--- title: "Irrigation interval with AMC, RAM graph" subject: "Irrigation and Drainage Engineering" chapter: 2 key: "semester_6/irrigation_and_drainage_engineering:interval-ram75-graph" source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/interval-ram75-graph site: BCT Engineering --- # Irrigation interval with AMC, RAM graph Irrigation and Drainage Engineering · Chapter 2: Irrigation Water Requirements - Asked in: 2075 Baisakh, 5+2 marks - 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/interval-ram75-graph - Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-2 ## Question The field capacity of soil is 40%, Permanent wilting point is 20%, Density of soil is 1.2 gm/cc, effective root depth is 90 cm, ET crop is 10 mm/day. Calculate the irrigation interval (IR) if the readily available moisture (RAM) is 75% of available soil moisture capacity and show AMC, RAM and irrigation interval on graph of available moisture and time. ## Short answer Available moisture capacity is 216 mm. With 75% RAM (162 mm), irrigation interval is 16.2 days (about 16 days). ## Answer **Given:** FC = 40%, PWP = 20%, $\gamma_d = 1.2$, $D_r = 900$ mm, $ET_c = 10$ mm/day, RAM = 75% of AMC. **Step 1: Available moisture capacity (AMC)** $$ AMC = \frac{40-20}{100}\times 1.2\times 900 = 216\ \text{mm} $$ **Step 2: Readily available moisture (RAM)** $$ RAM = 0.75\times 216 = 162\ \text{mm} $$ **Step 3: Irrigation interval** $$ IR = \frac{RAM}{ET_c} = \frac{162}{10} = 16.2\ \text{days}\approx 16\ \text{days} $$ **Graph of available moisture against time** ```text Avail. moisture (mm) 216 |= = = FC (AMC = 216 mm, full) = = = = |\ | \ RAM = 162 mm used | \ (75% of AMC) 54 |- -\- - - - - - - - - - - - - - - - limit of RAM | \ slope = ET = 10 mm/day | 0 |===================================== PWP 0 16.2 time (days) irrigate again at 16 days ``` Moisture falls from AMC (216 mm available, at FC) linearly at 10 mm/day. When 162 mm (RAM) has been used, i.e. after 16.2 days, the available moisture is 54 mm; irrigation is given to bring it back to FC. The remaining 25% of AMC (54 mm) is kept as reserve so the crop does not reach PWP. **Answer: irrigation interval = 16.2 days, say 16 days.** ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. AMC = A. 162 mm B. 54 mm C. 216 mm D. 180 mm Answer: C. 0.2 x 1.2 x 900. (Key point: AMC) 2. On the graph, what remains when RAM has been used? A. 0 B. 216 mm C. 162 mm D. 54 mm reserve (25% of AMC) Answer: D. 216-162. (Key point: Reserve) 3. The slope of the depletion line is A. ET_c = 10 mm/day B. RAM C. 1.2 D. 16.2 mm/day Answer: A. Moisture falls linearly. (Key point: Slope) 4. The interval is A. 21.6 days B. 16.2 days, about 16 C. 5.4 days D. 162 days Answer: B. 162/10. (Key point: Interval) ## Close variants set in other exams - [Irrigation interval: FC 40%, PWP 20%](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Ainterval-fc40-pwp20-root90) — Irrigation and Drainage Engineering, chapter 2, asked 4× - [Irrigation interval: FC 60%, PWP 25%](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Ainterval-fc60-pwp25-ram85) — Irrigation and Drainage Engineering, chapter 2, asked 4× ## Related in Irrigation and Drainage Engineering - [AMC, RAM, depth and interval; headworks depth](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Amoisture-depth-interval-etc9) — Irrigation and Drainage Engineering, chapter 2, asked 2× - [AMC, RAM and interval: FC 80%, PWP 35%](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Amoisture-fc80-pwp35-root60) — Irrigation and Drainage Engineering, chapter 2, asked 2× - [Irrigation interval: FC 32%, PWP 13%](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Ainterval-fc32-pwp13-root130) — Irrigation and Drainage Engineering, chapter 2 - [Irrigation interval: FC 28%, PWP 13%](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Ainterval-fc28-pwp13-root70) — Irrigation and Drainage Engineering, chapter 2, asked 2× - [Depth and frequency of irrigation](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Adepth-frequency-fc80-pwp30) — Irrigation and Drainage Engineering, chapter 2 - [Irrigation interval: FC 27%, PWP 14%](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Ainterval-fc27-pwp14-depth75) — Irrigation and Drainage Engineering, chapter 2, asked 2×