MODELING GROUNDWATER USE BY PHREATOPHYTES: FROM SIMPLE TO COMPLEX APPROACHES (Invited Presentation)

Upland phreatophytes provide a key link between groundwater and the land surface, having potentially significant consequences for both ecosystems and climate. Here, we address an important disconnect in their modeling: while sophisticated methods are available to predict the use of groundwater by vegetation, they are rarely applied for groundwater management.

In this study, we use two contrasting modeling frameworks to show the impact of uptake by phreatophytes on water availability. The first simulates groundwater uptake in upland ecosystems using an Earth system model, PARFLOW.CLM. This model resolves uptake and hydraulic redistribution at the sub-hourly scale and accounts for changes in soil moisture, depth to groundwater, precipitation, and evaporative demand. We find that in some ecosystems, nearly 80% of the water lost to evapotranspiration (ET) during dry months comes from groundwater sources. Modeling experiments show that changing the depth to the water table, either through initiation or cessation of pumping, can alter these rates by up over 1.0 mm/d. The second simulates Edwards Aquifer in Central Texas, which is a traditional MODFLOW-based package that was originally created as part of the state’s Groundwater Availability Modeling (GAM) system. By adding in simple formulations of groundwater uptake by the region’s phreatophytes, we show that extraction by vegetation could potentially rival pumping rates by permitted users. Their addition dramatically decreased springflow rates, and their spatial distribution led to pronounced disparities in subsurface water storage across the region. However, we found that the model was highly sensitive to recharge rates, and the values assumed by model developers were extremely uncertain. Calibration of the model had functionally included these losses in the annual water budget, but did not account for correlating seasonal peaks in pumping and plant uptake.

We conclude that while phreatophyte uptake can clearly alter groundwater availability at a regional scale, many models intended for groundwater management will not be able to resolve their influence unless better recharge estimates are available. We discuss how lessons learned from Earth system modeling could be applied to fill this gap.