Methodology to Assess the Effects of Rice Cultivation Under Flooded Conditions on van Genuchten’s Model Parameters and Pore Size Distribution

Abstract

The effects of rice (Oryza sativa L.) cultivation under flooded conditions on soil's physical-hydraulic properties were studied in this article, using a new methodology based on a combined analysis on soil water retention curve (WRC) and pore size distribution (PSD). WRC analysis was carried out through the changes of van Genuchten's model parameters, the characteristics of WRC at the inflection point, and the specific water capacity curve. Analysis of PSD was performed on the volume changes of porosity fractions through a detailed pore size classification, while different pore size classifications based on their hydraulic and structural characteristics were also used. The methodology was applied using a small dataset obtained from fine-textured Entisol soils which were subjected to rice cultivation under flooded conditions in Axios River plain (Northern Greece). Measurements of WRC were obtained at four depths of the soil profile from two fields, before and after the growing season of rice. The analysis indicated that the van Genuchten's model parameters (θ s, a, and n) and the WRC characteristics at the inflection point (pressure head h i, pore equivalent diameter D i, and slope S i) significantly changed after the growing season following similar patterns, along the soil profile in both fields. The parameters θ s, a, D i, and S i were decreased, while n and h i were increased. The h i and a were the most sensitive parameters, while the values of (h i and 1/a) in each layer before and after the growing season for each field were linearly correlated and shifted to higher values because of compaction, indicating that it could be applied as a tool to evaluate the degree of soil compaction to similarly textured soils. The peaks of the specific water capacity curves were compressed (lower values of slope S i) and shifted to lower water potentials (h i) that corresponded to pores of equivalent diameter D i between 2 and 6 μm. The soils had few structural pores (>9 μm) and low air-filled porosity (>30 μm) before the growing season, which presented accessory reduction after the growing season in both fields. Total porosity was reduced at the expense of structural porosity along the soil profile, while the pore size class of 5-3 μm was identified as the threshold where the smaller pores' volume started to increase in all layers of both fields. The results indicated that the changes in the WRC and the PSD follow specific trends, which can be used in future studies to model temporal variability of soil's physical-hydraulic properties

New Equations for the Determination of Soil Saturated Hydraulic Conductivity Using the Van Genuchten Model Parameters and Effective Porosity

“…Startsev and McNabb () and Aschonitis et al . () showed that the changes in soil physical properties can easily be detected by the most sensitive parameters of the van Genuchten model and porosity attributes. These are: the a parameter, the slope S i of the water retention curve at the inflection point and the effective porosity Φ e .…”

“…The absolute value of S i has been considered a significant indicator of soil physical conditions concerning the degree of soil compaction (Dexter, ; Han et al ., ; Guimarães Santos et al ., ). The slope S i (cm ‐1 ) of θ ( h ) at the inflection point is given by (Aschonitis et al ., ): …”

Modeling the effects of different irrigation water salinity on soil water movement, uptake and multicomponent solute transport

“…Cores of 5 cm height and 5 cm diameter were collected in thin-walled metal rings using sampling drill. The soil cores were wetted from below to saturation and the weights, before and after drying at 105 o C, were measured to determine the gravimetric saturated water content (Aschonitis et al, 2012). The bulk densities were measured at the same cores on dry weight.…”

Analysis of Temporal Variation of Soil Salinity during the Growing Season in a Flooded Rice Field of Thessaloniki Plain-Greece

“…Rice is being cultivated in the region for more than 50 years. A rotation program with three consecutive years of rice crop interrupted by one or two years of maize, fodder, or cotton, is the common practice in the area [5,10]. The soils in the region are mostly silty clay, poorly drained, classified as Typic Xerofluvents [42] under Mediterranean climate conditions, according to Soil Survey Staff [43].…”

“…Long-term submerged soils undergo physical changes which lead to the formation of a highly compacted layer below the root-zone [10], called "plow sole layer" and is considered as the major factor controlling infiltration rate and drainage in paddy rice fields [11][12][13][14]. Furthermore, infiltration can potentially be blocked by air entrapment due to surge flooding and shallow water table [15].…”