Development of a New Methodology for Estimating Groundwater Evapotranspiration

Abstract

Escalating demands on freshwater resources coupled with decreasing supplies dictates the need for improved groundwater modeling techniques and a more direct linkage between groundwater and riparian vegetation. This study describes the development and application of a new methodology and associated software model to simulate riparian and wetland evapotranspiration (RIP-ET). Traditional groundwater modeling, based on a quasi-linear relationship between evapotranspiration (ET) rates and groundwater elevation, has been used to predict changes in regional groundwater levels, and thus riparian vegetation potential. Our new approach uses multiple non-linear, segmented flux curves that more accurately reflect plant eco-physiology, thus simulating riparian and wetland ET in a manner that more realistically reflects the complexity inherent in these ecosystems. By decoupling evaporation and transpiration, both processes are more accurately defined and quantified. Plant functional groups (PFGs) based on water tolerance ranges and rooting depths are used to describe 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. For each group, the curve simulates transpiration declines that occur both as water levels decline below rooting depths and as waters rise to levels that produce anoxic soil conditions. Each of the multiple transpiration curves reflects a particular plant functional group, thus reflecting the variability in vegetative conditions.The RIP-ET model also improves accuracy by more effectively dealing with spatial issues of plant and water table distribution. ET can now be calculated by delineating the area of all plant assemblages (or habitat types), assigning accurate elevations to the individual polygons and then applying multiple ET curves to a single model cell. The ability to assign a unique surface elevation to each area, allows for a better representation of both ET and community/plant spatial relationships. Detailed information on the distribution of plant functional groups across land surface elevations effectively captures the range of ET responses across the topographic-hydrologic gradients.Case studies demonstrate that RIP-ET produces significantly different ET estimates and thus ecosystem interpretation than the traditional method. 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, improved basin scale water budgets and riparian vegetation water requirements. When combined with vegetation mapping and a supporting software (RIP-GIS), RIP-ET enables predictions of riparian vegetation response to changing water availability. 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.