A riparian evapotranspiration package for MODFLOW-2000 and MODFLOW-2005

摘要

A new version of an evapotranspiration package for the U.S. Geological Survey's groundwater -flow model,udMODFLOW, is documented. The Riparian Evapotranspiration Package (RIP-ET) provides flexibility in simulatingudriparian and wetland evapotranspiration (ET) not provided by the MODFLOW -2000 and MODFLOW 2005udtraditional Evapotranspiration (EVT) Package, nor by the MODFLOW-2000 Segmented FunctionudEvapotranspiration (ETS1) Package. This report describes how the package was conceptualized and providesudinput instructions, listings and explanations of the source code, and an example simulation.udTraditional approaches to modeling ET processes assume a piecewise linear relationship between ET flux rateudand hydraulic head. The RIP-ET replaces this traditional relationship with a segmented, nonlinear dimensionlessudcurve that reflects the eco-physiology of riparian and wetland ecosystems. Evapotranspiration losses from theseudecosystems are dependent not only on hydraulic head but on the plant types present. User -defined plantudfunctional groups (PFGs) are used to elucidate the interactive processes of plant ET with groundwater conditions.udFive generalized plant functional groups based on transpiration rates, plant rooting depth, and water toleranceudranges are presented: obligate wetland, shallow-rooted riparian, deep- rooted riparian, transitional riparian andudbare ground /open water. Plant functional groups can be further divided into subgroups (PFSG) based on plantudsize, density or other user defined field.udThe RIP -ET allows for partial habitat coverage and mixtures of plant functional subgroups to be present in audsingle model cell. Habitat areas are designated by polygons. A polygon can contain a mixture of PFSGs and bareudground, and is assigned a calculated land surface elevation. This process requires a determination of fractionaludcoverage for each of the plant functional subgroups present in a polygon to simulate the mixture of coverageudtypes and resulting ET. The fractional cover within a cell has two components: 1) the polygonal fraction of activeudhabitat in a cell, and 2) fraction of plant flux area in a polygon. The RIP -ET determines the ET rate for each plantudfunctional group in a cell, the total ET in the cell, and the total ET rate over the region of simulation.