Elsevier

Geomorphology

Volume 55, Issues 1–4, 30 September 2003, Pages 5-24
Geomorphology

Infiltration on mountain slopes: a comparison of three environments

https://doi.org/10.1016/S0169-555X(03)00129-6Get rights and content

Abstract

Water is well established as a major driver of the geomorphic change that eventually reduces mountains to lower relief landscapes. Nonetheless, within the altitudinal limits of continuous vegetation in humid climates, water is also an essential factor in slope stability. In this paper, we present results from field experiments to determine infiltration rates at forested sites in the Andes Mountains (Ecuador), the southern Appalachian Mountains (USA), and the Luquillo Mountains (Puerto Rico). Using a portable rainfall simulator–infiltrometer (all three areas), and a single ring infiltrometer (Andes), we determined infiltration rates, even on steep slopes. Based on these results, we examine the spatial variability of infiltration, the relationship of rainfall runoff and infiltration to landscape position, the influence of vegetation on infiltration rates on slopes, and the implications of this research for better understanding erosional processes and landscape change.

Infiltration rates ranged from 6 to 206 mm/h on lower slopes of the Andes, 16 to 117 mm/h in the southern Appalachians, and 0 to 106 mm/h in the Luquillo Mountains. These rates exceed those of most natural rain events, confirming that surface runoff is rare in montane forests with deep soil/regolith mantles. On well-drained forested slopes and ridges, apparent steady-state infiltration may be controlled by the near-surface downslope movement of infiltrated water rather than by characteristics of the full vertical soil profile. With only two exceptions, the local variability of infiltration rates at the scale of 10° m overpowered other expected spatial relationships between infiltration, vegetation type, slope position, and soil factors. One exception was the significant difference between infiltration rates on alluvial versus upland soils in the Andean study area. The other exception was the significant difference between infiltration rates in topographic coves compared to other slope positions in the tabonuco forest of one watershed in the Luquillo Mountains. Our research provides additional evidence of the ability of forests and forest soils to preserve geomorphic features from denudation by surface erosion, documents the importance of subsurface flow in mountain forests, and supports the need for caution in extrapolating infiltration rates.

Introduction

Although the high peaks tend to capture our attention, they comprise only a small portion of mountain terrain. As three-dimensional features extending thousands of meters in height and thousands of kilometers in length, mountains encompass a variety of microenvironments. A major challenge to geomorphologists is to identify the ranges of values and scales of spatial variability of geomorphic processes and their controls on mountain slopes. In this paper, we focus on forested mountain slopes in humid temperate to tropical regions. Soils on these slopes comprise the “skin” of the mountains, and their biophysical characteristics exert important control on rates of degradational processes. We seek to better understand the role of water as an agent of erosion, and the characteristics of slope surfaces that partition rainwater into surface and subsurface flow paths.

Precipitation and gravity are well established as major drivers of the geomorphic change that eventually reduces mountains to lower relief landscapes. Rainfall and water from snowmelt promote denudation in three ways:

  • (i)

    Weathering: moisture serves as a reactant and a transport agent in weathering processes in which slopes lose strength and rocks become fragmented.

  • (ii)

    Erosion: the erosive energy of water striking and flowing across the land surface entrains and transports particles downslope.

  • (iii)

    Mass wasting: water entering pore spaces in slope surface materials contributes to the potential for mass movement by adding mass, increasing pore water pressure, and reducing strength.

At the same time, however, especially in non-arid mountain regions and within the altitudinal limits of continuous vegetation, water promotes slope stability:

  • (i)

    Soil: water contributes to the development of soil, which stores moisture and promotes low energy, non-erosive, subsurface water movement.

  • (ii)

    Vegetation: plentiful moisture supports the growth of continuous and luxuriant vegetation, which, in turn, alters the moisture regime of slopes through interception and evapotranspiration and alters soil biophysical characteristics to better retain and drain moisture.

The role of water as a driver of erosion has received considerable scientific study. The more complex and less direct roles of water in promoting slope stability through its effects on vegetation have received less attention than they merit in geomorphology. Understanding the balance that determines whether water has a stabilizing or destabilizing effect on mountain slopes at micro- to subregional scales requires better understanding the role of the solum in integrating climatic, biologic, and geological components of the hillslope environmental system. Recent attention to mountain regions recognizes that the world's largest rivers originate in mountains and that at least half of the world's population depends on water flowing in or from mountains (Price, 1999). The importance of mountains as sources of fresh water further underscores the need for better understanding the water cycle, including infiltration processes, on mountain slopes.

In this paper, we present results from field experiments of rainfall runoff and infiltration in the Andes Mountains (Ecuador), the southern Appalachian Mountains (USA), and the Luquillo Mountains (Puerto Rico). We hypothesized that mid-scale (hillslope to km) differences in geologic, edaphic, topographic, and biotic conditions control the spatial variability of infiltration rates in forested mountain regions; and we expected relationships between infiltration rates and site factors to support a scientific basis for extrapolating infiltration rates, determined at points, to broader extents of montane forest. We also hypothesized that infiltration capacities (maximum rates) would be high enough to absorb rainfall and prohibit surface runoff during most rainfall events. Based on the results of our fieldwork in the three study areas, we examine the relationship of rainfall runoff and infiltration to landscape position, the spatial heterogeneity of infiltration, the influence of vegetation on infiltration, and the implications of this research for erosion and landscape change.

In the hydrologic cycle, rainwater returns to the atmosphere through evaporation and transpiration, remains on land (detention storage) and vegetation surfaces (as interception), or percolates into the soil. The movement of water into the soil, called infiltration (I), is generally measured indirectly. If evapotranspiration, detention, and interception are minor or absent, infiltration (I) can be calculated as I=R−RO, where R is rain and RO is a measure of runoff. Infiltration is not a single process but an assemblage of processes involving gravity and forces of molecular attraction between soil and water molecules. It integrates three independent processes: (i) entry through the soil surface, (ii) storage within the soil, and (iii) transmission through the soil (Dunne and Leopold, 1978). Infiltration rates are known to decline to a steady or quasi-steady state as a soil becomes increasingly moist over the period of a storm or experimental wetting. The widely used Philip equation (Philip, 1957) gives the infiltration rate (I) as a function of time t in the formI=A+Bt−1/2where A and B are constants that depend on the soil and its initial moisture distribution. A mainly represents the steady rate of infiltration under gravitational potential, and B is a time-dependent term representing the hydraulic potential gradient at the advancing wetting front. Rates of infiltration are usually compared by comparing the A (steady-state) term (Whipkey and Kirkby, 1978).

Environmental factors that control infiltration rates are rainfall rates, soil properties (including texture, pore characteristics, organic matter content, and structure), vegetation, land use, depth of soil, and initial moisture Betson, 1964, Dunne and Leopold, 1978. Most mountain slopes in humid regions are covered by forest, which contributes organic matter to the soil and increases soil drainage by promoting soil particle aggregation and supporting soil fauna. Organic litter protects the soil surface from compression and sealing by raindrop impact. Other environmental factors increasing infiltration rates in forests are those that create cracks and voids, such as earthworms and tree roots (Knapp, 1978). The effects of these factors all vary spatially.

As interest in using geographic information systems and modeling environmental processes across broader spaces grows, so does recognition of the spatial variability of infiltration rates and the difference between the net hydrologic performance of a slope compared to that of discrete points within it (Hawkins and Cundy, 1987). Jetten et al. (1993) found the sample variance of infiltration rates for tropical rainforest soils to be so large that it was not possible to predict infiltration rate as a simple function of soil properties. Loague and Gander (1990) analyzed 247 infiltration rate measurements at 25-, 5-, and 2-m spacings from a grassland catchment in Oklahoma. They found that variations in infiltration rates were not explained by soil texture and suggested that animal activity, vegetation, and climate strongly affected the distribution of infiltration rates.

In mountain environments, slope position may contribute to the spatial variability of infiltration rates. Woods et al. (1997) identified slope position as a significant control on spatial variability of subsurface runoff. Infiltration capacity has been shown to change with topographic position, but the trend of the change is not always the same. Grah et al. (1983) found infiltration capacity to decrease downslope. In contrast, Dunne et al. (1991) found it to increase downslope. Because infiltration is defined as the vertical movement of water into soil (Hillel, 1971) and most commonly measured on horizontal surfaces, studying the relationship of infiltration to slope position in mountain regions poses significant research challenges.

Knowledge of hillslope hydrology has been hampered by the lack of measurements of soil hydraulic properties, especially in the humid tropics (Bonell and Balek, 1993). Our research was undertaken to better document and understand apparent infiltration rates in soils in tropical and temperate mountainous regions. Specific research objectives were to explore the relationships of infiltration rates with vegetation type, soils, and slope position as a step towards better understanding the spatial variability of infiltration rates in forested mountain environments.

Section snippets

Study areas

The experiments and results come from three separate research projects: one on the lower eastern flank of the Andes Mountains in Ecuador, one in the southern Appalachian Mountains, and one in the Luquillo National Forest in Puerto Rico. General characteristics of the three research areas are given in Table 1. All are humid and forested; none are known to have been glaciated.

Field research at the Jatun Sacha Biological Reserve was conducted in June 1993 as part of a larger project investigating

Research methods

Each of the three studies discussed in this paper had a slightly different set of research questions and research design; yet all three involved measuring rainfall runoff and infiltration rates and used identical or similar research methods. Research methods for the three sites are summarized in Table 2. In Puerto Rico, in Tennessee, and at 21 of the Ecuadorian sites, we used a McQueen (1963)-type rainfall simulator with a ring infiltrometer to determine the infiltration rate of wet soils on

Results

At each study area, we found infiltration rates to vary considerably within site groups. Often, the full range of infiltration rates for a study area occurred within a single site. Table 3 summarizes the results of rainfall simulation experiments at all three study areas. At 34 of 72 sites in Tennessee, 15 of 54 in Puerto Rico, and 15 of 21 in Ecuador, the rate of rain applied was not enough to generate surface runoff. For those sites, we report that infiltration capacities exceed the rate of

Jatun Sacha, Andes

Infiltration rates in the tropical rainforests at Jatun Sacha and Puerto Rico are comparable in magnitude and local variability to those of forested sites in Tennessee. The most notable trend we observed at Jatun Sacha was that the alluvial soils had higher apparent infiltration rates than the older upland soils. Even in the upland, the most intense rainstorms of a typical year would only generate runoff at a small proportion of the off-trail, forested sites tested if all of the rain reached

Acknowledgements

The research was funded by the US Forest Service, the National Geographic Research and Exploration Fund, and a US EPA grant to Michael Huston at the University of Tennessee. We thank Fred Scatena, US Forest Service, for logistical support in the Luquillo Experimental Forest; and the Jatun Sacha Biological Reserve and the Oak Ridge Reservation for permission to work at those sites. We appreciate the assistance of Roger Clapp, who reviewed an earlier version of the manuscript, the UT Cartographic

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