--- title: "Soil moisture forms; factors affecting CWR" subject: "Irrigation and Drainage Engineering" chapter: 2 key: "semester_6/irrigation_and_drainage_engineering:soil-moisture-forms-cwr-factors" source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/soil-moisture-forms-cwr-factors site: BCT Engineering --- # Soil moisture forms; factors affecting CWR Irrigation and Drainage Engineering · Chapter 2: Irrigation Water Requirements - Asked in: 2062 Baisakh (old course), 10 marks - Video: [Irrigation Requirement of Crops - Water Requirement of Crops - Water Resource Engineering 1](https://www.youtube.com/watch?v=RPRK7hIVbHg) — Ekeeda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/soil-moisture-forms-cwr-factors - Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-2 ## Question Illustrate various forms of soil moisture and write down the factors affecting crop water requirement. ## Short answer Gravity water drains freely above field capacity. Capillary water between field capacity and permanent wilting point is available to plants; hygroscopic water is adsorbed and unavailable. Crop water requirement depends on climate, crop type and stage, soil, water table, effective rainfall, irrigation method and efficiency, agronomic practices, and salinity. ## Answer #### Forms of soil moisture Soil water exists in several forms, depending on how strongly it is held by the soil particles. ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which form of soil water is the main source for plants? A. Gravity water B. Capillary water C. Hygroscopic water D. Chemical water Answer: B. Held in small pores, the part above PWP is available. (Key point: Capillary water) 2. Gravity water drains out within A. Several months B. 15 minutes C. 1-3 days D. It never drains Answer: C. It moves down in the large pores above FC. (Key point: Gravity water) 3. What effect does hot, dry, windy weather have on crop water requirement? A. It reduces CWR B. It has no effect C. It only affects percolation D. It increases evapotranspiration and CWR Answer: D. High temperature and wind raise ET. (Key point: Climate effect on CWR) 4. How does a shallow water table affect CWR? A. Capillary supply reduces irrigation demand B. It increases demand C. It makes the soil saline only D. No effect Answer: A. Water rises by capillarity to the roots. 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