Hydrus-1D model for simulating water flow through paddy soils under alternate wetting and drying irrigation practice

Alternate wetting and drying (AWD) irrigation practice in paddy cultivation requires a safe soil moisture stress level in the root zone for which irrigation water savings can be maximized without compromising the yield. Determining safe soil moisture stress level in AWD practice experimentally is challenging and involves time and money. Mathematical models can be alternate tools for determining the safe soil moisture stress with least experimental data. In this study, field experiments were conducted on rice crop using thirty lysimeter plots during rabi seasons (January to April) of 2015-2016 and 2016-2017. Three irrigation treatments including irrigation under no-soil moisture stress, mild soil moisture stress (MS: 450 cm matric potential head) and severe soil moisture stress (SS: 700 cm matric potential head) were considered, and each treatment was replicated ten times. The ponding depth and matric potential head at 10, 40 and 70 cm soil depths were measured daily from all lysimeter plots. The measured field data were used to calibrate and validate Hydrus-1D model and simulate water flow under different soil moisture stress treatments. The water saving in MS and SS treatments was 23 and 27%, respectively. The deep percolation in MS and SS treatments was reduced by 27-30% and 30-35%, respectively. The safe ponding water depth required to minimize deep percolation loss in MS and SS treatments was 5 and 7 cm, respectively. The required ponding water depth was observed to be increased with the increased matric potential head.