22
A Note on Rice Irrigation
Estimating the irrigation requirement for
rice is exceptionally difficult. First, the ac-
tual evapotranspiration (ETa) for nearly all
the major crops is about 90 percent of the
reference crop of grass (ETP, in table B1),
but for rice, due mainly to land preparation
by flooding and the consequent exposed
surface of water, the ETa is about 110 per-
cent of grass. Thus, if the irrigated area of a
country is one-half rice, the country average
estimate is about right, but otherwise there
is a corresponding error. Unfortunately,
there are no international data on irrigated
area by crop, so adjustments for this factor
cannot be made. About 80 percent of the ir-
rigated area of Asia is in rice—so the error
could be significant, especially in Asia.
Second, an even more difficult problem
is that net evapotranspiration (NET) is not
the only—or, in many cases, not even the
most—important determinant of the irriga-
tion requirement for rice. Rice fields are kept
flooded primarily for weed control. This cre-
ates high percolation “losses” from the
fields. Thus in order to keep the fields
flooded, an amount of water that is several
times NET is often applied to the field. As
if this were not enough, many farmers also
like to have fresh water running through
their rice fields, rather than simply holding
stagnant water, in the belief that this in-
creases yield (and perhaps taste). There is no
scientific evidence for this belief except that
during very hot days running water may
beneficially cool the plant. On the other
hand, this practice flushes fertilizers out of
the rice fields and contributes to water pol-
lution. Whatever the reason, this common
practice leads to very high withdrawals of
water for rice irrigation—and, even with re-
cycling, a considerable amount of mismatch-
ing between water supply and demand.
Technological and managerial advances
in rice irrigation, especially with the use of
herbicides, have created the potential for ir-
rigating rice at much higher effectiveness;
but the problem lies in convincing farmers
to adopt these new methods.
Also, in light of recycling, one wonders
how the water withdrawals for irrigation
are actually estimated in the WRI database.
If the estimated “withdrawals” for irrigation
in a country are based on a field irrigation
requirement for rice that is several times
NET for the gross irrigated area in rice,
which may in fact be the case, this could
lead to a serious overestimation of actual
net withdrawals of water for irrigation in
the country. If this overestimation
possibility is true (and we suspect it is),
then the imputed ineffectiveness of irrigated
agriculture and hence the potential for
water savings in rice-intensive countries are
not as large as the data would indicate.
Of course, this same recycling effect may be
true for other crops as well, but the
magnitude of the error would not be
nearly so great. Clearly, this is an important
area for further research into the data set.
In the meantime, the calculations of
potential water savings from the irrigation
sector, especially in countries that have a
high percentage of their area in rice, must
be treated cautiously. Water requirements
for crops should be made on the basis
of NET, in the first approximation, with
the difference between this and the
irrigation requirement considered in light
of recycling within the basin. Perhaps the
best way to regard this problem is by
saying that countries with intensive rice
irrigation may have high potential for
transferring water from agriculture, if rice
irrigation is, in fact, highly ineffective from a
basin perspective.

32
1990 data First Season Second season Annual summary
Net Irr. Estima- Poten- NET Effec. Irr. Surplus Irr. % surp- Carry NET Irr. Surplus Ann Annual NET on
irrigated WITH ted tial effi. req. or inten- lus loss over to req. or irr gross irr. area
Country ID area depth crop assu. deficit sity between remai- deficit inten- Total Depth
(NIA) on NIA months Base = season ning sity
70% 50% season
(1,000 ha) km
3
m Months m % m m % m m m m m % km
3
m
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
World 245,067 2353
Countries 220,376 2086 1.37 10.6 0.2 69% 0.28 1.08 97% 50% 0.55 0.38 0.55 0.26 146% 892 0.28
% of total 90% 89%
Group 1 37,507 371 2.09 11.1 0.36 69% 0.52 1.53 99% 48% 0.79 0.65 0.94 0.40 138% 202 0.39
% of total 17% 18% 23%
Group 2 3,101 23 1.14 12 0.18 70% 0.26 0.88 95% 50% 0.45 0.41 0.58 0.19 148% 14 0.30
% of total 1% 1% 2%
Group 3 23,301 149 1.45 11 0.17 69% 0.24 1.21 97% 50% 0.62 0.43 0.62 0.25 156% 65 0.18
% of total 11% 7% 7%
Group 4 38,735 384 1.21 9 0.15 68% 0.22 0.99 98% 50% 0.50 0.22 0.32 0.25 144% 173 0.31
% of total 18% 18% 19%
Group 4 24,623 211 1.10 9 0.16 69% 0.23 0.87 95% 50% 0.44 0.17 0.26 0.30 130% 65 0.20
% of total 11% 10% 7%
China 47,965 463.39 0.97 10 0.18 60% 0.29 0.67 100% 50% 0.34 0.24 0.40 0.00 184% 181.2 0.21
% of total 22% 22% 20%
India 45,144 484.15 1.07 12 0.21 60% 0.35 0.72 100% 50% 0.36 0.48 0.81 0.00 145% 192.4 0.29
% of total 20% 23% 22%