Inverse methods in hydrogeology: Evolution and recent trends

• Track the evolution of the inverse methods over the last decades in hydrogeology.
• Discuss issues encountered in the inverse problem, e.g., handling multiGaussianity, and considering prior statistics.
• Reveal the development motivation of the inverse methods and their recent trends.

Parameter identification is an essential step in constructing a groundwater model. The process of recognizing model parameter values by conditioning on observed data of the state variable is referred to as the inverse problem. A series of inverse methods has been proposed to solve the inverse problem, ranging from trial-and-error manual calibration to the current complex automatic data assimilation algorithms. This paper does not attempt to be another overview paper on inverse models, but rather to analyze and track the evolution of the inverse methods over the last decades, mostly within the realm of hydrogeology, revealing their transformation, motivation and recent trends. Issues confronted by the inverse problem, such as dealing with multiGaussianity and whether or not to preserve the prior statistics are discussed.