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

Fig. 1.2. Water balance of a lowland rice field. C = capillary rise, E = evaporation, I = irrigation, O = overbund flow, P =percolation, R = rainfall, S = seepage, T = transpiration.pore volume, and to a large increase in micropores(Moorman and van Breemen 1978). A typicalvertical cross-section through a puddled rice fieldshows a layer of 0–10 cm of ponded water, a puddled,muddy topsoil of 10–20 cm, a plow pan thatis formed by decades or centuries of puddling, andan undisturbed subsoil (Fig. 1.2). Rice roots areusually contained within the puddled layer and aretherefore quite shallow. The plow pan reduces thehydraulic conductivity and percolation rate of ricefields dramatically.Because of its flooded nature, the rice field hasa water balance that is different from that of drylandcrops such as wheat or maize. The water balanceof a rice field consists of the inflows by irrigation,rainfall, and capillary rise, and the outflows bytranspiration, evaporation, overbund flow, seepage,and percolation (Fig. 1.2). Capillary rise is theupward movement of water from the groundwatertable. In nonflooded (aerobic) soil, this capillaryrise may move into the root zone and provide a cropwith extra water. However, in flooded rice fields,there is a continuous downward flow of water fromthe puddled layer to below the plow pan (called“percolation”; see below) that basically preventscapillary rise into the root zone. Therefore, capillaryrise is usually neglected in the water balanceof rice fields.Before the crop actually starts growing, waterinput is already needed for wet land preparation.After puddling, the field is usually left fallow andflooded for a few days (or 1 to 4 weeks) before theseedlings are transplanted. The amount of waterused for wet land preparation can be as low as100–150 mm when the turnaround time betweensoaking and transplanting is a few days only orwhen the crop is direct wet seeded. However, inlarge-scale irrigation systems that have poor watercontrol, the turnaround time between soaking andtransplanting can go up to 2 months and water inputsduring this period can reach 940 mm (Tabbalet al 2002). After crop establishment, the soil isusually kept ponded with a 5–10-cm layer of wateruntil 1–2 weeks before harvest. During both theturnaround time and the crop growth period, wateroutflows are by overbund runoff, evaporation, seepage,and percolation. During crop growth, wateralso leaves the rice field by transpiration. Of allwater outflows, runoff, evaporation, seepage, andpercolation are nonproductive water flows and areconsidered losses from the field. Only transpirationis a productive water flow as it contributes to cropgrowth and development.When rainfall raises the level of ponded waterabove the height of bunds, excess rain leaves therice field as surface runoff or overbund flow. Thissurface runoff can flow into a neighboring field,but, in a sequence of fields, neighboring fields willpass on the runoff until it is lost in a drain, creek,or ditch.Evaporation leaves the rice field directly fromthe ponded water layer. Transpiration by rice plants