Exploring options for water savings in lowland rice using a modelling approach

Water-saving irrigation regimes are needed to deal with a reduced availability of water for rice production. Two important water-saving technologies at field scale are alternately submerged-nonsubmerged (SNS) and flush irrigated (FI) rice. SNS allows dry periods between submerged soil conditions, whereas FI resembles the irrigation regime of an upland crop. The effects of these regimes on the water balance and water savings were compared with continuously submerged (CS) and rainfed (RF) regimes. The crop growth model ORYZA2000 was used to calculate seasonal water balances of CS, SNS, FI, and RF regimes for two locations: Tuanlin in Hubei province in China from 1999 to 2002 during summer seasons and Los Baños in the Philippines in 2002-2003 during dry seasons. The model was first parameterized for site-specific soil conditions and cultivar traits and then evaluated using a combination of statistical and visual comparisons of observed and simulated variables. ORYZA2000 accurately simulated the crop variables leaf area index, biomass, and yield, and the soil water balance variables field water level and soil water tension in the root zone. Next, a scenario study was done to analyse the effect of water regime, soil permeability, and groundwater table depth on irrigation requirement and associated rice yield. For this study historical weather data for both sites were used. Within seasons, the amount of irrigation water application was higher for CS than for any of the water-saving regimes. It was found that groundwater table depth strongly affected the water-yield relationship for the water-saving regimes. Rainfed rice did not lead to significant yield reductions at Tuanlin as long as the groundwater table depth was less than 20 cm. Simulations at Los Baños with a more drought tolerant cultivar showed that FI resulted in higher yields than RF thereby requiring only 420 mm of irrigation. The soil type determined the irrigation water requirement in CS and SNS regimes. A more permeable soil requires around 2000 mm of irrigation water whereas less permeable, heavy soil types require less than half of this amount. We conclude that water savings can be considerable when water regimes are adapted to soil characteristics and rainfall dynamics. To further optimize water-saving regimes in lowland rice, groundwater table dynamics and soil permeability should be taken into account.