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Soil Erosion Prevention and Modeling in Costa Rica
Brendan Luther
April 22, 2014
BE 322, Hydrology

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Table of Contents:
I. Abstract
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II. Introduction
………………………………………………………..…….………….…..3-4
III. Methods
………………………………………………………………………...….. 4-6
-Length Slope
-Soil Data
-Climate Data
-Initial Conditions
-Management Implementations
IV. Results and Discussion
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-Initial Conditions Model
-Management Implementation Model
V. Conclusion
…………………………………...…………………………………………..8
VI. References
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Abstract:
This project attempts to provide a solution to an Erosion problem faced by a 25 ha farm
in the mountains of Costa Rica. WEPP modeling software was used to predict erosion
rates of a 3 ha portion of the farm where squash was grown. Climate data had to be
changed in order to correctly model the farm in Costa Rica as WEPP climate data files
are only for the United States. It was found that significant impacts on soil erosion could
be made by the addition of a few management practices for the land. Even with the
management steps, the model still showed a soil erosion rate of 28 tons/ ha/ year which is
well above USDA standards.
Introduction:
Across the globe, as far back as history dates, soil erosion has been an issue.
Today, farmlands everywhere still face this problem, and it has become greatly
intensified (Owino, Class Lecture) There are many factors that give rise to erosion, such
as deforestation, overgrazing, planting annual crops on steep slopes, and continuing to
plant on soil that is no longer fertile. With human population rising exponentially, more
food is needed, but fecund land for farming is becoming harder to find (Owino, Class
Lecture). One way to obtain more farmland is deforestation, which unfortunately has a
paradoxical effect of contributing in large to unfertile soil (Carpenter et al.). This happens
because the soil becomes extremely exposed to wind and rain, causing the top layer of
rich nutrient soil to wash away into rivers and other bodies of water. Tropical areas that
receive a substantial amount of rainfall each year, such as Costa Rica and Nicaragua, tend
to face the problem of soil degradation more than most other areas (Carpenter et al.).
Once this starts to occur, it becomes especially difficult to regenerate because the topsoil
is lost. Not only is this a huge problem for farmers and food production, but also
increases the chance of water pollution, causing devastating effects to marine life (Owino.
Class Lecture).
As the area that this project focuses on relies so heavily on agriculture and fishing,
whether for business and trade or to provide food for one’s family, it is crucial that
farmers and landowners take the necessary steps to help prevent soil degradation as much
as possible. Because this is the majority of people’s livelihoods in developing countries,
focusing on erosion issues in Costa Rica appeared to be a pressing and very applicable
topic. There isn’t a great deal of information on Central America and erosion, but one
research paper investigates land use and erosion of Costa Rican Ultisol and the affects on

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soil chemistry and a certain type of fungi. According to this source, “Erosion exposes
subsoils that have different chemical characteristics than the topsoil”, causing the soil left
over to be less nutrient. The eroded Ultisols create a very stressful environment for plants,
causing a lack of growth. Different chemical characteristics such as pH and soil organic
matter can be monitored and analyzed to determine the degree of erosion and soil
degradation on different parts of the land.
This farm, located in southern Costa Rica outside a tow called San Vito, is broken
up into multiple land uses such as coffee bean trees, squash patch, grazing pasture, and
bare soil (Carpenter et al.). The steep slope of the land paired with the tremendous
amount of rainfall each year generates a high erosion rate, leaving the soil permanently
damaged. Although the lay of the land and the amount of rainfall cannot be controlled,
the techniques used for farming can be. With further research, improvements can be
made to ensure that the nutrient-rich topsoil remain on the farms to provide people with
food, instead of being carried away to pollute rivers.
The focus of this effort was on the 3 hectares of planted squash as it has the most
exposed soil and therefore likely the most erosion. Using the Water Erosion Prediction
Project software, more detailed options for which to project soil loss. As only limited
information was available about the site it was sometimes necessary to estimate and/or
pick arbitrary values to complete the calculations.
Methods
Length Slope:
Since the mountainous areas of Costa Rica are home to a relatively steep landscape
(Carpenter et al), the slope was shaped accordingly. A concave slope with an average
grade of 15% was created. To model it after the site area, the slope was fitted to a length
of 350 meters and the width 85.7 meters, giving a area value of 30,000 square meters or
three hectares.
Soil Data:
The soil was Typic Hapludult over top Ultisol Soil with a Clay Content of 17-26%, Silt
Content of 19-28% and a Sand Content of 48-64% and low organic matter content
(Carpenter et al.). As Hapludult soil was not an option in WEPP, Canisteo soil with
similar properties was chosen. The data for the Canisteo soil was further refined to be in
the mid ranges of the given percent contents.

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Rainfall Data:
The climate posed the greatest challenge for the program calculations. WEPP modeling
software has very detailed climate information however it is for locations within the
United States and its provinces. The solution to this problem is to pick a location in the
US (as the climates range far and wide) that is similar in climate to the desired location.
The issue with the location in Costa Rica however was that it averaged 4000 mm of
rainfall a year. That is almost twice the average yearly rainfall of anywhere in the
continental United States. To get around this, anew data file was created that began with
base climate data out of a weather station in the Everglades in Florida. This site had
similar solar radiation values and mean monthly temperature values as La Cuesta, Costa
Rica. Changing the amount of rainfall required playing with numbers until a satisfactory
yearly rainfall was achieved. The program does not allow you to simply change the total
rainfall number. It requires that you change the rainfall frequency of the months and the
rainfall intensity for the months. It is however, very feasible to make a brand new data
file for weather patterns around the world so long as the weather data is available.
Figure 1 shows the rainfall data changes for La Cuesta, Costa Rica.

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The data input changes for La Cuesta (which was the closest city to San Vito that had
available weather information), from the original Everglades data can be seen above in
Figure 1.
Initial Conditions Model
Figure 2: Land Management Practices for Initial Conditions
The squash grown in Costa Rice resembles a round melon rather than the thin yellow
squash we often see in the United States (Carpenter et al). Therefore watermelon was
used to better simulate the crop conditions on the squash farm. The Initial Conditions
were set to “After harvest of watermelon.” The Tillage was set to “Chisel Plow” which is
more likely to be used in Costa Rica with the steep slopes as opposed to conventional till.
Finally watermelon was planted as stated earlier to grow throughout the summer.
Altered Land Management Practices:
Figure 4: Altered Land Management Practices.
The changes made to the squash farm landscape can be seen in Figure 4. First
implemented raised contour cropping with ridges spaced 10 feet apart to reduced the
runoff velocity. Then rock rubble was added (a very available material in the mountains)
barriers along the raised contours and intermixed short grasses to retain soil and slow
runoff velocity but allow the squash vines to receive sunlight. Also added, was a 15 meter
buffer of tall grasses and coffee trees at the base of the hill. The goal was to reduce the
soil erosion yield to less than 10 tons per hectare per year which is mid range for the
acceptable soil loss stated by the USDA.

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Results and Discussion:
Figure 3: Results of Modeling with Initial Condition
The model with the initial management conditions was run and yielded some extreme
erosion values as can be seen in Figure 3. Even if the modeling software is only in a ball
park range of the actual erosion values, it is clear why the farmers can only use their land
for a short amount of time before it is ruined. The average yearly precipitation in
conjunction with exposed dirt and a steep slope result in very high amounts of erosion. If
this land is to be used in the future, management measures must be taken.
Figure 5: Shows soil loss values after implementation.
Figure 5 shows the soil loss data after the land management practices have been put in
place. As can be seen the soil yield was dramatically reduced by over 28 times. This
shows the ability of certain soil management plans to greatly impact the amount of soil
lost. Even with these measures, a way to reduce it to the desired value of 10 tons per
hectare per year was not found. Much was done changing the characteristics of the slope,
which yielded much less eroded soil, however the buffer did little to change the final
sediment yield. This suggests that something was not calculating correctly in the program.

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The buffer should allow the value of sediment yield to be within USDA standards should
it be constructed with heavy grasses and calculated correctly.
Conclusion:
It was possible to successfully model soil erosion in an international location during the
project even with the Wepp Weather Data Stations only available for locations in the
United States. As long as climate data about the site in question is available, the weather
station data in the WEPP modeling program can be altered accordingly. As for the farm,
there appears to be no way to successfully prevent erosion higher than 10 tons of soil loss
per hectare, per year growing squash on the hillsides, if the buffer cannot be introduced.
Even if the soil is trapped at the bottom of the hill preventing runoff into the rivers, it is
however still not usable by the squash plants therefore squash may not be feasible to
grow on site. After running erosion simulations and taking all pertinent factors into
account, the best option for the farmers is to focus on growing coffee trees and low
densities of cattle as these agricultural practices yield much lower erosion rates. In
addition, he/she should install the rock buffers, raised contouring, and grass buffers
similar to those outlined in the simulation to further reduce the ongoing soil erosion issue
References:
1. Carpenter, F. Lynn, Sergio Palacios Mayorga, Eduardo Gonzalez Quintero, and
Michelle Schroeder. "Land Use and Erosion of a Costa Rican Ultisol Affect Soil
Chemistry, Mycorrhizal Fungi and Early Regeneration." Land Use and Erosion of
a Costa Rican Ultisol Affect Soil Chemistry, Mycorrhizal Fungi and Early
Regeneration. N.p., n.d. Web. 22 Apr. 2014
2. Montgomery, David R. "Soil Erosion and Agricultural
Sustainability." PNAS104.33 (2007): n. pag. Web. 7 Apr. 2014.
3. Owino, Tom. Hydrology Class Lecture. January-May 2014. Clemson University
4. The Weather Channel.
<http://www.weather.com/weather/hourbyhour/graph/La+Cuesta+CSXX0047:1:
CS> n.d. Web. 01 Dec. 2015.

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