Contents & Foreword, Characterizing And ... - IRRI books

Modeling water availability,crop growth, and yieldof rainfed lowland ricegenotypes in northeast ThailandS. Fukai, J. Basnayake, and M. CooperCharacterizing and understanding the nature of the limitations imposed byrainfed lowland environments within a target region are important for developingan efficient rice breeding program. Major factors determining the environmentas it influences plant performance are soil properties and wateravailability. Rainfall is a major determinant of the water environment, but itsseasonal variability is high in northeast Thailand. Simulation models are usefulin determining many features of the paddy water environment, as theycan readily estimate water availability as a probability function by using pastrainfall patterns as inputs. The RLRice model was developed for simulatingthe paddy water balance and growth of cultivar KDML105 for the rainfedlowlands in Thailand.The model was used to quantify the water balance of paddies of theseveral locations used in the rainfed lowland rice breeding program in northeastThailand. Yearly variation in simulated yield at any location was relatedto variation in rainfall during crop growth. However, sensitivity analysis resultsshowed that simulated yield varied greatly with changes in componentsof the water balance, particularly the deep percolation rate, lateral watermovement, and initial water level at transplanting. Simulated yield was generallyassociated with the time of disappearance of water relative to flowering,and the depth of the free water level at flowering. The simulation resultsindicate a strong interaction of genotype and water environment through variationin water availability in relation to phenological development of each genotype.Progress in breeding for high-yielding cultivars for rainfed lowland rice is slow inmany countries, partly because genotypic ranking for yield varies greatly in differentenvironments as a result of large genotype-by-environment (G × E) interactions forgrain yield. Often, the G × E interaction variance for yield is several times greaterthan the genotype variance alone in rainfed lowland rice (Cooper et al 1999a). Thelarge interaction for yield is related to the large environmental variation within theModeling water availability, crop growth, and yield . . . 111