Chapter 6 Fetter Soil moisture and groundwater recharge
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Transcript of Chapter 6 Fetter Soil moisture and groundwater recharge
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 .
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
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.
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 moisture depend on:
1. Change in the amount
of precipitation.
2. Change in the amount
of evapotranspiration
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