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• A modified KC equation is proposed to calculate hydraulic conductivity in nanopores. • Influence of surface activity and pore size on nano-flow is modelled by MD simulations. • A specific function is proposed to quantify the adsorbed proportion based on MD simulation results. • The modified KC equation is valid to be applied to both sandy and clayey soils. Hydraulic conductivity is a critical parameter for studying the behavior of clay-water systems. However, accurately estimating saturated hydraulic conductivity k in clayey soil by using the Kozeny-Carman (KC) equation is challenging due to the neglect of its active surface properties and nanoscale pores. Clay surfaces can have diverse characteristics resulting from various physicochemical processes such as isomorphous substitution and are typically characterized by the Cation Exchange Capacity (CEC). These properties can significantly impact fluid transport through the clay matrix. To address this issue, this study modifies the KC equation by incorporating an adsorbed water proportion φ that considers the occurrence state of pore water and its correlation with pore diameter, making it applicable to clayey soils. Molecular dynamics (MD) models are established to investigate the influence of surface interaction (CEC) and nanopore size (r) on saturated hydraulic conductivity k of seepage flow in clayey soils. Based on the MD results, a specific function is proposed to quantify the adsorbed proportion φ , which is linearly related to the root of CEC and reciprocal of pore size (r), and then incorporated into a proposed hydraulic correction ratio k * to modify the classical KC equation. The modified KC equation is validated using experimental data from literature, showing a high R 2 value of 0.96. This result demonstrates that the proposed correction ratio k * can extend the application scope of the classical KC equation to clayey soils. [ABSTRACT FROM AUTHOR]

• Paired watershed is effective in studying runoff response to woody encroachment. • Rooting zone soil moisture increased after converting juniper woodland to grassland. • Juniper removal in central Great Plains resulted in fivefold increase in runoff. Mechanical removal of encroached juniper (Juniperus spp.) is a common practice to restore native grasslands. However, the hydrological responses to grassland restoration remain mostly unquantified for the climate transition zone in the southern Great Plains of the USA where ecosystem evapotranspiration is highly sensitive to the change of vegetation functional type. We used a paired watershed approach to directly quantify the impact of mechanical removal of eastern redcedar (Juniperus virginiana L., redcedar) and restoration to native prairie or establishment of switchgrass (Panicum virgatum L.) on root zone soil moisture and event-based runoff for eight years including three main phases - calibration, transition, and restored grassland in north-central Oklahoma, USA. Results showed that the root zone soil water storage on average increased 1.6 and 1.9 times for restored prairie and switchgrass, respectively, after juniper removal. The regression model estimation based on the relationships established in the calibration phases between event-based runoff from control watershed and the treatment watersheds found that the restored prairie and switchgrass production system increased annual runoff by 4.46 and 4.54 times, respectively. These results indicated that both soil moisture and runoff are highly responsive to land use change in the southern Great Plains. Reestablishment of herbaceous dominance by mechanically removing encroached woody species is closely followed by restoration of soil moisture dynamics and watershed runoff regime. [ABSTRACT FROM AUTHOR]