Evapotranspiration

As a leaf canopy covers a soil the rate of water loss becomes more closely related to transpiration rates. The potential transpiration rate represents the estimated loss of water from plants grown in moist soil with a full leaf canopy. It can be calculated from weather data (see Table 19.2).

Table 19.2 Potential transpiration rates. The calculated water loss (mm) from a crop grown in moist soil with a full leaf canopy, over different periods of time and based on weather data collected in nine areas in the British Isles

Area

April

May

June

July

Aug

Sept

Summer

Winter

Annual

Ayr

46

81

90

83

65

38

405

70

475

Bedford

50

78

89

91

80

43

430

70

500

Cheshire

53

75

83

88

76

44

420

80

500

Channel Isles

51

86

91

99

84

46

457

103

560

Essex (NE)

50

79

98

98

83

45

450

80

530

Hertford

49

79

91

94

80

43

435

75

510

Kent (Central)

50

79

93

96

83

44

445

65

510

Northumberland

44

64

81

76

60

34

360

70

430

Dyfed

46

75

84

81

74

44

405

105

510

As roots remove water it is slowly replaced by the water film equilibrium, but rapid water uptake by plants necessitates root growth towards a water supply in order to maintain uptake rates. At any point when water loss exceeds uptake, the plant loses turgor and may wilt. This tends to happen in very drying conditions, even when the growing medium is moist. Wilting is accompanied by a reduction in carbon dioxide movement into the leaf, which in turn reduces the plant's growth rate (see photosynthesis). The plant recovers from this temporary wilt as the rate of water loss falls below that of the uptake, which usually

The permanent wilting point (PWP) is the soil's water content when a plant growing in it does not regain turgor overnight.

Available water (to the plant) is the water held in the soil between field capacity and the permanent wilting point.

Soil consistency describes the effect of water on those physical properties of the soil.

occurs in the cool of the evening onwards. Continued loss of water causes the soil to reach the permanent wilting point because roots can extract no more water within the rooting zone.

When the soil has reached its permanent wilting point (PWP) there is still water in the smallest of the soil pores, within clay particles and in combination with other soil constituents, but it is too tightly held to be removed by roots. Typical water contents of different types of soil at their permanent wilting point (PWP) are given in Table 19.1.

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