## 1 Introduction  
Sanitary landfills (SLFs) are designed, constructed, operated, and maintained disposal facilities with control over the potential environmental impacts of their development and operation [1]. Landfilling was practiced for centuries, and initially, it was believed that the soil purified the leachate from the waste; however, several studies have shown that unregulated SLFs contaminate the groundwater [1]. Thus, SLFs are lined with impermeable layers to prevent the infiltration of harmful substances that come from the degradation of solid wastes into the surrounding soil. Furthermore, impermeable SLF covers prevent the increase in leachate volume due to the inflow of surrounding water into the SLF. Hence,proper design of a SLF cover is imperative to avoid large-scale environmental pollution due to improper practices, such as improper management of secondary leachate, which would lead to leachate leakage [2]. The low-permeability cover system may be composed of compacted clay, geomembranes, geotextiles, geosynthetic clay, geonets, or a combination of these materials. Among these materials, clay is the most abundant and most accessible. The effectiveness of these cover layers depends heavily on their hydraulic conductivity, which governs the movement of water and contaminants through the soil. The required hydraulic conductivity of a final cover in the United States is less than or equal to the hydraulic conductivity of the bottom liner or not greater than \(10^{-5}\) cm/s, whichever is lower [3]. Despite advancements in technologies, there remain challenges in ensuring the durability of clay covers and functionality over time, particularly under varying environmental conditions, such as wet-dry cycles. Hence, to establish complete flow models for unsaturated soils, it is imperative to analyze the water retention behavior together with the unsaturated hydraulic conductivity [4; 5]. In unsaturated soil, water is subjected to matric suction due to the physical affinity between the soil particle surfaces, capillary pores, and water [6]. In addition, uniform matric suction indicates that the soil is at equilibrium; hence, no moving force acts on the soil. However, if the matric suction is not uniform, water will flow spontaneously to where the matric suction is higher to equilibrate the potential. The pores of saturated soils are filled with water; thus, the water phase in this state is continuous, and the conductivity is at its maximum [6]. On the other hand, pores are filled with air in unsaturated soils; hence, the conductive portion of the soil's cross-sectional area diminishes [4]. Thus, the matric suction may be analyzed using the corresponding moisture content of the soil specimens. This analysis determines the water retention curve (WRC) or the relationship between matric suction and moisture content. The WRC shows the ability of the soil to maintain its stability relative to the change in matric suction [7]. It is also a measure of the amount of water stored in the pores of the soil at a given suction, and it describes the mechanical behavior of unsaturated soil [7; 8]. The WRC consists of a nearly horizontal line up to the air entry value (AEV), a rapid decrease in moisture content from AEV to the residual suction value (RSV), and a decrease from RSV to completely dry soil [9]. Stage I evaporation or the boundary effect zone is the stage wherein the soil pores are nearly saturated, and the change in moisture content is independent of soil suction up to the AEV [9]. AEV is the start of Stage II evaporation, wherein air enters the largest pores of the soil; on the other hand, the transition zone, or Stage III evaporation, is the stage when water migrates through the capillary network of soil [9]. RSV is the start of Stage IV evaporation, wherein the water phase in the soil becomes discontinuous and the pores begin to disconnect due to occluded air, while the residual zone or Stage V evaporation occurs under large suction values required to remove adsorbed water [9].