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

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By Saleh A. Al-Hassoun Associate Professor Department of Civil Engineering College of Engineering King Saud University Riyadh, Saudi Arabia shassoun@ksu.edu.sa 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 ). h =  + z  is matric potential (L) z is elevation head (L)

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  water content(L3/L3),  s saturated water content  e capillary fringe b minus of slope of the straight line approximate water retention curve on Log-Log scale plot. It is found to be 1.5. K hydraulic conductivity at field saturation and was determined by calibration in laboratory and found as 60 cm/hr., n =2 + 3/b

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

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

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).

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.

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