Interference of river level changes on riparian zone evapotranspiration estimates from diurnal groundwater level fluctuations

Groundwater evapotranspiration (ET) by phreatophytes is an important component of the water budget in riparian zones of many arid and semiarid environments. Diurnal fluctuations in groundwater levels have been used to estimate groundwater ET, but interference between nearby surface water (e.g., rivers, streams) and groundwater levels complicates the ET estimate. The objective of this study is to utilize spectral analysis to investigate the influence of surface water stage changes on the ET estimates in a riparian environment, thereby elucidating the potential caveats of using groundwater level diurnal fluctuations to estimate groundwater ET. The study is conducted in a stand of Tamarisk Ramossisima on the Colorado River near Blythe, California, USA. Results show that the most significant diurnal signal from the measurement of water levels of monitoring wells can be attributed to ET; however, distance of the groundwater level monitoring wells to the river directly impacts our ability to separate the two different signals from ET and river stage changes. Water level signals at groundwater monitoring stations close to the river (∼200 m in this study) are related to both ET and changes in river stages, which vary at frequencies other than diurnal. The spectral analyses of water level in time and frequency domains provide strong indications of the sensitivity of groundwater levels to fluctuation in the river stage. As a result, ET estimated from diurnal fluctuations of groundwater levels from the well near the river possesses large errors, even resulting in negative ET. No significant influence of river stage changes are observed on water levels at locations farther from the river (>800 m).