Current opportunities and challenges in developing hydro-climatic services in the Himalayas: report of pump priming project November 2019

The India-UK Water Centre (IUKWC) promotes cooperation and collaboration between the complementary priorities of NERC-MoES water security research. This report assesses the significant issues for hydro-climatic modelling and service development in the mountain regions of northern India. It is the main output from an IUKWC Pump Priming Project that ran from March to August 2018 and has been produced by an author team of climate scientist, hydrologists and glaciologist from India and the UK. It is found that although state-ofthe-art weather forecasting, climate, hydrological and glacier models are being used there are still substantial prediction uncertainties on all prediction timescales. There is a lack of detailed understanding of regional meteorological and hydrological processes, which results in potential misrepresentation of them in the models. Large-scale drivers of regional climate variability in the region have been identified but questions remain about their relevance on different timescales, their interaction, and their representation in global weather forecasting and climate models. Improving short-term predictions and climate change projections requires more meteorological, hydrological and glaciological observations in the Himalayas, improvements in data sharing, as well as additional efforts to integrate meteorological and hydrological modelling. There is also a need for improved communication of predictions to users, which should include their uncertainties. The report is intended for workshop participants, India-UK Water Centre Open Network members and stakeholders

Current practice and recommendations for modelling global change impacts on water resource in the Himalayas

Global change is expected to have a strong impact in the Himalayan region. The climatic and orographic conditions result in unique modelling challenges and requirements. This paper critically appraises recent hydrological modelling applications in Himalayan river basins, focusing on their utility to analyse the impacts of future climate and socio-economic changes on water resource availability in the region. Results show that the latter are only represented by land use change. Distributed, process-based hydrological models coupled with temperature-index melt models are predominant. The choice of spatial discretisation is critical for model performance due to the strong influence of elevation on meteorological variables and snow/ice accumulation and melt. However, the sparsity and limited reliability of point weather data, and the biases and low resolution of gridded datasets, hinder the representation of the meteorological complexity. These data limitations often limit the selection of models and the quality of the outputs by forcing the exclusion of processes that are significant to the local hydrology. The absence of observations for water stores and fluxes other than river flows prevents multi-variable calibration and increases the risk of equifinality. The uncertainties arising from these limitations are amplified in climate change analyses and, thus, systematic assessment of uncertainty propagation is required. Based on these insights, transferable recommendations are made on directions for future data collection and model applications that may enhance realism within models and advance the ability of global change impact assessments to inform adaptation planning in this globally important region

Seasonal cycle of precipitation over major river basins in South and Southeast Asia: a review of the CMIP5 climate models data for present climate and future climate projections

We review the skill of thirty coupled climate models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) in terms of reproducing properties of the seasonal cycle of precipitation over the major river basins of South and Southeast Asia (Indus, Ganges, Brahmaputra and Mekong) for the historical period (1961–2000). We also present how these models represent the impact of climate change by the end of century (2061–2100) under the extreme scenario RCP8.5. First, we assess the models' ability to reproduce the observed timings of the monsoon onset and the rate of rapid fractional accumulation (RFA) slope — a measure of seasonality within the active monsoon period. Secondly, we apply a threshold-independent seasonality index (SI) — a multiplicative measure of precipitation (P) and extent of its concentration relative to uniform distribution (relative entropy — RE). We apply SI distinctly over the monsoonal precipitation regime (MPR), westerly precipitation regime (WPR) and annual precipitation. For the present climate, neither any single model nor the multi-model mean performs best in all chosen metrics. Models show overall a modest skill in suggesting right timings of the monsoon onset while the RFA slope is generally underestimated. One third of the models fail to capture the monsoon signal over the Indus basin. Mostly, the estimates for SI during WPR are higher than observed for all basins. When looking at MPR, the models typically simulate an SI higher (lower) than observed for the Ganges and Brahmaputra (Indus and Mekong) basins, following the pattern of overestimation (underestimation) of precipitation. Most of the models are biased negative (positive) for RE estimates over the Brahmaputra and Mekong (Indus and Ganges) basins, implying the extent of precipitation concentration for MPR and number of dry days within WPR lower (higher) than observed for these basins. Such skill of the CMIP5 models in representing the present-day monsoonal hydroclimatology poses some caveats on their ability to represent correctly the climate change signal. Nevertheless, considering the majority-model agreement as a measure of robustness for the qualitative scale projected future changes, we find a slightly delayed onset, and a general increase in the RFA slope and in the extent of precipitation concentration (RE) for MPR. Overall, a modest inter-model agreement suggests an increase in the seasonality of MPR and a less intermittent WPR for all basins and for most of the study domain. The SI-based indicator of change in the monsoonal domain suggests its extension westward over northwest India and Pakistan and northward over China. These findings have serious implications for the food and water security of the region in the future

Spatio-temporal changes in terrestrial water storage in the Himalayan river basins and risks to water security in the region: a review

Nearly one-fifth of the Earth's accessible freshwater is stored in the densely-populated, alluvial floodplains of the Brahmaputra, Ganges, Indus, Irrawaddy and Meghna River Systems in the Himalayan region where extreme hydrological conditions exist due to the seasonal variability in terrestrial water storage (TWS). Groundwater storage (GWS) – a hidden resource underneath the land surface, plays a critical role in sustaining irrigated agriculture in these river basins, particularly during the dry season when rice crops are generally grown in irrigated lands across South Asia. Although monitoring of groundwater levels has been operational in the region for a number of decades, a basin-wide comprehensive assessment of GWS is lacking in most river basins. The NASA's Gravity Recovery and Climate Experiment (GRACE) twin satellites offer an opportunity to map basin-wide changes in GWS where in-situ observations are limited in time and space. GRACE-derived assessments of GWS vary substantially in these basins and have not been reconciled with in-situ observations in most cases. Recent declining trends in GWS over the Himalayan river basins are attributed primarily to over-abstraction of groundwater due to dry-season irrigation. Seasonal variability in terrestrial water is likely to increase or become unpredictable in the future as a result of increased climate variability. The consequent impacts may potentially threaten the security of water supply and food in the region, where there is currently a growing demand for food grains from irrigated agriculture, energy, and domestic and industrial water supplies

Seasonality of the hydrological cycle in major South and Southeast Asian river basins as simulated by PCMDI/CMIP3 experiments

In this study, we investigate how PCMDI/CMIP3 general circulation models (GCMs) represent the seasonal properties of the hydrological cycle in four major South and Southeast Asian river basins (Indus, Ganges, Brahmaputra and Mekong). First, we examine the skill of the GCMs by analysing their performance in simulating the 20th century climate (1961–2000 period) using historical forcing (20c3m experiment), and then we analyse the projected changes for the corresponding 21st and 22nd century climates under the SRESA1B scenario. The CMIP3 GCMs show a varying degree of skill in simulating the basic characteristics of the monsoonal precipitation regimes of the Ganges, Brahmaputra and Mekong basins, while the representation of the hydrological cycle over the Indus Basin is poor in most cases, with a few GCMs not capturing the monsoonal signal at all. While the model outputs feature a remarkable spread for the monsoonal precipitation, a satisfactory representation of the western mid-latitude precipitation regime is instead observed. Similarly, most of the models exhibit a satisfactory agreement for the basin-integrated runoff in winter and spring, while their spread is large for the runoff during the monsoon season. For the future climate scenarios, most models foresee a decrease in the winter P − E over all four basins, while agreement is found on the decrease of the spring P − E over the Indus and Ganges basins only. Such decreases in P − E are mainly due to the decrease in precipitation associated with the western mid-latitude disturbances. Consequently, for the Indus and Ganges basins, the runoff drops during the spring season while it rises during the winter season. Such changes indicate a shift from rather glacial and nival to more pluvial runoff regimes, particularly for the Indus Basin. Furthermore, the rise in the projected runoff, along with the increase in precipitation during summer and autumn, indicates an intensification of the summer monsoon regime for all study basins

Climate change vs. socio-economic development : understanding the future South Asian water gap

The study assesses combined impacts of climate change and socio-economic development on the future water gap in the Indus, Ganges, and Brahmaputra (IGB) river basins until the end of the 21st century. The IGB provides about 900 million people with water resources used for agricultural, domestic, and industrial purposes. Environmental flow requirements (EFRs) have not been considered in most future assessments on climate change induced or socio-economic development-induced changes in water supply and demand in the region. The paper proposes a model simulation incorporating factors of the hydrological cycle.UK Government’s Department for International Developmentcore funds of ICIMODNetherlands Organisation for Scientific Research (NWO

Historical Climate Trends over High Mountain Asia Derived from ERA5 Reanalysis Data

The climate of High Mountain Asia (HMA) has changed in recent decades. While the temperature is consistently increasing at a higher rate than the global warming rate, precipitation changes are inconsistent, with substantial temporal and spatial variation. Climate warming will have enormous consequences for hydroclimatic extremes. For the higher altitudes of the HMA, which are a significant source of water for the large rivers in Asia, often trends are calculated using a limited number of in situ observations mainly observed in valleys. This study explores the changes in mean, extreme, and compound-extreme climate variables and their seasonality along the full altitudinal range in HMA using daily ERA5 reanalysis data (1979–2018). Our results show that winter warming and summer wetting dominate the interior part of HMA. The results indicate a coherent significant increasing trend in the occurrence of heatwaves across all regions in HMA. The number of days with heavy precipitation shows more significant trends in southern and eastern basins than in other areas of HMA. The dry period occurrence shows a distinct demarcation between lower-and higher-altitude regions and is increasing for most basins. Although precipitation and temperature show variable tendencies, their compound occurrence is coherent in the monsoon-dominated basins. These changes in indicators of climatic extremes may imply substantial increases in the future occurrence of hazards such as floods, landslides, and droughts, which in turn impact economic production and infrastructure

Potential impacts of climatic warming on glacier-fed river flows in the Himalaya

The Himalayan region is one of the most highly glacierised areas on Earth. Regarded as the “water towers” of Asia, the Himalayas are the source of several of the world’s major rivers. The region is inhabited by some 140 million people and ten times as many (~1.4 billion) live in its downstream river basins. Freshwater from the mountains is vital for the region’s economy and for sustaining the livelihoods of a fast-growing population. Climatic warming and the rapid retreat of Himalayan glaciers over recent decades have raised concerns about the future reliability of mountain melt-water resources, leading to warnings of catastrophic water shortages. Several previous studies have assessed climate change impacts on specific glacier-fed rivers, usually applying meso-scale catchment models for short simulation periods during which glacier dimensions remain unchanged. Few studies have attempted to estimate the effects on a regional scale, partly because of the paucity of good quality data across the Himalaya. The aim of this study was to develop a parsimonious grid-based macro-scale hydrological model for the Indus, Ganges and Brahmaputra basins that, in order to represent transient melt-water contributions from retreating glaciers, innovatively allowed glacier dimensions to change over time. The model initially was validated over the 1961-90 standard period and then applied in each basin with a range of climate-change scenarios (sensitivity analysis- and climate-model-based) over a 100-year period, to gain insight on potential changes in mean annual and winter flows (water availability proxies) at decadal time-steps. Plausible results were obtained, showing impacts vary considerably across the region (catchments in the east appear much less susceptible to glacier retreat effects than those in the west, due to the influence of the summer monsoon), and, in central and eastern Himalayan catchments, from upstream to downstream (effects diminish rapidly downstream due to higher runoff from non-glaciated parts)

HI-AWARE consortium final technical report 2018

Funded by the UK Government’s Department for International Development and the International Development Research Centre, Canada.How to develop timely adaptation measures and approaches to respond to rising temperatures, seasonal shifts in glacier and snowmelt induced runoff, and increased frequency of extreme events in the HKH mountains and floodplains in order to improve the resilience of livelihoods of the poorest and most vulnerable women, men and children in the region

Potential impacts of climatic warming on glacier-fed river flows in the Himalaya

The Himalayan region is one of the most highly glacierised areas on Earth. Regarded as the “water towers” of Asia, the Himalayas are the source of several of the world’s major rivers. The region is inhabited by some 140 million people and ten times as many (~1.4 billion) live in its downstream river basins. Freshwater from the mountains is vital for the region’s economy and for sustaining the livelihoods of a fast-growing population. Climatic warming and the rapid retreat of Himalayan glaciers over recent decades have raised concerns about the future reliability of mountain melt-water resources, leading to warnings of catastrophic water shortages. Several previous studies have assessed climate change impacts on specific glacier-fed rivers, usually applying meso-scale catchment models for short simulation periods during which glacier dimensions remain unchanged. Few studies have attempted to estimate the effects on a regional scale, partly because of the paucity of good quality data across the Himalaya. The aim of this study was to develop a parsimonious grid-based macro-scale hydrological model for the Indus, Ganges and Brahmaputra basins that, in order to represent transient melt-water contributions from retreating glaciers, innovatively allowed glacier dimensions to change over time. The model initially was validated over the 1961-90 standard period and then applied in each basin with a range of climate-change scenarios (sensitivity analysis- and climate-model-based) over a 100-year period, to gain insight on potential changes in mean annual and winter flows (water availability proxies) at decadal time-steps. Plausible results were obtained, showing impacts vary considerably across the region (catchments in the east appear much less susceptible to glacier retreat effects than those in the west, due to the influence of the summer monsoon), and, in central and eastern Himalayan catchments, from upstream to downstream (effects diminish rapidly downstream due to higher runoff from non-glaciated parts)