Evaluating water table response to rainfall events in a shallow aquifer and canal system

摘要

Shallow aquifers typically have greater hydrologic connectivity and response to recharge and changes in surface water management practices than deeper aquifers and are therefore often managed to reduce the risk of flooding. Quantification of the water table elevation response under different management scenarios provides valuable information in shallow aquifer systems to assess indirect influences of such modifications. The episodic master recession method was applied to the 15-min water table elevation and NEXRAD rainfall data for 6 wells to identify water table response and individual rainfall events. The objectives of this study were to evaluate the effects of rainfall, water table elevation, canal stage, site-specific characteristics, and canal structure modification/water management practice on the fluctuations in water table elevations using multiple/stepwise multiple linear regression techniques. With the modification of canal structure and operation adjustment, significant difference existed in water table response in the southern wells due to its relative downstream position regarding the general groundwater flow direction and the structural modification locations. On average, water table response height and flood risk were lower after than before the structure modification to canals. The effect of rainfall event size on the height of water table response was greater than the effect of antecedent water table elevation and canal stage on the height of water table response. Other factors including leakance of the canal bed sediment, specific yield, and rainfall on i-1day had significant effects on the height of water table response as well. Antecedent water table elevation and canal stage had greater and more linear effects on the height of water table response after the management changes to canals. Variation in water table response height/rainfall event size ratio was attributed to difference in S-y, antecedent soil water content, hydraulic gradient, rainfall size, and run-off ratio. After the structure modification, water table response height/rainfall event size ratio demonstrated more linear and proportional relationship with antecedent water table elevation and canal stage.