Environmental Engineering Reference
In-Depth Information
Figure 8.2
(right) Radiation budget components for 30 July
1971 at Matador, Saskatchewan (50° N) over a 0·2 m stand of
native grass. Inputs to the surface have been plotted as
positive and outputs as negative to assist interpretation. K is
short-wave radiation, L is long-wave radiation and Q* is net
radiation. The arrows indicate the direction of radiation flow.
Source: After Ripley and Redmann (1976).
than dry soils. However, if the soil contains a lot of water, the large heat capacity of the
water will prevent the soil warming despite heat being conducted from the surface. For
most agricultural crops a balance is needed so that soils warm up fairly quickly at depth
and are neither too wet nor too dry. This is achieved when the moisture content of the soil
is about 20 per cent.
THE NATURE OF HEAT TRANSFER FROM THE SOIL
During the day, the ground surface gets hotter through absorbing the sun's energy and
there is a positive radiation balance (Figure 8.2). The layer of air in contact with the
ground becomes warm by conduction. If this were the only mechanism of heat transfer, it
would take a very long time before even the lowest 1 m of air was warmed. The daytime
maximum temperature at that height would not occur until about 9.00 p.m. Clearly this
cannot be the only process transferring heat, although it is the most important in the
lowest few millimetres, where temperature gradients are extreme. Above that level, the
effect of heating the air causes it to become buoyant through being less dense than its
surroundings, and so it rises, carrying heat with it. Cooler air then moves in to take its
