--Deforestation and hydrologic disturbance in the Tropics--(FINAL)

This topic submitted by Jaleann M. Matos-McClurg ( geojmm79@yahoo.com) at 1:34 AM on 5/19/04.

A view from the air--Where have the forests gone? (Sierpe River Area, SW Costa Rica)

Tropical Field Courses -Western Program-Miami University



A. Background

Currently there are many tropical countries clearing their forests. The clearing of forests causes habitat fragmentation, loss of biodiversity, climatic change, pollution problems and hydrological disturbance. From a macro-scale perspective, deforestation as a problem to society is complex in nature. There is no question to whether deforestation usually affects the climate system dynamics, atmospheric composition, and other ecosystem processes (Sanchez-Azofeifa et al, 2001; Zhang et al, 2001; Kricher, 1997). Rosero-Bixby & Palloni argues that deforestation is a phenomenon caused by physical phenomena alone although it also a human product. This paper will focus only on the physical aspect of deforestation specifically on hydrological processes, although I will keep myself aware of the human dimension involved in it.

Stream flow and base flow are two main hydrological processes impacted by deforestation. Deforestation alters surface cover conditions. Surface cover conditions are dependent on changing land-use and land-cover patterns over time in a given geographical area. The accelerated changes due to a decrease of pervious cover alter the way water is stored and cycled from the atmosphere to the hydrosphere. I will discuss the temporal land-cover trends of Costa Rica and other tropical countries and generalize from three case studies in the Tropics (Venezuelan Andes, Himalayas and Mexico) about what types of hydrological disturbances are significant. Finally, I will synthesize the literature and suggest future research concerned with tropical deforestation and environmental degradation.

B. Causes of deforestation

Mather et al (1998) emphasized the prominence of population as a surrogate of forest loss. They firmly suggested that temporal population growth is only one of many other driving forces of deforestation worldwide, which operates at multiple scales of analysis. Because of the prominence of population as a predictor of forest loss in the literature, it is the most frequently invoked variable used to explain human-related change in the environmental degradation of water resources. Furthermore, Mather et al (1998) recognized that the relationship between population and forest loss is not a simple one. They suggested that the emergence of political ecology has undermined the notion of “pressure-of-population-on-resources view”. Not until the last decade, political ecology provides with the theoretical framework to consider other drivers of forest loss, which are not purely physical in nature. For example, other factors involved in forest loss are the access to the forest, land resources, uneven distribution of income and land, access to credit and capital, rural poverty, international markets, land-use policy, management and tilting policies, inappropriate and unsuitable technologies, technological changes, economic growth and social realities (Mather, 1998; Rosero-Bixby & Palloni).

Sanchez-Azofeifa et al (2001) highlighted the importance of land-use policy to preserve forests. They found that much of Costa Rica’s biodiversity for the tropical and pre-montane forest life zones has already been lost forever due to the inefficacy of land-use policies in place between 1986 and 1991. The authors urged for a comprehensive forest management strategic plan if the remaining Costa Rican forests are to survive beyond the next few decades.

C. Deforestation in Costa Rica

The forest cover in Costa Rica declined 50% between 1940 and 1984 (Sader & Joyce, 1988). The United Nations Food and Agricultural Organization estimated that Costa Rica’s deforestation rate was 3.2 percent/yr between 1976 and 1982 (FAO, 1990). This high deforestation rate ranked the country as fifth in the world (in terms of percentage).

Land-use/land-cover change dynamics of in Costa Rica during the past 30 years are generally driven by the expansion of crop and cattle production (Sanchez-Azofeifa, 2000). Provinces with the least forest cover during 1991 are Guanacaste (3%), Alajuela (16%), Puntarenas (19%), San Jose (30%), and Heredia (41%) while Cartago (68%) and Limon (60%) had the highest forest cover (Sanchez-Azofeifa et al, 2001). Limon province has high forest cover due to large forest tracks in the Tortuguero and Talamanca region, which has high slopes, therefore limiting overexploitation practices. In the other hand, Cartago province has high forest cover due to the presence of several conservation areas designated for water resources protection. Of the remaining forest, 70% remains outside of protected areas (Sanchez-Azofeifa et al, 2001).

D. Agents of deforestation

The agents of deforestation typically cited in the literature are logging companies,
and farmers who clear their land for logging, cattle ranching or agriculture (cash crops—bananas) (Rosero-Bixby & Palloni; Sharma et al, 2000; Allan et al, 2002; Kricher, 1997; Viramontes & Descroix, 2003).


E. Consequences of deforestation in the hydrological flow regime: A dynamic understanding of processes

**Case Study 1: Himalaya - (54,000km2 basin) Source: (Sharma et al, 2000)**

i. Underlying causes of deforestation
- Association between forest loss, road access, and population growth.
- The extent of land disturbance was relatively high in relation to road and building density.

ii. Hydrological impacts
-Forest loss increases water and sediment yield and can result in greater runoff from rain events, causing bank scouring, failure of bridges and roads, and habitat deterioration.

iii. Lesson learned: Colonization and deforestation can be episodic, with periods of rapid land conversion followed by periods of relative stability.

**Case Study 2: Venezuelan Andes – (262-988 km2 basins) Source: (Allan et al, 2001)**

i.Underlying causes of deforestation
- Debate between land-use and climatic change on hydrology. Which one is the definite underlying force?

ii. Hydrological impacts
- Increasing tendency of temperature and precipitation coincident with temporal population growth.

**Case Study 3: Mexico – (4660-7130km2 basins) Source: (Descroix & Viramontes, 2003)**

i. Underlying causes of deforestation

- Forest cover threatened by short-term overexploitation due to clandestine harvesting.
- Non-authorized harvesting and strong market demand for wood led to overexploitation.

ii. Relevant information
- Land-use trends: forest (-), pasture (+) and bare soil (+) due to increased density and area of livestock.

iii. Environmental consequences
- High erosion rates
- Soil loss
- Increase runoff coefficient
- Water balance changes
- Increasing flood risk
- Decreasing annual water supply

iv. Hydrological impacts
-The proportion of base flow of the total annual runoff decreased from more than 30% in the 1970’s to less than 25% in the 1990’s. Inversely, the proportion of flood flow increased significantly.
-The temporal variability of stream flow is increasing; this should be a consequence of the soil water storage reduction in the basin, due to soil and vegetation degradation.
-Baseflow recession index increased due to excessive deforestation patterns.
-Two-day recession index increased due to excessive deforestation patterns.
-Decrease water storage.
-Higher values of peak flows.
-Lower low-flows.
-This study found that increasing runoff and erosion were due to the development of new soil surface features, particularly crusted soils that drastically enhance runoff.
-The most important degradation factor is overgrazing, because trampling cows make the soil bulk density increase and reduce hydraulic conductivity.

F. Future research

* Consider issues related to geographical scale (Mather et al 1998).
* Examine the different methodological and conceptual approaches to measure deforestation (Mather et al, 1998).
* Distinguish between proximate and underlying drivers of deforestation and carefully choose surrogate and significant variables (Mather et al, 1998).
* In the Costa Rican context, Sanchez-Azofeifa et al (2001) emphasized the need for new conservation and preservation strategies in the country concerned with the impacts of deforestation on biodiversity loss.

G. Conclusion: The study of the causal relationship between forest loss and population is not a simple one and requires further scholarly work since in the real world land-use land-cover changes are human-driven forces of global change.

H. References

--Journal Articles--

Allan, J.D. and others. 2002. Land use in watersheds of the Venezuelan Andes: a comparative analysis. Conservation Biology, 16(2):527-538.

FAO, 1990. Forest resources assessment 1990: Tropical Countries. FAO Forestry Paper 112, Rome, Italy. 59 p.p.

Kricher, J. 1997. A Neotropical Companion:an introduction to the animals, plants and ecosystems of the New World Tropics. Second Edition Revised and Expanded. 451p.p.

Mather, A.S., Needle, C.L. and J. Fairbairn. 1998. The human drivers of global land-cover change: the case of forests. Hydrological Processes, 12:1983-1994.

Sanchez-Azofeifa, G.A. 2000. Land-use ad cover change in Costa Rica. In C. Hall (Ed.) Quantifying sustainable development, p.p. 473-501. Academic Press, New York, New York.

Sanchez-Azofeifa, G.A., Harriss, R.C. and D. L. Skole. 2001. Deforestation in Costa Rica: a quantitative analysis using remote sensing imagery. Biotropica, 33(3)378-384.

Sader, S., and Joyce, A. 1988. Deforestation rates and trends in Costa Rica, 1940 to 1983. Biotropica, 20:11-19.

Sharma, K.P., Moore, B., Vorosmarty, C.J. 2000. Anthropogenic, climatic, and hydrologic trends in the Kosi Basin, Himalaya. Climatic Change, 47(1-2):141-165.
Viramontes D, Descroix L. 2003. Changes in the surface water hydrologic characteristics of an endoreic basin of northern Mexico from 1970 to 1998. Hydrological Processess, 17(17): 1291-1306.
Zhang, H., Henderson-Sellers, A., and McGuffie K., 2001. The Compounding Effects of Tropical Deforestation and Greenhouse Warming on Climate. Climatic Change 49: 309-338.

--Web Sources--

Rosero-Bixby L. and Palloni, A. Population and deforestation in Costa Rica. Retrieved on 4/15/2004. URL: http://www.ssc.wisc.edu/cde/cdewp/96-19.pdf

FACTS AND NUMBERS OF COSTA RICA. Retrieved on 4/25/2004. http://Inmotico.com



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