Simulating riparian evapotranspiration: a new methodology and application for groundwater models

This paper describes the development and application of a new methodology to simulate riparian and wetland evapotranspiration (ET) in groundwater models. Traditional approaches for modeling ET are based on quasi-linear relationship between ET flux rate and hydraulic head (groundwater elevation). The approach presented here uses multiple non-linear, segmented flux curves that reflect the ecophysiology of the plant species in these systems. Five plant functional groups (PFGs) based on water tolerance ranges and rooting depths are used to elucidate the interactive processes of plant transpiration with groundwater conditions. ET flux rate curves set the extinction and saturation extinction depths and define the group's ET flux rate as a function of water table depth relative to the ground surface. The calculated ET loss from a riparian or wetland system is dependent on the contributing area of each plant functional subgroup present and water table conditions. The new methodology requires a fractional coverage for each of the PFGs present within the groundwater model cell and allows for more accurate assignment of land surface elevations. Model results for a case-study show significant differences in predicted ET and subsequent depth to groundwater between the new and the traditional approaches. The development of physiologically based transpiration curves combined with the traditional linear curve for bare soil/open water results in more accurate determinations of riparian ET and improved basin scale water budgets. The use of PFGs in combination with the new RIP-ET package provides an explicit link between groundwater and riparian/wetland habitat conditions and offers an opportunity to better manage and restore riparian and wetland systems.