Quantifying the impact of soil and climate variability on rainfed rice production

Methods and sampling strategies for measurement of soil hydraulic functions in puddled and non-puddled rice soils are discussed. A method was developed to measure in situ water percolation rate and soil water pressure head gradients in puddled soils. The number of measurements needed to estimate field-average infiltration rates was determined using a sequential t -test. It was concluded that sampling strategies for soil hydraulic functions and the choice of spatial interpolation techniques are best based on preliminary simulation and sensitivity analyses of the impact of soil heterogeneity and weather variability on yield variability and associated cost/benefit calculations. Several possibilities for water saving in rice-based cropping systems are discussed. Field and laboratory experiments indicated that the use of a pre-tillage operation before reflooding a dry, cracked rice field may allow earlier transplanting and reduce the risk of late drought. Maintaining shallow depths of ponded water and construction of extra bunds after puddling the field, close to bunds that are left undisturbed from year to year, may also reduce water use. Drought stress responses of two lowland rice cultivars to temporary drought at different growth stages were studied. Drought in the vegetative phase delayed flowering and maturity. Drought in the reproductive phase resulted in large yield reductions. The following morphological and physiological responses to soil moisture content were quantified: (1) rate of leaf production, (2) rate of leaf rolling, (3) rate of senescence and (4) relative transpiration rate. A simulation model for rainfed rice growth was developed, validated and used to estimate regional rice yield losses due to drought in a province of the Philippines, as a function of soil and climate variability.The results of this study are discussed in relation to the increasing use of simulation models in rice growing countries for estimation of potential and water limited yields, priority setting in research and extrapolation of new technologies.