Anne Coudrain

Relationship between precipitation phase and air temperature: comparison between the Bolivian Andes and the Swiss Alps / Relation entre phase de précipitation et température de l'air: comparaison entre les Andes Boliviennes et les Alpes Suisses

Abstract Determining the precipitation phase—rain or snow—is an important factor in modelling discharge in mountainous basins. In a study carried out in the outer tropical Andes Cordillera of Bolivia, half-hourly determination of precipitation phase was obtained by applying a suitable expert system, taking 11 meteorological parameters into consideration that are measured over 21 months at an altitude close to 4800 m. Straightforward relationships between the determined precipitation phase and observed air temperature were analysed in histograms that contain percentage occurrences of snowfall, rainfall and mixed precipitation events for 0.5°C air temperature increments. The graph shows a nearly identical distribution of percentage occurrences of snowfall in the Andes to that on a 1600-m high site in the Swiss Alps. This result suggests that, for hydrological modelling purposes in the outer tropical Andes, the same rain/snow threshold temperature as in the compared Swiss site can be applied.

Calcul de la pluie sur le bassin versant du lac Titicaca pendant l'Holocène

The water levels of a lake situated in an endorheic catchment make it possible to calculate the associated rainfall rate on the basis of the water balance over the whole catchment. Evolution during the Holocene of water levels in Lake Titicaca (Bolivia), previously published, shows that in the most arid period, between 8 O00 yr and 4 O00 yr BP, the average level was 50 m lower than today. The calculated rainfall associated with this low level is 635 k 50 mmyr-I i.e. about 18 %lower than the present amount. (O Academie des sciences / Elsevier, Paris.) rain amount / endorheic catchment / Holocene / Titicaca / Bolivia