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Rainfall-Runoff and Water-Balance Models for Management of the Fena Valley Reservoir, Guam

Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2004-5287


By Chiu W. Yeung


Prepared in cooperation with the
U.S. Department of the Navy


This report is available as a pdf.



The U.S. Geological Survey's Precipitation-Runoff Modeling System (PRMS) and a generalized water-balance model were calibrated and verified for use in estimating future availability of water in the Fena Valley Reservoir in response to various combinations of water withdrawal rates and rainfall conditions. Application of PRMS provides a physically based method for estimating runoff from the Fena Valley Watershed during the annual dry season, which extends from January through May. Runoff estimates from the PRMS are used as input to the water-balance model to estimate change in water levels and storage in the reservoir.

A previously published model was calibrated for the Maulap and Imong River watersheds using rainfall data collected outside of the watershed. That model was applied to the Almagosa River watershed by transferring calibrated parameters and coefficients because information on daily diversions at the Almagosa Springs upstream of the gaging station was not available at the time. Runoff from the ungaged land area was not modeled. For this study, the availability of Almagosa Springs diversion data allowed the calibration of PRMS for the Almagosa River watershed. Rainfall data collected at the Almagosa rain gage since 1992 also provided better estimates of rainfall distribution in the watershed. In addition, the discontinuation of pan-evaporation data collection in 1998 required a change in the evapotranspiration estimation method used in the PRMS model. These reasons prompted the update of the PRMS for the Fena Valley Watershed.

Simulated runoff volume from the PRMS compared reasonably with measured values for gaging stations on Maulap, Almagosa, and Imong Rivers, tributaries to the Fena Valley Reservoir. On the basis of monthly runoff simulation for the dry seasons included in the entire simulation period (1992-2001), the total volume of runoff can be predicted within -3.66 percent at Maulap River, within 5.37 percent at Almagosa River, and within 10.74 percent at Imong River. Month-end reservoir volumes simulated by the reservoir water-balance model for both calibration and verification periods compared closely with measured reservoir volumes. Errors for the calibration periods ranged from 4.51 percent [208.7 acre-feet (acre-ft) or 68.0 million gallons (Mgal)] to -5.90 percent (-317.8 acre-ft or -103.6 Mgal). For the verification periods, errors ranged from 1.69 percent (103.5 acre-ft or 33.7 Mgal) to -4.60 percent (-178.7 acre-ft or -58.2 Mgal). Monthly simulation bias ranged from -0.19 percent for the calibration period to -0.98 percent for the verification period; relative error ranged from -0.37 to -1.12 percent, respectively. Relatively small bias indicated that the model did not consistently overestimate or underestimate reservoir volume.




Purpose and Scope


Description of Study Area





Runoff Characteristics

Rainfall-Runoff Model

Description of Precipitation-Runoff Modeling System

Background Data

Runoff Data

Climate Data



Physiographic Data

Selection of Calibration and Verification Periods

Model Building Using the Modular Modeling System

Watershed Delineation

Characterization and Delineation of Hydrologic Response Units

Model Parameterization

Model Calibration, Verification, and Results

Calibration Objective

Water-Budget Adjustments

Parameter Adjustments

Simulation Results

Dry Season Runoff Simulation

Comparison to Previous Model

Model Uncertainties

Water-Balance Model

Description of Model

Model Development and Data

Model Calibration, Verification, and Results

Model Uncertainties

Application and Evaluation of the Two-Step Modeling Procedure

Summary and Conclusions

References Cited

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Suggested citation:


Citation: Yeung, C.W., 2005, Rainfall-runoff and water-balance models for management of the Fena Valley Reservoir, Guam: U.S. Geological Survey Scientific Investigations Report 2004-5287, 52 p.


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