Water management in irrigated rice - Rice Knowledge Bank ...

fertilized vegetables were included in the croppingsystem (Bouman et al 2002). In traditionalrice systems, relatively few herbicides are used aspuddling, transplanting, and the ponding of waterare effective weed control measures. Mean biocideuse in irrigated rice varies from some 0.4 kg activeingredients (a.i.) ha −1 in Tamil Nadu, India, to 3.8kg a.i. ha −1 in Zhejiang Province, China (Bouman atal 2002). In the warm and humid conditions of thetropics, volatilization is a major process of biocideloss, especially when biocides are applied on thesurface of water or on wet soil (Sethunathan andSiddaramappa 1978). The relatively high temperaturesfurther favor rapid transformation of the remainingbiocides by (photo)chemical and microbialdegradation, but little is known about the toxicity ofthe residues. In case studies in the Philippines, meanbiocide concentrations in groundwater underneathirrigated rice-based cropping systems were one totwo orders of magnitude below the single (0.1 µgL −1 ) and multiple (0.5 µg L −1 ) biocide limits for safedrinking water, although temporary peak concentrationsof 1.14−4.17 µg L −1 were measured (Boumanet al 2002). As for nitrogen, however, biocidesand their residues may be directly transferred toopen water bodies through drainage water flowingoverland out of rice fields. The potential for waterpollution by biocides is greatly affected by fieldwater management. Different water regimes resultin different pest and weed populations and densities,which farmers may combat with different amountsand types of biocides. Residual biocides interactdifferently with soil under different water regimes(Sethunathan and Siddaramappa 1978).4.3 Effects of water scarcityWater scarcity affects not only the ability of ricefields to produce food but also the environmentand the other ecosystem services of rice fields.Increasing water scarcity is expected to shift riceproduction to more water-abundant delta areas,and to lead to less flooded conditions in rice fieldsand to the introduction of upland crops that do notrequire flooding. These changes will have environmentalconsequences and will affect the traditionalecosystem services of the rice landscape.Rice that is not permanently flooded tendsto have more weed growth and a broader weedspectrum than rice that is permanently flooded(Mortimer and Hill 1999). It is expected that watershortages will lead to more frequent use of herbicides,which may increase the environmental loadof herbicide residues. With less water, the numbersand types of pests and predators (e.g., spiders) maychange as well as predator-pest relationships. Thepossible shift in the use of pesticides by farmersin response to these changes, and what this meansfor the environment, is as yet unknown. Moreleaching of nitrate is expected with increased soilaeration (either with growing rice under nonfloodedconditions, or with the shift to upland crops) thanunder flooded conditions. Less methane emissionsare expected under aerobic conditions than underflooded conditions, but higher nitrous oxide emissionsare expected (Bronson et al 1997a,b). However,the relative emissions of these greenhouse gases varywith environment and management practices (Dittertet al 2002). Flooded rice is effective in leachingaccumulated salts from the soil profile, and thechange to more aerobic conditions may result inincreased salinization.There is little information on how waterscarcity will affect the ecosystem services of ricelands listed in Chapter 4.1. There is a growing recognitionthroughout the rice-growing world that abetter understanding of the ecosystem services ofthe rice environment is needed. Although somemethodologies exist to measure and estimate differentservices of agricultural systems, quantifyingand valuing the positive and negative externalitiesstill presents a major challenge. In many countries,relevant data at the appropriate geographic levelare not available.37