--- title: "Necessity and methods of drainage of irrigated land" subject: "Irrigation and Drainage Engineering" chapter: 9 key: "semester_6/irrigation_and_drainage_engineering:drainage-necessity-methods" source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/drainage-necessity-methods site: BCT Engineering --- # Necessity and methods of drainage of irrigated land Irrigation and Drainage Engineering · Chapter 9: Water Logging and Drainage - Asked in: 2078 Poush; 2079 Jestha, 3 marks; 2065 Kartik (old course), 7 marks (asked 3 times) - Video: [Drainage Requirements of Irrigated lands](https://www.youtube.com/watch?v=0qqlVN4ZYNE) — UCESTH JNTUH - 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/drainage-necessity-methods - Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-9 ## Question Explain why the drainage of irrigated land is necessary. Describe the various methods (types of drainage systems) employed for such drainage. ## Short answer Drainage prevents waterlogging, maintains soil aeration, controls salinity, and removes excess water. Methods include surface open drains (field, collector, main), subsurface tile/mole drains, vertical pumping from wells, and interceptor drains along canals or slopes. ## Answer **Drainage** is the removal of excess surface water and groundwater from land so that the crops have suitable soil moisture and aeration. Irrigation without drainage raises the water table and leads to water logging and salinity. #### Why drainage of irrigated land is necessary - To prevent **water logging**: the water table must be kept below the root zone (at least 1.5 m below ground). - To maintain soil aeration, so that roots get oxygen and soil bacteria can work. - To prevent **salinity and alkalinity** by leaching and by removing the salts brought up by capillary action. - To remove excess rainfall (floods) quickly, especially in the Terai paddy areas. - To allow cultivation, machinery work and timely sowing, and to raise the soil temperature. - To control mosquito-borne diseases and to protect roads, canals and buildings. #### Methods of drainage ```text Drainage |-- Surface drainage (open drains) |-- Subsurface drainage (tile, mole, vertical) '-- Interceptor / relief drains ``` 1. **Surface drainage:** removal of excess surface water through open drains. Field drains collect water from the fields, collector drains gather it from several field drains, and main (outfall) drains carry it to a natural stream. Land levelling and grading help. It suits flat fields and heavy rainfall, as in Terai paddy fields. 2. **Subsurface drainage:** lowering of the water table by buried drains. - *Tile (pipe) drains:* perforated pipes or clay/concrete tiles laid at 1.5–2.5 m depth with laterals and collectors; the spacing is found from Donnan's/Hooghoudt's formula. - *Mole drains:* unlined cylindrical channels formed in clay soil by a mole plough, 0.5–1 m deep. - *Open ditches (deep drains)* to control the water table over large areas. 3. **Vertical drainage:** pumping from tube wells to lower the water table where the aquifer is permeable. The pumped water can be used for irrigation. 4. **Interceptor drains:** deep drains laid along the foot of a canal or hill slope to intercept seepage before it reaches the fields. 5. **Natural drainage improvement:** cleaning and straightening natural streams. The choice depends on the soil permeability, depth of the impervious layer, rainfall, slope and cost: surface drains in flat, low-permeability, high-rainfall areas; tile or vertical drainage where a high water table is the problem. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is drainage of irrigated land necessary? A. To keep the water table below the root zone, give aeration and prevent salinity B. To increase water table height for crops C. To increase seepage from canals D. To keep soil permanently saturated Answer: A. Without drainage the water table rises, causing water logging and salinity; drainage maintains aeration and leaches salts. (Key point: Need for drainage) 2. How does surface drainage work? A. Buried perforated pipes lower the water table B. Open field, collector and main drains carry excess water to a natural stream C. Tube wells pump groundwater D. Deep drains intercept canal seepage only Answer: B. Field drains feed collector drains, which feed main (outfall) drains leading to a natural stream. (Key point: Surface drainage) 3. Which is a subsurface drainage method? A. Field drains on the surface B. Land levelling C. Tile (pipe) drains laid at 1.5-2.5 m depth D. Flap gates at outlets Answer: C. Subsurface drainage uses buried drains - tile, mole or deep ditches - to lower the water table. (Key point: Subsurface drainage types) 4. What is the purpose of interceptor drains? A. To pump irrigation water B. To store flood water C. To raise the water table D. To intercept seepage along the canal or hill foot before it reaches the field Answer: D. Deep drains along the foot of a canal or slope catch seepage before it enters cultivated land. (Key point: Interceptor drains) ## Related in Irrigation and Drainage Engineering - [Short note: Design of drainage of irrigated land](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Adrainage-irrigated-land-design-note) — Irrigation and Drainage Engineering, chapter 9 - [Reservoir capacity with irrigated areas](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Areservoir-capacity-areas-79j) — Irrigation and Drainage Engineering, chapter 2 ## Related in other subjects - [Domestic water supply and drainage systems](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Awater-supply-drainage) — Building Technology, chapter 12