USGS

 

SIR 2004-5195

Prepared in cooperation with the
St. Johns River Water Management District


2004

 

A Method for Simulating Transient Ground-Water Recharge in Deep Water-Table Settings in Central Florida by Using a Simple Water-Balance/Transfer-Function Model

Andrew M. O’Reilly


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Abstract
Introduction
    Purpose and Scope
    Hydrologic Conditions in Central Florida
Development of the Water-Balance/Transfer-Function Model
    Conceptual Model
    Water-Balance Module
    Transfer-Function Module
Application of the Water-Balance/Transfer-Function Model
    Field Site in West Orange County
        Estimation of Recharge by Analysis of Water-Table Fluctuations
        Calibration of Water-Balance/Transfer-Function Model
    Evaluation of the Transfer-Function Module by Comparison with VS2DT
        Application of the Variably Saturated Flow Model VS2DT
        Comparison of the Transfer-Function Module with VS2DT
        Linearity Assumption
    Hypothetical Sites
Suggestions for Use of the Water-Balance/Transfer-Function Model
    Relative Importance of Precipitation, Surface Runoff, and Evapotranspiration
    Model Calibration
Model Assumptions and Limitations
Summary
References
Appendix 1–Documentation of Water-Balance/Transfer-Function Model Input and Output
Appendix 2–Example Problem

Abstract

A relatively simple method is needed that provides estimates of transient ground-water recharge in deep water-table settings that can be incorporated into other hydrologic models. Deep water-table settings are areas where the water table is below the reach of plant roots and virtually all water that is not lost to surface runoff, evaporation at land surface, or evapotranspiration in the root zone eventually becomes ground-water recharge. Areas in central Florida with a deep water table generally are high recharge areas; consequently, simulation of recharge in these areas is of particular interest to water-resource managers. Yet the complexities of meteorological variations and unsaturated flow processes make it difficult to estimate short-term recharge rates, thereby confounding calibration and predictive use of transient hydrologic models.

A simple water-balance/transfer-function (WBTF) model was developed for simulating transient ground-water recharge in deep water-table settings. The WBTF model represents a one-dimensional column from the top of the vegetative canopy to the water table and consists of two components: (1) a water-balance module that simulates the water storage capacity of the vegetative canopy and root zone; and (2) a transfer-function module that simulates the traveltime of water as it percolates from the bottom of the root zone to the water table. Data requirements include two time series for the period of interest—precipitation (or precipitation minus surface runoff, if surface runoff is not negligible) and evapotranspiration—and values for five parameters that represent water storage capacity or soil-drainage characteristics.

A limiting assumption of the WBTF model is that the percolation of water below the root zone is a linear process. That is, percolating water is assumed to have the same traveltime characteristics, experiencing the same delay and attenuation, as it moves through the unsaturated zone. This assumption is more accurate if the moisture content, and consequently the unsaturated hydraulic conductivity, below the root zone does not vary substantially with time.

Results of the WBTF model were compared to those of the U.S. Geological Survey variably saturated flow model, VS2DT, and to field-based estimates of recharge to demonstrate the applicability of the WBTF model for a range of conditions relevant to deep water-table settings in central Florida. The WBTF model reproduced independently obtained estimates of recharge reasonably well for different soil types and water-table depths.

 


Suggested Citation:

O’Reilly, A.M., 2004, A Method for Simulating Transient Ground-Water Recharge in Deep Water-Table Settings in Central Florida by Using a Simple Water-Balance/Transfer-Function Model: U.S. Geological Survey Scientific Investigations Report 2004-5195, 49 p.

U.S. Department of the Interior,
U.S. Geological Survey
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Altamonte Springs, FL 32714

aoreilly@usgs.gov


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