How much water does rice need? - adron.sr

How much waterdoes rice use?by Bas BoumanRice Today examines this often-asked(and often poorly answered) questionJose Raymond PanaliganMany people ask thequestion, “How muchwater does it take toproduce 1 kg of rice?”The answer to this question lies inthe definition of “water use” and of“rice.” We can identify three types ofwater “use”—through transpiration,evaporation, and a combinationof seepage and percolation—at,respectively, three scales of rice—theplant, the crop, and the field.The rice plant “uses” waterthrough the process of transpiration,which cools the plant and drivesthe upward sap flow—which carriesessentialnutrients—fromroots to leaves.This is a “real”water use, sinceonce the plant hastaken up waterand released it tothe atmospherethroughtranspiration,that amountof water is nolonger availablefor reuse by thatsame plant inthe same growthcycle. Transpiredwater enters theSOglobal water cycle and will eventuallyreturn to the earth as rain or snow.The rice crop comprisesthe plants and underlying soil.Besides transpiration from theplants, water leaves the cropfrom the soil underneath throughevaporation. Like transpiration,evaporated water is “lost” andcannot be used again by that samecrop in the same growth cycle. Thiscombined water use by a rice cropis called “evapotranspiration.”In rice fields, water is oftenponded to ensure there is plentyfor the crop to take up. BesidesPlow layerI R TPuddledsoilSubsurface soilevapotranspiration, outflows ofwater from a field occur throughseepage and percolation: sidewardand downward water flows throughthe soil and bunds out of the field.For an individual farmer, these arereal losses as well, and she considersthe total combined outflows byevapotranspiration, seepage, andpercolation as water use by her ricefield (see figure). The farmer needsto ensure sufficient irrigation (tocomplement rainwater if rainfallis insufficient) to match all theseoutflows. At a larger spatial scale,however, seepage and percolationFloodwaterCPEBundSOGroundwaterWater balance of a puddled rice field: C = capillary rise; E = evaporation; I = irrigation; O = over-bund flow; P = percolation; R = rainfall;S = seepage; T = transpiration.28 Rice Today January-March 2009