Watershed Plan for Heredia, Costa RicaWhitney Thomas, Matthew O’Malley, William Brown

Biological Systems Engineering, Virginia Tech

Rio Tibas WatershedGIS analysis

Best Management Practices

Conclusions

Acknowledgements: Dr. Theo Dillaha - Biological Systems Engineering Mr. Luis Gámez - Public Utilities Company of Heredia S.A., Costa Rica

Heredia

Introduction•Problem Statement: The Public Utilities Company of Heredia S.A. charges all customers a green fee in order to pay land owners in the Heredia watershed to keep their lands forested or to reforest existing crop land. Currently any land owner that wishes to take part in the program may enter into a contract and all participants receive the same stipend. The company would like to distribute funds based on the amount of erosion and resulting total suspended solids each land parcel contributes.

•Design Objective: The goal of this study is to identify the land areas that contribute the most to erosion and total suspended solids in the Rio Tibas and Rio Segundo Sub-Watersheds of the Heredia Basin and determine the BMP options that will help lower TSS to 10 mg/L at the watershed outlets.

Rio Segundo WatershedGIS analysis

Design Results:The highest areas of erosion were found to be occurring in urban areas near the watershed outlet. The areas with medium potential erosion were found throughout the middle and lower regions of the watershed where majority of slopes are 0-15% and the main land use is permanent crops. Here BMPs such as vegetative filter strips are planned for the high and medium erosive areas 50 m from the stream and intercropping can be used 50 m and further to reduce erosive potential.

The Rio Segundo Watershed is located in the western region of the Heredia watershed. Currently, only locations in the upper portion of the watershed have been reforested as part of the Procuencas Project. An analysis of the present situation within the Rio Segundo watershed was assessed using GIS and the USLE to determine whether reforestation from previous years has reduced erosion and to assess where critical areas of erosion are occurring.

The Rio Tibas Watershed is located in the Eastern region of the Heredia watershed. Currently, the only locations involved in the Procuencas Project are in the upper mountainous portion of this watershed. The areas that were previously reforested in this upper region have shown to be efficient at reducing erosion. An analysis of the present situation within the Rio Tibas watershed was assessed using GIS and the USLE.

The Procuencas Project is implemented by the Public Utilities Company of Heredia, S.A. as an incentive to promote better water quality throughout the Heredia Watershed. Landowners are paid to keep their land forested or to reforest barren land or cropland. The goal of this program is to improve water quality through flow regulation, water filtration, erosion control and sedimentation, and maintaining the hydrological functions provided by forests.

Procuencas Project

• Forested areas located in the Upper reaches of each watershed should continue to be protected, as they have proven to be effective at reducing erosion and soil loss•The majority of the focus of the Procuencas Program should be protecting new land in the Rio Segundo Watershed because it has more land with high erosion rankings.•Implementation of the above BMP’s should help to reduce soil loss. Vegetative Filter Strips have found to be about 75% efficient at reducing sediment and Intercropping has a similar efficiency.•Future field work should be done in Costa Rica to establish the validity of the USLE values used.

The same mountain in Rio Segundo before (left) and 5 years after (right) reforestation.

Vegetative Filter Strip•Vegetative filter strips were chosen out of many possible BMPs because they require little maintenance and cost little to install.•Filter strip length (parallel to flow) should always equal the width of the contributing drainage area.•The limits of the lateral slope for embankment are 2%-15%.

WfMIN = Q/q*Where: WfMIN = minimum filter strip width

perpendicular to flow (feet)

q = (0.00236/n)(Y1/5)(S1/2)*Where:

q = discharge per foot of width of filter strip (cfs/ft)

Y = allowable depth of flow (inches)S = slope of filter strip (percent)n = Manning’s “n” roughness coefficient

Design Results:The highest areas of erosion were found to be occurring in the middle regions of the watershed closest to the streams. These regions contain mostly permanent crop areas and urban areas with slopes ranging from 0-15%. Assuming that all of these crop areas implement vegetation filter strips within 50 m of the stream and intercropping in the permanent crop areas 50 m or more from the stream the results indicate that the erosion will have been reduced to low erosion levels. The only remaining areas of high erosion are occurring in the Urban areas.

•Costa Rica loses 860 million tons of top soil annually.

•The population of Heredia is about 125,000 people in an area of 2657 square kilometers.

• The Public Utilities Company has reforested 1000 hectares of land thus far and distributes drinking water to all of Heredia.

Cost AnalysisCurrently all land owners are paid the same amount regardless of how much erosion their land produces. The funds to contribute to the Procuencas Project have been divided so that high erosive values were multiplied by 1.5, medium by 0.75, and low by 0.25. These multipliers were decided based on erosion classification and percent of watershed.

Land ErosionClassification

Present Value (₡/ha/yr)Percent of

Watersheds

Suggested Value (₡/ha/yr)

Reforestation Keep Forest Reforestation Keep Forest

High 45,000 47,720 6.76 67,500 71,580

Medium 45,000 47,720 15.44 33,750 35,790

Low 45,000 47,720 77.79 11,250 11,930

Costa Rica

Heredia Watershed

IntercroppingIntercropping is the practice of cropping twocrops at the same time in the same place.Some examples of intercropping in tropicalareas include; maize-bean, maize-pumpkin,and sugarcane-bean. A case study fromsouthern China showed that intercroppedcassava in Arachis pentoi reduced soil loss 4-fold. Intercropping has a similar effect to the vegetation filter strip but it is placed within crop fields.