Hydraulic head gradients governing groundwater movement and surface water-groundwater exchange is a fundamental physical principle of hydrogeology, making this common knowledge that requires no citation.
The claim states that direct recharge of groundwater from river water is governed by hydraulic head gradients. This is a foundational, axiomatic principle of fluid mechanics and hydrogeology (Darcy's Law dictates that fluid flows from higher to lower hydraulic head). While papers such as [1] discuss hydraulic head gradients governing exchange fluxes, the underlying premise is a basic physical truism of hydrology and does not require empirical citation to establish.