Soil Moisture and

Groundwater Recharge

prepared by

Safaa A. EL Derawi

Al-Azhar University-Gaza

Master Program of Water and Environmental

Science

Introduction

The uppermost layer of the earth may contain a three phase

system of material:

solid

liquid

gaseous

The earth layer that

contain the three phase

system is called the:

vadose zone also know

as zone of aeration, and

unsaturated zone .

Porosity and water content of soil

Porosity

The void ratio of the soil

is the ratio of volume of the voids to the volume of the solids.

V= Vv + Vs ,

n= Vv/V

n= Vv / Vv +Vs divide by Vs

V= Vv + Vs ,

e= Vv/Vs

e= Vv / V - Vv divide by V

The gravimetric water content

The volumetric water content

The saturation ratio

The dry bulk density :

Bulk Soil = Particle volume + pore volume

The particle density:

Porosity can be calculated in practice:

Soil Water Measurement

Measures mass water content

Take field samples

weigh(Wm)

oven dry at 105 C

weigh ( Ws) until a constant mass is

obtained Ww=Wm-Ws

Gravimetric water content = Wm-Ws / Ws

and Vw = (Wm - Ws / w )/V

Volumatric water content =Vw/V

Electrical Resistance Blocks & Meters

Capillarity and the capillary fringe

Capillary forces :

The forces acting on soil

moisture in the

unsaturated zone,

attributable to molecular

attraction between soil

particles and water.

The capillary rise in cm is :

Capillary Rise

Capillary fringe :

The zone of aeration is best defined as the zone where the

soil moisture is under tension

Pore-water tension in the vadoes zone

Fluid pressures in the vadose zone are negative

The negative pressure head is measured in the field

with a tensiometer

Tensiometer for Measuring Soil Water Potential

Porous Ceramic Tip

Vacuum Gauge (0-100 centibar)

Water Reservoir

Variable Tube Length (12 in- 48 in)

Based on Root Zone Depth

soil water is the water in the vadoes zone, that is available to

growing plants.

Soil moisture depend on:

1. Change in the amount

of precipitation.

2. Change in the amount

of evapotranspiration

Types of soil water

Field Capacity - Wilting Point

Theory of Unsaturated Flow

The infiltration of water into the unsaturated can be

considered in terms of:

Gravity potential, Z

Moisture potential, ψ

Darcy’s law is valid for flow in the unsaturated zone.

Is not constant , is a function of the volumetric water

content

Idealized curves showing relationships of volumetric water

content, hydraulic conductivity, and soil-moisture head

Water-Table Recharge

When the front of

infiltrating water reaches

the capillary fringe, it

displaces air in the pore

spaces and cause the water

table to rise.

Rate of Recharge

The rate of water-table recharge depends on:

1. Thickness of the unsaturated zone

(The thinner zone, the faster rise of water table)

2. The vertical unsaturated hydraulic conductivity

3. The permeability .

May generate a localized ground-water mound