Lab Soils 456 on  irrigating an Alfalfa field

Introduction

An alfalfa dead level field is border irrigated. The field is 300 ft wide and 400t long  and the farmers can open all the 6 slide gates from a trapezoidal canal at the head of the field or he can open two slide gates and irrigate a third of the field at a time. He would like to minimize the amount of time he spends in the field because he gets water from the irrigation district at night and would like to open the water and then set the alarm clock to check the water at the end of the irrigation time. However, he also would like to get a uniform application of water over the field. The flow to the cannel is controlled by Manning's equation because the field is a mile and a half from the main irrigation district  turnout. The slope of the supply cannel is 0.00001ft of drop /ft of length of the cannel. The cannel is trapezoidal in shape with base of 1 ft  and side slopes of 1:1. The water is at a high of 3 ft in the cannel.   The soil is a clay loam. See http://www.lmnoeng.com/manningn.htm for Manning's roughness coefficient. 

Question

How many gates should he open at a time.

Solution

First calculate the flow rate in the cannel using Manning equation. The wetted perimeter of the water in the cannel is 1ft +2 * (3^2+3^2)^0.5=9.4 ft . Area of the water is 1ft*3ft + 3ft*3ft= 12ft^2. Using Manning's Equation  and a n value of  0.03, the flow is 2.2 ft^3/sec. 

Using the basin border model calculate optimal width of the field for the given flow and how many gates should he open..

 

Extra credit activity.  Evaluative the width of the boarders for optimal operation if the field was a  a graded border with the slope 0.0005. Down load the graded border model and run and determine the optimal width.