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

and macro scales, rice environments are generally classified as ecosystemsand subecosystems, which are then characterized in terms of the prevailingbiophysical conditions using the key parameters and at the lower scales. Forexample, at the micro level, the analysis is simply a more detailed descriptionof the same, or similar, biophysical parameters. Such an analysis conveysthe same output (more detailed interpretation at the micro level) when,in fact, at different levels, it is expected to fulfill different objectives.Some important parameters that are often overlookedThe following parameters are often overlooked in characterization.● Groundwater table information. The groundwater information needed to determinethe nature of water fluctuation, to interpret the duration and severityof drought, and to understand the groundwater contribution by capillary riseand its simulation is missing in most of the characterization studies reviewedin this chapter. This is probably because of the difficulty in gathering theinformation, which, by its own nature, is highly variable over time and heterogeneousover space. However, rice yields are very sensitive to groundwatertable depth, especially when it fluctuates within 1 m from the soilsurface (Wopereis 1993). Exclusion of this term may result in a 30% underestimationof yields at 4 t ha –1 and a 90% underestimation at 1 t ha –1 (Boltonand Zandstra 1981).● Surface hydrology. The information on sustained surface water depth, whichis highly dynamic, is also sparse in most of the studies reviewed in thischapter, probably for the same reason: the difficulty in gathering this information.Rice yields are also sensitive to surface flooding if it occurs at theactive tillering stage and the surface water depth patterns determine, to agreater extent, both the nature and severity of effects of hydrological stresson the crop and crop management practices.● Seepage and percolation (S&P) rates. These are the main components ofwater balance and they strongly influence the presence or disappearance ofsurface water. In drought-prone areas, one of the reasons for frequent waterstress is high S&P rates, and this affects yields significantly. Using cropsimulation, Fukai et al (1995) estimated that a 2 mm d –1 reduction in S&P,from 6 to 4 mm d –1 , would increase rice yields by more than 60% in Thailand.● Soil fertility mapping. Mapping units delineated in soil maps show polygonsof soil type/series and are based on soil genesis and classification. But existingsoil maps explain only 0–27% of the variances for the soil fertility parametersthat directly influence yield (Oberthur et al 1995). To be more relevantto increasing rice yields, technology development should focus onparameters that may limit yield (macro- and micronutrients, cation exchangecapacity, texture).● Biotic stress profile. Biotic stresses (weeds, insect pests, diseases) are asimportant in rainfed systems as in the irrigated one. Savary et al (1997) showedCharacterizing rainfed rice environments: an overview . . . 27