Irrigation Training Manual: Planning, Design, Operation and Management of Small-Scale Irrigation Systems (Peace Corps, 1994, 151 p.)
 Training session
 Section 5: Assessing irrigation water requirements
 (introduction...) Exam: Section 5 - Calculating water requirements

### (introduction...)

* Estimating Net Crop Water Needs
* Estimating the Efficiency of Irrigation Systems
* Estimating Gross Daily Irrigation Requirements and Design Capacity
* Exam: Section 5 (Calculating Water Requirements)

Session Topic: Estimating Net Crop Water Needs

Session Goal: Trainees will be able to estimate net crop water requirements using local crop data.

Session Objectives:

(1) Trainees will be able to define crop water use and the factors that determine the crop water deficits that are compensater for through irrigation.

(2) Trainees will be able to estimate net irrigation requirements using local crop and climate data.

Overview: Estimating net irrigation requirements is an essential prerequisite for the design and management of farm irrigation systems. Trainees will now apply skills learned in estimating water supplies and evaluating soil conditions to determine how much water must be made available to support specific crops.

 Session Activities: Time 60 Min. Trainer will begin session by describing several crops and then ask Trainees to describe the factors that determine how much water is needed to support these crops. Trainer should list these factors on a flip chart and then begin a brief lecture that links the factors identified by the Trainees to the concepts of evapotranspiration, reference crop evapotranspiration, crop coefficients, dependable and effective rainfall, ground water contribution, and soil water storage. Trainer should be able to describe each of these concepts graphically, assigning mathematical symbols for each concept. 60 Min. Trainer then uses one or two basic examples to demonstrate the use of equations and simple techniques for quantifying evapotranspiration, effective rainfall, and net irrigation requirements. Next, trainer pins up local crop, temperature, and rainfall data, and assigns each Trainee two or three specific crops. Trainees are to use the local data to estimate the net irrigation requirements for each crop. Trainees compare results at the conclusion of the exercise, and trainer describes the role of estimating net crop water needs in planning an irrigation system design.

Trainer Notes: Trainers need to obtain local data before the session. Climatic data should be available from government or university sources. Crop data may be scarce, and it may be necessary for the trainers to assemble this information from their own field observations and professional experience.

Materials Required:

* calculator
* local data on crop types, length of growing season, crop development stages
* local data on maximum and minimum temperatures, monthly rainfall

Selected References:

Chapter 4, Irrigation Reference Manual: Estimating Irrigation Requirements International Irrigation Center Modules #7, #8, and #9

Session Topic: Estimating the Efficiency of Irrigation Systems

Session Goal: Trainees will be able to estimate how efficiently water is being conveyed and applied to fields.

Session Objectives:

(1) Trainees will be able to define application and conveyance efficiencies and list the factors that determine the efficiency of an irrigation system.

(2) Trainees will be able to estimate the expected irrigation efficiency as a function of irrigation method used.

(3) Trainees will be able to estimate conveyance efficiencies as a function of soil type and canal conditions.

Overview: Irrigation efficiencies must be estimated correctly if crop water needs are to be met by the irrigation system. Thus it is important that Trainees be able to estimate efficiency with some accuracy. This session represents a good linkage with the session on estimating crop water requirements to begin to show how to estimate total project water needs.

 Session Activities: Time 45 Min. Trainer begins session with a profile drawing of an irrigation canal and irrigated field and an inflow of water on one end. Be certain to include some plants and debris in the drawing alongside the canal. Trainer asks Trainees to describe all of the ways water moves through this profile, adding to the drawing the concepts of infiltration, soil water movement, and evapotranspiration. Trainer describes the goal of irrigation as supplying limited water supplies to the greatest possible area for crop cultivation. Trainees then list all of the factors that must be taken into account to determine how efficiently water is being moved through conveyance channels and onto fields. Trainer then links this description of factors to concepts in simple equations that can be used to estimate canal and field efficiencies. Trainer uses one or two simple examples to show how these efficiencies are calculated. 75 Min. Trainer leads Trainees into the field and instructs groups of three to record visual observations of soils, irrigation method, and canal conditions. Each group should then estimate canal and field application efficiencies.

Trainer Notes: Trainer may want to have Trainees measure inflow and outflows from a canal if time permits and if Trainees have already had sessions enabling them to do this calculation. Measuring flows will give them practice in measuring conveyance efficiencies.

Materials Required:

* calculator
* flow measurement equipment if efficiencies are to be measured
* transport

Selected References:

Chapter 4, Irrigation Reference Manual: Estimating Irrigation
Requirements
International Irrigation Center Modules #29, #30, and #31

Session Topic: Estimating Gross Daily Irrigation Requirements and Design Capacity

Session Goal: Trainees will be able to estimate daily irrigation requirements as a depth of water to be applied and use this information to determine the amount of water required to support an irrigation system.

Session Objectives:

(1) Trainees will be able to calculate the gross daily irrigation requirements for a system as a depth of water to be applied and as a flow rate required.

Overview: The gross water requirements of an irrigation system must be known before we can determine whether the available water supply is sufficient to meet the crop needs. This information is necessary to estimate canal and pipeline design capacities and effectively manage an overall irrigation system. This session builds on the experiences Trainees have had in the previous sessions on estimating net crop water requirements and estimating canal and field efficiencies.

 Session Activities: Time 45 Min. Trainer provides some basic data about a proposed irrigation system: flow rate from an available stream, estimated crop water requirements, dimensions and materials for conveyance channels, and annual monthly rainfall. Trainer asks Trainees if they feel they have enough information here to design and manage the system. If not, Trainees should identify additional information they feel is necessary to facilitate a system design. Trainer records this information and uses it to derive equations that can be used to estimate gross water requirements, emphasizing estimates of both water depth and flow rates to meet the system needs. Trainees use the same data as were distributed in the sessions on estimating net crop water requirements and estimating canal and field efficiencies to calculate the gross depth of water required to satisfy a given local crop and the design flow rate required on a per area basis.

Trainer Notes: This session should immediately follow and build on the previous session plans in this section so that the concepts tie together easily.

Materials Required:

* calculator
* local climatic and crop data
* results from previous sessions

References:

Chapter 4, Irrigation Reference Manual: Estimating Irrigation
Requirements
International Irrigation Center Modules #38, #39, and #40

### Exam: Section 5 - Calculating water requirements

1. List 5 factors that you must quantify in order to determine the design capacity of an irrigation system.

Crop water use
Effective rainfall
Irrigation efficiencies
Hours per day of operation
Area to be irrigated

2. In February, at a station with a latitude of 14°N, daily maximum average temperatures are 28°C and average daily minimum temperatures are at 15°C. What ETo might you expect?

Ra = 13.6 mm/day (from Table 4.1, Irrigation Reference Manual), Td = 28 - 15 = 13°C
T° C = 1/2(28 + 15) = 21.5
ETo = 0.0023 x Ra x Td1/2 x (T° C + 17.8) = 0.0023 x 13.6 x 131/2 x (21.5 + 17.8) = 4.43 mm/day

3. With low wind and high humidity conditions, what maximum Kc might you expect for a lettuce crop at mid-season?

Kc is approximately that of cabbage and equals
0.95 for low advective conditions (from Table 4.3, Irrigation Reference Manual).

4. What ETc might you expect at mid-season for the conditions of Problems 2 and 3?

ETc = Kc x ETo = 0.95 x 4.43 = 4.2

5. What ETc might you expect initially in the season for the lettuce crop if ETo is 5 mm per day?

ETc = 0.4 x 5.0 = 2.0 mm/day

6. If your mean rainfall is 50 mm for February and is 80% effective, what is the average monthly effective rainfall (Pe) in mm/month? mm/day?

 Pe = Pmean x effectiveness = 50 mm x 0.8 = 40 mm/month = 40/28 = 1.4 mm/day

7. What is the net daily water requirement (Ire) with the ETc of Problem 4 and the Pe of Problem 6? Assume Wb and Gw = 0.

Irn = ETc - Pe = 4.2 - 1.4 = 2.8 mm/day

8. What is the gross daily irrigation requirement (Irg) for the example of Problem 7 if the efficiency of application is 70% and the conveyance efficiency is 80%?