Assessment of the hydrological components in the glacierized Dudh Koshi river catchment (Nepal)

International audienceThe Himalayan water resources, vital for 800 million people, come mainly from the monsoon and from the melting of the cryosphere. The impact of climate change on these resources, especially on the cryosphere, is a major issue in the Himalayan range. In this context, assessment of the rainfall, snowmelt and ice-melt components of the water balance is crucial. Consequently, a distributed conceptual hydrological model (HDSM) was developed to estimate the contribution of each component to the Dudh Koshi river flow from 2001 to 2005. The Dudh Koshi river basin (3700 km2), with 14% of glacierized area and Mount Everest as the highest peak, is located in eastern Nepal. The snow-cover area, calibrated with satellite data (MOD10A2), and the daily runoff are correctly simulated by the model. Nevertheless, to obtain these results the ice degree-day factor is overestimated (~36 mm °C–1 d–1), leading to an ice-melt contribution around 60% of annual discharge, against 5% in the literature. This overestimation offsets underestimation around 80% of precipitation, especially solid precipitation. After correction of the precipitation, the contributions of rainfall, snowmelt and ice melt represent, respectively, 63%, 9% and 29% of the Dudh Koshi annual discharge from 2001 to 2005. These results highlight large uncertainties in the hydro-climatic data of the Dudh Koshi river basin, which limit understanding of the hydrological and cryospheric processes. They also underline the limited influence of the glacier melting in the context of climate change on the annual Dudh Koshi river’s water resources, since the monsoon rainfall is the main component of the flow

Conceptual rainfall-runoff model with a two-parameter, infinite characteristic time transfer function

International audienceA two-parameter transfer function with an infinite characteristic time is proposed for conceptual rainfall-runoff models. The large time behaviour of the unit response is an inverse power function of time. The infinite characteristic time allows long term memory effects to be accounted for. Such effects are observed in mountainous and karst catchments. The governing equation of the model is a fractional differential equation in the limit of long times. Although linear, the proposed transfer function yields discharge signals that can usually be obtained only using non-linear models. The model is applied successfully to two catchments, the Dud Koshi mountainous catchment in the Himalayas and the Durzon karst catchment in France. It compares favourably to the linear, non-linear single reservoir models and to the GR4J model. With a single reservoir and a single transfer function, the model is capable of reproducing hysteretic behaviours identied as typical of long term memory effects. Computational efficiency is enhanced by approximating the infinite characteristic time transfer function with a sum of simpler, exponential transfer functions. This amounts to partitioning the reservoir into several linear subreservoirs, the output discharges of which are easy to compute. An efficient partitioning strategy is presented to facilitate the practical implementation of the model

Geostatistical Estimation of Daily Monsoon Precipitation at Fine Spatial Scale: Koshi River Basin

International audienceThe use of appropriate space and time scales is fundamental to model the water budget in mountainous regions and to give appropriate replies to the initial requests. However, at a daily scale, the determination of precipitation behavior is not an easy task due to its high variability in mountainous areas. Seven years (2001–2008) of accurate precipitation maps (1 km ground resolution) have been produced for the monsoon season over the Koshi River basin (Nepal) to be used for hydrological modeling. Due to field and topographical constraints, the geostatistical method of ordinary cokriging interpolation (OCK) was used to compute precipitation grids over a 57,800  km2 basin with a rain gauge network made of 47 stations. Using elevation as a covariable, regionalization models were run to produce 976 daily precipitation grids. They describe temporal and spatial variability close to observed data. Comparisons of the OCK results to an Aphrodite’s reference grid (resolution of 25 km) show that the OCK grids are characterized by a higher spatial variability. Both OCK and Aphrodite data sets underestimate observations, with OCK grids showing the best fit to observed data. However, the OCK method, with an eventual adaptation of the cokriging model, appears consistent for situations where the resolution of the precipitation’s spatial distribution is insufficient and an alternate explaining variable such as elevation is available

Improving Knowledge about Snow by Crossing Qualitative and Quantitative Data from the Everest Region (Nepal)

International audienceIn the Nepalese region of Mount Everest, snow data is fragmentary and unreliable, as in many mountainous regions of the world. To get a better understanding of solid precipitation and how it has evolved over the last decades, and thus to improve our knowledge of hydro-climatic variations, we crossed quantitative data (measured, spatialised and simulated) produced from work in hydrology, with qualitative data (field interviews) derived from work in human geography. Here we present an innovative comparison method based on three key factors: seasonality, trends and notable events. Although quantitative and qualitative data show a similar decrease in snowfall in the Upper Khumbu Valley, which is taken as a case study, there are discrepancies that highlight the uncertainties present in the two data sets. This method of crossing data from different disciplines proves very useful for mountain areas where meteorological stations and precise surveys are lacking, and where the wealth of local people’s knowledge is not sufficiently taken into account.Dans la région népalaise de l’Everest, les données sur la neige sont fragmentaires et peu fiables comme dans de nombreuses régions montagneuses du monde. Pour mieux appréhender les précipitations solides et leur évolution au cours des dernières décennies, et ainsi améliorer la connaissance des variations hydroclimatiques, nous avons croisé des données quantitatives (mesurées, spatialisées et simulées) issues de travaux en hydrologie, et des données qualitatives (entretiens sur le terrain) issues de travaux en géographie humaine. Nous présentons ici une méthode de comparaison innovante basée sur trois clés de lecture communes : la saisonnalité, les tendances et les événements notables. Si de façon similaire les données quantitatives et qualitatives mettent en évidence la diminution des chutes de neige dans la vallée du haut Khumbu prise comme cas d’étude, des divergences soulignent les incertitudes présentes dans les deux jeux de données. Ce travail de croisement de données issues de disciplines différentes s’avère d’une grande utilité pour les zones de montagne où les stations météorologiques et les relevés précis font défaut, et où les connaissances de la population sont riches, mais insuffisamment prises en compte

Mieux appréhender la neige par le croisement de données qualitatives et quantitatives dans la région de l'Everest (Népal)

International audienceDans la région népalaise de l'Everest, les données sur la neige sont fragmentaires et peu fiables comme dans de nombreuses régions montagneuses du monde. Pour mieux appréhender les précipitations solides et leur évolution au cours des dernières décennies, et ainsi améliorer la connaissance des variations hydro-climatiques, nous avons croisé des données quantitatives (mesurées, spatialisées et simulées) issues de travaux en hydrologie, et des données qualitatives (entretiens sur le terrain) issues de travaux en géographie humaine. Nous présentons ici une méthode de comparaison innovante basée sur trois clés de lecture communes : la saisonnalité, les tendances et les évènements notables. Si de façon similaire les données quantitatives et qualitatives mettent en évidence la diminution des chutes de neige dans la vallée du Haut Khumbu prise comme cas d'étude, des divergences soulignent les incertitudes présentes dans les deux jeux de données. Ce travail de croisement de données issues de disciplines différentes s'avère d'une grande utilité pour les zones de montagne où les stations météorologiques et les relevés précis font défaut, et où les connaissances de la population sont riches mais insuffisamment prises en compte

Water budget on the Dudh Koshi River (Nepal) : uncertainties on precipitation

Although vital for millions of inhabitants, Himalayan water resources remain currently poorly known, mainly because of uncertainties on hydro-meteorological measurements. In this study, the authors propose a new assessment of the water budget components of the Dudh Koshi River basin (3720 km(2), Eastern Nepal), taking into account the associated uncertainties. The water budget is studied through a cross analysis of field observations with the result of a daily hydrological conceptual distributed snow model. Both observed datasets of spatialized precipitations, interpolated with a co-kriging method, and of discharge, provided by the hydrological agency of Nepal, are completed by reanalysis data (NCEP/NCAR) for air temperature and potential evapotranspiration, as well as satellite snow products (MOD10A2) giving the dynamics of the snow cover area. According to the observation, the water budget on the basin is significantly unbalanced; it is attributed to a large underestimation of precipitation, typical of high mountain areas. By contrast, the water budget simulated by the modeling approach is well balanced; it is due to an unrealistic overestimation of the glacier melt volume. A reversing method led to assess the precipitation underestimation at around 80% of the annual amount. After the correction of the daily precipitation by this ratio, the simulated fluxes of rainfall, icemelt, and snowmelt gave 63%, 29%, and 8% of the annual discharge, respectively. This basin-wide precipitation correction is likely to change in respect to topographic or geographic parameters, or in respect to seasons, but due to an insufficient knowledge of the precipitation spatial variability, this could not be investigated here, although this may significantly change the respective proportions for rain, ice or snow melt. 2015 Elsevier B.V. All rights reserved

The impact of climate change on water availability in Eastern Nepal: a presentation of the project methodology taking into account the vaious origins of water

International audienceThe paper reflects the main methodological aspects of the PAPRIKA Project based on the following objectives: (i) to contribute to a more accurate assessment of glacier retreat, snow cover and climate change in Koshi Basin, Nepal; (ii) to have a better understanding of the contribution of glacier and snow melting to water availability; (iii) to correlate the results with local people's perceptions of climate change and their socio-economic impact. For this, the paper: highlights the fact that the water used by the population comes from different origins (glacier melting, snow melting, frost, rain) the combination of which varies between the four main landscape units: high, middle and low mountains, and finally the Terai plain; describes the methodology adopted to observe and analyse current as well as future environmental changes in the atmosphere, cryosphere and hydrosphere; shows that, for each origin, different reasons may explain the changes in water availability, and thus the impact on agriculture and the different water usages