Geoscience Reference
In-Depth Information
Irrigation is the other side of the coin from drainage. On a hot sunny day in Bri-
tain, a crop will draw about 10 tons or 2240 gallons of water an acre from the soil in
transpiration (about 25,000 litres/ha). If this water is not replaced, the crop may even-
tually wilt and finally die. The optimum condition for crop growth is when the soil
al soil moisture deficit below field capacity when growth and yield are reduced. This
point can be reached quite quickly on light sandy soils in East Anglia which do not
hold much water at field capacity, and where there is little rainfall to keep the level
topped up.
Irrigation is an obvious answer, and could be more widely used to extend the
range and profitability of crops. It depends, of course, on an adequate source of water
and a means of monitoring the soil water deficit. An open lysimeter is one method;
rather crude and expensive to install but easy to use. The other method involves keep-
ing a water balance sheet. Here, a running tally of soil moisture deficit (SMD) is kept
by entering daily rainfall, and subtracting the water lost in evaporation and transpira-
tion (evapotranspiration), thus:
SMD (today) = SMD (yesterday) + evapotranspiration - rainfall
Agrometeorologists can now provide weekly 'potential evapotranspiration' fig-
ures for any particular district based on weather data from the nearest weather station.
The 'actual evapotranspiration' figures used in the equation above are then calculated
from a knowledge of the particular soil - its moisture holding capacity and so on -
and the state of growth of the crop concerned. For irrigation purposes, a plan might
specify, for example, that 25mm of water should be applied whenever the calculated
soil moisture deficit reached 30mm.
Figure 60
shows, in graphical terms, the water
balance sheet for maincrop potatoes over a nine week period from the last week of
May to the end of July.
