SPIS Toolbox - Calculate Water Requirements

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4. Calculate Water Requirements

Understanding when, where, and how much water is needed for agricultural production and other uses is crucial for efficient water management.

Crop Water Requirements

Crop water requirements are defined as the total amount of water needed to meet the water loss through evapotranspiration. In other words, it is the amount of water needed by the various crops to grow optimally.

The crop water requirement always refers to a crop grown under optimal conditions, i.e. a uniform crop, actively growing, completely shading the ground, free of diseases, and favourable soil conditions (including fertility and water). The crop thus reaches its full production potential under the given environment.

The crop water need mainly depends on:

• Climate: in a sunny and hot climate crops need more water per day than in a cloudy and cool climate
• Crop type: crops like maize or sugarcane need more water than crops like millet or sorghum
• Growth stage of the crop: fully grown crops need more water than crops that have just been planted.

Table 2: Impact of climate conditions on crop water needs (Source: FAO 1989)

Climate Factor	Crop water need
	High	Low
Temperature	hot	cool
Humidity	low (dry)	high (humid)
Windspeed	windy	little wind
Sunshine	sunny (no clouds)	cloudy (no sun)

Nevertheless, real-life conditions are rarely optimal and there are many other factors that also influence evapotranspiration rates. Factors such as soil salinity, poor land fertility, limited use of fertilizers and chemicals, lack of pest and disease control, poor soil management and limited water availability at the root zone may limit crop development and reduce evapotranspiration.

Other factors that affect evapotranspiration are groundcover and plant density. Cultivation practices and the type of irrigation system can alter the microclimate, affect the crop characteristics, and affect the wetting of the soil and crop surface.

Irrigation Water Requirements

Irrigation water requirements refer to the water that must be supplied through the irrigation system to ensure that the crop receives its full crop water requirements. If irrigation is the sole source of water supply for the plant, the irrigation requirement will always be greater than the crop water requirement to allow for inefficiencies in the irrigation system. If the crop receives some of its water from other sources (rainfall, water stored in the ground, underground seepage, etc.), then the irrigation requirement can be considerably less than the crop water requirement.

In humid climates, precipitation and soil moisture can be sufficient to ensure satisfactory growth in rainfed agriculture. In arid climates or during extended dry seasons, however, irrigation is necessary to compensate for the evapotranspiration (crop transpiration and soil evaporation) deficit due to insufficient or erratic precipitation.

In order to understand how much irrigation water is required, an analysis of the water balance is needed. There are three levels to such an analysis.

The first is the balance of agricultural and other demands within a watershed. This helps establish the safe yield of water from various sources and thereby how much irrigation is possible within sustainable margins (see module SAFEGUARD WATER).

Another water balance perspective is at farm (or command area) level. Fields are often not irrigated individually, but they share the water delivered through a canal or well. They also often share drainage channels. A water balance at the farm looks at the access to water, priority uses, timing and duration of irrigation. This is the basis for efficient design of the system and delivery of services.

A third perspective looks at the water requirements of crops in a field. Providing the crop with irrigation at the appropriate time and in the appropriate quantity requires experience and will depend on climate, rainfall, soil and crops stage, as well as the field irrigation system and irrigation technology used.

Special computerized irrigation programmes, such as the FAO CROPWAT programme, may be used to advise farmers on water supply and irrigation schedules for the given climatic conditions, crop, soil and field irrigation method.

The SAFEGUARD WATER – Water Requirement Tools and the IRRIGATE – Soil Tool may be useful in roughly assessing the crop water supply needed.

Irrigation Scheduling

Once the crop water and irrigation requirements have been calculated, the next step is the preparation of field irrigation schedules. Three parameters have to be considered in preparing an irrigation schedule:

• The daily crop water requirements
• The soil, particularly its total available moisture or water-holding capacity
• The effective root zone depth

Plant response to irrigation is influenced by the physical condition, fertility and biological status of the soil. Soil condition, texture, structure, depth, organic matter, bulk density, salinity, sodicity, acidity, drainage, topography, fertility and chemical characteristics all affect the extent to which a plant root system penetrates into and uses available moisture and nutrients in the soil. Many of these factors influence the water movement in the soil, the water-holding capacity of the soil, and the ability of the plants to use the water. The irrigation system used should be able to function under all or most of these conditions.

In the field, the actual value may vary from site to site, season to season and even within the season. Within the season, it varies depending on the type of farm and tillage equipment, number of tillage operations, residue management, type of crop, and water quality.

Soils to be irrigated must also have adequate surface and subsurface drainage, especially in the case of surface irrigation. Internal drainage within the crop root zone can either be natural or from an installed subsurface drainage system.

What Crops are Best Suited for Solar-powered Irrigation?

There are no crops that are particularly suitable (or unsuitable) for solar-powered irrigation as long as the irrigation method can meet crop water requirements and is compatible with farming practices, climate, water resources, and other agronomic aspects.

The cropping pattern should be such that the selected crop can be successfully grown under the prevailing climate and soil conditions, and the irrigation system should be compatible with crops and agricultural practices. Furthermore, adequate attention should be given to the selection of the crops and cropping calendar. Crops should be marketable at economic prices.

Selection of Suitable Crops for Irrigation

Specific agronomic aspects to be considered include the following:

• Cropping calendar of present common crops grown in the area during the wet and dry seasons, indication of seasonal hazards (drought, floods, pests and diseases)
• New crops with good potential to be introduced under irrigation
• Crops for self-sufficiency and (household/ national) food security
• Crops destined for the market
• Experience, motivation and priorities given by farmers in selection of the crops.

The use of crops or varieties with greater resilience to dry spells is preferable. This can also help farmers to adapt to changing temperatures and rainfall patterns. Increased agricultural diversification, including better integration of trees, crops, fish and livestock can reduce risk and increase the resilience of farming systems.

Some crops are sensitive to the way water is applied to them. Systems, which wet the whole crop, such as sprinkler irrigation, may have undesirable consequences, such as leaf burn, fruit spotting and deformation, crown rot, and other. These considerations would influence the choice of the irrigation method (Savva and Frenken 2004).

As a rule, most vegetable (and other row) crops have shallow effective root zone depth and respond better to low moisture depletion levels. They are therefore well suited for localized, drip irrigation, which is often linked to PV powered pumps.

It is important to note that plant breeding and biotechnology can help by increasing the harvestable parts of the biomass, reducing biomass losses through increased resistance to pests and diseases, reducing soil evaporation through vigorous early growth for fast ground cover, and reduced susceptibility to drought (FAO 2012).

In selecting suitable crops, farmers need to ensure that they have access to agricultural inputs, such as quality seeds, fertilizers, pesticides and tools, as well as credit to buy the necessary inputs.

Suitable Agricultural Practices and Inputs

• Present agricultural practices of common crops grown in terms of inputs, labour and tools;
• New or improved agricultural practices to be introduced for the irrigated crops to ensure optimum production levels
• Assessment of inputs required for optimal production in terms of quality seed, organic and inorganic fertilizers, tools, availability of inputs, and access to credit.

Outcome/Product

• Understanding of what affects crop water requirements
• Recognizing the difference between crop water requirements and irrigation water requirements
• Incorporating the different perspectives of water requirements into the irrigation plan
• Incorporating the necessary parameters in irrigation scheduling and drainage design

Data Requirements

• Crop rotations planned
• Schedule of planting and harvesting
• Consumption requirements of other water users in the basin
• Future climate scenarios for the area

People/Stakeholders

• SPIS advisors
• Farmers
• Irrigation managers, water user groups or farmer organization

Important Issues

• The CWR and IWR need to be resilient to future climate scenarios
• Irrigation schemes require planning at multiple scales, from basin to farm to individual crop
• The soil health and type are key to calculating the water requirements
• Boundaries on seasonal water availability should influence the crop choice balancing CWR, other user requirements, and water availability
• Soil drainage should be an upfront concern and consideration
• Choice of crops should also factor in other availability of other inputs such as labour, fertilizer, tools, herbicides, etc.

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