Prediction and Control of Salt
Accumulation in the Upper Root Zone
Under Sub­Surface Drip Irrigation

H. Fujimaki*, T. Amano**, C. Kanatsuka**, M. Inoue** Arid Land Research Center, Tottori University, Japan**University of Tsukuba, Japan

IntroductionSub­surface drip irrigation (SSDI) is
expected to play a key­role in arid regions as
the ultimate water saving method.

One of the drawback of SSDI: the accumulation of salt to the upper rootzone and it cannot be leached out by water from emitter.

Introduction

Introduction
Upward Leaching Method
1.Application of much waterenough to make wetting front
reach to the soil surface
2.Scraping of surface layer after salt
accumulates due to evaporation

Objectives
To develop a simulation model for predicting salt
accumulation and leaching under sub­surface drip irrigation: i.
e. two dimensional movement of water, heat and solute aswell as evapotranspiration and root water uptake, considering
the water vapor movement and effect of salt crust on
evaporation.
To test the validity of the model by comparison withexperimental results
To evaluate the upward leaching method to removeaccumulated salt in the upper root zone.

60 cmMaterials and Methods

Porous pipe

40 cm

7/16: seeding
8/15: saturation with tap water
9/ 1: irrigation with 5000ppm CaCl2
solution started
irrigation depth = ET
9/25: upward leaching with 1.7 cm
10/1: scraping and sampling

40 cm

soil: Masa loamy sandplaced in a greenhouseplant: soybean

Electricalbalance

TDR
Thermocouples

Drainage tube under suction control

Numerical Simulation
Governing equation for water flow∂ q lx ∂ q lz
∂ q vx ∂ q vz
∂=−−S
∂t
∂x
∂z
∂x
∂z
∂
q lx =−K∂x∂q lz =−K−1∂z



−1w

q vx=−a   h r D va−1w

q vz =−a   h r D va{{ v * ∂ w
Rv T s ∂ x
v * ∂  w
Rv T s ∂ z∂ v * ∂ T s∂Ts ∂ x∂ v * ∂ T s∂Ts ∂z}}: volumetric water content
ql : liquid water flux (cm/s)
qv : water vapor flux (cm/s)
K: hydraulic conductivity (cm/s)
: pressure head (cm): matric potential (cm)w: water potential(cm)a: air­filled porosity
: tortuosity
Dva : water vapor diffusion coefficientin air (cm2/s)
: enhancement factor for thermalvapor diffusion
Tls : soil temperature (K)v* : saturated water vapor density(g/cm3)hr : relative humidity
Rv : the gas constant for water vapor(4697 cm/K)
S : root water uptake (s­1 )Numerical Method: Alternative Direction Implicit FDM (ADI)with Celia(1990)'s mass­conservative iteration scheme

Transpiration Rate
Potential Transpiration Rate(Trp )T rp =W E p K cW: width of the target region (cm)
Ep: ET by Penman equation (cm/s)
Kc: crop coefficient for transpiration
K c =a kc [1−exp b kc  T dt ]c kc

aKc , bKc : plant­specific parameters
Actual Transpiration Rate(Tr)drt

WT r =∫0 ∫0 S dxdz
S =T rp  w  s

αw: reduction coefficient for water stress
αs : reduction coefficient for salinity stressRoot Water Uptake
van Genuchten's response function (1987)

= w  s =1 1
50

p11×1 oo 50

fitting parametersreduction due to salinity stress
reduction due to water stressp2

Governing equation for solute movement

∂ q sx ∂ q sz
∂ c=− Sc∂t∂x∂z∂c∂cq sx =− D xx− D xz
q lx c∂x∂z∂c
∂cq sz=− D zz
− D xz
q lz c∂z∂x
2

 D xx = D iw s 
 D zz= Diw s 
 D xz = D zx =2

L q lx T q lz

∣q l∣
 L q 2lz  T q 2lx∣q l∣L −T  q lx q lz

∣ql∣

c : solute concentration (mg/cm3)
qs : solute flux (mg/cm2/s)
Sc: sink/source for solute (mg/cm3/s)
D : dispersion coefficient (cm2/s)
Diw : diffusion coefficient in free water(cm2/s)
s : tortuosity factor for ionic diffusion
λL : longitudinal dispersivity (cm)
λT : traversal dispersivity (cm)Conditions for Numerical Simulationonroot density distribution: measured one
lower BC: daily­measured cumulative outflow
Thermal vapor diffusion was incorporated by plainly inter/extrapolating
measured soil temperature.
Aerodynamic resistance:
given from hourly wind velocity at the nearest weather station withcorrection factor such that aerodynamic resistance gives daily
evaporation rate from wet soil beside the lysimeter.

Hydraulic Properties :independently measured using the hanging water method, the vapor
equilibrium method, the transient evaporation method=sat −n m[ 1−   ]m

{ [ 1−ln−m 1ln−0 1 

K = K sat

 
 sat

]}
2

Solute Transport Properties :independently measured using the half cell method
and the long column method (c vs z).

Results and Discussion

Distribution of salt content per unit bulk volume of soilat the end of the experiment 2 (Masa).

computationtime: 88 min
at 1.6GHz CPU

mass balanceerror = 0.3%

Underestimated K?
compensated root
water uptake?

mass balanceerror = 0.7%

16 % of salts
supplied withirrigation
accumulated totop 0.5 cm

Conlusion
Software for predicting two dimensional movement ofwater and solute considering the water vapor movement
has been developed.
Simulated values were in fair agreement, but further
investigation is required to improve accuracy.

Salts in the upper root zone was transported to the soil
surface and a part of them was effectively removed byscraping top 0.5 cm layer.

Thank you for your attention

Atmospheric Boundary Condition**
 vs hrs − va hra
E=r a r sc
w
h rs≈ h re =exp
Rv T ρv* : saturated water vapor density (g/cm3)
hr : relative humidity
ra : aerodynamic resistance (s/cm)
rsc : salty crust resistance (s/cm)
ψw: water potential(cm)Rv : the gas constant for water vapor (4697 cm/K)

helghtWater vapor density
at reference height, va *hrararsc vs * hrs