NUMERICAL MODELING TO AQUIFER RECHARGE TO PREFERENTIAL FLOW--POWERFUL INSIGHTS OF RICK HEALY (Invited Presentation)

Rick Healy’s total commitment to accuracy, objectivity, painstaking thoroughness, and moreover his fundamental insights into the interaction of earth and water, shine forth in his original research and numerous review articles, instructional materials, and overviews.

Rick was an enthusiastic creator of versatile numerical models for the state and movement of water in the subsurface, notably his VS2D (Variably Saturated in 2 Dimensions) family of models. For more than three decades he maintained these as state-of-the-art tools with all sorts of expanding capabilities—water, solute, and heat transport in many forms and variations. Rick was totally attentive to user needs. Time after time, starting from words like “Rick, I need a code that will handle [a specific process—solute transport, heat transport, chemical reactions, point-source input, etc.]” there would soon be a new version to rigorously accomplish the needed task, and a new letter (T, H, I, R, etc.) appended to the name of the code. From reliance on line-by-line tabular input and output in the 1980s (late stages of the punched-card era), Rick kept the code current and later pushed it into the future with a sleek and user-friendly visual interface.

Among the many topics of his hydrologic expertise, aquifer recharge stands out. Besides creating many tools for recharge estimation, Rick put his deep understanding of the relevant hydrologic processes into syntheses, reviews, and basic text materials, the highlight of which is his 2010 book, Estimating Groundwater Recharge. His knowledge was thoroughly comprehensive, from the nuts and bolts of essentially every recharge-estimation method ever devised, to the intricacies of physical process and connections to every facet of hydrology.

Another vital subsurface water topic that Rick dove into is preferential flow. Besides demonstrating and quantifying it in field experiments, he directly incorporated it into predictive models. In 2015 he developed VS2DSR, which included my own source-responsive model based on preferential flow in the form of films. He surprised me by finding water-balance flaws in my published formulation, and developed ways to correct these. This changed my fundamental thinking about the role of macropore water content in quantifying preferential flow, which I now look back on as the most important of many insights I gained from Rick Healy.