Example of the calculation of monthly water requirements.
Calculation of soil moisture deficits from average evapotranspiration and rainfall
data for a typical sandy loam soil in a temperate region in the northern hemisphere.
All figures are in mm.
Month Rainfall
(P)E* P–E Storage Excess DeficitMarch
April
May
June
July
August
September
October
November
December88
47
49
40
53
51
50
57
88
100
16
45
80
105
100
88
40
16
+72
+2
-31
-65
-47
-37
+10
+41
+88
+100
50
50
19
0 0 0 0 0 9
50
+72
+2
0 0 0 0 0 0 0
59
0
0
0
46
93
130
120
79
0
0
E* = Evapotranspiration
Controlling the Temperature of Crops
The practice of cooling of crops and protecting them from frost is often associated with
irrigation since the same overhead systems can be used to alter the temperature of crops:
- Cooling of Crops—in very hot weather it has been proved that some crops benefit
from being cooled with water, while benefiting from irrigation at the same time. Cooling
of crops normally uses less water than protecting crops from frost.
The critical temperature at which many plants benefit from cooling appears to be
32 °C, and ideally water should be applied until the temperature falls to this. In practice
however, some crops in some climates should only be cooled for a few hours per day,
and allowed to remain at high temperatures—above 32°C—for the rest of the day. In
fact the sugar content of some crops may fall if the plants are maintained at 32°C. - Frost Protection—to avoid plant damage when frosts occur water can be applied by
sprinklers to crops if they are at a vulnerable stage of their life cycle, such as flowering.
The flowers are protected in a remarkable way, governed by basic laws of physics.
When water changes into ice, heat is released and the local air temperature also rises as
a result. At the same time, ground heat is held to some extent by the fog or mist that is
created.
When the temperature then rises to above zero, heat is absorbed and the ice melts.
The heat is regained from the moisture in the air—if there had been little or no moisture
in the air heat would have been taken instead from the plants themselves, leading to
frost damage by breakdown of their exploded cells.