Modelling soil hydrothermal regimes in pigeon pea under conservation agriculture using Hydrus-2D

Abstract

Limited information is available about the effects of conservation agriculture (CA) practices on soil temperature moderation and distribution of water in the soil profile during crop growth, particularly on the changes in the components of water and energy balance. Hence, the objective of this study was to assess soil hydrothermal regimes and water and energy balance components in pigeon pea (Cajanas cajan) grown under CA in a pigeon pea-wheat (Triticum aestivum) cropping system using the Hydrus-2D model. There were seven treatments: permanent broad bed (PBB), PBB with crop residue (PBB + R), permanent narrow bed (PNB), PNB with crop residue (PNB + R), zero tillage (ZT), ZT with crop residue (ZT + R), and conventional tillage (CT). Results during the seventh year of the experiment showed that the PBB, PBB + R, PNB, PNB + R and ZT + R treatments significantly reduced surface soil bulk density (BD), increased field saturated hydraulic conductivity (Ksat) and improved soil water retention over the CT. The Ksat values obtained as the output of the Rosetta-Lite model which is implemented in Hydrus-2D) were very low. Hence, experimentally measured Ksat values were optimized along with parameters α and n that were obtained as output of the Rosetta Lite model, through inverse modelling (IM). The model predicted daily changes in profile soil water content (SWC) with reasonable accuracy (R2 = 0.77, RMSE = 0.012; n = 84). Soil water balance simulated from the model indicated higher cumulative transpiration (CTr), lower cumulative evaporation (CE) and higher soil water retention in the PNB + R and PBB + R plots than CT. Computed values of thermal conductivity (λ) obtained from the observed soil temperature (ST) data at different SWC values showed significant correlations with those optimized through IM. In general, Hydrus-2D model over-predicted the ST values during a simulation period of 10 days. However, reasonably accurate (R2 = 0.91, RMSE = 1.41 °C; n = 102) predictions were observed for the 0–20 cm soil layer using the optimized values. It was also observed that PBB + R and PNB + R treatments improved soil hydrothermal regimes, root growth, radiation interception, leaf area index and biomass production of the pigeon pea crop. Hydrus-2D used in this study could also satisfactorily simulate the temporal changes in energy balance components, such as soil heat flux (G) and evaporative heat flux (LE). Hence, this model may be adopted for evaluating arable management practices for characterizing different components of water and energy balance in pigeon pea.