Human impacts on headwater fluvial systems in the northern and central Andes

Human impacts on headwater fluvial systems in the northern and central Andes

Geomorphology 79 (2006) 249 – 263 www.elsevier.com/locate/geomorph Human impacts on headwater fluvial systems in the northern and central Andes Carol...

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Geomorphology 79 (2006) 249 – 263 www.elsevier.com/locate/geomorph

Human impacts on headwater fluvial systems in the northern and central Andes Carol P. Harden Department of Geography 304 Burchfiel Geography Building, University of Tennessee, Knoxville, TN, 37996-0925, USA Received 25 August 2005; received in revised form 6 June 2006; accepted 6 June 2006 Available online 17 August 2006

Abstract South America delivers more freshwater runoff to the ocean per km2 land area than any other continent, and much of that water enters the fluvial system from headwaters in the Andes Mountains. This paper reviews ways in which human occupation of high mountain landscapes in the Andes have affected the delivery of water and sediment to headwater river channels at local to regional scales for millennia, and provides special focus on the vulnerability of páramo soils to human impact. People have intentionally altered the fluvial system by damming rivers at a few strategic locations, and more widely by withdrawing surface water, primarily for irrigation. Unintended changes brought about by human activities are even more widespread and include forest clearance, agriculture, grazing, road construction, and urbanization, which increase rates of rainfall runoff and accelerate processes of water erosion. Some excavations deliver more sediment to river channels by destabilizing slopes and triggering processes of mass-movement. The northern and central Andes are more affected by human activity than most high mountain regions. The wetter northern Andes are also unusual for the very high water retention characteristics of páramo (high elevation grass and shrub) soils, which cover most of the land above 3000 m. Páramo soils are important regulators of headwater hydrology, but human activities that promote vegetation loss and drying cause them to lose water storage capacity. New data from a case study in southern Ecuador show very low bulk densities (median 0.26 g cm− 3), high organic matter contents (median 43%), and high water-holding capacities (12% to 86% volumetrically). These data document wetter soils under grass than under tree cover. Effects of human activity on the fluvial system are evident at local scales, but difficult to discern at broader scales in the regional context of geomorphic adjustment to tectonic and volcanic processes. © 2006 Elsevier B.V. All rights reserved. Keywords: Human impact; Soil erosion; Fluvial geomorphology; Soil moisture; Andes

1. Introduction The Andes Mountains are the primary headwater region of the continent of South America, which delivers more freshwater runoff to the ocean per km2 land area than any other continent. Assessments of all natural freshwater resources indicate that the natural E-mail address: [email protected] 0169-555X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2006.06.021

internal freshwater resources of South America are second only to those of Asia (Table 1, FAO, 2003a). Rivers in the Andes, like rivers in mountain regions across the globe, respond to gradients, material properties, and inputs of water and sediment. Unlike most mountain drainage basins, however, those of the tropical Andes have been population centers for millennia and are locations of major cities. The rich history of human habitation, especially in the northern Andes, and the

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Table 1 World water resources by continent (FAO, 2003a) Continent

Total water resources km3 yr− 1

Asia 12,461.0 South America 12,380.0 North and Central 7443.1 America Europe 6619.4 Africa 3950.2 Oceania 910.7 World 43,764.3

% of world freshwater resources 28.5 28.3 17.0 15.2 9.0 2.1 100

rapid economic development of Andean countries during the past half-century make this an especially interesting region in which to examine effects of human activities on fluvial geomorphology. Although physical processes by which humans affect runoff, erosion, and sedimentation are not unique to the Andes, examination of the intensity and variety of human uses of Andean landscapes offers a different perspective on human im-

pacts on fluvial systems from that obtained in the northern hemisphere. Most (85%) of the continent of South America drains to the Atlantic Ocean. Brazil, the largest country, contains the most fresh water, but mean annual internal freshwater resources per area are greater in Colombia (1.85 m/yr), Ecuador (1.52 m/yr), Peru (1.26 m/yr), and Venezuela (0.79 m/yr) than in Brazil (0.63 m/yr)(FAO, 2003a). Focusing on the five tropical countries of Venezuela, Colombia, Ecuador, Peru, and Bolivia that form the Andean headwaters of the Amazon (Fig. 1), this paper reviews deliberate and unintentional impacts of human activities on headwater rivers in the northern and central Andes, and highlights human activities that increase overland rainfall runoff and cause more sediment to enter river channels. Zooming in from the spatial scales of a region and countries to the scales of a watershed and small plots, it investigates the unusual characteristics and hydrologic importance of páramo soils of the northern Andes and comments on the relative importance of human impacts at different spatial scales.

Fig. 1. Map of the northern and central Andes.

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2. Regional setting 2.1. The physical setting The cordilleras of the Andes divide the Pacific from Atlantic drainages and capture and regulate the flow of water for most of the continent of South America. On the Atlantic side, the Amazon River basin comprises 34% of the land area of South America. Other major drainage basins are the Orinoco (Atlantic), Magdelena (Caribbean), and Paraná (Atlantic). The highest peaks reach to nearly 7000 m above sea level. On the Atlantic and Pacific sides, relief is dramatic and hillslopes and river gradients are steep. Westward-flowing rivers from southern Ecuador to central Chile deliver water to arid lands as they flow from mountain to ocean. Most of the region is tectonically active, and sections of it are volcanically active. Rainfall tends to increase with elevation. Moisture reaches the mountains primarily from the east, but rainfall in some InterAndean valleys and on the western flank of the cordillera is also influenced by weather originating in the Pacific. ENSO effects are evident, although small differences in the intensity or penetration of ENSO events change ENSO influences in the higher mountains (TarrasWahlberg and Lane, 2003). The Magdelena River basin of Colombia has generally received less rain in El Niño years and more in the La Niña years (Restrepo and Kjerfve, 2000). Similarly, pulses of deposition from Andean headwaters, recorded in floodplain sediment cores in the Beni and Mamore river basins, two Bolivian tributaries of the Amazon, were associated with La Niña events (Aalto et al., 2003). On a longer timescale, sediment cores extracted from one high, Amazon-draining lake on the western cordillera in Ecuador showed periodic episodes of high sedimentation rates that correlated well with Holocene ENSO fluctuations (Rodbell et al., 1999). The highest peaks are snow-capped. Below the snowline (ca. 4800–5000 m in Ecuador and Colombia) and above the upper limits of trees and most cultivated land (ca. 3000–3500 m), the northern Andes are characterized by the páramo environment. The páramo ecosystem, and, in particular, páramo soil, is considered to be the principal regulator of the terrestrial hydrological system of the northern Andes (Podwojewski and Poulenard, 2000; Hofstede, 2001). These soils, in Ecuador, Colombia, and Venezuela, consist of a very black, highly organic epipedon (A, Ah, and/or O horizons) discontinuously overlying an unrelated, inorganic surface (Fig. 2). Because mineral particles in páramo soils are eolian in origin, páramo soils near to and downwind from active volcanoes may be 1–2 m thick, while soils farther from ash sources or on glaciated surfaces may be only 20–

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30 cm deep. Páramo vegetation varies, but is most commonly grass (Calamagrostis sp., Stipa sp., and Agrostis sp.), with some shrubs in less disturbed sites. Organic matter decomposes very slowly in the moist, cool conditions that result from high elevation, frequent cloudiness, fog interception, and plentiful rainfall (ca. 1000–2000 mm yr− 1). Typical mean annual temperatures are 10–12 °C (Medina and Turcotte, 1999). Páramo soils are classified as Andosols or Histosols, depending on the organic matter content, which is typically around 30%. Low bulk-densities make them very sensitive to disturbance from humans and livestock. As in the páramo, the scarcity of trees in the drier puna grasslands of the central Andes is thought to result from anthropogenic burning and forest removal (Gade, 1999). Because the Andes are still tectonically active (Norabuena et al., 1998), the physical setting includes active volcanism, ongoing uplift, earthquakes, and high magnitude mass movements. Uplift has caused rivers to incise (Safran et al., 2005) and denudation rates to be high (Aalto et al., 2006). From Colombia to Bolivia, ten volcanoes have erupted since 1964 (Table 2), and six others erupted earlier in the 20th century. Volcanoes contribute sediment to fluvial systems by direct input, by producing bare slopes vulnerable to erosion processes, by blanketing the surrounding landscape with ash, and by oversteepening

Fig. 2. Exposure of black páramo soil, under tussock grasses and perched on a fine-textured deposit, much lighter in color, Cajas National Park, Ecuador.

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Table 2 Volcanoes active since 1964

locally increasing the bed slope and causing rapid incision through the deposit.

Country

Volcano name

Recent eruption

Bolivia/Chile Colombia Colombia Colombia Ecuador Ecuador Ecuador Ecuador Peru Peru

Irruputuncu Galeras Nevado del Ruiz Purace Guagua Pichincha Reventador Sangay Tungurahua Sabancaya Ubinas

1995 2002 1991 1977 2004 2003 2004 2004 2003 1969

Source: Smithsonian Institution (2005).

slopes, thus, making them vulnerable to landsliding. Historical reports indicate that a 1773 eruption of Tungurahua Volcano (Ecuador) dammed the Pastaza River, and that the same river had been dammed three times by eruptions in 1918 (Zevallos, 1996). Ash, ice, and water in combination create lahars, which fill and alter river channels for long distances. Lahars produced by the 1985 eruption of Nevado del Ruiz volcano in Colombia moved a total of about 9 × 107 m3 of lahar slurry to areas up to 104 km from the source (Pierson et al., 1990). In Ecuador, a lahar from an 1877 eruption of Cotopaxi volcano followed a river valley to the Pacific Ocean, over 250 km away (Hall, 1977). Mass movements are important in delivering sediment to the river channels of the Andes (Table 3). On the east flanks of the Andes in Bolivia, where anthropogenic effects on erosion processes appear to have been minimal (Aalto et al., 2006), landsliding is widespread (Safran et al., 2005). In this region of steep slopes, mass movements are readily triggered by wet conditions and by earthquakes. The rugged topography favors landsliding and the formation of landslide dams. In an assessment of mass movements (rock and earth slides, debris avalanches, debris and mud flows) triggered by two earthquakes about 25 km north of Reventador Volcano in northeastern Ecuador, Schuster et al. (1996) found the greatest amount of property destruction to have been caused by flood surges of the main rivers, which had been near flood stage before large volumes of landslide debris were added to them. The largest flood surges were caused by the breaching of temporary landslide debris dams. In the case of the large 1993 slope failure at La Josefina (southern Ecuador), 30 × 106 m3 of debris filled the channel of the Paute River for 1 km of its length (Plaza-Nieto and Zevallos, 1994). The engineered but catastrophic release of the landslide dam 33 days later re-formed the channel downstream,

2.2. Population trends In the northern and central Andes, the cordillera of the Andes is not a single spine, but a region of multiple ranges with elevated valleys and plateaus. In the north, changing patterns of subduction of the Nazca plate under the South American plate have built parallel ranges, with high InterAndean valleys between. This pattern is seen in Colombia, where three parallel north–south-trending ranges define the landscape, and in much of Ecuador, where a relatively well-defined InterAndean valley is flanked by two parallel ranges. Farther south, the highest peaks lie east of the Altiplano. In this tropical region, higher elevation valleys and plains have long been favored for human settlement—daytime temperatures are pleasant, health-threatening insects and other disease vectors are rare, and soils support productive agriculture where moisture is adequate. Today, the InterAndean valleys contain major towns and cities, including Bogota, Colombia (2640 m), and Quito, Ecuador (2850 m). Cusco, Peru is at 3250 m, and La Paz, Bolivia between 3300 and 3600 m. Bogotá had 6.8 million inhabitants in Table 3 Recent, major mass movements Year Country

Location

Comments

1962 Peru

Nevados Huascaran Nevados Huascaran Mayanmarca rockslide

13 × 106 m3, 400–500 persons killeda 30–50 × 106 m3, 18,000 killed; triggered by M 7.7 earthquakea 1.6 × 106 m3, created 150-m high dam on the Mantaro River, 450 persons killed.b 90 × 106 m3, killed over 23,000 in Armero; triggered by volcanic eruption.c 75–110 × 106 m3, ∼ 1000 persons killed, earthquake-triggered rock and earth slides, debris avalanches, and debris and mud flows; most deaths from floods.d 30 × 106 m3, N35 killed, debris dam on Paute River.e Thousands of slides in 250 km2 area, earthquake triggered, 270 dead, 1700 missing.a

1970 Peru 1974 Peru

1985 Colombia Nevado del Ruiz lahars 1987 Ecuador

1993 Ecuador

Reventador mass movements

La Josefina rockslide 1994 Colombia Paez landslides a b c d e

USGS, 2005. Martinez et al., 1995. Pierson et al., 1990. Schuster et al., 1996. Plaza-Nieto and Zevallos, 1994.

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2003, Quito had 1.3 million (in 2001), Cusco had over 300,000 (in 2002), and La Paz had 789,585 (in 2001) (Citypopulation, 2006). Many contemporary settlements occupy sites with long histories of human use. In spite of significant rates of international emigration, populations of Andean countries continue to grow and to become more urbanized (Table 4). Fertility rates remain high by North American standards, ranging from 2.6 (Colombia and Peru) to 3.9 (Bolivia), compared to 1.9 (children per woman) in the United States (Earthtrends, 2003). Population increased dramatically between 1950 and 2005 in the five countries of Bolivia, Columbia, Ecuador, Peru, and Venezuela, although not all of the increase was in the Andean highlands. 3. Deliberate human impacts on the Andean fluvial system As populations have grown and economies developed, countries have deliberately altered fluvial systems to take advantage of water resources. The two most significant intentional interventions are dams and water withdrawls. Major dams have been built for hydroelectric power generation and to store water for irrigation (Table 5). Abundant water and dramatic drops in elevation have allowed the northern Andean countries to become dependent on hydroelectric power. In Venezuela, 60% of the power is hydroelectrically generated (Earthtrends, 2003). At least 60% of the electric power of Ecuador is generated at a single large dam, the Daniel Palacios Dam on the Paute River (Hofstede, 2005). By trapping sediment and altering flow, dams profoundly influence downstream reaches. Water withdrawls for irrigation are more widespread than dams. In Bolivia, Ecuador, and Peru, more than 80% of surface water withdrawls are for agricultural irrigation Table 4 Population growth and urbanization by country Country

Population1 Annual population growth1

Population Percent % increase1 urban2

Bolivia Colombia Ecuador Peru Venezuela

Table 5 Dams by country, as reported by FAO (1998–2002) Country

Number of dams

Reservoir storage

Bolivia Colombia

5 large 26 large (N25 × 106 m3) 90 medium and small 12 N3 96

nd 9.1 km3 3.4 km3 7.5 km3 2.7 km3 157 km3

Ecuador Peru Venezuela

(Table 6). Some irrigation systems, such as those used in Peru by the Incas and in northern Ecuador by preHispanic populations (Knapp, 1991), pre-date the Spanish conquest by at least hundreds of years. The amount of water withdrawn for irrigation increased dramatically in the second half of the 20th century as a result of population growth, land reform, and government efforts to intensify agriculture. In Colombia, modern projects for public irrigation were initiated in 1936, and, in Bolivia, a commission began planning for public irrigation projects in 1938 (FAO, 1998–2002). Irrigation withdrawls in the Andean countries expanded following agrarian reform, which occurred from the 1950s (Bolivia) through the late 1960s (Peru). Irrigation has typically been small-scale (micro-irrigation), as landholdings are small (b 5–10 ha) (e.g., White and Maldonado, 1991), and hundreds of irrigation districts manage the withdrawl and distribution of irrigation water. Internationally available data on irrigation withdrawls, which aggregate data by country, reveal major increases in irrigation over recent decades, but do not separate Andean data from that of lowland drainages. Abstraction of water for irrigation reduces in-stream flows, especially in drier seasons. The extent to which abstraction affects geomorphically important flow levels in the Andes remains unexamined. Water added to the landscape by irrigation can reduce soil erosion by

Table 6 Water withdrawls by country and sector (FAO, 2003a) Country

% growth 2002

Rural Areas

Urban Areas

1950–2002 1965 1989

8,705,000 43,495,000 13,112,000 26,523,000 25,093,000

0.3 0.0 0.2 0.7 2.0

3.8 2.7 4.0 2.6 2.1

221 246 425 248 392

40 54 37 52 70

51 69 55 70 84

Source: 1Earthtrends, 2003, 2Valladares and Prates Coelho, 1993.

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Internal renewable surface water km3

Bolivia 277 Colombia 2112 Ecuador 432 Peru 1616 Venezuela 700

Total Withdrawls annual Agriculture Domestic Industry withdrawl as % of renewable water 0.3% 0.5% 4.3% 1.2% 0.8%

87% 37% 82% 86% 46%

10% 59% 12% 7% 44%

3% 4% 6% 7% 10%

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improving protective vegetative cover, but too much water added can destabilize slopes and water added rapidly on steep slopes causes soil erosion. 4. Human activities that increase soil erosion and sediment in rivers Sources of sediment in rivers include hillslopes and river channel erosion. Sediment is added to the fluvial system by overland flow, mass movement, deliberate dumping, or anthropogenic reworking of sediment stored in channel beds. All of these occur in the Andes. Studies across the globe have linked increases in sediment yields of rivers with changes in land use (e.g., Ostry, 1982; Walling, 1983). Forest clearance for cultivation increases sediment yields by two to three orders of magnitude (Ongley, 1996). Agriculture can also be a primary source of eroded sediment (Bennett, 1939). Irrigation increases river sediment loads, by reducing the flow and corresponding dilution effect, and also by conveying water across steep slopes with high erosion potential. Analysis of air photos from 1976 and 1989 and a field survey in 1999 of a 900-ha catchment in the southern Ecuadorian Andes found new gullies, which were attributed to poor construction and management of irrigation infrastructure (Vanacker et al., 2003b). Gully formation was observed to be a consequence of the spillover of water from open canals and irrigation reservoirs and of mismanagement of extra irrigation water. In a similar study of a different Ecuadorian watershed, the proximity of gullies to the river appeared to control differences in suspended sediment concentrations between two subcatchments (Vanacker et al., 2003a).

Mining activities contribute suspended sediment as well as metal pollutants. Since the Spanish conquest, the Andes has been known for deposits of gold, silver, and other valuable metals. Gold mining remains active, and gold extraction more than doubled in Bolivia, Colombia, and Venezuela between 1950 and 1985 (United Nations, 1990). Gold production in the Portovelo–Zaruma mining district of western Andean Ecuador increased 80% between 1994 and 1999 (Tarras-Wahlberg and Lane, 2003). Copper, iron, lead, manganese, and zinc are also extracted in large quantities (United Nations, 1990). In 1991, a database of geological deposits in the Andes that have already been mined, are currently being mined, or are under evaluation for mining contained over 3300 records (BRGM, 2001). Although effects of mining on fluvial systems in the Andes have been little studied, mines are typically located in remote areas and mining regulations not well enforced (Tarras-Wahlberg and Lane, 2003). Excavations on steep slopes send debris cascading into streams and also increase the frequency of landslides. Failures of tailings impoundment also send debris into rivers. In one study of the effects of gold mining in the Puyango River basin on the western flank of the Andes in Ecuador (Tarras-Wahlberg and Lane, 2003), concentrations of suspended sediment derived from mining were apparent in drier years, but represented only a small proportion of the estimated sediment yield in wet years. Restrepo and Kjerfve (2000) cited gold mining in the Cauca basin of Colombia as an important contributor to high sediment concentrations in the Magdelena River, which has one of the highest sediment yields on the continent. Another form of mining that has affected fluvial systems is the in-stream removal of sand, gravel, and river

Fig. 3. Homemade weir catches marketable sand at high flow in a headwater stream.

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rocks for construction purposes (Fig. 3). In-stream extraction was widely practiced in Andean streams in Ecuador in the early 1990s (Harden, 1993) but has diminished greatly because of recognition of the environmental consequences and enactment and enforcement of new regulations. In-stream mining suspends sediment at all flow levels rather than only during runoff events. In mined river systems, then, rainfall is not a good predictor of suspended sediment loads. Steep gradients promote fluvial transport of sediments. Dams and reservoirs trap sediment, calling attention to the magnitude of sediment loads when reservoir storage capacity is lost. The Daniel Palacios dam on the Paute River in Ecuador originally, in 1983, contained 120 × 106 m3 of storage capacity, but that capacity had been reduced to 95 × 106 m3 by 1993 because of sedimentation. The mean annual rate of sedimentation in the reservoir increased from 2.47 × 106 m3 to 2.76 × 106 m3 after May, 1993, when the release of the large landslide dam on the Paute River at La Josefina mobilized additional sediment (Zevallos and Jerves, 1996). An expensive dredging program has been used to maintain storage because of the important hydroelectric power plant associated with this dam. Mass movement hazards are increasing globally as population pressure and economic development push people onto steeper lands and natural vegetation is removed for cultivation and other purposes (Vanacker et al., 2003c). Although landsliding is a natural adjustment to the crustal shortening occurring in the Andes, not all of the major Andean landslides have been completely natural events. Two of 11 causal factors listed in an analysis of the 1993 La Josefina rockslide were excavation at the toe and diversion of drainage for mining purposes (Plaza-Nieto, 1996). Higher in the Paute River basin, locals attribute the 2003 reactivation of a former landslide (Soroche) to alterations in agricultural drainage above the failure (Fig. 4). 5. Land use effects on rainfall runoff One of the most widespread human impacts to the fluvial system in the Andes, and in other mountain regions, is increasing the proportion of rainfall that reaches rivers as surface runoff. Changes in the rates of runoff-generation occur as unintended consequences of nearly all human activities. Forest cover, agricultural practices, grazing, urbanization, and road construction have important and spatially extensive effects on runoff, which then controls rates of erosion and sediment movement. These effects are not unique to the Andes, but are especially interesting in the Andes because of the intensity of human occupation

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Fig. 4. Soroche landslide in the upper Machángara River tributary of the Paute River in southern Ecuador.

and the special natural and cultural characteristics of Andean highland environments. Forest clearance is part of the legacy of human occupation of the Andean region. Contemporary deforestation is occurring primarily in areas, generally on the external margins of the Andes, where forests remain, or in dry climates, where tree removal promotes desertification (Ministerio de Desarrollo Sostenible y Medio Ambiente, 1996). InterAndean valleys appear to have been cleared even before the Spanish Conquest, although experts disagree about whether certain areas, which today are marginally dry or have thin soils, ever supported trees (Acosta-Solis, 1977; Ellenburg, 1979; White, 1985; Gade, 1999; Sarmiento and Frolich, 2002). Photos taken in the late 19th and early 20th centuries in the province of Tungurahua, Ecuador show far less tree cover than exists there today (Banco Central, 1984). The introduction of Eucalyptus trees in the 1860s led to the reforestation of cleared areas and the present-day dominance of Eucalyptus throughout the InterAndean region (Dickinson, 1969). Forest cover continues to be lost, even while reforestation programs are adding trees. Between 1990 and 2000, Bolivia lost forest cover at an average rate of 0.3% per year, Colombia at 0.4%, Ecuador at 1.2%, Peru at 0.4%, and Venezuela at 0.4% (FAO, 2003b). Much of this

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change occurred on external flanks of the Andes, and some occurred in lowlands rather than in Andean regions. Studies of land-cover change have documented transitions from native trees to plantations of exotic species, principally Eucalyptus and Pinus, without changing the overall extent of tree cover in the study area. Vanacker et al. (2003a) detected profound changes in land cover between 1962 and 1995 in the Machángara watershed in southern Andean Ecuador, which included an overall gain in total wooded area even though 41% of the secondary native woodland was cleared during that period. They attributed the changes to land reform programs and population growth, and the net gain to reforestation with Eucalyptus on formerly degraded lands. Gentry and Lopez-Parodi (1980) linked deforestation in the upper Amazon watershed in Peru and Ecuador to increased high water levels in the Amazon River at Iquitos and in Peruvian tributaries upstream of Iquitos. The illicit drug trade, specifically of coca in Peru, Bolivia, and Colombia, has been identified as a contributor to deforestation in the tropical Andes (U.S. Department of State, 2001). Illicit coca production has typically involved a slash-and-burn approach in which fields are abandoned after two to three growing seasons and new fields cleared to gain fertility and evade authorities. If growers have moved onto the land from other environments and are unfamiliar with local conditions, coca cultivation is more likely to promote runoff and soil loss. The U.S. State Department (2001) estimated that a minimum of 2.4 Mha of forest were cleared for coca production in the Andean region over the previous 20 yrs. Globally, research has demonstrated that forest clearance leads to less rainfall interception, less infiltration, less evapotranspiration, and more surface runoff (e.g., Bosch and Hewlett, 1982). Loss of forest cover, thus, increases storm hydrograph volumes and shortens lag times to peak discharges. Although forest removal typically increases runoff, reforestation does not necessarily reverse the trend and increase rainfall infiltration (Harden and Mathews,

Fig. 5. Increase of cattle in northern and central Andean countries.

2000). Soils are vulnerable to erosion following forest clearance, so cleared mountain slopes may readily lose soil, and, thus, the ability to absorb rainwater, between the times of clearing and reforestation. Many examples of degraded locations, at which Eucalyptus trees planted for reforestation out-competed the understory, exist in the region. Such scenarios result in trunks of trees rising from bare surfaces, which continue to erode and degrade, and to which little or no new organic matter is added. Inbar and Llenera (2000) suggested that the massive reforestation of Eucalyptus in highland Peru in 1976 was less effective than ancient terraces in preventing soil erosion. Where efforts have been made to replace páramo vegetation with pine trees to increase carbon sequestration, pines have been observed to reduce water yields and dry the soils (Hofstede, 2001). Other human activities that have altered rates and patterns of rainfall infiltration and runoff are those that cause soil compaction. Among these are the effects of livestock grazing, increased tractor use in Andean agriculture, growth of road networks, and urbanization. Grazing and trampling pressures increased greatly after the Spanish brought cattle, sheep, and horses to the Andes in the early 1500s. Compared to the native camelids (llamas, alpacas, vicuñas), hoofed animals of European origin are heavier and exert more force per foot (White and Maldonado, 1991; Gade, 1999). When heavy animals, e.g., cattle, are confined to a limited area, their weight compacts the underlying soil, and reduces infiltration capacity (Hofstede et al., 2002). The absence of a freezing winter season means that trampled soils do not have an annual period of recuperation, so infiltration capacities remain low from year to year. Grazing has been locally intense during the five centuries since Europeans arrived, and, where conditions have allowed, cattle numbers have increased in recent years (Fig. 5, FAO, 2006). Contemporary agriculture can increase or decrease rates of soil infiltration and rainfall runoff. Although clearing forested land for cultivation usually has the effect of increasing runoff, tilling cleared land increases infiltration capacities (Harden, 1991). The weight of tractors compacts the soil, so increased tractor use over the past half-century (Fig. 6, FAO, 2006) can be expected to have increased rates of runoff in Andean farmlands. The preferential generation of runoff on roads and footpaths accelerates erosion on hillslopes (Harden, 1992). The location of roads near streams allows increased runoff to discharge quickly from the land to the fluvial system. Land reform programs in the 1960s and 1970s in Ecuador led to more land ownership of steeper hillsides and higher elevation fields. At the same time, land became more parcelized (Vanacker et al., 2003a). Because boundaries between parcels interrupt

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Fig. 6. Increase in tractor use since 1955, by country.

overland flow, parcelization can reduce soil and water loss from agricultural plots. The interrelated factors of plot steepness and length, tillage practices, and the distribution of croplands on hillsides make the hydrologic effects of cultivation difficult to generalize. Cessation of a land use that exacerbates runoff or sediment production does not necessarily mean return to a previous state of infiltration and sediment stability. In the dry central Andes of Peru, where annual rainfall is only 350 mm, Inbar and Llenera (2000) found that abandonment of agricultural terraces increases rates of soil erosion and sediment yields. More than 2 Mha of highland Peru are estimated to have been terraced, but 91% of terraces in high areas have been abandoned (Inbar and Llenera, 2000). Plants do not grow naturally on the terraced slopes in this dry environment. In more humid Ecuador, where slopes were not terraced, previously cultivated sites that had been abandoned or left in long-term fallow generated significantly more runoff than sites under cultivation (Harden, 1996). Degraded soil, lack of moisture, and ongoing grazing stresses cause abandoned Andean farmlands to continue to be sources of runoff and sediment for many years (Harden, 2001). An additional factor that has reduced the ability of the soil to absorb and hold rainwater has been the wholesale removal of peaty soil. In Bolivian, turf has been sold as fuel (Godoy, 1990); in the puna of Peru, peat soil has been mined for fuel and for horticultural purposes (Llerena, 1987); and, in Ecuador, this author has observed peat mining for greenhouses that produce flowers for export. 6. Human impacts on the moisture storage capacity of páramo soil Soils play a key role in determining whether rainfall is absorbed by or shed from a site; they also store and release water. Human activities that cause soil compaction reduce the available pore space in the soil and thus reduce the

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ability of the soil to store water. Soil moisture storage capacity is also reduced when soil is lost or the organic matter content of a soil decreases. The páramo soils, which cover 35,000 km2 of the northern Andes (Hofstede, 2005), are especially vulnerable to changes that reduce pore volume. With low bulk density and high organic matter content, páramo soils are viewed as enormous sponges, which feed and regulate flow to the fluvial system (Luteyn, 2005). Water retention capacity of undisturbed páramo soils is extremely high, reaching values of more than 100% at the wilting point, and strongly correlated with the organic matter content (Buytaert et al., 2005; Luteyn, 2005). Poulenard et al. (2003) reported water contents in epipedons of páramo Hydric Melanudands at 1500 kPa ≥ 1000 g kg− 1 and attributed high porosities to the abundance of organic colloids. Loss of vegetative cover promotes drying, which irreversibly reduces pore space and the water-holding capacity of the páramo soil (Poulenard et al., 2003). Studies have shown páramo soils to become crusted and even hydrophobic following disturbance (Poulenard et al., 2001). Fire is the principal human disturbance affecting the hydrology of páramo environments. Today, páramos are primarily burned to remove old grass and promote the growth of tender new shoots as food for cattle. Gade (1999) reported that any grassy site in the high Andes is probably burned at least once each 5 yrs. Similarly, Hofstede (2005) suggested that only the most remote or most protected páramo sites are not affected by livestock. Páramos are also burned to clear land for cultivation, improve hunting, or implement local belief systems (e.g., bring rain, deter evil spirits) (Hofstede, 2001). Grass páramos have been used as grazing lands at least since the arrival of cattle, sheep and horses with the Spanish in the 1500s, so widespread burning, coupled with trampling and vegetation removal by grazing, would have reduced moisture storage and increased rates of runoff in páramo environments over the last five centuries. These practices have been so widespread that the region lacks control sites for comparative studies. 7. Case study of soil moisture To more closely examine the soil moisture conditions in the headwater region and to investigate differences in soil moisture between grass- and tree-covered highland sites, a study was conducted of surface soil moisture characteristics in and near the 50 km2 Llaviucu watershed, in the western cordillera of the Andes near Cuenca, Ecuador. The Llaviucu watershed is of special interest because it yields 20–30% of the water used by the city of

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Cuenca and because most of it lies within Cajas National Park (Fig. 7). Elevations in the watershed range from 3000 to 4300 m. Vegetation is predominantly grass páramo, with occasional small stands of Polylepis trees at high elevations (Fig. 8), tropical montane forest vegetation below 3400 m on the steep flanks of the glacial trough of the lower watershed, and imported pasture grasses on the trough floor. The national park is managed by the Municipality of Cuenca through a corporation with major leadership by ETAPA (Empresa Pública Municipal de Telecomunicaciones, Agua Potable y Alcantarillado y Saneamiento de Cuenca), the local utility company. In addition to the usual goals of protecting the natural environment and fostering tourism, recreation, and environmental education, this park is also managed to maximize water storage and dry season river flow. Cattle grazing in the park has successfully been reduced from tens of thousands to a very small number, and the corporation must now decide whether to continue to burn the páramo regularly or allow vegetative succession to occur. Evidence in the region indicates that trees, which presently occur in isolated islands up to 4300 m (Gade, 1999), may become dominant in the absence of burning (White and Maldonado, 1991), but the water resources effects of such a change are not known.

For this study, pairs of soil moisture study plots were established at elevations between 3163 m and 3527 m to compare the effects of grass and tree cover where other factors–elevation, location, slope, parent material, soil development–were the same. A HydroSense (Campbell Scientific) Time Domain Reflectometer (TDR) was deployed in 4 m2 plots at 23 sites to obtain 5–15 replications of in situ measurements of volumetric moisture content (VMC) at each site. The 12-cm TDR probes integrated VMC along their length. In each plot, a 12-cm deep soil sample was collected between one set of TDR probe holes for laboratory determination of gravimetric moisture content (GMC), and a second sample (0–12 cm deep), taken within 1 m of the first, was extracted for bulk density determination. The sand replacement method was used in the field to determine in situ volume for bulk density calculations. All soil samples were air-dried and then oven-dried (24 h at 105–110 °C) and weighed. GMC samples were weighed before drying, so that GMC could be calculated as the ratio of the mass of water (mwater = wet soil mass − oven-dried soil mass) to the mass of oven-dried soil (GMC =mwater /msoil). Loss-on-Ignition (LOI), interpreted as an approximate measure of the organic material (% by mass), was determined as the percentage of the sample mass

Fig. 7. Location of Cajas National Park, Ecuador.

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Fig. 8. Páramo grassland in Cajas National Park, Ecuador.

remaining after burning the oven-dried sample at 550 °C for 2 h. All sampling was done in June and July of 2004, a relatively moist year. Volumetric moisture content is the ratio of the volume of water to the volume of soil. As volume is the ratio of mass to density, VMC may also be expressed as VMC ¼ ðmwater =qwater Þ=ðmsoil =qsoil Þ:

ð1Þ

Using soil bulk density as ρsoil, and setting the density of water (ρwater) at 1 g ml− 1, the three values for VMC, GMC and bulk density at each site can be related as GMC ¼ ðVMC⁎1 g ml1 Þ=ð bulk densityÞ:

ð2Þ

The resulting data show extremely low bulk densities (median 0.26 g cm− 3), high organic matter contents of surface horizons (median 43%), and high water-holding capacities (median GMC 1.52 g g− 1; VMC ranges from 12% to 86%). In other words, the mass of water in these soils is typically 1.5 times the dry mass of solid soil material. Measurements from paired plots showed that soils under grass cover consistently had higher VMC than soils under trees (Table 7), and field observations showed that soils under trees contained more macropores compared to soils under grass cover. In all pairs, soil under tree cover was less dense and drier than soil under grass. A comparison of soil properties at four páramo soils sites, three that had burned recently and one that had not, showed little or no difference in bulk density or VMC between the four (Table 8).

Although bulk densities in the Llaviucu watershed study were very low, generally good agreement (same order of magnitude, most within 50%) between measured and calculated values of soil moisture and field observation provided confidence in the results. This comparison between grass- and tree-covered soils was limited by the small size of the database and by the difficulty of finding sites at which all other factors were equal, as trees were observed to typically occupy steeper and rockier sites. These initial results suggest that treecovered sites store less moisture than grass-covered sites and raise the question of whether páramo soils only behave as “sponges” under grass cover, which may, in turn, be an artifact of anthropogenic fire management. Lightning-caused fires have not been reported in páramos, although lightning strikes have occasionally been observed (Luteyn, 2005). The lack of difference in soil moisture and soil bulk density between recently burned páramo sites and an unburned, but similarly grassy site was not unexpected, as the longer-term history of all grass páramo sites appears to include relatively frequent fires. Results from this study suggest that, not the fire, but the establishment of woody plants in the absence of fire would increase evapotranspiration rates and soil drainage (through macropore formation), causing páramo soil to lose its capacity as sponge. A small sample can only be illustrative, however. A further confounding factor in the moisture storage capacity of páramo soils is the depth of soil at a given location. Soil in much of the Llaviucu watershed is thin

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Table 7 Data from paired plots in Llaviucu watershed, Ecuador Vegetation Altitude Moisture GMC (m) 0–12 cm g g− 1 PAIR 1 Grass 1 Grass 2 Wood 1 Wood 2 PAIR 2 Grass 3 Grass 4 Wood 3 Wood 4 PAIR 3 Grass 5 Wood 5 Wood 6 PAIR 4 Grass 6 Wood 7

Bulk density 0– Moisture VMC 12 cm g cc− 1 average 0–12 cm %

VMC range of obs values %

VMC ratio grass: wooded

GMC g g− 1 calculated as VMC/BD

GMC ratio of obs/calc

3.4 1.9 5.0

0.4 0.9 0.2

0.6 0.7 1.3 1.1

0.8 1.0 0.5 0.8

2.2 3.3 3.0

0.7 0.4 0.8

0.6 0.4

2.6 2.5

2.8 3170 3170 3175 3175

1.52 1.80 1.06 nd

0.20 0.34 0.04 nd

68 65 20 27

48–76 53–79 8–34 13–34

3248 3248 3260 3260

0.49 0.70 0.73 0.89

0.54 0.48 0.09 0.11

35

18–57

12

8–24

3170 3176 3176

1.45 1.31 2.50

0.32 0.06 0.05

71 20 15

64–76 6–28 10–27

3163 3273

1.45 1.10

1.08 1.06

60 47

53–65 38–58

2.8

4.0

1.3

(20–30 cm deep), compared to soils at other high elevation páramo sites less affected by glaciers. Although it is presently below the lower limit of permanent ice, the upper Llaviucu watershed was under part of a larger ice cap (Rodbell et al., 2002), and the lower Llaviucu watershed is the trough-shaped valley of an outlet glacier. Field observations made in the Llaviucu catchment documented the flashiness of runoff on thin, quickly saturated soils in this glacially scoured valley. The flashiness of runoff in the watershed and relative dryness of its wooded soils, even in a wet time, can be readily explained, but are contrary to conventional expectations. This underscores the importance of localized differences and the need for more such case studies, which should help management make informed decisions for this national park in its effort to sustain and Table 8 Bulk density, Loss-On-Ignition (LOI), volumetric soil moisture (VMC) and gravimetric soil moisture (GMC) of páramo soils at four sites in Cajas National Park, southern Ecuador Vegetation

Elev. Bulk LOI VMC m Density g % avg. cc− 1 %

VMC min. %

VMC max. %

GMC g g− 1

Páramo burned 1 Páramo burned 2 Páramo burned 3 Páramo unburned

3465 0.47

74

72

76

153

80

78

82

211

84

83

85

238

86

84

88

252

3467 0.37 3469 0.28 3527 0.27

29.3 32.5 42.2 48.5 54.5 58.5 57.6

improve downstream water resources as well as better understand the páramo in other areas. 8. Summary and conclusions Ways in which Andean people and their activities affect the flow and sediment loads of mountain rivers are essentially the same as on other continents. The tropical location of the northern Andes, belief systems that motivate certain land-management strategies, and the dominance of grass páramo distinguish the Andes from other steep, volcanically active mountain regions of the world. Beyond deliberately engineered changes of river impoundments and water withdrawls, human activities have many more subtle and unintentional geomorphic effects on the fluvial system. Forest clearance, grazing, agriculture, roadways, and urbanization increase the proportion of rainfall that flows to the channel network during storms, and steep slopes give overland flow the energy to erode and move fine sediments. Human interventions that destabilize slopes, from mining, to tree removal, to problems with irrigation canals, contribute to triggering mass movements, the largest of which dam rivers and subsequently serve as fluvial sediment sources. Páramo soils, which are hydrologically important in the northern Andes, are particularly sensitive to drying, which occurs when vegetation is removed by burning, grazing, or tilling. The Llaviucu case study verified the low bulk density and high moisture-holding capacities of páramo and highland soils and showed that soils under grass cover

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hold more moisture in a moist season than soils under trees. At the regional scale, natural processes of uplift and denudation, volcanic activity, steep slopes, and mass movement create a geomorphically active environment in the northern and central Andes. Where human settlement is sparse and relief is high, such as on the external flanks of the Andes, the impacts of human activities are negligible compared to the magnitudes of natural processes and adjustments in the fluvial system. A recent study in the Upper Beni River basin in the Bolivian Andes found no significant change in rates of erosion, determined from 10Be analysis of quartz grains, over the last millions of years (Safran et al., 2005). Likewise, a broader study of rates of erosion from 47 rapidly eroding drainage basins in the Bolivian Andes concluded that anthropogenic disturbance was minimal (Aalto et al., 2006). Given the great distance and the number of sediment sinks in the Amazon basin between the Andes and the Atlantic Ocean, it is unlikely that human impacts in the Andean headwaters are noticed at the mouth of the Amazon. On the scale of 103 km2 in human-dominated landscapes of the InterAndean valleys and on a temporal scale of years to centuries, unintended human impacts on soil erosion and runoff are evident, if not well documented. These are visible as truncated soils, reservoir sedimentation, stream incision, increased duration of stream turbidity, and accelerated rates of mass movement where people have steepened slopes through construction and mining. At the scale of plots (10 m2) definite differences in soil properties are associated with differences in land cover and land use. Many Andean river valleys are narrow and steep, so changes in slope stability or changes that cause soil to erode or rainfall to run off are rapidly transmitted to the river system. The land uses today follow a legacy, begun long before the Colonial era, of forest clearance, agriculture, and urbanization in the Andean region. Land uses have changed as populations have increased, local and global economies and technologies have changed, and land reforms have been implemented. Human impacts in these mountains may now be more intensive and extensive, but they are not new. Acknowledgements The author completed the case study in Cajas National Park with the support from the Fulbright Commission, U.S. Department of State, with collaboration of faculty and students from the University of Cuenca, Ecuador, and with permission from Cajas

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