1. GCP 2011 General Research Meeting
21-25 September,2011
Hyderabad, India
Characterization of drought-prone
rainfed rice ecosystems of
the Mekong Region
Inthavong, T., Linwattana, G., Touch, V.,
Pantuwan, G, Jongdee, B., Mitchell,
J.H., and Fukai, S.
(NAFRI, BRRD, CARDI, UQ)

2. Objectives
1. Development of Soil Water Balance Model
and use it for quantifying field water
availability during the growing season
2. Identify the spatial pattern of drought prone
areas in the Mekong Region.
3. Estimation of yield reduction by water stress
4. Conclusions

3. Rainfed lowland rice production area
Large areas of rainfed lowland areas with drought-prone and drought- &
submergence-prone environments in the Mekong region
 65% of total rice land in Cambodia, 44% in Laos, 56% in Thailand
% Irrigated Rainfed lowland Upland Deepwater
Total F D DS S MD
Cambodia 16 75 7 22 43 0 3 1 8
Indonesia 54 35 20 4 0 3 8 11 0
Laos 14 65 22 22 22 0 0 21 0
Malaysia 66 21 15 0 0 5 1 12 1
Myanmar 30 59 24 6 0 15 14 4 7
Philippines 67 30 15 5 1 3 6 3 0
Thailand 20 74 6 38 18 9 3 2 4
Vietnam 53 39 15 8 0 12 4 5 3
F: favorable, D: drought-prone, DS: drought- and submergence-prone, S: submergence-prone, MD: medium-deep
Source: Mackill et al. (1996), IRRI (2005)

4.  Rainfall distribution
(i) uncertainty in the onset of the rainy season that can affect
timely sowing and transplanting.
(ii) late season drought affects on reproductive stage of
plant growth and development.

5. • Coarse soils
CL (4.5%)
LL (11.5%)
SL (38.1%)
LS (41.3%)
SA (4.5%)
Proportion of % soil texture types distributed throughout
the Savannakhet province
Soil characteristics

6. Top paddy
Middle paddy
Bottom paddy
Groundwater
table/head
Downward
movement
Lateral
movement
Groundwater flow
Runoff
(catchment)
Toposequence
» A sequence of
paddy fields on
slopping land

7. Development and use of a soil water balance
model (SWB) for quantifying:
 weekly rice field water storage
spatial variation in field water availability
 water stress development during rice growing
period

8. Rainfall
Transpiration
Evaporation
Runoff
Seepage
Under-bund
Percolation
Lateral movementDownward movement
Quantifying water balance components

9. Surface layer
D
Subsoil layer
DP
bund
Surface soil water content (Wsurface)
within 0 - 20cm
Subsoil water content (Wsubsoil) between 20 -
100cm where Dsubsoil equals to deep
percolation (DP)
The total amount of water storage in two
layers (surface + subsoil)
+
=
Development of a 2-layer soil water balance model
Lateral water
movement

10. Quantifying
downward water
movement (D)
Groundwater
tube
Perched water
tube
Percolator

11. Percolation on different soils (clay content)
Large variation in percolation across locations could be
explained by clay content of the soil.
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
0 10 20 30 40 50
Downwardwaterflow(mm/day)
Clay content (%)
D= 18.7/clay -0.16 (r=0.67) D measured from
NEThailand
D predicted for
these three regions
D measured from
southern Laos
D measured from
CARDI (Cambodia)
When there is standing water in the field.
When field water storage decrease from soil Saturate to FC
When there is no standing water, there is no downwater loss.

12. Generate climate input
Spatial interpolation function in GIS
(Variography and kriging)
Rainfall data (1985-2008)
325 met and hydrological stations:
33 met. and 10 hydro. Stations (Laos)
169 met. (North and North-East Thailand)
94 stations (Cambodia)
- Rainfall
- Crop Evapotranspiration (ETc)
Thailand
Laos
Cambodia

13. Crop Evapotranspiration (ETc)
ocsc ETKKET 
   
 2
2
34.01
273
900
408.0
U
eeU
T
GR
ET
dan
o







 
 airdryWFCW
airdryWsurface
s
SS
SW
K
__
_



+ Crop coefficient (Kc)
(Initial stage Kc=1.05,development stage Kc=1.2,
late season stage Kc=0.6-0.9) (Allen et al.,1998)
+Water stress coefficient (Ks)
+ Reference Evapotranspiration (ETo)

14. Potential evapotranspiration (Penman-Monteith equation)
Week 15 Week 25
Week 35 Week 40

15. Soil profile samples

16. Schematic diagram for quantifying free water level and water stress
development based on lowland water balance model
Soil data
Clay
%
Downward
(D)
Climate data
Rainfall ETc
FIELD WATER BALANCE MODEL: W(t)= W(t-1)+RF(t)-ETc(t)–D(t)-L(t)-R(t)
Determination of LGP, SGP, EGP
Daily free water level
Estimation probability of drought occurrence
Sat, FC,
WP, Air
(Saxton & Rawls)
•Point based (daily)
•Gridded surface
(weekly)

17. Soil water balance model for quantifying LGP

18. Duration for the length of growing period (LGP)

19. Start of growing period (SGP) End of growing period (EGP

20. The prediction of water availability can be
made with weather forecasting
Weather Forecast Division
Department of
Meteorology and
Hydrology
0
50
100
150
200
250
300
350
400
450
500
550
1 2 3 4 5 6 7 8 9 10 11 12
Months
mm
2000
2001
2002
2003
2004
Mean

21. Results of weekly water availability prediction
made at two different times in 2010.
Forecast standing water level for
week 28 (9-15 July 2010)
Forecast standing water level for
week 41 (8-14 Oct 2010)

22. 0
10
20
30
40
50
60
70
80
90
100
-40 -35 -30 -25 -20 -15 -10 -5 0
Ralative water level (cm)
Grainyieldreduction
(%)
Early
Medium
Late
Y = -1.68X; r2 = 0.80
Linear (Y = -1.68X; r2
= 0.80)
Y=-1.68X; r2 =0.80 (cm)1.68WL-(%)reductionYield Rel
-100.00
-50.00
0.00
50.00
100.00
150.00
200.00
250.00
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51
Weeks
mm
Rainfall Depth of standing water Field water storage
Soil water at FC Soil water at WP
StartLGP wk19 End LGP wk41
Flowering
Khanthaboury (wet
season rice, 1988)
Estimation of Yield limited by water stress
• Water stress around
flowering (3 week
before and after)
Ouk et al.,(2006)
Start LGP End LGP LGP Flowering date Wlrel(mm) %yield reduction
19 43 25 17-Sep -74.1 12

23. Using SWB in conjunction with GIS can provide:
 a geographical dimension of soil hydrological patterns for
various rice growing environments.
 identify the spatial pattern of drought stress that is likely to
occur from long term climate data.
 identify strategies for plant breeding and geographical targeting
of improved varieties with particular drought tolerance or
drought avoidance characteristics.
To provide guidelines for practical advice to the rice farmers
and researchers for the determination of appropriate crop
management strategies (e.g. time of planting, selection of
varieties) and policy makers for investment decisions.
Conclusion

24. THANK YOU!