Soil moisture forms; factors affecting CWR

Forms of soil moisture

Soil water exists in several forms, depending on how strongly it is held by the soil particles.

  Moisture content (%)
 |  Saturation  ........ gravity water (drains out)
 |  ---------------------------------- FC (1/3 atm)
 |    capillary water    available
 |                       moisture
 |  ---------------------------------- PWP (15 atm)
 |    capillary water    (not available)
 |  ---------------------------------- hygroscopic coefficient (31 atm)
 |    hygroscopic water  (not available)
 |  ---------------------------------- oven dry

1. Gravity (free) water: water in the large pores above field capacity, which moves down under gravity. It drains in 1-3 days and is not stored for plant use; any excess causes waterlogging.
2. Capillary water: water held in the small pores by surface tension, between field capacity and the hygroscopic coefficient. This is the main source of water for plants. The part above PWP is the available moisture.
3. Hygroscopic water: a thin film held on the soil particles by strong adhesive forces, absorbed from the humidity of the air (tension above 31 atm). It is not available to plants.
4. Chemical (combined) water: water in the chemical structure of minerals; unavailable.

Soil moisture constants

• Saturation capacity: all pores are filled with water.
• Field capacity: the upper limit of capillary water.
• Permanent wilting point: the lower limit of available water; the plant wilts permanently.
• Available moisture: $FC - PWP$; readily available moisture: the part of it extracted without stress.
• Moisture equivalent and hygroscopic coefficient are related constants defined in the laboratory.

Factors affecting crop water requirement

1. Climate: temperature, humidity, wind speed and sunshine; hot, dry and windy weather increases $ET$.
2. Crop type: rice and sugarcane need much more water than wheat or pulses; leaf area and root depth matter.
3. Growth stage and crop period: the mid-season stage needs the most water; the length of the crop period decides the total.
4. Soil: texture, water-holding capacity, infiltration and percolation losses.
5. Water table depth: a shallow water table supplies capillary water and reduces demand.
6. Rainfall: effective rainfall reduces the irrigation need.
7. Method of irrigation and efficiency: drip uses less water than flooding.
8. Cultural practices: time and method of sowing, tillage, mulching, weed control and plant density.
9. Salinity: the need for leaching raises water demand.
10. Land preparation and season: puddling for rice, and the kharif or rabi season.