7/31/2019 RT Vol. 8, No. 1 How much water does rice use

1/2

by Bas Bouman

28 Rice TodayJanuary-March 2009

How much water

does rice use?

Many people ask thequestion, How muchwater does it take toproduce 1 kg of rice?

The answer to this question lies inthe denition of water use and ofrice. We can identify three types ofwater usethrough transpiration,evaporation, and a combinationof seepage and percolationat,respectively, three scales of ricetheplant, the crop, and the eld.

The rice plantuses waterthrough the process of transpiration,which cools the plant and drivesthe upward sap owwhich carriesessentialnutrientsfromroots to leaves.This is a realwater use, sinceonce the plant hastaken up water

and released it tothe atmospherethroughtranspiration,that amountof water is nolonger availablefor reuse by thatsame plant inthe same growthcycle. Transpiredwater enters the

global 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 felds, water is oftenponded to ensure there is plentyfor the crop to take up. Besides

evapotranspiration, outows ofwater from a eld occur throughseepage and percolation: sidewardand downward water ows throughthe soil and bunds out of the eld.For an individual farmer, these arereal losses as well, and she considersthe total combined outows byevapotranspiration, seepage, andpercolation as water use by her riceeld (see gure). The farmer needsto ensure sufcient irrigation (tocomplement rainwater if rainfallis insufcient) to match all theseoutows. At a larger spatial scale,however, seepage and percolation

Rice Todayexamines this often-asked

(and often poorly answered) question

S

Bund

I R T

E

O

O

S

Puddledsoil

C PSubsurface soil

Plow layer

Groundwater

WaTER balanCE o pudded rice ed: C = cpiry rise; E = evportio; I = irrigtio; O = over-ud fow; P = percotio; R = ri;S = seepge; T = trspirtio.

Floodwater

JoseRaymondPanaligan

7/31/2019 RT Vol. 8, No. 1 How much water does rice use

2/2

1Haefele SM, Siopongco JDLC, Boling AA, Bouman BAM, Tuong TP. 2008. Transpiration efciency of rice (Oryza sativa L.). Field Crops Research.(In press.)

2Mom R. 2007. A high spatial resolution analysis of the water footprint of global rice consumption. Master thesis, University of Twente, Enschede,Netherlands.

3Falkenmark M, Rockstrm J. 2004. Balancing water for humans and nature: the new approach in ecohydrology. Earthscan, London, UK. 247 p.

4Chapagain AK, Hoekstra AY. 2004. Water footprint of nations. Value of water research report series No. 16. Delft (Netherlands): UNESCO-IHE. 76 p

5Zwart SJ, Bastiaansen WGM. 2004. Review of measured crop water productivity values for irrigated wheat, rice, cotton and maize. Agric. WaterManagement 69:115-133.

ows from one eld enter thegroundwater or creeks and drains,from where other farmers mayreuse the water to irrigate otherelds. This is in contrast to thewater losses by evapotranspiration,which cannot be recaptured.

Rice plant water use(by transpiration)Pot experiments and greenhousestudies carried out at theInternational Rice ResearchInstitute (IRRI) have shown thatrice plants growing under a rangeof water applications transpired5001,000 liters of water to produce1 kg of rough (unmilled) rice.

1This

is at the high end of comparablecereals such as wheat and barley.

Rice crop water use (byevapotranspiration)The estimated water use byevapotranspiration of all rice eldsin the world is some 859 cubickilometers per year.

2With a global

rough rice production of around600 million tons, it takes an averageof 1,432 liters of evapotranspiredwater to produce 1 kg of roughrice. This is roughly the same asthe world-average water use ofwheat, but higher than that ofmaize and barley (see Table 1).

The variability in water use

by evapotranspiration by ricecrops is large. Table 2 summarizesexperimental data from well-managed lowland eld experimentswith rice.

By comparison with total globalwater use, Table 3 puts the worldrice water use by evapotranspirationinto perspective. Producing theworlds rice accounts for 1213%of the amount of evapotranspired

water used to produce all of theworlds food (food crops andgrass and fodder for livestock).

Rice feld water use (to accountor evapotranspiration plusseepage and percolation)On average, about 2,500 litersof water need to be supplied (byrainfall and/or irrigation) to a riceeld to produce 1 kg of rough rice.These 2,500 liters account for allthe outows of evapotranspiration,seepage, and percolation. Thisaverage number is derived from

a large number ofexperimental data atthe individual eldlevel across Asia.Variability is large,ranging from around800 liters to morethan 5,000 liters. Thisvariability is causedby crop management

(such as varietyplanted, fertilization

Rice TodayJanuary-March 2009 29

Table 1. World-average water use by evapotranspiration of major

nonrice grain crops (liters of water per kg of grain).

Source Wheat Maize Barley

Falkenmark and Rockstrm, 20043

1,480 1,150 1,000

Chapagain and Hoekstra, 20044

1,300 900

Table 2. Liters of evapotranspired water needed to produce 1 kg of

rough rice.

source mnu meu maxu

Zwart an Bataanen, 20045

625 909 1,667

Table 3. Total global water use (cubic kilometers of

water per year).

source Chapagan

an Hoektra,

20044

Falkenark an

Rocktr, 20043

Total 7,450 8,160

Foo 6,390 7,200

inutry 716 780

doetc 344 180

regime used, and pest and diseasecontrols adopted), weather, andsoil properties. At the eld level,water inputs to rice elds are 23times those of other major cereals.

Although rices water productivityin terms of evapotranspirationis similar to that of comparable

cereals such as wheat, rice requiresmore water at the eld level thanother grain crops because of highoutowsin the forms of seepageand percolationfrom the eld.However, because these outowscan often be captured and reuseddownstream, rices water-useefciency at the level of irrigationsystems (which comprise many elds)may be higher than that at the eldlevel. Nevertheless, around one-quarter to one-third of the worlds

developed freshwater resources areused to irrigate rice (which, it mustbe remembered, is the staple food foralmost half the worlds population).

Rice production must be viewedin the light of the emerging watercrisis, as climate-change-inducedshifts in rainfall patterns combinewith the diversion of irrigation waterfor urban and industrial uses. Asagricultural water scarcity increases,there is a growing need for water-saving technologies such as aerobicrice (varieties that grow well inunooded elds; seeHigh and dryon pages 28-33 ofRice Today Vol. 6,No. 4) and more efcient irrigationregimes such as alternate wetting anddrying (see The big squeeze on pages26-31 ofRice Today Vol. 7, No. 2).

Dr. Bouman is a senior waterscientist and head of the Crop and

Environmental Sciences Division atIRRI.