Elsevier

CATENA

Volume 47, Issue 2, 2 April 2002, Pages 101-116
CATENA

Hydrological characteristics of slope deposits in high tropical mountains (Cordillera Real, Bolivia)

https://doi.org/10.1016/S0341-8162(01)00179-5Get rights and content

Abstract

The hydrological processes within slope deposits are not well known, particularly for high tropical mountains with climatic conditions giving seasonal contrasts in the hydrological cycle. Measurements have been taken in a high Andean valley of the Cordillera Real (Bolivia) with the aim to determine the role of the slope deposits in the framework of a hydraulic modelling. Different transfer time measurements combined with morphometrical and sedimentological analysis were carried out on three talus slopes and two moraines during the rainy season. After injection of salt, the electrical conductivity of the water in slope deposits springs is monitored to estimate transfer velocities.

The hydrological behaviour of these slope deposits depends on the season. In the dry season, all the water infiltrates and there is no visible outlet. During the wet season, one part soaks in and gives springs down slope. The lag times estimated by the measurement of tracer velocity demonstrate that these slope deposits delayed the flow depending on the type of deposit. The delay is at least 24 h in the case of the talus slopes and more than 48 h in the case of lateral moraines.

Introduction

The last few years have seen many investigations which aim to define precisely the geometrical and sedimentological characteristics of slope deposits in high tropical mountains and to propose genetic modelling. In this context, numerous studies were carried out in the case of talus slope deposits Albjär et al., 1979, Francou, 1988, Francou, 1991, Van Steijn et al., 1995, Jomelli and Francou, 2000.

Nevertheless, hydrological processes within slope deposits in high mountains have generated very little interest. Paradoxically, permafrost slope formations, have often been studied, including rock glaciers Tenthorey, 1992, Hinzman et al., 1993, Barsch, 1996.

Further knowledge of the hydrological processes of slope deposits without permafrost, like talus slopes or morainic deposits is needed. Parriaux and Nicoud, 1990, Parriaux and Nicoud, 1993 published on hydrologic processes in glacial and fluvio-glacial deposits. Some specific studies can be mentioned on response times in fluvio-glacial formations Frohlich and Kelly, 1985, Davit and Looser, 1993 and estimations of groundwater recharge Johansson, 1987, Harte and Winter, 1995. Jointly, chemical or isotopic observations Van de Griend et al., 1986, Hamid et al., 1989, Gelhar et al., 1992 made it possible to identify various types of circulation systems and dispersivity within basal moraines and fluvio-glacial deposits. Other studies deal with the influence of a stone cover on the runoff yield of arid talus slopes Jung, 1960, Yair and Lavee, 1976. From a qualitative point of view, the results of laboratory experiments are not realistic enough to be a substitute for field studies (Gelhar et al., 1992). Studies have mostly been focused on glacial and fluvio-glacial formations. Moreover, no study has been realised in high tropical mountains where the seasonality of the climate gives important contrasts in soil water content. From a quantitative point of view, the lack of systematic analysis based on a large sampling of deposits makes it difficult to apply and extrapolate results on other geographical contexts. The parameters controlling the hydrological characteristics of slope deposits like the length, width, slope, texture or structure have to be precise to understand the variability of the results.

The lack of knowledge on the hydrologic role of slope deposits makes it difficult to model the flow processes within a high altitude valley supplied by glaciers, snow cover and rainfall. All the flow produced by snow and ice melting, as well as, but to a lesser degree, by rainfall, has to pass through the slope deposits although their surface area is much smaller than the upstream basin.

Thus, the aim of this article is to define the influence of the slope formations on the water pathways. Do they play a negligible role or in the case of response mechanisms, how long do they delay the flow? When the daily time step is chosen in the modelling, do we have to consider the influence of slope deposits?

Section snippets

The study area

The study is located in the upper Rio Zongo Valley (16°S, 68°W, Fig. 1A) on the Eastern part of the Andean chain, about 50 km north of La Paz, capital of Bolivia. The basin where this study is carried out extends over an area of 95.2 km2; it is dominated by summits reaching 5000–6000 m asl (Huayna Potosi, 6088 m asl). Some of these are capped by glaciers that supply the Rio Zongo. The slope deposits range between 3500 and 4500 m asl and are under the climatic influence of humid air coming up

Methods

Some tracing methods into aquifers are good estimates of hydraulic characteristics Freyberg, 1986, Leblanc et al., 1991, Gelhar et al., 1992. However, these methods cannot be used here because of difficult field conditions (high elevation) and locally poor analysis support and facilities. The use of salt (NaCl) as tracer seemed to be adequate for our purpose. It is cheap, easily carried and with low impact on the environment and humans when dilute. In order to determine the role of these slope

Results

The results of the experiments consist in two complementary approaches: (i) a field observation of the flow processes identifying the flow path, and (ii) the computing of the time needed by the tracer to flow through the slope deposit.

Discussion

Four points emphasised by the method used and by the results of the experiments have to be discussed: (i) the role of the clear water injected continuously after the salt mixture; (ii) the two phases of the response (response time and rising time); (iii) the differences between talus slopes and lateral moraines, and (iv) the transfer velocities within the slope deposit.

Conclusion

The conductivity experiments undertaken on three talus slopes and two lateral moraines in high tropical mountains without permafrost lead to the following conclusions.

⋅ These formations play a significant role in the waterflow transfer modelling towards the outlet. Surrounding the rock faces with summits capped by snow and glaciers, they are the only way for all the water flowing down the valley. The minimum delay is at least 24 h in the case of talus slopes with a length of 200 m, which

Acknowledgements

We would like to thank O. Slaymaker, H. Van Steijn and an anonymous referee for their constructive comments.

This study was conducted within the framework of the IRD Programme GREAT ICE, at University San Andres of La Paz (Bolivia), Maison des Sciences de l'Eau (Montpellier, France) and Laboratoire de Géographie Physique de Meudon (Paris, France). The authors are very grateful to J.P. Chazarin, R.C Salinas, R. Fuertes, R. Pillco, J.M. Leblanc, T. Condom, C. Santana, C. Pacena, T. Treuil for

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