Simulation of the Hydrologic Effects of Afforestation in the Tacuarembo River Basin, Uruguay
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Date
2005-08-02
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Abstract
The Soil and Water Assessment Tool (SWAT) was used to simulate the hydrology of two small catchments in northern Uruguay. The modeling study was intended to complement a long-term field research project that is evaluating the hydrologic and water quality effects of the conversion of pasture to pine plantation.
The field study takes a paired catchment approach to evaluate the small watershed scale effects of afforestation. The control and treatment catchments were monitored for a three year pretreatment period during which the land use was grassland with livestock grazing (July 2000 to June 2003). Subsequently, the treatment catchment was planted with loblolly pine (Pinus taeda) in July 2003 (57% afforested). The control catchment (pasture) and treatment catchment (pine trees) will be continuously monitored from planting through tree maturation and harvesting. The control and treatment catchments are 69 hectares and 108 hectares, respectively.
The objective of the modeling study was to predict the hydrologic effects of converting native pasture to pine plantation in Uruguay, South America. Supporting objectives of the research included accurately simulating the hydrologic response of the two catchments during the pretreatment period, improving upon the understanding of the hydrologic processes occurring on the two catchments and evaluating the dependency of the difference in evapotranspiration between grass and pine trees to factors such as weather variability, vegetative rooting depth, soil properties and groundwater characteristics.
The SWAT model of the two catchments was calibrated and validated using the monitoring data from the pretreatment period. Model input parameters were either based on measured properties, published values or adjusted during calibration. Two modeling scenarios were developed as possible explanations for the higher base flows in the treatment catchment observed during the pretreatment period.
The SWAT model predicted outflows from the catchments reasonably well as compared to observed outflows during the years with above average rainfall (July 2000 — June 2003). The model overpredicted flows during the dry year (July 2003 — June 2004). The annual flow volumes were within +/- 13% for each of the years in the calibration and validation period, except for the control catchment during the dry year, which ranged from 77 - 86% for the two modeling scenarios. Model efficiency, E, for daily outflow volumes was greater than 0.86 for each catchment under each scenario, indicating a good fit between simulated and observed results.
A 34-year weather data set was compiled in order to run a long-term continuous simulation using the calibrated model. Three land uses were simulated: native grassland, pasture with livestock grazing and afforestation of the treatment catchment. The conversion of the catchments from the baseline pasture condition to native grassland or afforested condition resulted in a predicted reduction in outflow volume from the catchments. The reduction in mean annual flow volumes for the grassland ranged from 15% to 18%, and for the pine treatment from 23% to 27%. However, certain hydrologic processes may be underrepresented in the model, including the effect of deep rooted pine trees and the evapotranspiration of the pine trees during dormancy.
A sensitivity analysis was conducted on selected model parameters to evaluate the impact on predicted effect of land use conversion. The maximum effect was determined by maximizing the pine trees ability to withdraw water from the shallow aquifer. The maximum predicted effect on mean annual outflow volume for the afforestation of the treatment catchment ranged from 30% to 36%.
The results of this study were an initial attempt to evaluate the hydrologic effects of the conversion of pasture to pine plantation in northern Uruguay. Further refinement of the SWAT model, additional field investigations and improved understanding of pine tree evapotranspiration are recommended to improve the accuracy of the predicted hydrologic effects.
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Keywords
afforestation, modeling, SWAT, hydrology, evapotranspiration, grassland
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Degree
MS
Discipline
Biological and Agricultural Engineering
