Pôle Alpin Risques Naturels (PARN) Alpes–Climat–Risques Avec le soutien de la Région Rhône-Alpes (2007-2014)
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Réf. Pratatp & al. 2015 - A

Référence bibliographique
Bhanu PRATAP, D.P. DOBHAL, Manish MEHTA, Rakesh BHAMBRI, Influence of debris cover and altitude on glacier surface melting: a case study on Dokriani Glacier, central Himalaya, India. Annals of Glaciology 56(70) 2015 doi: 10.3189/2015AoG70A971 PDF

Abstract : Most of the central Himalayan glaciers have surface debris layers of variable thickness, which greatly affect the ablation rate. An attempt has been made to relate debris-cover thickness to glacier surface melting. Thirty stakes were used to calculate ablation for debris-covered and clean ice of Dokriani Glacier (7 km2 ) from 2009/10 to 2012/13. Our study revealed significant altitude-wise difference in the rate of clean and debris-covered ice melting. We found a high correlation (R2 = 0.92) between mean annual clean-ice ablation and altitude, and a very low correlation (R2 = 0.14) between debris-covered ice melting and altitude. Debris-covered ice ablation varies with variation in debris thickness from 1 to 40 cm; ablation was maximum under debris thicknesses of 1–6 cm and minimum under 40 cm. Even a small debris-cover thickness (1–2 cm) reduces ice melting as compared to that of clean ice on an annual basis. Overall, debris-covered ice ablation during the study period was observed to be 37% less than clean-ice ablation. Strong downwasting was also observed in the Dokriani Glacier ablation area, with average annual ablation of 1.82 m w.e. a–1 in a similar period. Our study suggests that a thinning glacier rapidly becomes debris-covered over the ablation area, reducing the rate of ice loss.

Mots-clés
 climate change, debris-covered glaciers, glacier ablation phenomena

Organismes / Contact

Authors / Auteurs :

  • Bhanu PRATAP, Centre for Glaciology, Wadia Institute of Himalayan Geology, Dehra Dun, India
  • D. P. DOBHAL, Centre for Glaciology, Wadia Institute of Himalayan Geology, Dehra Dun, India
  • Manish MEHTA, Wadia Institute of Himalayan Geology, Dehra Dun, India
  • Rakesh BHAMBRI, Wadia Institute of Himalayan Geology, Dehra Dun, India

(1) - Paramètre(s) atmosphérique(s) modifié(s)
(2) - Elément(s) du milieu impacté(s)
(3) - Type(s) d'aléa impacté(s)
(3) - Sous-type(s) d'aléa
  Glacier    

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
India central Himalaya Dokriani Glacier     2009-2013

(1) - Modifications des paramètres atmosphériques
Reconstitutions
 
Observations

 

Modélisations
 
Hypothèses
 

Informations complémentaires (données utilisées, méthode, scénarios, etc.)



(2) - Effets du changement climatique sur le milieu naturel
Reconstitutions
 
Observations

 We found a strong positive correlation between mean annual ablation of clean ice and elevation (R2 = 0.92) during the study period. The maximum clean-ice melting (4.9 m w.e. a–1) was observed near the snout between 4000 and 4200 m a.s.l. from 2009/10 to 2012/13. The clean-ice melting decreases with increasing elevation and reaches 0.34 m w.e. a–1 at 4900–5000 m a.s.l. near the ELA

Conversely, inconsistent ablation was observed in debriscovered ice where debris thickness ranged from 1 to 40 cm. Ablation measurements also show that debris-covered ice had a lower melt rate than clean ice (Fig. 3). Annual ablation measurements in areas between 3900 and 4400 m a.s.l. from the initial visit in October 2009 to the end of the 2013 ablation season show that the difference in ablation rates of debris-covered and clean ice varies between 27% and 44% (average 37%) on Dokriani Glacier. Furthermore, we found that even a small debris-cover thickness (1–2 cm) retards melting compared to that of clean ice on an annual basis. Thus, no evidence was found to support increased ablation under a fine debris thickness compared to clean ice.

The debris that particularly accumulates in the lower ablation zone due to substantial clean-ice melting now reduces the ice melt. These results imply that the lower part of glacier response becomes less sensitive to climate. Therefore, we conclude that the substantial icesurface loss in the recent warm period is gradually increasing the debris cover in the ablation zone and insulating ice melt underneath.

 

Au cours de la période d’étude, nous avons pu mettre en evidence une importante relation entre l’ablation moyenne annuelle de la glace « propre » et l’augmentation du gradient altitudinal (R² = 0.92). Le taux de fonte le plus important (4.9 m e.e. par an) a été observé au niveau de la langue terminale du glacier, entre 4000 et 4200m (sur la période 2009 -2013). Le taux de fonte de la glace propre diminue avec l’augmentation de l’altitude pour atteindre un minimum de 0.34m e.e par an entre 4900 et 5000m, au niveau de la Ligne d’Equilibre Glaciaire.

A l’inverse, on observe une ablation irrégulière sur les zones de glace où l’épaisseur de la couverture détritique est comprise entre 1 et 40 cm. Les différentes mesures d’ablation mettent également en évidence un taux de fonte plus élevé dans les zones où la glace est « propre » que dans les zones où la glace est recouverte de matériel sédimentaire. Les mesures annuelles, réalisées entre 2009 et 2013, mettent en évidence une différence des taux d’ablation, entre la glace « propre » et la glace retrouvée sous une couverture détritique supérieure à 1cm, allant de 27% à 44%.

Nous avons également observé, en comparant une zone exempte de couverture détritique avec une zone recouverte par des débris, que même une couverture détritique peu importante, entre 1 et 2 cm, ralentissait le taux de fonte.

Cependant, nous n’avons pu trouver aucune preuve du rôle d’une fine couche détritique sur l’augmentation des vitesses d’ablation.

Les débris accumulés sur la partie basse de la zone d’ablation du glacier, en raison de la fonte rapide de la glace « propre », contribuent désormais à ralentir les vitesses de fonte. Nous pouvons donc conclure, d’après ces résultats, que la partie basse du glacier réagit désormais moins rapidement aux variations climatiques, et ce en raison de la couverture détritique nouvellement formée.

Modélisations
 
Hypothèses
 

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 

To determine the effects of a supraglacial debris cover of varying thickness on glacier ice melt, ablation was measured for four consecutive years (2009/10–2012/13). At the end of October every year during the study period, 30 ablation stakes were drilled into the debris-covered and clean-ice portions of Dokriani Glacier to quantify ablation. Ablation at each stake was monitored at 15 day intervals during the ablation season, and winter melting was calculated based on cumulative melting from 1 November to 30 April in order to determine net winter, net summer and monthly ablation.

Afin de déterminer les effets de la couverture détritique, d’épaisseur diverse, sur la vitesse de fonte du glacier, les taux d’ablation ont été mesurés pendant 4 ans, entre 2009 et 2010 puis entre 2011 et 2012. Chaque année durant cette période, à la fin du mois d’octobre, 30 sondes ont été utilisées pour mesurer les taux d’ablation, à la fois dans les zones de glace « propre » et dans les zones recouvertes par du matériel sédimentaire.

Sur chaque zone de forage l’ablation était surveillée tous les 15 jours pendant la période de fonte. La fonte hivernale, quant à elle, était mesurée à partir de la fonte cumulée entre le 1er Novembre et le 30 avril. Cette double mesure permettait de mettre en évidence les taux de fontes mensuels pour les périodes hivernales et estivales.


(3) - Effets du changement climatique sur l'aléa
Reconstitutions
 
Observations
 
Modélisations
 
Hypothèses
 

Paramètre de l'aléa
Sensibilité du paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 
 
 

(4) - Remarques générales

(5) - Syntèses et préconisations
 

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