Carol P. Harden

Infiltration on mountain slopes: a comparison of three environments

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.

INCORPORATING ROADS AND FOOTPATHS IN WATERSHED-SCALE HYDROLOGIC AND SOIL EROSION MODELS

Hydrologic and soil erosion models, even distributed models, require some degree of generalization of land surface characteristics. Because generalization typically depends on the areal extent of parameter values, surfaces that affect geomorphic/erosional processes out of proportion to their areal extent require special consideration. Increased erosion and sediment yield on roads and trails is frequently reported, but the interactive process relationships between such highly compacted and neighboring, less compacted surfaces have received little research attention. This paper reviews watershed-scale modeling strategies and presents the results of rainfall simulation experiments conducted on paired path and non-path sites in highland Ecuador and East Tennessee. The results show rural roads and footpaths to be the most active runoff-generating components of inhabited mountain landscapes and provide preliminary quantitative and qualitative bases for incorporating the runoff and erosion-initiating effects of ...

Compensation for ecosystem services: an evaluation of efforts to achieve conservation and development in Ecuadorian páramo grasslands

Ecosystem services programmes have been advocated for their potential to join conservation and poverty alleviation efforts, integrate working landscapes, and provide a flow of ecosystem services upon which populations rely. Ecuadorian paramo grasslands have rapidly become the focus of compensation for ecosystem services (CES) programmes intended to conserve hydrologic services, carbon sequestration and biodiversity. This paper reviews CES programmes in Ecuadorian paramos using a combination of semi-structured interviews with project personnel, policy makers and community leaders involved in CES programme development, document analysis, and archival research. Findings indicate that, in some cases, CES schemes can support local development, with potential to contribute to poverty alleviation; however, measures of programme effects on poverty were lacking. The programmes fell across the spectrum of activity-reducing to activity-enhancing, with some functioning as protected areas and others integrating working landscapes; however, designation of land as protected did not necessarily imply more restrictive use. Finally, these cases all reflect scenarios in which limited information is available linking land use with ecosystem services production and underscore the idea that adequate understanding of ecosystem production functions continues to be a barrier to development of effective programmes, particularly where the provision of multiple ecosystem services is anticipated.

Framing and Reframing Questions of Human–Environment Interactions

Ways in which geographers have framed research on human–environment interactions have changed over time. This review emphasizes the limitations of previous ways of framing human–environment research and indicates new opportunities to be pursued by reframing the research questions. It begins with the research and influence of W. M. Davis and follows with research framed as environmental determinism, human ecology, natural hazards, human impacts on the environment, and sustainability. Studies of interactions between people and environments are central to geography, but such studies have dominantly been one-sided as a result of the type of relationship studied or the perspectives (physical or social) brought by the investigators. Awareness of the nature of nature and the dynamic, interactive behavior of biophysical and human systems has the potential to bring new perspectives to the traditional human–environment dichotomy. Because many of the worlds important problems involve interactions between people and environments, geographers are encouraged to turn their attention to this core area of the discipline. Research opportunities include studies of the effects of environmental change on human populations, including the complex web of interactions and feedbacks involved; studies of how environmental services are valued and managed; and other studies that provide knowledge to support more sustainable human–environment interactions, especially in an urbanizing world.

Understanding Human–Landscape Interactions in the “Anthropocene”

This article summarizes the primary outcomes of an interdisciplinary workshop in 2010, sponsored by the U.S. National Science Foundation, focused on developing key questions and integrative themes for advancing the science of human–landscape systems. The workshop was a response to a grand challenge identified recently by the U.S. National Research Council (2010a)—“How will Earth’s surface evolve in the “Anthropocene?”—suggesting that new theories and methodological approaches are needed to tackle increasingly complex human–landscape interactions in the new era. A new science of human–landscape systems recognizes the interdependence of hydro-geomorphological, ecological, and human processes and functions. Advances within a range of disciplines spanning the physical, biological, and social sciences are therefore needed to contribute toward interdisciplinary research that lies at the heart of the science. Four integrative research themes were identified—thresholds/tipping points, time scales and time lags, spatial scales and boundaries, and feedback loops—serving as potential focal points around which theory can be built for human–landscape systems. Implementing the integrative themes requires that the research communities: (1) establish common metrics to describe and quantify human, biological, and geomorphological systems; (2) develop new ways to integrate diverse data and methods; and (3) focus on synthesis, generalization, and meta-analyses, as individual case studies continue to accumulate. Challenges to meeting these needs center on effective communication and collaboration across diverse disciplines spanning the natural and social scientific divide. Creating venues and mechanisms for sustained focused interdisciplinary collaborations, such as synthesis centers, becomes extraordinarily important for advancing the science.

Effects of Land-Use Change on Water in Andean Páramo Grassland Soils

Mountain environments, including the Andean páramo grasslands of Ecuador, are important water source areas. They are often sites of programs and policies intended to achieve multiple management objectives, such as carbon sequestration, biological conservation, and water resource protection; yet such environments are often data poor. This creates challenges for programs that compensate landowners for protecting ecosystem services and uncertainty regarding which land uses are compatible. For example, does afforestation for carbon sequestration complement or hinder efforts to protect water resources in Andean páramos? We compared characteristics of high-elevation soil profiles at sites in two Ecuadorian páramo study areas and measured soil–water properties to assess whether changes in land use affected the soil hydrology. Using a space-for-time substitution, we compared soils in plantations of pines and Polylepis racemosa in grasslands with different grazing and burning regimes. Methods included soil description in soil pits; soil moisture measurement in soil pits and across surface transects; tracer studies of soil–water movement; and laboratory determination of bulk density, particle size, and humic acid composition. Of the land uses examined, only afforestation significantly affected soil moisture, whereas soil properties did not differ among grassland burning and grazing regimes. The results suggest that afforestation of páramos hinders the production of water and they underscore the need for further investigation to inform the trade-offs needed in managing páramo landscapes to support multiple ecosystem services.

Rates and Processes of Streambank Erosion in Tributaries of the Little River, Tennessee

Erosion pins were installed on 32 banks in five tributaries of the Little River, eastern Tennessee, to quantify the contributions of streambanks to stream sediment loads and better understand the processes of streambank erosion. In the first two years of monitoring, erosional losses from streambanks were readily measurable, with a median loss of 1 cm bank-1. Streambanks lost material through subaerial erosion processes as well as by mass movements and removal of sediment by hydraulic forces. Although the highest median erosion rates occurred below the water line, erosion also occurred on the upper banks. Initial results demonstrate that channel bank instability is widespread and channel banks are potentially important sources of sediment in these tributaries. Two observations made in this study, of streambank erosion independent of hydraulic forces and of bank undercutting active during drought, call attention to gaps in knowledge that affect models of streambank processes and efforts to improve water quality in headwater streams.

Fish functional traits correlated with environmental variables in a temperate biodiversity hotspot.

The global biodiversity crisis has invigorated the search for generalized patterns in most disciplines within the natural sciences. Studies based on organismal functional traits attempt to broaden implications of results by identifying the response of functional traits, instead of taxonomic units, to environmental variables. Determining the functional trait responses enables more direct comparisons with, or predictions for, communities of different taxonomic composition. The North American freshwater fish fauna is both diverse and increasingly imperiled through human mediated disturbances, including climate change. The Tennessee River, USA, contains one of the most diverse assemblages of freshwater fish in North America and has more imperiled species than other rivers, but there has been no trait-based study of community structure in the system. We identified 211 localities in the upper Tennessee River that were sampled by the Tennessee Valley Authority between 2009 and 2011 and compiled fish functional traits for the observed species and environmental variables for each locality. Using fourth corner analysis, we identified significant correlations between many fish functional traits and environmental variables. Functional traits associated with an opportunistic life history strategy were correlated with localities subject to greater land use disturbance and less flow regulation, while functional traits associated with a periodic life history strategy were correlated with localities subject to regular disturbance and regulated flow. These are patterns observed at the continental scale, highlighting the generalizability of trait-based methods. Contrary to studies that found no community structure differences when considering riparian buffer zones, we found that fish functional traits were correlated with different environmental variables between analyses with buffer zones vs. entire catchment area land cover proportions. Using existing databases and fourth corner analysis, our results support the broad application potential for trait-based methods and indicate trait-based methods can detect environmental filtering by riparian zone land cover.

UPLAND EROSION AND SEDIMENT YIELD IN A LARGE ANDEAN DRAINAGE BASIN

This preliminary effort toward developing a sediment budget for the primarily agricultural 3,531 km2 upper and middle drainage basin of the Rio Paute in Andean Ecuador focuses on the geographic locations of potential upland nonpoint sediment sources and compares upland soil erosion estimates based on the Universal Soil Loss Equation (USLE) with others extrapolated from rainfall simulation experiments. Although the USLE-based erosion estimates are consistently higher, spatial patterns of high and low erosion rate classes for the two approaches are relatively similar, differing primarily in urban areas and in peripheral high elevation zones. Relating sediment yield data to the upland erosion rates suggests that 17 to 41 percent of the eroded material leaves the basin annually. [Key words: geomorphology, sediment budget, soil erosion, GIS, Ecuador.]

The human-landscape system: challenges for geomorphologists

Deliberately or indirectly, most of the terrestrial surface has been affected by the actions of human beings. For that reason, geomorphologists have needed to broaden their scope of inquiry to encompass the human-landscape system. Four themes related to human actions emerge in recent research in geomorphology: (1) human impacts on geomorphic systems, (2) human-landscape feedbacks, (3) geomorphic hazards and (4) stratigraphic markers of anthropogenic origin. The importance of humans as geomorphic agents challenges geomorphologists and their collaborators to move beyond unidirectional cause-and-effect (human impacts), and develop new research frameworks that better integrate the ongoing interactions between people and landscapes.