Short green crops

Short green crops, up to a metre or so in height, supplied with sufficient water and exposed to similar solar radiation conditions, all have a similar net radiation (Rn) balance. This is largely because of the small range of albedos, 20 to 30 per cent for short green crops compared with 9 to 18 per cent for forests. Canopy structure appears to be the primary reason for this albedo difference. General figures for rates of energy dispersal at noon on a June day in a 20-cm high stand of grass in the higher mid-latitudes are shown in Table 12.1.

Figure 12.7 Energy balance components for a melting snow cover at Bad Lake, Saskatchewan (5I°N) on 10 April 1974.

Source: Granger and Male. Modified by Oke (1987). By permission of Routledge and Methuen & Co, London, and T.R. Oke.

Figure 12.7 Energy balance components for a melting snow cover at Bad Lake, Saskatchewan (5I°N) on 10 April 1974.

Source: Granger and Male. Modified by Oke (1987). By permission of Routledge and Methuen & Co, London, and T.R. Oke.

Figure 12.8 Energy fluxes over short grass near Copenhagen (56°N). (A) Totals for a day in June (seventeen hours daylight; maximum solar altitude 58°) and December (seven hours daylight; maximum solar altitude 11°). Units are W m-2. (B) Seasonal curves of net radiation (Rn), latent heat (LE), sensible heat (H) and ground-heat flux (G).

Figure 12.8 Energy fluxes over short grass near Copenhagen (56°N). (A) Totals for a day in June (seventeen hours daylight; maximum solar altitude 58°) and December (seven hours daylight; maximum solar altitude 11°). Units are W m-2. (B) Seasonal curves of net radiation (Rn), latent heat (LE), sensible heat (H) and ground-heat flux (G).

Source: Data from Miller (1965); and after Sellers (1965).

Table 12.1 Rates of energy dispersal (W m-2) at noon in a 20-cm stand of grass (in higher mid-latitudes on a June day).

Net radiation at the top of the crop 550

Physical heat storage in leaves 6

Biochemical heat storage (i.e. growth processes) 22

Received at soil surface 200

Figure 12.8 shows the diurnal and annual energy balances of a field of short grass near Copenhagen (56°N). For an average twenty-four-hour period in June, about 58 per cent of the incoming radiation is used in evapotranspiration. In December the small net outgoing radiation (i.e. Rn negative) is composed of 55 per cent heat supplied by the soil and 45 per cent sensible heat transfer from the air to the grass.

We can generalize the microclimate of short growing crops according to T. R. Oke (see Figure 12.9):

1 Temperature. In early afternoon, there is a temperature maximum just below the vegetation crown, where the maximum energy absorption is occurring. The temperature is lower near the soil surface, where heat flows into the soil. At night, the crop cools mainly by long-wave emission and by some continued transpiration, producing a temperature minimum at about two-thirds the height of the crop. Under calm conditions, a temperature inversion may form just above the crop.

Crop Height And Transpiration

Figure 12.9 Temperature and windvelocity profiles within and above a metre-high stand of barley at Rothamsted, southern England, on 23 July 1963 at 01:00 to 02:00 hours and 13:00 to 14:00 hours.

Figure 12.9 Temperature and windvelocity profiles within and above a metre-high stand of barley at Rothamsted, southern England, on 23 July 1963 at 01:00 to 02:00 hours and 13:00 to 14:00 hours.

noon sunset rn=671 LE-706 h = 59 | rn = 21 le=150h=150

I 14

soil

co 400-z

O cr

co 400-z

B

// // // // //

\ \ \ ^^

// s/

G_ \

hour

Figure 12.10 Energy flows involved in the energy diurnal balance of irrigated Sudan grass at Tempe, Arizona, on 20 July 1962.

Source: After Sellers (1965).

2 Wind speed. This is at a minimum in the upper crop canopy, where the foliage is most dense. There is a slight increase below and a marked increase above.

3 Water vapour. The maximum diurnal evapotranspiration rate and supply of water vapour occurs at about two-thirds the crop height, where the canopy is most dense.

4 Carbon dioxide. By day, CO2 is absorbed through photosynthesis of growing plants and at night is emitted by respiration. The maximum sink and source of CO2 is at about two-thirds the crop height.

Finally, we examine the conditions accompanying the growth of irrigated crops. Figure 12.10 illustrates the energy relationships in a 1-m high stand of irrigated Sudan grass at Tempe, Arizona, on 20 July 1962. The air temperature varied between 25 and 45°C. By day, evapotranspiration in the dry air was near its potential and LE (anomalously high due to a local temperature inversion) exceeded Rn, the deficiency being made up by a transfer of sensible heat from the air (H negative). Evaporation continued during the night due to a moderate wind (7 m s-1) sustained by the continued heat flow from the air. Thus evapotranspiration leads to comparatively low diurnal temperatures within irrigated desert crops. Where the surface is inundated with water, as in a rice paddy-field, the energy balance components and thus the local climate take on something of the character of water bodies (see B, this chapter). In the afternoon and at night the water becomes the most important heat source and turbulent losses to the atmosphere are mainly in the form of the latent heat.

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