Abstract
A modified Water-Table Fluctuation (WTF) method is developed to quantitatively characterize the regional groundwater discharge patterns in stressed aquifers caused by intensive agricultural pumping. Two new parameters are defined to express the secondary information in the observed data. One is infiltration efficiency and the other is discharge modulus (recurring head loss due to aquifer discharge). An optimization procedure is involved to estimate these parameters, based on continuous groundwater head measurements and precipitation records. Using the defined parameters and precipitation time series, water level changes are calculated for individual wells with fidelity. The estimated parameters are then used to further address the characterization of infiltration and to better quantify the discharge at the regional scale. The advantage of this method is that it considers recharge and discharge simultaneously, whereas the general WTF methods mostly focus on recharge. In the case study, the infiltration efficiency reveals that the infiltration is regionally controlled by the intrinsic characteristics of the aquifer, and locally distorted by engineered hydraulic structures that alter surface water-groundwater interactions. The seasonality of groundwater discharge is characterized by the monthly discharge modulus. These results from individual wells are clustered into groups that are consistent with the local land use pattern and cropping structures.
Highlights
Human activities in many regions have greatly altered groundwater systems [1,2]
Short term water-table fluctuations can be caused by changes in temperature or atmospheric pressure [24], air trapped in the medium [30,31], and temporary horizontal flow interferences, etc
These processes are not related to the conceptual model of Water-Table Fluctuation Regression (WTFR), and have no effect on the intrinsic aquifer property β and long-term pumping strength indicator DM
Summary
Human activities in many regions have greatly altered groundwater systems [1,2]. In areas where groundwater is greatly exploited, such as areas with intensive agricultural pumping, many previously connected aquifer systems have evolved into multiple isolated hydrogeological units. Vertical water exchanges including infiltration and pumping have become the dominant flow patterns in those highly stressed unconfined aquifers [3,4]. In such systems, groundwater recharge and discharge are critical components for understanding fluid and contaminant transport [5]. Recharge to unconfined aquifers is derived from the downward infiltration from the land surface of precipitation, irrigation water, and/or seepage from surface water bodies.
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