Long‐Term Changes in Runoff Generation Mechanisms for Two Proglacial Areas in the Swiss Alps I: Overland Flow

Abstract

In many areas of the world, the surface of the earth is changing rapidly. This can affect the partitioning of precipitation into overland flow (OF) and infiltration. OF can reach the stream quickly and thus strongly influences the streamflow response to precipitation. It can also cause surface erosion. However, our knowledge of the changes in OF responses during landscape evolution is still limited. To investigate how hillslope aging effects OF, we studied three plots on four different aged moraines (several decades to ∼13.5 thousand years old) at a silicate and carbonate proglacial area in the Swiss Alps. We used sprinkling experiments to determine OF characteristics (such as the runoff ratio and timing) and used tracers (δ2H and NaCl) to identify the mixing of rainfall and soil water. Sediment concentrations and turbidity measurements provided an estimate of OF‐driven soil erosion rates. The OF ratios were largest (42%) for the oldest moraines because the clay‐rich layer at 20–40 cm below the surface caused saturated OF. However, OF occurred more frequently on the youngest moraines due to the high stone cover. Soil and vegetation development increase the soil water retention capacity and the pre‐event water fractions in OF for the old moraines, but decreased the suspended sediment yield. The results show that OF characteristics and sediment transport change markedly during landscape evolution. This needs to be taken into account when simulating runoff and erosion responses for rapidly changing Alpine areas, and can—together with the outcomes for subsurface flow (see companion paper Maier et al., 2021, https://doi.org/10.1029/2021WR030223)—be used to improve landscape evolution models.

Long‐Term Changes in Runoff Generation Mechanisms for Two Proglacial Areas in the Swiss Alps II: Subsurface Flow

“…The results of the field campaigns at both study areas suggest that the age of a moraine impacts hillslope runoff generation (Figures 1 and 12). SSF (this study) and OF (companion paper, Maier & van Meerveld, 2021) on the old moraines are both due to saturation and mixing of rainfall with stored water. The accumulation of fine material and clay on the oldest moraine reduces vertical percolation and promotes saturation and lateral flow above this layer (cf.…”

“…Figure 1), and the implementation of these feedbacks in landscape evolution models (Temme et al., 2017; Tucker & Hancock, 2010). This study and the accompanying study on OF (Maier & van Meerveld, 2021) are a step to establish a data basis to test and help to improve landscape evolution models. It provides valuable information on the characteristics of the moraines (see also Hartmann, Semenova, et al., 2020; Maier et al., 2020; cf.…”

“…We determined the vertical saturated hydraulic conductivity K sat for three different depths (0-5, 5-20, and 20-40 cm below the soil surface) at three locations at each plot based on the steady state infiltration rate. For the surface (0-5 cm), we used a Double-Ring-Infiltrometer with an inner diameter of 20 cm; the constant water level was 15-25 cm (see Maier and van Meerveld [2021] for more details). For the subsurface, we used a Constant Head Permeameter (Amoozometer; Amoozegar, 1992Amoozegar, , 1989.…”

“…The water in the pool was thoroughly stirred for at least 10 min to ensure that it was fully mixed. For a more detailed description on the plot setup and additional instrumentation (including overland flow collectors), see Maier and van Meerveld (2021).…”

“…At each study area, we selected four moraines between ∼30 and ∼13,500 years in age (Table 1; Figure 2 in Maier & van Meerveld, 2021 [companion paper]). Their names reflect their approximate age, that is, the 10‐ky moraine is about 10,000‐years old, whereas the 160‐y moraine is ∼160 years old and was formed during the little ice age (i.e., soil development began in ∼1860).…”