• N K Thakur

      Articles written in Journal of Earth System Science

    • Geomagnetic calm intervals and anomalous solar cycle 20

      K G Sontakke N K Thakur G K Rangarajan

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      Long period variations in the occurrence of prolonged intervals of calm magnetic field conditions are studied using index Ap of magnetic activity. The solar-cycle variation in occurrence is compared with the sunspot number. Anomalous behaviour for solar cycle 20, observed in other solar parameters, are shown to be manifested in the occurrence frequency of quiet intervals. Spectral characteristics of occurrence indicates a dominant long period variation of about 30 years and a more feeble 11-year oscillation

    • Assessment of different topographic corrections in AWiFS satellite imagery of Himalaya terrain

      V D Mishra J K Sharma K K Singh N K Thakur M Kumar

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      The influence of topographic effects in optical satellite imagery is not investigated very extensively in the Himalayan terrain.The topographic variability causes a problem of differential illumination due to steep and varying slopes in rugged Himalayan terrain. Therefore,topographic corrections are essential for qualitative and quantitative analysis of snow cover applications.The present paper discusses the implementation of different topographic correction models on AWiFS sensor onboard IRS P6 satellite images and the qualitative and quantitative comparative analysis in detail.Both the Lambertian and non-Lambertian assumptions have been considered in the present analysis with the aim to explore best suitable empirical model for rugged terrain.The main topographic methods implemented are:

      C-correction

      Minneart corrections

      Civco ’s modi fied version of cosine correction

      two-stage normalization and

      slope matching technique.

      Lambertian assumptions are found to be very unrealistic over Himalayan terrain as these lead to either underestimation or overestimation of physical parameters significantly both on sunlit slopes as well as the slopes away from the Sun.This problem is overcome by considering non-Lambertian assumption.Minneart constant and C-correction coefficients for all AWiFS satellite bands are estimated using regression analysis.All the results due to topographic effects are investigated qualitatively and quantitatively using four criteria namely visual analysis,validation with field measurements (in-situ observations),spectral re flectance of training samples of snow on the south and north aspects and graphically.The visual analysis confirms the minimization of three dimensional relief effects in two-stage normalization and slope matching methods and retrieves some of the information under mountain shadow.Due to the very bright surface of snow fields there is likely to be more diffuse reflected light in these areas than over darker vegetated surfaces.The qualitative analysis in other methods does not extract any information on shady slopes.The quantitative validation of topographic results in satellite imagery with in-situ observations shows underestimation of spectral reflectance of snow significantly except for slope matching technique. It is also apparent that although all the topographic methods correct the reflectance of training snow samples on the south and north aspects but most acceptable values are achieved using slope matching.The results obtained from graphical analysis reveal that mean reflectance after all topo- graphic corrections are independent of illumination.This study also suggests that the suitability of topographic models can not be concluded as successful based on single criterion.Slope matching technique is the only technique which satisfies all the four criteria successfully and produces the best result for Himalayan terrain.

    • Monitoring and evaluation of seasonal snow cover in Kashmir valley using remote sensing, GIS and ancillary data

      H S Negi N K Thakur Rajeev Kumar Manoj Kumar

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      Seasonal snow cover is a vital natural resource in the Himalaya. Monitoring of the areal extent of seasonal snow cover is important for both climatological studies as well as hydrological applications. In the present paper, snow cover monitoring was carried out to evaluate the region-wise accumulation and ablation pattern of snow cover in Pir Panjal and Shamshawari ranges of Kashmir valley. The study was carried out for the winter period between November and April of 2004–05, 2005–06 and 2006–07, using multi-temporal WiFS sensor data of IRS-1C/1D satellites. The study shows reduction in the areal extent of seasonal snow cover and rising trend of maximum temperature in three winters for the entire Kashmir valley. This has been validated with 20 years (1988– 89 to 2007–08) climatic conditions prevailed in both ranges of Kashmir valley. Region-wise study shows the spatial and temporal variability in seasonal snow cover within Kashmir valley. Advance melting was observed in Banihal and Naugam/Tangdhar regions than Gurez and Machhal regions. Different geographical parameters of these regions were studied to evaluate the influence on snow cover and it was observed that altitude and position of region with respect to mountain range are the deciding factors for retaining the seasonal snow cover for longer duration. Such region-wise study of snow cover monitoring, can provide vital inputs for planning the hydropower projects, development in habitat areas, recreational and strategic planning in the region.

    • Monitoring of Gangotri glacier using remote sensing and ground observations

      H S Negi N K Thakur A Ganju Snehmani

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      In this study, Gangotri glacier was monitored using Indian Remote Sensing (IRS) LISS-III sensor data in combination with field collected snow-meteorological data for a period of seven years (2001–2008). An overall decreasing trend in the areal extent of seasonal snow cover area (SCA) was observed. An upward shifting trend of wet snow line was observed in the beginning of melt period, i.e., in May and dominant wet snow conditions were observed between May and October. Snow meteorological parameters collected in the Gangotri sub-basin suggest reduction in fresh snowfall amount during winter, increase in rainfall amount during summer, decrease in snowfall days, increase in rainfall days and rising trend of average temperature. The prevailing wet snow condition on glacier has caused scouring of slopes which led the excessive soil/debris deposition on the glacier surface. This was observed as one of the major factor for activating fast melting and affecting the glacier health significantly. Apart from climatic conditions, terrain factors were observed for changing the glacio-morphology. The significant changes on the glacier surface were observed in the regions of abrupt slope change. The above factors affecting the Gangotri glacier health were also validated using high resolution satellite imageries and field visit. A deglaciation of 6% in overall area of Gangotri glacier was observed between the years 1962 and 2006.

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