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Groundwater investigation techniques 151

ρ ce + e )
(
∆H + p sat act
r
ET = sfc eq. 5.9
⎧ γ (r + r )⎫
⎪
⎪
λ ⎨ ∆ + sfc aero ⎬
⎩
⎭
⎪ r aero ⎪
where, further to the symbols applied in equation
5.7, ρ = density of water, λ = latent heat of vaporiza-
tion, c = specific heat capacity of water, r = surface
p sfc Fig. 5.10 Sketch of the occurrence of water within unsaturated
resistance, r = aerodynamic roughness. material showing both soil grains coated with a film of adsorbed
aero
The Penman–Monteith formula is used as the basis water and soil pores filled with capillary water.
for the national computerized system, morecs, the
United Kingdom Meteorological Office Rainfall and
Evaporation Calculation System (Thompson et al. which the polar water molecules are attracted to
1981). morecs provides an areal-based estimate of the charged surfaces of soil particles. Osmosis is often
evapotranspiration which supplements the Penman ignored but acts to retain water in the soil as a result
approach with simulation of soil water flux and a of osmotic pressure due to solutes in the soil water.
consideration of local vegetation cover. The model is This occurs particularly where there is a difference in
based on a two-layer soil and provides estimates of solute concentration across a permeable membrane
areal precipitation (P), PE, actual evapotranspiration such as the surface of a plant root, making water less
(AE), soil moisture deficit (SMD) and hydrologically available to plants, especially in saline soils, and is
effective rainfall (P − AE − SMD) for 40 × 40 km grid of importance when considering irrigation water
squares on a weekly basis. quality (Section 6.2.2).
Capillary forces result from surface tension at
the interface between the soil air and soil water.
5.4 Soil water and infiltration Molecules in the liquid are attracted more to each
other than to the water vapour molecules in the air,
Understanding soil water distribution, storage and resulting in a tendency for the liquid surface to con-
movement is important in hydrology in predicting tract. This effect creates a greater fluid pressure on
when flooding will occur and also in irrigation the concave (air) side of the interface than the convex
scheduling. In hydrogeology, understanding infiltra- (water) side such that a negative pressure head,
tion of water in the unsaturated zone is a necessary indicated by −ψ (in centimetres or metres head of
prerequisite to quantifying groundwater recharge to water), develops relative to atmospheric pressure.
the water table. The branch of hydrology dealing The smaller the neck of the pore space, the smaller
with soil water and infiltration is studied in detail by the radius of curvature and the more negative the
soil physicists and suggested further reading in this pressure head. In soil physics, the negative pressure is
topic is provided at the end of this chapter. often termed the suction head or tension head and
describes the suction required to obtain water from
unsaturated material such as soils and rocks of the
5.4.1 Soil moisture content and soil unsaturated zone. With increasing moisture content,
water potential the larger pore spaces become saturated and the
radius of curvature of the menisci increases creating a
Water contained in the soil zone is held as a thin film more positive pressure head (i.e. the suction head is
of water adsorbed to soil grains and also as capillary reduced). Close to the water table, the pore space is
water occupying the smaller pore spaces (Fig. 5.10). fully saturated but the pressure head is still negative
The main forces responsible for holding water in the as a result of water being drawn up above the water
soil are those of capillarity, adsorption and osmosis. table by the capillary effect (see Fig. 2.14). Hence, the
Adsorption is mainly due to electrostatic forces in measurement of soil moisture content and pressure