Origin of Paddy Fields and Functions for Environmental Remediation in the Urbanized Areas of Yangtze River Delta - Environmental Contamination: Health Risks and Ecological Restoration - page 404

local.governments.are.on.high.alert.in.order.to.prevent.and.control.looding,.
and.water.lodging.in.Huihe,.Yangtze,.and.Pearl.Rivers..The.Yangtze.River.
Delta.in.China.is.a.loodplain.with.an.average.altitude.of.2-2.5.m.above.sea.
level,.where.the.Taihu.Lake.is.in.the.center.with.a.water.network.system.con-
necting.the.whole.region..Since.the.prehistoric.times,.fertile.soils.on.lat.land.
and.annual.warm.temperatures.have.provided.good.conditions.for.rice.cul-
tivation..There.is.approximately.250.×.104.hm 2 .of.irrigated.paddy.ields.that.
account.for.more.than.70%.of.the.total.arable.land.of.the.Delta.(Cao.and.Lin.
2006,.Xu.et.al..1998,.Zhao.et.al..1991)..The.growing.stage.of.rice.is.when.the.
looding. period. of. the. Yangtze. River. and. the. time. of. the. monsoon. season.
coincide..Paddy.ields.can.usually.keep.a.layer.of.surface.water.at.a.depth.of.
7-10.cm.for.growing.rice,.but.it.can.retain.water.up.to.a.depth.of.15.cm.within.
the. ields. if. needed,. during. the. rainstorm. season. (Figure. 18.8).. Therefore,.
approximately.1500.m 3 /hm 2 .more.water.can.be.stored.in.paddy.ields.than.
in.upland.ields,.in.which.the.former.can.impound.a.total.of.approximately.
37.5.×.109.m 3 .of.water.from.the.Yangtze.River.Delta.(250.×.104.hm 2 ),.which.is.
equal.to.about.85%.of. the.total.water.that.can.be.impounded.by.the.Taihu.
Lake.(44.3.×.109.m 3 )..In.addition.to.the.standing.water.layer.in.paddy.ields,.
there. is. also. a. water. saturated. proile. within. 0-60.cm. depth,. which. shows.
paddy.soils.hold.about.50%.more.water.content.than.that.found.in.upland.
soils..In.total,.there.is.a.difference.of.about.75.×.109.m 3 .between.the.water.stor-
age.proiles.of.paddy.soils.and.upland.soils.in.this.delta.region..Altogether,.
paddy.soils.of.the.Yangtze.River.Delta.can.impound.112.5.×.109.m 3 .of.water,.
which.is.2.5.times.higher.than.the.water.reserve.in.Taihu.Lake.(Cao.2007,.Cao.
and.Lin.2006,.Cao.and.Zhou.2008,.Xu.et.al..1998)..Paddy.ields.and.soils.play.
a.very.important.role.in.the.prevention.and.control.of.looding.and.water-
lodging.disasters.by.storing.excess.water..If.half.of.the.rice.paddy.ields.were.
to.be.lost.(see.Table.18.1),.then.the.surplus.water.of.56.25.×.109.m 3 .in.the.region.
would.have.essentially.lost.its.containment,.which.may.lead.to.catastrophic.
looding.and.water-lodging.consequences.

Structure of irrigated rice field

Surrounded by field dam

Stand-
water flue
out only
when or
after
rainstorm

Stand-water covered the surface

Plow layer
stopped
leaching
or sub-
flow

FIGURE 18.8
Schematic.diagram.of.paddy.ields.and.runoff.features.