Short-term effect of tillage methods, residue levels, and forward speeds on soil-water characteristic curve (SWCC): A case study on the eastern soils of Karun River, Khuzestan province, Iran.

The current study was conducted to investigate the effect of tillage methods, residue rates, and forward speeds on the soil-water characteristic curve (SWCC) of Haploustepts soil over the course of one crop year (2014-2015). The treatments consisted of conventional mechanized tillage (CT: moldboard plough+disc) and reduced tillage (RT1: chisel peker+plough and RT2: combined tillage), different surface residues, including three levels of no residue, 40% residue, and 80% residue, and forward speeds at three levels: low (4 km/h), normal (7 km/h), and high (10 km/h). The experimental water retention data were fitted to uni-modal van Genuchten (termed uni-modal vG) and bi-modal Dexter (termed bi-modal Dex) models. No significant impact was observed on different physical parameters, except for parameter n. In the slope at the inflection point of SWCC, 11.8% and 8.9% reductions were observed in CT and RT1 treatments, respectively, compared to RT2. Based on the results,  measured under CT tended to be higher than that of other tillage treatments. Residual covers and higher forward speeds tended to increase both  and . Changes in PSD were more pronounced in larger (macro) and medium (meso) pore diameter classes. The highest value of structural void ratio as transmission pores was observed in RT2. This finding indicates that with respect to , , , and  values, the soil PSD descriptive system is a bi-peak distribution such as H-L; therefore, due to the hierarchical nature of soil structure, van Genuchten equation cannot appropriately describe multi-modal soils inherently.