Modified Approach to Reduce GCM Bias in Downscaled Precipitation: A Study in Ganga River Basin

Trend analysis of hydrological parameters of Ganga River

Ganga River basin, being the largest river basin in India, adheres with its social and spiritual importance for the country. Although many studies have been conducted on the Ganga basin, however, trend observed in flow and sediment yield in Ganga River has been scarcely studied. Alteration in flow pattern and sediment transport in rivers brings remarkable impact on river geomorphology and entire ecosystem of the region. The present paper attempts to identify trends observed in Ganga River in terms of annual maximum and annual minimum discharges, water and sediment yield during monsoon season. The trend analysis was accomplished by performing the Mann-Kendall (M-K), Sen’s slope, and innovative trend analysis at various gauging sites along the river for the monsoon months. Significance of trend tests were tested at 5% significance level. Innovative trend analysis (ITA) revealed non-monotonicity within the time series and provided more detailed understanding of changes in hydrological changes. While observed annual maximum discharge showed a negative trend at almost all the sites, annual minimum discharge showed positive trend at gauging sites upstream of confluence of Yamuna River. Data of discharge and sediment load for monsoon months also revealed declining trends at most of the gauging sites. Water and sediment yields at all sites except Gandhighat showed a negative trend. Decline in trend of sediment and water yield is more pronounced in Western Ganga Plain (WGP) as compared to Eastern Ganga Plain (EGP). Incorporating these trends can be helpful for various water management projects in the future. Both climatic factors and human intervention appear to be responsible for the alteration in flow pattern of Ganga River.

An Analysis of Long-Term Rainfall Trends and Variability in the Uttarakhand Himalaya Using Google Earth Engine

This paper analyses the spatio-temporal trends and variability in annual, seasonal, and monthly rainfall with corresponding rainy days in Bhilangana river basin, Uttarakhand Himalaya, based on stations and two gridded products. Station-based monthly rainfall and rainy days data were obtained from the India Meteorological Department (IMD) for the period from 1983 to 2008 and applied, along with two daily rainfall gridded products to establish temporal changes and spatial associations in the study area. Due to the lack of more recent ground station rainfall measurements for the basin, gridded data were then used to establish monthly rainfall spatio-temporal trends for the period 2009 to 2018. The study shows all surface observatories in the catchment experienced an annual decreasing trend in rainfall over the 1983 to 2008 period, averaging 15.75 mm per decade. Analysis of at the monthly and seasonal trend showed reduced rainfall for August and during monsoon season as a whole (10.13 and 11.38 mm per decade, respectively); maximum changes were observed in both monsoon and winter months. Gridded rainfall data were obtained from the Climate Hazard Infrared Group Precipitation Station (CHIRPS) and Precipitation Estimation from Remotely Sensed Information Using Artificial Neural Networks-Climate Data Record (PERSIANN-CDR). By combining the big data analytical potential of Google Earth Engine (GEE), we compare spatial patterns and temporal trends in observational and modelled precipitation and demonstrate that remote sensing products can reliably be used in inaccessible areas where observational data are scarce and/or temporally incomplete. CHIRPS reanalysis data indicate that there are in fact three significantly distinct annual rainfall periods in the basin, viz. phase 1: 1983 to 1997 (relatively high annual rainfall); phase 2: 1998 to 2008 (drought); phase 3: 2009 to 2018 (return to relatively high annual rainfall again). By comparison, PERSIANN-CDR data show reduced annual and winter precipitation, but no significant changes during the monsoon and pre-monsoon seasons from 1983 to 2008. The major conclusions of this study are that rainfall modelled using CHIRPS corresponds well with the observational record in confirming the decreased annual and seasonal rainfall, averaging 10.9 and 7.9 mm per decade respectively between 1983 and 2008, although there is a trend (albeit not statistically significant) to higher rainfall after the marked dry period between 1998 and 2008. Long-term variability in rainfall in the Bhilangana river basin has had critical impacts on the environment arising from water scarcity in this mountainous region.