Sharad Joshi

A comprehensive approach and methods for glacial lake outburst flood risk assessment, with examples from Nepal and the transboundary area

Like other mountainous areas, Nepal is highly vulnerable to glacial lake outburst floods (GLOFs), and this vulnerability has increased due to climate change. Risk reduction strategies must be based on a comprehensive risk assessment. A comprehensive methodological approach for GLOF risk assessment is described and illustrated in case studies of the potential GLOF risk posed in Nepal by four glacial lakes, one located in China. People, property and public infrastructure (including hydropower plants, roads and bridges) are vulnerable, and there is a need to integrate GLOF risk reduction strategies into national policies and programmes.

Preliminary results of mass-balance observations of Yala Glacier and analysis of temperature and precipitation gradients in Langtang Valley, Nepal

Abstract Monitoring the glacier mass balance of summer-accumulation-type Himalayan glaciers is critical to not only assess the impact of climate change on the volume of such glaciers but also predict the downstream water availability and the global sea-level change in future. To better understand the change in meteorological parameters related to glacier mass balance and runoff in a glacierized basin and to assess the highly heterogeneous glacier responses to climate change in the Nepal Himalaya and nearby ranges, the Cryosphere Monitoring Project (CMP) carries out meteorological observations in Langtang Valley and mass-balance measurements on Yala Glacier, a debris-free glacier in the same valley. A negative annual mass balance of –0.89m w.e. and the rising equilibrium-line altitude of Yala Glacier indicate a continuation of a secular trend toward more negative mass balances. Lower temperature lapse rate during the monsoon, the effect of convective precipitation associated with mesoscale thermal circulation in the local precipitation and the occurrence of distinct diurnal cycles of temperature and precipitation at different stations in the valley are other conclusions of this comprehensive scientific study initiated by CMP which aims to yield multi-year glaciological, hydrological and meteorological observations in the glacierized Langtang River basin.

Multiresolution image fusion using edge-preserving filters

Abstract. We propose two approaches of multiresolution image fusion using multistage guided filter and difference of Gaussians (DoGs). In a multiresolution image fusion problem, the given multispectral (MS) and panchromatic (Pan) images have high spectral and high spatial resolutions, respectively. One can obtain the fused image using these two images by injecting the missing high frequency details from the Pan image into the MS image. The quality of the final fused image will then depend on the method used for high frequency details extraction and also on the technique for injecting these details into the MS image. Specifically, we have chosen the guided filter and DoGs for detail extraction since these are more versatile in applications involving feature extraction, denoising, and so on. The detail extraction process in the fusion approach using a guided filter exploits the relationship between the Pan and MS images by utilizing one of them as a guidance image while extracting details from the other. The final fused image is obtained by adding the extracted high frequency details to the corresponding MS image. This way, the spatial distortion of the MS image is reduced by consistently combining the details obtained using both MS and Pan images. In the fusion method using DoGs, the high frequency details are extracted in the first and second levels by subtracting the blurred images of the original Pan. The extracted details at both DoGs are added to the MS image to obtain the final fused image. Advantages and disadvantages of each method are discussed and the comparison of the results is shown between the two. The results are also compared with the traditional and the state-of-the-art methods using the images captured using different satellites such as Quickbird, Ikonos-2, and Worldview-2. The quantitative assessment is evaluated using the conventional measures as well as using a relatively new index, i.e., quality with no reference which does not require a reference image. The results and measures clearly show that there is promising improvement in the quality of the fused image using the proposed approaches.

Lemthang Tsho glacial Lake outburst flood (GLOF) in Bhutan: cause and impact

BackgroundThe Hindu Kush Himalayan (HKH) region being seismically active and sensitive to climate change is prone to glacial lake outburst flood (GLOF). The Lemthang Tsho GLOF breached in the evening of 28 July 2015 innorth-western Bhutan is reminds of the looming threat, and stresses the need to have good risk management plan. The need to understand the physical processes in generating GLOF to is therefore imperative in order to effectively manage the associated risk. The paper therefore assesses the cause and impact of the Lemthang Tsho GLOF event using field and remote sensing data.ResultsThe collapse of near vertical wall of supraglacial lake triggered by 2 days of incessant rainfall, opened up the englacial conduit resulting in emptying of interconnected supraglacial lakes into Lemthang Tsho. The5.1 magnitude earthquake epicentered 187 km to southeast in the Indian state of Assam in the morning (7:10 am Bhutan Standard Time) of the same day is unlikely to have played any role in triggering the event. The estimated volume of water unleased is 0.37 million m3, with peak discharge estimated to be ranging from 1253 to 1562 m3/s, and velocity of 7.14–7.57 m/s. The impact was minimal and confined up to 30 km downstream from the lake. The flood took lives of 4 horses, washed away 4 timber cantilever bridges, 148 pieces of timber, damaged 1 acre of land, and washed away 1 km of trail. The team also monitored 3 out of 25 identified critical glacial lakes and downgraded the risk of all 3 critical glacial lakes based on the finding. This brings the number of critical glacial lakes in Bhutan to 22.ConclusionThe threat of GLOF still looms large in the Himalaya, particularly in view of impact of climate change and frequent seismic activities. There is a need for good risk management practices which starts fromidentification of critical glacial lakes, to prioritize in-depth investigation. As the present list of critical glacial lakes are largely based on inventory done over a decade based on topographic maps some of which datedback to 1960s, we need to revisit the critical glacial lakes and assess the risk considering recent changes. The new assessment needs to consider supraglacial lakes as one of the criteria in evaluating the GLOF risk, as highlighted by the Lemthang Tsho GLOF.

Remote sensing based glacial lake inventory in the Hindu Kush-Himalaya region

Glacial lake inventory is the main method to investigate the glacial lakes in remote area and provides required information for glacier risk management and climate change research. A glacial lake inventory based on Landsat TM/ETM+ images has been carried in Hindu-Kush Himalaya regions, and 20204 glacial lakes with total area of 1955.75 km2 are documented by this inventory. This paper introduced the method and material and discussed the merits and demerits of the method. Landsat based glacial lake inventory is effective method for large scale area, but more detail inventory with high resolution satellite images is necessary for glacier risk management and glacial lake change detection. The distribution characteristics is also analyzed by this paper, obvious regional difference was found by this inventory, the formation and distribution of glacial lake are controlled by terrain, glaciation and conditions. The selection of assessment method and criteria need to consider the regional feature of glacial lakes and their environment.

Consistent pan-sharpening based on multistage joint and dual bilateral filters

In this paper, we propose a new pan-sharpening method with better noise rejection capabilities. It is based on consistent combination of large and small scale features obtained from the decomposition of high spectral resolution multi-spectral (MS) and high spatial resolution panchromatic (PAN) images. In the decomposition process, MS and PAN images are used to extract the features with the help of joint and dual bilateral filters, respectively. These filters take care of the relationship between MS and PAN images and decompose them into a base layer (large-scale) and a detail layer (small-scale). Since the joint bilateral filter (JBF) preserves the edges of the auxiliary image, it is used for decomposition of MS images where the different layers are computed by using the PAN image as an auxiliary image. In a similar manner, the different layers of the PAN image are obtained by using dual bilateral filter (DBF) which preserves the edges of both (MS and PAN) input images. This process is extended to multistage decomposition to obtain a bilateral image pyramid. The base and detail layers of both MS and PAN images obtained at various stages are combined using a weighted sum, respectively. Finally, the computed weighted sum of detail layer (small-scale) of the PAN image is fused separately to the weighted base layers (large-scale) of the MS images. The proposed method is tested over the images obtained from Quickbird and Worldview-2 satellite sensors. The experimental results are compared with the widely popular methods and recently proposed bilateral filter based methods. The experimental results demonstrate that the proposed method performs better for all satellite sensors.

An edge preserving multi-resolution image fusion: Use of joint bilateral filter

In this paper, we propose a new fusion method based on an edge preserving joint bilateral filter. Here, a multistage form of joint bilateral filter is used to fuse high spectral resolution multispectral (MS) and high spatial resolution panchromatic (PAN) images. In the detail extraction process, both PAN and MS images are used to obtain the available high frequency details. First, the details of PAN image are extracted by using MS image as an auxiliary input to the joint bilateral filter. Similarly, PAN image is used as an auxiliary image to joint bilateral filter to extract the high frequency features from the MS image. This process is extended to multistage form in order to obtain the meaningful details from PAN as well as MS images. Next, these extracted details are first weighted with an average gain and then fused separately with the four MS image bands i.e. red (R), green (G), blue (B) and near infra-red (NIR) to obtain four consistent image bands. The proposed method is tested over images obtained from Quick-bird satellite. The experimental results are compared with the widely popular methods as well as recently proposed methods.

Expansion of moraine-dammed glacial lake in the central Himalayas from 1977 to 2009

The glacial lakes from 1977 to 2009 in the Nepal-China border region were investigated using satellite imagery. 247 ice-contact or ice-proximal glacial lakes with total area of 126.3km2 in the study area were identified in 2009. The expansion rate of growing lakes are approximately <70m/year in length and 0.12 km2/year in area since1977, respectively. The glacial lake growth rate keeps increase from 1977. The expansion rate of growing lake on southern side is greater than that on northern side.