WATER STORAGE IN THE SOIL
SOIL MANTLE AND WATER:
Soil Mantle is upper/ outermost part of earth up to the depth of 30-35 km. Soil being a zone of weathered rock material is subjected to seasonal variation in moisture content. Soil mantle in any catchment provides water for stream flow.
Water received on the soil surface is called Precipitation. This water is distributed according to the following illustration:
- Water infiltrated is important to flood danger and control.
- Surface runoff causes erosion, sedimentation and Siltation, and floods.
- Soil characteristics, vegetation type by land use affect the absorption and hydrological behavior of catchment.
SOIL MOISTURE EQUILIBRIUM POINTS:
- Saturation Point
- Field Capacity
- Wiling Point
- Available Soil Moisture
When precipitation occurs if is absorbed by the soil. The soil has macro- and micro-pores. When both the macro and micropores of the soil are filled with water and there is no more capacity to be absorbed. Such a condition is called Saturation Point.
Field Capacity (F.C)/ Capillary Capacity:
When drainage is complete or becomes infinitely slow, the soil has reached a level of moisture which is called F.C.
Moisture content at which permanent wilting of plants occurs. The wilting point represents the soil moisture level when plants cannot interact water from the soil and only the hygroscopic water is available. It is that condition when both types of pores (ie macro and micro) are devoid of water.
Available Soil Moisture:
The difference b/w the moisture content at field capacity and the wilting point is called available moisture. It represents the useful storage capacity of the soil and the maximum water available to plants.
HYDROLOGIC CHARACTERISTICS OF SOIL:
- Mineral soil originates from weathering of rocks. Continuous weathering adds more mineral soil (regolith) making soil depth
- The process of soil formation is balanced by erosion. Where occurs vegetation, organic matter is added increasing vol of soil more than weathering of rocks.
Soil depth is most important in relation to the hydrologic point as soil depth determines the storage capacity of water by the soil.
- The more deep soil has more storage capacity. Depth’s linear content depends upon:
- Storage Capacity
- Soil type (2.5 cm deep clay and 15 cm deep sand store 7.5 mm water)
- Soil particle size/ pores and their size. Pores occupying generally ½ of soil mass contain water. More airless water and vice versa.
- Pore type and size
- Capillaries having a diameter less than 0.05 mm
- Non-capillary pores dia less than 0.05 mm
- Non-capillary pores have less storage capacity because water moves freely under gravitational pull and adhesive forces are weak.
- Capillary pores retain and store more water as they have dia less than 0.05 mm. These are filled for more periods due to strong adhesive forces.
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