1.1 Relationship of this manual to foregoing training manuals

1.2 Some definitions

1.3 Summary of the manual

Training Manual 3, "Irrigation Water Needs", gives an introduction to crop water needs and discusses the influence of climate, crop type and growth stage. It also discusses how much of the crop water need can be met by rainfall and/or irrigation applications. Further, Manual 3 introduces the special requirements of growing rice, where water is needed to soak the soil before preparing the land and to establish a water layer on the field. Finally, Training Manual 3 looks at the irrigation water needs within a parcel of land at the plant level. This amount of water is presented as the crop's consumption per day or per month, regardless of how the supply is organized.

Training Manual 4, "Irrigation Scheduling", explains how irrigation water is normally delivered at the field, not continuously or even every day, but at intervals of several days. The manual discusses the symptoms of water shortages and the impact of drought on yields. It describes three methods of determining the timing for the next irrigation application. A separate chapter is devoted to irrigation scheduling of rice crops. The basic concepts of irrigation efficiency are explained in an Annex. Thus with the help of Manuals 3 and 4, field technicians can determine the irrigation water needs at the crop level as well as estimate the proportions in which it should be supplied.

In Training Manual 5, "Irrigation Methods", the discussion shifts from the crop level to the field level and the different methods of applying water at the field level are described.

This Training Manual, which is the sixth in the series and entitled "Scheme Irrigation Water Needs and Supply", takes the subject of irrigation water needs two steps further: from the level of an individual plant or crop to that of a field, and from an individual field to an entire scheme comprised of many fields and various crops.

With the help of this manual, field technicians and extension agents will be able to compare the irrigation needs of a given area with the amount of water that is available in that area. Thus, this manual focuses on a scheme's irrigation water needs, on the one hand, and its water supply on the other. To make a reasonably accurate prediction of an area's irrigation need, the manual will introduce the concepts of a cropping calendar and cropping pattern. Although the more complex subjects, such as river hydrology, are beyond the scope of these manuals, technicians will be able to learn the basics of predicting a scheme's water requirements and assessing the amount of water that is available from a specific water source. They will also learn how to cope when a given situation is likely to change. In addition, the manual provides a set of five water management methods to help technicians cope with different situations of water shortage.

A number of terms will be used in this training manual which refer to the quantity and flow of water for irrigation. All these terms are listed and defined below and their interrelationship is indicated in the diagram in Annex I.

**IN**_{net}**: Net Irrigation Need ** (mm/day, mm/month, l/s/ha)

This is the crop's irrigation need, expressed as a layer of water in millimetres per day, or month, or any other period of time. Water losses are not included in theIN_{net}. This water layer can and often will be converted into a continuous flow of water per unit area, and is then often expressed in litres per second per hectare (8.6 mm/day =1.0 l/s/ha). Training Manual 4 contains a conversion table which converts mm per day into litres per second per hectare. This table is again attached to this manual as Annex II. How theIN_{net}is determined is explained in Training Manual 3.

**IN**_{gross}** Gross Irrigation Need ** (mm/day, mm/month, l/s/ha)

This is the crop's irrigation need plus the water needed to compensate for all losses that occur between the intake and delivery of the irrigation water to the crop. The irrigation efficiency,e, is a measure of all the losses.

IN_{gross }is expressed as a water layer in mm/day or month, or as a continuous flow of water per unit area in l/sec/ha. The formula appears below.

**IN**_{gros}_{s}** = IN**_{net}** **x 100/e

**SIN**_{net}**: Net Scheme Irrigation Need** (l/s)

This is the net irrigation need, not per hectare, but for the whole irrigation area. Therefore theSIN_{net}is no longer expressed in mm/day, but rather as litres per second (l/s). The formula is shown below.

**SIN**_{net }**= IN**_{net}** **x Scheme Area (l/s)

**SIN**_{gross}**: Gross Scheme Irrigation Need** (l/s)

This is the irrigation need for the entire area plus the extra water needed to compensate for losses during delivery. The formula is shown below.

**SIN**_{gross}** ***=*** IN**_{net}** **x Scheme area = (**IN**_{net} x 100/e) x Scheme area (l/s)

**SIN**_{op}**: Operational Scheme Irrigation Need ** (l/s)

This is the actual flow of water which should be delivered to the scheme according to a predetermined irrigation schedule. The total volume of water for the entire irrigation season is the same forSIN_{gross}andSIN_{op}, but the operational factors make the flow ofSIN_{op}larger thanSIN_{gross}.Generally, irrigation is carried out not every hour of every day, but only during specific hours of certain days. Thus, the larger flow of water for SIN is needed to compensate for the times when irrigation water is not being delivered. This timing, or scheduling of irrigation is calledoperational criteria(T_{op}).

**SWS: Scheme Water Supply ** (l/s, m^{3}/s)

This is the flow of water that can be withdrawn for irrigation at the water source. For schemes that depend on pumped water, theSWScan never be greater than the capacity of the pump installed. For schemes that do not depend on pumps, theSWScan not exceed the capacity of the intake facility. The scheme water supply is expressed in litres per second. Sometimes, if water flows are large, the supply will be expressed in m^{3}per second.

In discussing the various kinds of water sources and differences in the availability of water, this manual provides the important link between the availability pattern of water and the irrigation need of a scheme. The two must be reconciled with each other. The gross scheme irrigation need, **SIN**_{gross}, is estimated in the first instance, in a very simple way. The net irrigation need, **IN**_{net}, is given as a standard figure. Then the **IN**_{net}** **of all the individual fields are integrated into the net scheme irrigation need, **SIN**_{net}. How much water should be added to the **SIN**_{net} to make up for losses during the transport, distribution and application of water to the field is also discussed and added to the equation to determine the **SIN**_{gross}. **SIN**_{gross} is still expressed as a continuous required flow of water. The effect of the** operational criteria,** which are criteria that describe the duration of the irrigation supply per day and per week, on the actual required water supply is also discussed. The simple calculation method of the **SIN**_{gross} is replaced by a somewhat more complex method which is based on a cropping calendar and a cropping pattern. Both of these concepts are explained in the manual. Finally, the manual discusses how to match the calculated scheme irrigation need with the available water supply.