two models (model 1: 5 inputs, 1 output and model 2: 4 inputs, 1 output) and two scenarios (scenario 1: estimate wind speeds and scenario 2: forecasting wind speeds) were developed for the study. From scenario 1, it was found that at both the buoy locations the model 1 outperformed model 2 in estimating observed wind speeds and RH had noticeable influence on the model performance. Persistence Method (PM) was chosen as base method for comparing the wind speed forecasts. From scenario 2, at AD07, model 1 forecasts were accurate than other two models and at CB02, the PM forecasts were most accurate. However, it was found that the model 1 forecasts at CB02 were closer to PM. Altogether, the model 1 performance was higher than model 2 indicating the error in forecasts due to absence of RH observations. The study concludes that the model performance was enhanced by incorporating RH observations as an input to the ANFIS model. The RMSE of forecasted wind speeds up to three time steps, at AD07 and CB02 would be approximately lower by 37% and 14% respectively.ii Further, the study examines the performance of ANFIS and Wavelet-ANFIS (WT+ANFIS) hybrid techniques to forecast wind speeds for multiple time steps at the same buoy locations (AD07 and CB02) in the Arabian Sea. The forecast accuracy of ANFIS and WT+ANFIS were compared with PM. The RMSE for the testing dataset at AD07 and CB02 using ANFIS model was found to be 1.3 m s-1 and 1.26 m s-1 for 1st (t+1) time step respectively. The RMSE for WT+ANFIS model at AD07 and CB02 was obtained as 1.5 m s-1 and 1.20 m s-1 for 1st (t+1) time step respectively. It was observed at CB02, the WT+ANFIS model forecast was closest to PM. At AD07, an ANFIS and WT+ANFIS model performance was almost similar and found to be better than PM. In general, the WT+ANFIS model outperformed ANFIS and PM for multiple time steps. Thus, the analysis establishes that WT+ANFIS hybrid method has the potential to be a complementary tool in obtaining short-term offshore wind speed forecasts. In the offshore region the scarcity of in situ wind data in space proves to be a major setback for wind power potential assessments. Satellite data effectively overcomes this setback by providing continuous and total spatial coverage. The satellite data needs to be validated at the study area before conducting WRA study. Hence the work centers on estimating the performance of Oceansat–2 scatterometer (OSCAT)– derived wind vector using in situ data from buoys (id– AD02 and CB02) at different locations in the Arabian Sea. For the validation of OSCAT winds, the buoy winds are required to be extrapolated to height of 10 m and are known as Equivalent Neutral Winds (ENW). A comparative study among three methods- power law, logarithmic and Liu– Katsaros–Businger (LKB) method for estimating the ENW for buoys is carried out. OSCAT winds were closest to ENW estimated by the Liu–Katsaros–Businger (LKB) method. The spatial and temporal windows for comparison were 0.5° and ±60 minutes, respectively. The monsoon months (June–September) of 2011 were selected for the study. The root mean square deviation for wind speed is less than 2.5 m s−1 and wind direction is less than 20°, and a small positive bias is observed in the OSCAT wind values. From the analysis, the OSCAT wind values were found to be consistent with in situ-observed values. Furthermore, wind atlas maps were developediii with OSCAT winds, representing the spatial distribution of winds at a height of 10 m over the Arabian Sea. Satellite-based regional scale offshore wind power resource assessment was carried out for the Karnataka state, which is located on the west coast of India. OSCAT wind data and GIS based methodology were adopted in the study. The real time ship based observations is considered in the present work, to assess the accuracy of OSCAT wind data. The INCOIS Realtime All Weather Station (IRAWS) data provides greater spatial coverage than conventional buoy setup. Probably, this is the first attempt to validate OSCAT data using IRAWS dataset, which offered greater number of collocated observation points and hence provided better assessment. Wind speed maps at 10 m, 90 m and wind power density maps using OSCAT data were developed to understand the spatial distribution of winds over the study area. Bathymetric map was developed based on the available foundation types and demarking various exclusion zones to help in minimizing conflicts. The wind power generation capacity estimation performed using REpower 5 MW turbine, based on the water depth classes was found to be 9,091 MW in Monopile (0-35 m), 11,709 MW in Jacket (35-50 m), 23,689 MW in Advanced Jacket (50-100 m) and 117,681 MW in Floating (100-1000 m) foundation technology. In Indian scenario, major thrust may be given for wind farm development in Monopile region. Therefore, as first phase of development for 10% of the estimated potential in this region, 116% of energy deficit for FY 2011-12 could be met. Also, up to 79% of the anticipated energy deficit for the FY 2014-15 of the Karnataka state of India could be achieved.Item Assessment of Variable Source Area Hydrological Models in the Upper Cauvery Basin, Karnataka, India(National Institute of Technology Karnataka, Surathkal, 2016) B. C, Kumar Raju; Nandagiri, LakshmanWith increased availability of spatial data-sets of catchment characteristics and hydrometeorological variables, distributed hydrological models are being applied to solve a variety of problems related to catchment hydrology and water resources management. However, experimental results obtained in recent decades have shown the possiblity of existence of runoff generation mechanisms other than the conventional infiltration-excess (Hortonian) mechanism. In particular, it has been shown that Variable Source Area (VSA) mechanism of runoff generation may prevail in humid steeply sloping and well vegetated watersheds. Accordingly, efforts have been made by previous researchers to incorporate this mechanism into distributed hydrological models and their performances have been evaluated in mostly humid temperate regions and not so much in humid tropical regions. The primary objective of the present study was to compare the performances of hydrological models which incorporate the Variable Source Area (VSA) mechanism of runoff generation with that of the Soil and Water Assessment Tool (SWAT) which employs the conventional infiltration-excess mechanism of runoff generation. One of the VSA based model used, SWAT-VSA, has been proposed by earlier researchers as a re-conceptualization of the SWAT model and uses a topography-based wetness index to identify source areas and simulates runoff in a manner consistent with VSA hydrology. In the present study, the topography-based wetness index was replaced with a Modified Normalized Difference Water Index (MNDWI) derived from satellite imagery resulting in a new VSA model version, SWAT-MNDWI. Performance evaluation of the models was carried out through their application in two humid tropical watersheds (Hemavathi – 2974 km2; Harangi – 538.8 km2) located in the Upper Cauvery River Basin (36,682 km2), India wherein previous studies have shown the existence of VSA hydrology. The other aspects addressed in this study include: assessment of significance and magnitude of trends in historical records of observed hydrometeorological variables in the Upper Cauvery Basin, evaluation of uncertainties associated with streamflowii predictions of the 3 hydrological models and simulation of the hydrologic impacts of hypothetical land use/land cover (LU/LC) changes in the Hemavathi and Harangi watersheds. The present study examined the significance and magnitude of trends in the monthly rainfall (33 rain gauges), maximum and minimum temperature (6 climate stations) and streamflow at 4 gauge sites in the Upper Cauvery Basin for the historical 30 year period 1981-2010. The statistical parameters - Coefficient of Variation (CV) and percentage departure were calculated for average monthly values separately for 3 decades. The Seasonal-Kendall and Sen’s slope estimator were used to calculate significance and magnitude of trends in rainfall, temperature and streamflow data. Detrended Fluctuation Analysis (DFA) method was used to detect long-term persistence in the time series data. As expected, the CV of rainfall shows a large variation in the month December to March, while the percentage departure also varies during these months for different decades. But there was no significant trend found for all rain gauge stations and sub basins except for the Arkavathi sub basin. For maximum temperature there was not much variation except in the months of May and June at the Hassan climate station. Statistically significant trend was observed in maximum temperature for Chikmagalur and Hassan stations. The CV of minimum temperature shows a large variability from November to March for all climate stations and also a significant increasing trend for Hassan and Bangalore stations, while for Madikeri a decreasing trend was observed with a variation of -0.16 0C/year. There was not much variation found for streamflow except in K M Vadi gauge site and T.Narasipur gauge site which showed a significant decreasing trend of -0.778 m3/s/ year. Long range dependence analysis revealed a weak persistence for both rainfall and streamflow of the basin. Using relevant data inputs pertaining to rainfall, climate, elevation, Land use/Land Cover (LU/LC) and soils, the SWAT, SWAT-VSA and SWAT-MNDWI models were applied separately to both watersheds using a daily time step. Models were calibrated for the historical period 2000-2003 and validated for the period 2004-2006 using observed daily streamflow records at the watershed outlets. The comparative assessment focused specifically on the following aspects for the six cases considerediii (3 models applied to 2 watersheds): 1) sensitivity of model parameters 2) accuracy of daily streamflow predictions at the watershed outlets 3) predictions of spatially and temporally averaged annual water balance components 4) differences in spatial patterns of source areas of surface runoff. Sensitivity analysis indicated that for the SWAT model, Curve Number (CN) was the most important parameter while for the VSA based models, parameters related to the unsaturated zone and shallow groundwater were important, a result consistent with the runoff mechanism incorporated in the models. The accuracies of streamflow prediction as determined from scatter plots and model performance statistics were more or less similar both in calibration and validation for all the three models with the models performing better in the forested Harangi watershed. Overall, the SWAT-MNDWI model proved to be the best one in simulating daily streamflow with Nash-Sutcliffe efficiency (ENS) of 0.85, coefficient of determination (R2) of 0.88, percentage bias (PBIAS) of 13.2% and root mean square error (RMSE) of 37.48 m3/s for the Hemavathi watershed and corresponding values of 0.88, 0.88, 1.09% and 16.67 m3/s for the Harangi watershed. All three models simulated spatially and temporally averaged major water balance components in a consistent manner resulting in a residual error of <5% of annual rainfall in the annual water balance. However, evapotranspiration loss as a percentage of rainfall appeared unreasonable (27% - 32%) for the wet Harangi watershed probably on account of it being predominantly forested. The spatial patterns of surface runoff generation were somewhat similar for the SWAT-VSA and SWATMNDWI models, but completely different for the SWAT model, again a result consistent with the runoff generation mechanism adopted. Overall results of this study have demonstrated that models incorporating VSA hydrology, and in particular the SWAT-MNDWI model proposed in this study, provide accurate and convenient tools for distributed hydrologic modelling in humid tropical watersheds. This study also focuses on assessing uncertainties associated with SWAT-MNDWI, SWAT-VSA and SWAT models using SWAT-CUP (Calibration and Uncertainty Programs) tool. Two multi-objective uncertainty techniques (Generalized Likelihood Uncertainty Equation (GLUE) and Sequential Uncertainty Fitting algorithm (SUFI-2)) were tested for the Hemavathi and Harangi watersheds. The goodness-of-fit and efficiency of the models have been tested using ENS as the objective function. GLUEiv and SUFI-2 techniques yielded good results in minimizing the differences between observed and simulated streamflows at the outlets of the Hemavathi and Harangi watersheds. The results show that GLUE performance was slightly better than the SUFI-2 technique for all models for both the watersheds during calibration and validation periods. The 95PPU estimated by the GLUE and SUFI-2 techniques are very close to each other and larger than 45% (P-factor) for all models for both the watersheds during calibration and validation periods. For GLUE, R-factor values during the validation phase for the Hemavathi watershed were 0.35, 0.38 and 0.34 for the SWAT-MNDWI, SWAT-VSA and SWAT models respectively with corresponding values for the Harangi watershed being 0.41, 0.39 and 0.40. It should be noted that that both GLUE and SUFI-2 cannot accurately quantify the prediction uncertainty of SWAT-MNDWI, SWAT-VSA and SWAT models. Overall results indicated that the GLUE technique applied on the SWAT-MNDWI model performed best in quantifying the prediction uncertainty of streamflow at the outlets of both watersheds. In order to simulate the hydrologic impacts of LU/LC changes in the study area, two hypothetical LU/LC change scenarios were formulated for Hemavathi and Harangi watersheds. The SWAT-MNDWI, SWAT-VSA and SWAT models were used to simulate the hydrologic responses under these scenarios. Values of average annual water balance components and their percentage change with respect to reference results were calculated for both watersheds using the three models. Additionally, an effort was also made to construct the Flow Duration Curves (FDCs) using daily streamflow values generated under each scenario. Differences in optimal parameters of an empirical model for the FDC, magnitudes of flow quantiles, high flow index and low flow index were computed for each scenario. For the Hemavathi watershed, with increase in agricultural land there is increase in water yield predicted by all three models. With increase in forest cover there is decrease in water yield predicted by SWAT-VSA and SWAT models while for SWAT-MNDWI an increase in water yield was found. For Harangi watershed, with increase in agricultural land or forested area there is decrease in water yield for all three models except SWAT-VSA model in scenario I. Both the scenarios appeared to have significant impacts on the runoffv regime as indicated by significant changes in FDC model parameters, flow quantiles and flow indices. Overall results of this study provide useful inputs with regard the magnitude and direction of likely future changes in important hydrometeorological variables which can be used to prepare plans for mitigation and adaptation to climate change in the Upper Cauvery Basin. The present study has demonstrated an overall methodology for application, performance evaluation and uncertainty analysis of distributed hydrological models using a variety of ground-based inputs and satellite data within a GIS framework. Since previous studies in similar watersheds in the Western Ghats region have identified VSA as a dominant mechanism of runoff generation, the spatial patterns obtained with the SWAT-VSA and SWAT-MNDWI models provide information which will prove to be extremely useful in soil and water conservation measures and in identifying source areas of non-point pollution. The SWAT-MNDWI model proposed in this study is particularly attractive since it employs satellite imagery to accurately identify areas of different wetnesses within the watershed and integrates this information into a distributed hydrological model. As the results of this study have demonstrated, such a modelling approach using VSA hydrology provides an accurate and convenient tool for distributed hydrologic modelling and impact assessment of LU/LC changes in humid tropical watersheds.