Unsaturated flow is an important factor that affects groundwater motion. Among various drainage models, the nonlinear Hillslope-storage Boussinesq (HSB) model has been commonly used to predict water flux along a slope. In this study, we improved this model by considering lateral flow in the unsaturated zone. Using modified van Genuchten functions, we analytically expressed the concept of equivalent propagation thickness in the vadose zone. This analytical expression was then incorporated into the HSB model to reflect two different stages of the drainage process and to simulate the hillslope drainage process more accurately. The model results indicated that lateral flow has significant effects in the unsaturated zone during the hillslope drainage process. Even in sandy aquifers, the amount of water contributed by the unsaturated zone is a key factor that enables a decrease in the water table during the middle and late stages of the process. A comparison between the measured and simulated results based on both convergent-type and divergent-type hillslope drainage processes revealed that the thickness of the saturated zone decreases as the unsaturated flow increases. This study emphasizes the necessity of considering unsaturated flow in the HSB model to improve the accuracy of predicting groundwater outflow rates and develop more accurate hydrographs. The concept of equivalent propagation thickness also provides a criterion for assessing the importance of unsaturated lateral flow for future drainage research.

Recession flow analysis is usually conducted to infer hydraulic parameters of hillslope aquifers. Various Boussinesq equation‐based models, both linear and nonlinear, have been used to analyze the recession curves for sloping aquifers, with a focus on the long‐time recession behavior. Based on a modified Boussinesq equation with capillarity incorporated, we demonstrate the significant effect of unsaturated flow on the recession curve, which result in three (instead of two) power law regimes with two transition points (instead of one) corresponding to the formation of a fully unsaturated zone at the adjacent area of the upslope boundary and across the whole domain, respectively. The results show that the power of the second and third recession regime is variable, depending on the slope angles, soil types, and hillslope geometries. The unsaturated flow effects also lead to the absence of drastic drop of −dQ/dt at the transition between the first and second regime, which was predicted by previous numerical models but has not been observed in the field or laboratory experiments. These findings have important implications for recession flow analysis in studies of hillslope aquifers.

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Carbon capture and storage (CCS) technology has been considered as an important method for reducing greenhouse gas emissions and for mitigating global climate change. Three primary options are being considered for large-scale storage of CO 2 in subsurface formations: oil and gas reservoirs, deep saline aquifers, and coal beds. There are very many large saline aquifers around the world, which could make a big contribution to mitigating global warming. However, we have much less understanding of saline aquifers than oil and gas reservoirs. Several mechanisms are involved in the storage of CO 2 in deep saline aquifers, but the ultimate goal of injection of CO 2 into the aquifers containing salt water is to dissolve the CO 2 in the water. So it is important to study the solubility trapping and sensitivity factors of CO 2 in saline aquifers. This paper presents results of modeling CO 2 storage in a saline aquifer using the commercial reservoir simulator ECLIPSE. The objective of this study was to better understand the CO 2 /brine phase behavior (PVT properties) and quantitatively estimate the most important CO 2 storage mechanism in brine-solubility trapping. This would provide a tool by performing theoretical and numerical studies that help to understand the feasibility of CO 2 geological storage. A 3-dimensional, 2-phase (water/gas) conceptional reservoir model used finite, homogenous and isothermal formations into which CO 2 is injected at a constant rate. The effects of main parameters were studied, including the vertical to horizontal permeability ratio k v /k h , salinity, and residual phase saturations. The results show that the vertical to horizontal permeability ratio has a signifi cant effect on CO 2 storage. Moreover, more CO 2 dissolves in the brine at lower k v /k h values.