Specify the important definitions: Reference crop evapotranspiration (ET0) (slide 07 – 13 )

ET0 is the rate of evapotranspiration from a large area, covered by green grass, 8 to 15 cm tall, which grows actively, completely shades the ground and which adequately provided with water (FAO). ET0 is the rate of evapotranspiration from a large area, covered by green grass, 8 to 15 cm tall, which grows actively, completely shades the ground and which adequately provided with water (FAO).

Reference evapotranspiration (ET0), sometimes incorrectly referred to as potential ET, is a representation of the environmental demand for evapotranspiration and represents the evapotranspiration rate of a short green crop (grass), completely shading the ground, of uniform height and with adequate water status in the soil profile. It is a reflection of the energy available to evaporate water, and of the wind available to transport the water vapour from the ground up into the lower atmosphere. Actual evapotranspiration is said to equal reference evapotranspiration when there is ample water. Some US states utilize a full cover alfalfa reference crop that is 0.5 m in height,rather than the short green grass reference, due to the higher value of ET from the alfalfa reference.

Evaporation and transpiration occur simultaneously and there is no easy way of distinguishing between the two processes. Apart from the water availability in the topsoil, the evaporation from a cropped soil is mainly determined by the fraction of thesolar radiation reaching the soil surface. This fraction decreases over the growing period as the crop develops and the crop canopy shades more and more of the ground area.

When the crop is small, water is predominately lost by soil evaporation, but once the crop is well developed and completely covers the soil, transpiration becomes the main process. Factors affecting evapotranspiration Weather parameters, crop characteristics, management and environmental aspects are factors affecting evaporation and transpiration.

 

Specify the important definitions: Crop factor (KC)

The crop type, variety and development stage should be considered when assessing the evapotranspiration from crops grown in large, well-managed fields. Differences in resistance to transpiration, crop height, crop roughness, reflection, ground cover and crop rooting characteristics result in different ET levels in different types of crops underidentical environmental conditions.

Crop coefficient is the ratio between crop ET and potential ET. Crop coefficient depends on soil cover, soil moisture and crop height. At early stage crop covers only a fraction of soil and covers as it matures. The rate at which plants grow and cover the soil depends on the crop. In the early stage main component of ET is evaporation, while under fully covered condition transpiration is the main component. ET of a crop depends on plant height and leaf area index. Crop coefficients vary with relative humidity and wind velocity. For most of the crops, Kc value increases from a low value at the time of emergence to a maximum value during the period when the crop reaches flowering and

declines as the crop approaches maturity.

 

Specify the influence of various factors on water use (slide 07 – 3-4 )

Main physical characteristics (4)

Temperature

Sediment

Color

Odor;

Main chemical characteristics (6)

1. Most common anions and cations

2. The total salt concentration

3. The relative quantities of dissolved sodium salts and magnesium salts in water: Na

content (%), sodium adsorption ratio (SAR), ratio Mg/(Mg+Ca) (%).

4. The phenolphthalein alkalinity

5. The carbonate / bicarbonate content

6. The specific pollutants(BOD, COD, TOC, heavy metals, pesticides, microorganisms,

pahs, petroleum derivatives, pcbs, detergents, antibiotics, etc)

 

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