Effects of Frozen Soil on the Groundwater Recharge in Alpine Areas

DANIEL BAYARD1 AND MANFRED STAHLI2 1EPFLausanne, GEOLEP, ENAC, 1015 Lausanne, Switzerland, 2Swiss Federal Research Institute WSL, Zurcherstrasse 111, 8903 Birmensdorf, Switzerland

7.1 INTRODUCTION

Frozen ground is one of the most special features of alpine regions. At lower altitudes, soil frost forms seasonally, depending on weather, topography, snow cover and other surface properties, whereas at higher altitudes (and latitudes) a frost layer may persist year-round (permafrost). Soil frost not only makes great demands on buildings, construction and roads but it also strongly influences the water cycle.

During the last decades, numerous research studies have been carried out demonstrating that at the local scale soil frost may drastically reduce or in the worst case impede soil water flow. Laboratory studies using hydraulic (Burt and Williams 1976) or air permeameters (Seyfried and Murdock 1997) were used to estimate the reduction in hydraulic conductivity due to soil freezing, which could be several orders of magnitude. Obviously, the ice content (or air-filled porosity) of the soil plays a decisive role for the hydraulic conductivity, with lower values for ice-rich soil material. However, it is important to stress that frozen soil is not a priori impermeable. Laboratory studies (e.g. Stadler et al. 2000) and field measurements on small delimited plots (e.g. Johnsson and Lundin 1991; Stahli et al. 1999) clearly demonstrated that water infiltration into frozen soils takes place if at the beginning of the snowmelt the soil contains air-filled pores. Whereas the soil frost effect on the water flow is quite well investigated at the local scale, we know much less about the importance of frozen soil on the hydrology of larger areas. In studies in which for several years the snowmelt runoff from small catchments was compared with soil frost indicators (Shanley and Chalmers 1998), there was no clear evidence for faster and larger runoff in winters with deep soil frost. One major reason is that deep soil frost usually coincides with a shallow snow cover. With regard to the groundwater recharge during snowmelt, very few studies have been published that illuminate the role of the frozen ground. Thorne et al. (1998) presented data from the Canadian Shield showing the groundwater recharge with respect to soil frost conditions for two winters.

Whereas the hydrology of seasonally frozen soils for the most part has been studied experimentally, there are only few examples of numerical simulation models that describe in detail the water infiltration into frozen soils. Most existing soil water transfer models do not include frost effects. One of the first models coupling heat and water fluxes of a layered soil profile was suggested by Harlan (1973). More recently, the models SOIL (Jansson and Halldin 1979), which in the meantime has been renamed and further developed to COUP (Jansson and Karlberg 2001), and SHAW (Flerchinger and Saxton 1989) extended Harlan's concept to detailed soil-vegetation-atmosphere transfer (SVAT) schemes including a process description of soil freezing and thawing, as well as formation and ablation of the snow cover.

Climate and Hydrology in Mountain Areas. Edited by C. de Jong, D. Collins and R. Ranzi © 2005 John Wiley & Sons, Ltd

Since these models treat only one-dimensional water flow, they have been predominantly tested ''at the plot scale''. However, some applications of these models were run in a quasi 2-d mode, representing hillslope runoff with a sequence of coupled profiles (Stahli et al. 2001). Larger-scale groundwater models that include soil frost processes are mostly conceptual (Cherkauer and Letten-maier 1999; Koren etal. 1995; De Gaetano et al. 1996). Ippisch (2001) implemented a soil frost routine in a three-dimensional water and heat flow model, but the coupling between the soil and the snow cover was not integrated.

In alpine ski-resorts, tourism, artificial snow production and hydropower supply generate an increasing demand for water, especially during the winter-spring season. Therefore, it is important to thoroughly study the recharge of the aquifer during the snowmelt and to illuminate the effect of a frozen soil surface on the timing and the amount of ground water recharge. This was the starting point for setting up an extensive field experiment in the southern Swiss Alps that will be presented in the following chapter. Our objectives were

• to explore the local effect of seasonally frozen ground on the snowmelt discharge from alpine slopes to examine the impact of the spatial and altitudinal variability on frost and snowmelt, so as to regionalize obtained results to investigate the key processes that influence groundwater recharge during snowmelt periods to determine the large-scale effect of soil frost on aquifer recharge to identify soil frost situations that might entail hydrological risks.

7.2 THE HANNIGALP/GD ST BERNARD EXPERIMENT

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