Paddy soil cracks: characteristics and their impact on preferential flow

Zhongbin ZHANG and Xinhua PENG

Institute of Soil Science, Nanjing, CAS

15, April, 2015

Sponsored by NFSC and CAS

Rice production and water scarcity

Bouman et al., 2007

Rice is the most important staple food for nearly half of the world's population. 1 kg rice requires 2-3 ton water, much higher than other cereals Paddy soils mainly distribute in East Asia, South Asia and Southeast Asia where

drought disasters usually happen

Tillage management (puddling) in Paddy field

Plowing Leveling

Consequences:

1. Control weeds2. Reduce permeability (develop plow pan)3. Prepare soft rice bedsoft rice bed (easy transplanting)

Low bulk densityhigh shrinkage capacityin plow layer

Harrowing

Due to puddling at over-saturated condition, paddy soil easily crack under drying condition (Yoshida and Adachi 2001)

The cracks (area, width or depth) could serve as pathways for preferential flow, governing the rate and the velocity of water and solute transport (Liu et al., 2003; Sander and Gerke, 2007)

Understanding of the relation between crack characteristics and water percolation in paddy soil is critical for water saving management.

Paddy soil cracks and preferential flow

Two paddy fields with different cultivation years:20 years (Young paddy field, YPF)>100 years (Old paddy field, OPF)

Two water management methods:alternate flooding and drying (AFD) continous flooding (CF)

Four treatments:YPF-AFD YPF-CFOPF-AFDOPF-CF

Experiment site and treatments

Depth Bulk density SOC Sand Silt Clay Vermiculite Hydromica Chlorite Kaolinite

cm g/cm3 g/kg

0-15 1.36 9.79 31.4 35.4 33.2 31 14 20 31

15-25 1.49 5.3 38.1 29.7 32.2 28 18 18 31

0-15 0.95 21.7 35.7 44.1 20.1 36 8 22 30

15-25 1.59 12.4 39.0 40.5 20.5 36 10 22 26

YPF

OPF

Paddy soils——— % ————————————— % ————————————

Soil properties of two paddy fields

Plow layer: 0-15 cm; Plow pan: 15-25 cm

The young soil was higher in clay content, and lower in organic carbon than the old paddy soil.

Puddling on cracking in paddy fields

Ultrasonic wave was used to simulate puddling in the field.Soil cracking generally increased with increasing simulated puddling intensity.

4/20 5/4 5/18 6/1 6/15 6/29 7/13 7/270.1

0.2

0.3

0.4

0.5

0.6

YPF-AFD YPF-CF OPF-AFD OPF-CF

Soil

wat

er c

onte

nt

(cm

3 cm

-3)

Date

Sampling date

0

50

100

150

200

250

300

Rai

nfal

l (m

m)

Rainfall

Alternate flooding and drying (AFD) on soil shrinkage

4/20 5/4 5/18 6/1 6/15 6/29 7/13 7/27

0.8

1.0

1.2

1.4 YPF-AFD YPF-CF OPF-AFD OPF-CF

Bul

k de

nsity

(g

cm

-3)

Date

During the whole early rice： 6 AFD cycles

Intensity of AFD:

YPF > OPF

Bulk density increased with AFD

YPF > OPF

Alternate flooding and drying (AFD) on soil shrinkage

Shrinkage capacity (COLE) decreased with AFD, probably

resulting from the increase of bulk density.

7-1 7-2 7-36-12

7-7 7-8 7-10 7-11

AFD on soil cracking (YPF-AFD)

7-1 7-2 7-36-12

7-8 7-9 7-10 7-11

AFD on soil cracking (OPF-AFD)

Relation between crack area and soil water content

6/7 6/21 7/5 7/190

4

8

12

16

20

0.0

0.1

0.2

0.3

0.4

0.5

Cra

ck a

rea

den

sity

(%

)

Date

OPF-AFD cracks YPF-AFD cracks

Soi

l wat

er c

onte

nt

(cm

3 cm

-3)

OPF-AFD SWC YPF-AFD SWC

0.2 0.3 0.4 0.5 0.60

2

4

6

8

10

12

YPF-AFD OPF-AFD

Cra

ck a

rea

den

sity

(%

)Soil water content (cm3 cm-3)

Greater stability of soil structure in OPF than in YPF.

Cracks-induced percolation

0.0

0.2

0.4

0.6

0.8

1.0

1.2

4/20 5/4 5/18 6/1 6/15 6/29 7/13 7/270.0

0.2

0.4

0.6

0.8

1.0

1.2

Wat

er in

filt

rati

on r

ate

(mm

h-1

) YPF-AFD YPF-CF

Date

OPF-AFD OPF-CF

Cracks increased percolation in YPF but not in OPF

1st dry

Examples of dye tracer sites in the four plots

20

15

10

5

00 10 20 30 40 50

20

15

10

5

00 10 20 30 40 50

YPF-AFD-1 YPF-AFD-2 YPF-AFD-3 YPF-AFD-4

Area density of the largest crack (%)

Soi

l dep

th (

cm)

OPF-AFD-1 OPF-AFD-2 OPF-AFD-3 OPF-AFD-4

Characteristics of soil cracks in 3D

Preferential flow caused by cracks (dye tracer)

Cracks induced preferential flow. However, preferential flow was reduced considerably by the presence of plow pan.

Conclusions

Soil cracking increases with puddling intensity, probably resulting from increase of clay-sized aggregates. AFD decreases shrinkage capacity and influences soil cracking patterns.

Cracks geometry (2D and 3D) is different in the YPF and the OPF. Therefore the effect of cracks on infiltration was different in the two soils.

Cracks induced preferential flow in the plow layer. However, the preferential flow was reduced considerably by the presence of plow pan.

Thanks for your attention