--- title: "Forms of soil moisture with diagram" subject: "Irrigation and Drainage Engineering" chapter: 2 key: "semester_6/irrigation_and_drainage_engineering:soil-moisture-forms-diagram" source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/soil-moisture-forms-diagram site: BCT Engineering --- # Forms of soil moisture with diagram Irrigation and Drainage Engineering · Chapter 2: Irrigation Water Requirements - Asked in: 2065 Kartik (old course), 8 marks - Video: [Types of soil water and capillarity](https://www.youtube.com/watch?v=gdpDEb91NLk) — Concept Civil Engineering - 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-diagram - Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-2 ## Question Describe with the help of a diagram the various forms of soil moisture available to a plant. ## Short answer Water exists as gravitational (drains out), capillary (available between FC and PWP), hygroscopic (unavailable), and chemical water. FC is at ≈1/3 atm, PWP at 15 atm; available moisture = FC – PWP; readily available is the stress‑free portion. ## Answer 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is true of gravity water? A. It is the main source for plants B. It sits above FC, drains in 1-3 days and excess causes waterlogging C. It is held at 31 atm D. It is part of mineral structure Answer: B. Gravity water moves down and is not stored for plants. (Key point: Gravity water) 2. Which water is the main source for plants? A. Hygroscopic water B. Chemical water C. Capillary water held by surface tension in small pores D. Gravity water Answer: C. The part of capillary water above PWP is available moisture. (Key point: Capillary water) 3. Why is hygroscopic water unavailable? A. It drains by gravity B. It is too far below the water table C. It is only found in clay D. It is held by strong adhesive forces at tension above 31 atm Answer: D. Tension is greater than the roots can overcome. (Key point: Hygroscopic water) 4. Available moisture is the range between A. FC and PWP B. Saturation and FC C. PWP and oven dry D. Saturation and PWP Answer: A. AM = FC - PWP. (Key point: Soil moisture constants) ## Related in Irrigation and Drainage Engineering - [Soil moisture forms; factors affecting CWR](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Asoil-moisture-forms-cwr-factors) — Irrigation and Drainage Engineering, chapter 2 - [Field capacity, available moisture definitions](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Adefine-fc-am-ram-deficiency) — Irrigation and Drainage Engineering, chapter 2 - [Readily available moisture, PWP, interval](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Armc-pwp-irrigation-interval-terms) — Irrigation and Drainage Engineering, chapter 2 - [Soil moisture tension and related terms](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Asoil-moisture-tension-terms) — Irrigation and Drainage Engineering, chapter 2 - [Soil-moisture-irrigation relationship](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Asoil-moisture-irrigation-relationship) — Irrigation and Drainage Engineering, chapter 2, asked 2× - [Frequency of irrigation from soil moisture](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Afrequency-of-irrigation-soil-moisture) — Irrigation and Drainage Engineering, chapter 2 ## Related in other subjects - [Mass haul diagram and soil cement base](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Amass-haul-soil-cement-base-construction) — Transportation Engineering II, chapter 3 - [Combined plant components and T-s diagram](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-components-ts) — Power Plant Equipment, chapter 8, asked 4× - [Causes of moisture variation in subgrade](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Amoisture-causes) — Transportation Engineering I, chapter 4, asked 13× - [Digester volume for sludge, moisture 92%](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asludge-digester-volume-92) — Sanitary Engineering, chapter 8, asked 2×