Describe minidisk infiltrometer ( slide 02 – 8-9)

Because the Mini Disk Infiltrometer is a tension Infiltrometer, it measures the unsaturated hydraulic conductivity of the medium it is placed on at different applied tensions. Flow through an unsaturated soil is more complicated than flow through continuously saturated pore spaces. Macrospores generally fill with air, leaving only the finer pores to accommodate water movement. Therefore, the hydraulic conductivity of the soil is strongly dependent on the detailed pore geometry, water content, and differences in matric potential.

The Mini Disk Infiltrometer measures the hydraulic conductivity of the medium it is placed upon. Because the Infiltrometer has an adjustable suction (0.5 to 7 cm) you can get additional information about the soil by eliminating macro pores with an air entry value smaller than the suction of the Infiltrometer. This is done by controlling the infiltration with a small negative pressure or suction. When the water is under tension or suction, it does not enter macro pores such as cracks or wormholes, but goes further into and through the soil as determined by the hydraulic forces in the soil.

 

Переносной, вода в трубке. Работа основана на suction. Воздух может попасть только сверху.

 

9. Specify and the important definitions: saturated water content, field capacity, Wilting point, Permanent wilting point (s 7/11) , total available water capacity (slide 02 – 16-24)

Saturated conductivity occurs when all the pores, including the large ones (such as cracks or wormholes), are filled with water. Macrospores flow, however, is extremely variable from place to place, and therefore difficult to quantify. Infiltrating water under a tension prevents the filling of the macrospores and gives a hydraulic conductivity characteristic of the soil matrix, and is less spatially variable.

Unsaturated soil hydraulic conductivity is a function of water potential and water content of the soil. The decrease in conductivity as the soil dries is due primarily to the movement of air into the soil to replace the water. As the air moves in, the pathways for water flow between soil particles becomes smaller and more tortuous, and flow becomes more difficult.

• Saturated water content: soil is 100% saturated with water.

• Field Capacity (FC) is the amount of soil moisture or water content held in the soil after excess water has drained away and the rate of downward movement has decreased. Usually field capacity reaches after 3 days of draining. In the ground water areas where ground water is deep, the water capacity of the upper layers is practically the same as the minimum water capacity (0.33 bar-2.5pf).

• Permanent wilting point (PWP) or wilting point (WP) is defined as the minimal point of soil moisture the plant requires not to wilt. If moisture decreases to this or any lower point a plant wilts and can no longer recover its turgidity when placed in a saturated atmosphere for 12 hours.

• Saturated – FC = gravitation water

 

FC-WP=Total available water (TAW) is the range of available water that can be stored in soil and be available for growing crops.  As the water content above field capacity cannot be held against the forces of gravity and will drain and as the water content below wilting point cannot be extracted by plant roots, the total available water in the root zone is the difference between the water content at field capacity and wilting point.

Although water is theoretically available until wilting point, crop water uptake is reduced well before wilting point is reached. Where the soil is sufficiently wet, the soil supplies water fast enough to meet the atmospheric demand of the crop, and water uptake equals ETc. As the soil water content decreases, water becomes more strongly bound to the soil matrix and is more difficult to extract. When the soil water content drops below a threshold value, soil water can no longer be transported quickly enough towards the roots to respond to the transpiration demand and the crop begins to experience stress. The fraction of TAW that a crop can extract from the root zone without suffering water stress is the readily available soil water:

RAW = p TAW (83)

 

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