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BRAE 340 Irrig. Water Management Name: HW 9 - Drainage & Salinity (only odd numbered problems will be graded) [C] 1. Over the course of an irrigation season, an irrigation system applies a gross amount of 35.5 inches. Pre-infiltration losses are 2.3 inches; leaching for salt removal is 2.8 inches; deep percolation due to poor timing is 4.3 inches; deep percolation due to non-uniformity is 5.1 inches; and the remainder is used to satisfy crop ET. Rainfall is negligible. What is the total amount of water that must be drained away to prevent build up of the water table? A) 10.5 inches B) 8.7 inches C) 12.2 inches D) 13.8 inches E) 15.5 inches Solution: Drainage must remove all deep percolation, including beneficial leaching for salt removal Deep Percolation = 2.8 in + 4.3 in + 5.1 in = 12.2 in [A] 2. Over the course of an irrigation season, an irrigation system applies a gross amount of 40 inches. Pre-infiltration losses are 3 inches; total deep percolation is 9.0 inches, of which 4 is beneficial for salt removal; and the remainder is used to satisfy crop ET. Rainfall = 0. What is the total amount of water that must be drained away to prevent build up of the water table? A) 9.0 inches B) 7.4 inches C) 5.1 inches D) 10.0 inches E) 6.4 inches Solution: Drainage must remove all deep percolation, including beneficial leaching for salt removal Deep Percolation = 9.0 in = total DP [B] 3. Peak crop ET = 0.25 in/day; LR = 4 %; pre-infiltration loss = 0. With good management, AE = 75 %. Rainfall = 0. A drainage system should be able to handle what drainage coefficient? A) 0.108 in/day B) 0.097 in/day C) 0.124 in/day D) 0.084 in/day E) 0.070 in/day Solution: Gross = net / [AE/100 x (1-LR)] = 0.25 in/day / [(75/100) x (1-(4/100))] = 0.347 in/day Deep Percolation = gross - ETc = 0.347 in/day - 0.25 in/day = 0.097 in/day All deep perc (including leaching) must be drained away. So, drainage coefficient = 0.097 in/day [A] 4. Peak crop ET = 0.29 in/day; LR = 4 %; pre-infiltration loss = 0. With good management, AE = 76 %. Rainfall = 0. A drainage system should be able to handle what drainage coefficient? A) 0.107 in/day B) 0.081 in/day C) 0.118 in/day D) 0.093 in/day E) 0.059 in/day Solution: Gross = net / [AE/100 x (1-LR)] = 0.29 in/day / [(76/100) x (1-(4/100))] = 0.397 in/day deep perc. = gross - ET = 0.397 in/day - 0.29 in/day = 0.107 in/day All deep perc (including leaching) must be drained away. So, drainage coefficient = 0.107 in/day Solution Key Prob. 5-13: Assume the depth to the impermeable barrier and the root zone depth which must remain free of saturation are known and fixed. If the item listed is INCREASED, determine whether the required drain spacing will [I] Increase or [D] Decrease. ... View Full Document