Some ET Definitions

Evaporation is the when liquid water is converted to vapour due to the addi­tion of energy. Evaporation of water occurs from any free water surface, such as a lake. Evaporation requires energy, which is mostly supplied by solar radiation, and to a lesser extent by air temperature. Evaporation will slowly decrease to zero as the humidity of the air above the water surface increases. Thus, wind is critical to carry moist air away, and replace it with drier air. In the context of soil evaporation, plants will shade the soil, reducing solar radiation and reducing evaporation.

Transpiration is the evaporation of liquid water from the plant tissues. Plants primarily lose water through pores in their leaves called stomata. Different plant species are able to regulate their stomata openings to greater or lesser degrees. Thus, different plants will transpire at different rates. Similar to evap­oration, the overall rate is determined by solar radiation, humidity and the wind. However, transpiration also depends on the status of the crop, cultiva­tion practices, etc.

Evapotranspiration (ET) is the combination of both evaporation and transpi­ration on vegetated surfaces. Both processes occur simultaneously and can­not be separated. In the case of crops sown in bare soil, after sowing ET is 100% evaporation but as the plants grow and shade the soil, the transpiration begins to dominate.

Reference ET (ET0) is the maximum ET that can be extracted from a stand­ard grass surface (reference crop) that is fully watered. The ET0 is a climate parameter and is dependent only on the climate variables. It is NOT depend­ent on the vegetation or crop. ET0 is defined by the Penman-Montieth method. The most widely used reference document for the Penman-Montieth method is the FAO-56 document by Allen et al (1998).

The units of ET are normally the same as precipitation, eg mm/day. Evapo­transpiration is a significant loss of water. One hectare is 10,000 m2. So an ET rate of 1mm/d is equivalent to 10 m3 of water per hectare per day.

Table 21.1          Average ET0 for different climatic regions in mm/day (from FAO-56)

Regions

Mean daily temperature

Cool (~10C)

Moderate (20C)

Warm (>30C)

Sub-/Tropics

 

humid/sub-humid

2-3

3-5

5-7

arid/semi-arid

2-4

4-6

6-8

Temperate

 

humid/sub-humid

1-2

2-4

4-7

arid/semi-arid

1-3

4-7

6-9

Leaf Area Index (LAI)

The Leaf Area Index (LAI) is defined as the one-sided, area of leaves above a unit area of the ground surface. Generalised time varying functions of the LAI for most crops and types of vegetation are available in the literature. In MIKE SHE, you must specify the temporal variation of the LAI for each vegetation type during the growing seasons to be simulated. Different climatic conditions from year to year may require a shift of the LAI curves in time but will gener­ally not change the shape of the curve. Typically, the LAI varies from 0 to a about 7 for forests and up to about 10 for dense plantation forests.

Root Depth

The root depth is defined as the maximum depth of active roots in the root zone.