In turn, FT cycles also influence these hydrological variables. Understanding these cycles in cold regions is crucial owing to their vital roles in agricultural and ecological environments, because they have profound influences on soil water distribution, heat balance, and spring farming and plant germination 3. To enhance the collective knowledge of soil FT processes, a more comprehensive monitoring regime of hydrological variables, such as soil water content ( SWC), soil salt content, soil temperature, frost depth, groundwater levels, are required. Soil freeze–thaw ( FT) cycles is a constantly repeating process of material exchange and energy transfer that occurs in the surface soil and also extends downward into the deeper soil layers 1, 2. Landscapes also affected soil water and salt migrations during FT processes, with the trend being AS land > LT grassland > farmland. Therefore, the FT processes contributed to the surface soil salinisation and alkalisation. The freeze-induced upward redistribution and enrichment of soil water and salt caused the rise and expansion of the soil salification layer, which was the main source of explosive accumulations of surface salt in springtime. Results indicated that the strongest freezing process occurred in AS land, which was characterised by the deepest frost depth (165 cm) and highest freezing rate (3.58 cm/d), followed by LT grassland, and then farmland. The spatiotemporal distributions of soil water and salt, frozen depths and soil temperatures were examined at depths of 0–200 cm in three typical landscapes (farmland, Leymus chinensis ( Trin.) Tzvel ( LT) grassland and alkali-spot ( AS) land) from October 2015 to June 2016. This study aimed to explore the soil FT characteristics and their influences on soil water and salt migrations to clarify the underlying mechanism of the springtime soil salinisation in the western Songnen Plain, China. Therefore, understanding the mechanisms behind soil salinisation during winter and spring is crucial for management strategies effectively alleviating this. We will show how that metrics can be used in a very efficient way for representing soil heterogeneities in simulation models.Seasonally freeze-thaw ( FT) processes affect soil salinisation in cold and arid regions. The metrics provided a very efficient tool to quantify the observed behaviour, depending on depth and soil heterogeneity: Different soils differed primarily with respect to the extent of damping per depth interval rather than to the kind of damping.
In all cases, the intrinsic dimensionality in fact was very close to unity, confirming our hypothesis.
SOIL TEXTURE HYDROLOGICAL PROCESSES SERIES
In a last step, the same approaches were applied to 55 time series of observed soil water content from 15 sites and different depths. In a second step, model outputs for heterogeneous soils were analysed. The latter provided a metrics for the extent of transformation ("damping") of the input signal. The intrinsic dimensionality of that matrix was assessed using the Correlation Dimension and a non-linear principal component approach. A matrix of time series of soil matrix potential and soil water content at 10 cm depth intervals was set up. In a first step, we analysed the output of a soil hydrological model, based on the Richards equation, for homogeneous soils down to 5 m depth for different soil textures. Different soils differ only with respect to the extent of transformation of input signals. That means, the way how the soil transforms any hydrological input signals is the same for different soil textures and structures. We hypothesize that the intrinsic dimensionality of soil hydrological processes, which is induced by spatial heterogeneities, actually is very low and that soil hydrological processes in heterogeneous soils follow approximately the same trajectory. In practice, however, data are hardly available at the required spatial resolution, and accounting for observed heterogeneities of soil and aquifer structure renders models very time and CPU consuming. Soils and aquifers usually exhibit substantial spatial heterogeneities at different scales that can, in principle, be represented by corresponding parameterisations of the models.
Thus, the Darcy or the Richards equation can be applied to model water fluxes in the saturated or vadose zone, respectively. Hydrology is based on the observation that catchments process input signals, e.g., precipitation, in a highly deterministic way.
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