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Techniques and Methods 6–A39

Office of Groundwater, Transboundary Aquifer Assessment Program

RIP-ET: A Riparian Evapotranspiration Package for MODFLOW-2005

By Thomas Maddock III and Kathryn J. Baird, University of Arizona; R.T. Hanson, U.S. Geological Survey; and Wolfgang Schmid and Hoori Ajami, University of Arizona

Thumbnail of and link to report PDF (9 MB)Abstract

A new evapotranspiration package for the U.S. Geological Survey’s groundwater-flow model, MODFLOW, is documented. The Riparian Evapotranspiration Package (RIP-ET) provides flexibility in simulating riparian and wetland transpiration not provided by the Evapotranspiration (EVT) or Segmented Function Evapotranspiration (ETS1) Packages for MODFLOW 2005. This report describes how the RIP-ET package was conceptualized and provides input instructions, listings and explanations of the source code, and an example.

Traditional approaches to modeling evapotranspiration (ET) processes assume a piecewise linear relationship between ET flux and hydraulic head. The RIP-ET replaces this traditional relationship with a segmented, nonlinear dimensionless curve that reflects the eco-physiology of riparian and wetland ecosystems. Evapotranspiration losses from these ecosystems are dependent not only on hydraulic head, but on the plant types present. User-defined plant functional groups (PFGs) are used to elucidate the interaction between plant transpiration and groundwater conditions. Five generalized plant functional groups based on transpiration rates, plant rooting depth, and water tolerance ranges are presented: obligate wetland, shallow-rooted riparian, deep-rooted riparian, transitional riparian and bare ground/open water. Plant functional groups can be further divided into subgroups (PFSGs) based on plant size, density or other characteristics.

The RIP-ET allows for partial habitat coverage and mixtures of plant functional subgroups to be present in a single model cell. RIP-ET also distinguishes between plant transpiration and bare-ground evaporation. Habitat areas are designated by polygons; each polygon can contain a mixture of PFSGs and bare ground, and is assigned a surface elevation. This process requires a determination of fractional coverage for each of the plant functional subgroups present in a polygon to account for the mixture of coverage types and resulting transpiration. The fractional cover within a cell has two components: (1) the polygonal fraction of active habitat (excluding area of bare ground, dead trees, or brush) in a cell, and (2) fraction of plant type area or bare ground area in a polygon. RIP-ET determines the transpiration rate for each plant functional group and evaporation from bare ground/open water in a cell, the total ET in the cell, and the total ET rate over the region of simulation.

First posted March 5, 2012

For additional information contact:
Director, California Water Science Center
U.S. Geological Survey
6000 J Street, Placer Hall
Sacramento, California 95819
http://ca.water.usgs.gov

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Suggested citation:

Maddock, Thomas, III, Baird, K.J., Hanson, R.T., Schmid, Wolfgang, and Ajami, Hoori, 2012, RIP-ET: A riparian evapotranspiration package for MODFLOW-2005: U.S. Geological Survey Techniques and Methods 6-A39, 76 p.



Contents

Acknowledgments

Abstract

1.0 Introduction

2.0 Plant Functional Groups

3.0 Plant Functional Subgroups

4.0 Transpiration Flux Curves

5.0 Transpiration Flux Curve Linear Interpolation

6.0 Development of Plant Functional Group Curves

7.0 Plant Functional Subgroup Coverage

8.0 Fractional Coverage

9.0 Land-Surface Elevation

10.0 Transpiration Calculations

11.0 Summary

12.0 Input Instructions for RIP-ET

13.0 Explanation of Variables Read by the RIP-ET Package

14.0 Example

15.0 Code Documentation for the RIP-ET Package

16.0 References Cited

Appendix A. Plant Functional Subgroup Curves


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