A novel physical model demonstrating critical zone structure and flow processes in headwaters for teaching and research purposes

Critical zone structures, such as vegetation, topography, soil, and bedrock, are first-order controls that shape runoff generation in headwater catchments. However, the critical zone beneath our feet, where the subsurface water occurs, is invisible, and until recently, it is still not a completely clear world (Grant & Dietrich, 2017). As the underlying surface of a watershed has huge heterogeneity at different scales, it is difficult to characterize the subsurface flow path of transient runoff (Weiler et al., 2006). Hence, it is almost impossible to reproduce runoff formation in critical zones in classroom teaching and short-term field trips. Contrary to field research, the physical model is a practical means for theoretical verification and law discovery by reducing spatial heterogeneity (Etkina, 2002). In particular, the models with fully controlled terrain and soil properties may have great potential to deepen our understanding of the critical zone. Soil troughs (with variable slopes) have been widely used to investigate hydrological processes. For example, the slope-variable soil trough at Hohai University is crucial for studying the effects of vegetation cover on runoff response. In contrast to such soil troughs, the landscape evolution observatory (LEO) is favoured by scholars because it provides convergent topography data. However, the topography adopted in theoretical studies has always been greatly simplified (Liu et al., 2016). Numerous studies have also shown, through in laboratory experiments, that erosion of backfilled soil, caused by rainfall and overland flow, is a very common phenomenon (Gevaert et al., 2014). This challenge inhibits physical models from employing variable and desired morphologies that reflect the complex characteristics of critical zones. To further facilitate education and research on the role of critical zones, we built a physical model with complex terrain (Shen et al., 2021). The key scope of the model was to abandon the traditional backfilled soil and seek a suitable permeable material. Currently, permeable bricks made of fine aggregate (sand), coarse aggregate (gravel), and cement have been widely used for pavements to improve infiltration in the field of low impact development (LID). Meanwhile, the permeable materials also have good compressive strength and bending strength. Generally, their maximum water holding capacity (approximately 13%) is much smaller than that of clay or loam. The saturated hydraulic conductivity (Ks) is more than one order of magnitude higher (Tang et al., 2019) because their principal particle components are far coarser than those of natural soils. According to the suggested ratios, we used aggregates that accounted for about 70% of total volume of concrete (Cai et al., 2018) and water cement ratio by weight ranging from 0.3 to 0.4 (Rahmani et al., 2020). However, the fine aggregate ratio was increased to approximately 0.8 to reduce permeability. Three cases with different aggregate cement ratios (1.83, 1.61 and 1.43, respectively; Table 1) were tested by altering the bulk density of the mixed material, ranging from 1.4 to 2.4 g/cm. Ks was selected as the only hydraulic indicator because it plays a key role in the seepage process. Permeabilities of the test materials closely match that of the natural soil, while the field capacity (FC) of the Received: 8 March 2021 Revised: 23 May 2021 Accepted: 26 May 2021