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

DiscussionThis simulation work shows the importance of maintaining standing water for a longtime period during the reproductive to grain-filling stages. This confirms the experimentalresults in northeast Thailand reported by Jearakongman et al (1995). Twoindicators can be used to estimate overall water availability and grain yield in a plantbreeding program: time of disappearance of standing water relative to flowering timeand free water level at flowering. The latter can be more readily determined if there isstanding water at flowering, but this may not be the case if standing water has alreadydisappeared by flowering. Recording the date of disappearance of standing water anddates of flowering of different genotypes should help characterize the water conditionsof each selection trial.The water balance in rainfed lowlands is complex and the time of disappearanceof standing water relative to flowering is determined by several factors, as demonstratedin the simulation studies here. For a given hydrological and agronomic condition,rainfall between transplanting and flowering affects the relative time of disappearanceof standing water. However, deep percolation rate, lateral water movement,and, to a lesser extent, initial water level at transplanting all affect water balance andhence grain yield. Sensitivity analyses of the influence of these factors on grain yield(Table 1) show interactions among these factors as well as with rainfall. These complexrelationships would affect the water balance of a lowland field used by a breedingprogram. Among these interacting factors, rainfall can be determined accuratelyand initial water level can be readily estimated if there is standing water at transplanting.The deep percolation rate and lateral water movement are important componentsin determining overall water balance, and a combined rate can be determined readilyby monitoring the field water level and using the estimated value of the evapotranspirationrate. A separate estimation of the deep percolation rate and lateral water movementwould make a more accurate estimation of water balance possible. This may beparticularly important in a toposequence where the lateral water movement componentis expected to be large.Lateral water movement is a difficult component of the water balance to determinebecause it is not constant throughout the growth period. The model estimatesthe net amount of lateral water movement from the coefficient C L , the amount ofrainfall, and soil water level, thus assuming that lateral water movement takes placeimmediately after a large rainfall event and only when the soil is saturated with water.The value of the coefficient may range from 0.5 to 1.5 as used in the sensitivityanalysis in the present work, as this would cause about 30 d of difference in the dateof disappearance of standing water in a toposequence. This difference is not uncommonin rainfed lowlands (Wade et al 1999b) and cultivars with different maturitygroups are often planted at different toposequence positions (Nesbitt and Phaloeun1997). The development of a method to determine lateral water movement appearsimportant for the characterization of rainfed lowland environments. Water loss throughunder-bund percolation may also be important in determining the water balance, andmay need to be considered in the model (Tuong et al 1994).Modeling water availability, crop growth, and yield . . . 127