By Saleh A. Al-Hassoun Associate Professor Department of Civil Engineering College of Engineering King Saud University Riyadh, Saudi Arabia [email protected].

By

Saleh A. Al-HassounAssociate Professor

Department of Civil EngineeringCollege of EngineeringKing Saud UniversityRiyadh, Saudi [email protected]

SIMULATION OF MOISTURE AND RECHARGE IN

SAND DUNE USING 'SWIM' MODEL

OBJECTIVES

Simulation of soil moisture distribution .

Estimation of aquifer recharge .

In sand dune in Qassim region in Saudi Arabia . Using :

Numerical Model named 'SWIM' .

What is “SWIM” ?

SWIM is an acronym for Soil Water Infiltration and Movement. It is a software package developed and published in 1990 by Ross

(SWIM V1).

SWIM allows addition of water to the system as precipitation and removal by run-off, drainage, evaporation from the soil surface and transpiration by vegetation.

It consists of a menu-driven suite of three programs that allow the user to simulate soil water balances using numerical solutions

1. SWIMFILE 2. SWIMEV 3. SWIMPLOT

SWIM V1

SWIM V2

Richard’s equation

q flux density (L/T),K hydraulic conductivity (L/T),h hydraulic head at position x in direction of flow (L).

θ volumetric water content (dimensionless), t time (T),S source strength (1/T ).

x

hKq

Sx

q

t

h = + z is matric potential (L)

z is elevation head (L)

Sx

z

xK

xt

Field Data % sand in all samples were between 90.44% and 99.06%.

K ranged from 1.32 x 10-3 cm/s(1.14 m/day) to 13.64 x 10-3 cm/s(11.78 m/day).

Precipitation was obtained using an automatic weather station for the whole season (annual avg. precipitation ~ 120 -150 mm.)

Evaporation in the study area was obtained on daily basis (annual avg. evaporation ~ 2500-3000 mm.)

Infiltration was measured using infiltrometers in the site.

Application of SWIM Model

666.0

132.0)(

water content(L3/L3),s saturated water contente capillary fringeb minus of slope of the straight line approximate water retention curve on Log-Log scale plot. It is found to be 1.5.

b

es

/1

)(

K hydraulic conductivity at field saturation and was determined by calibration in laboratory and found

as 60 cm/hr.,n = 2 + 3/b

n

s

KK

)(

MODEL RESULTS

At SDB1 (on Oct. 10, 1997) – Simulation started (on April 7,1997 )

0

0,01

0,02

0,03

0,04

0,05

0,06

0,07

0 0,5 1 1,5 2 2,5 3 3,5 4 4,5 5 5,5 6 6,5 7 7,5 8 8,5 9 9,5 10 10,5

11 11,5

12 12,5

13 13,5

Depth ( m )

Wat

er C

onte

nt (

Ө )

Field

Model

MODEL RESULTS

At SDB2 (on Oct. 17, 1999) Simulation started (on May 5 ,1997 )

0

0,005

0,01

0,015

0,02

0,025

0,03

0,035

0,04

0,045

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Depth ( m )

Wat

er C

onte

nt (

Ө )

Field

Model

Comparison of Water Contents

Comparison of Predicted at SDB 2 with the observed (actual) evolution of water content at site SDB 5 on same date (on Oct. 10, 1997).

0

0,005

0,01

0,015

0,02

0,025

0,03

0,035

0,04

0,045

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Depth ( m )

Wat

er C

onte

nt (

Ө )

Field

Model

Drainage Result

cumulative recharge at the end of 2 years was found as 30 mm.

cumulative rainfall during same period was recorded as 276 mm.

Aquifer recharge below the sand dunes in Qassim region

is about 11% of the total rainfall during study period.

CONCLUSIONS

Application of 'SWIM' model to estimate moisture and aquifer recharge in sand dunes reasonably well.

An acceptable value of recharge under dunes in the study area has been predicted by 'SWIM' model.

Recommendation

More field data from other sand dune areas should be collected and used in this model to prove its usefulness.

Thanks