Evapotranspiration and groundwater system dynamics in arid riparian zone

Background/Question/Methods

In areas where plants directly tap groundwater for their water supply, hydrographs from the water table typically display diurnal fluctuations superimposed on other larger and smaller trends during the plant growing season. In riparian zone, groundwater use by phreatophytes may also cause groundwater to fluctuate regularly in diurnal cycles. If phreatophytes use groundwater to any significant extent during the daylight-hour transpiration, groundwater table will decline. When transpiration significantly diminishes or ceases during the night, the water table will rebound through net inflow. Thus, twice per day, plant water use causes peak and trough in the groundwater level in the morning and evening, respectively. In this study, detrended fluctuation analysis is utilized to analyze the observed arid lower Colorado River riparian zone groundwater levels and nearby river stages to quantify fractal dynamics of riparian zone groundwater systems, particularly in relation to groundwater use by phreatophytes. The aim is to seek possible links between the scaling properties of groundwater level fluctuations in the riparian zones and the nearby river stages in relation to the proximity of groundwater wells to the river.

Results/Conclusions

For groundwater fluctuations recorded at wells far away from the river, it is found that there are two crossovers in the scaling plots: one at the time scale around 1 day which is the characteristic time scale of groundwater level fluctuation due to phreatophyte water use, and the other at about 3 days. For river stage and groundwater levels at wells close to the river, there is only one crossover in the scaling plots of water levels. For groundwater levels recorded far away from the river, variations of groundwater levels are persistent (i.e., the scaling exponent is larger than 1.50) at small time scales (less than 1 day) and large time scale (longer than about 3 days). At the  time scale between 1 day and about 3 days, the groundwater water level variations are close to white noise (i.e., the scaling exponent  is about 0.5). For river stages and groundwater levels at wells close to the river, the value of scaling exponent decreases as the scale increases.