Irrigation schedules - Scheduling of Irrigation and Fertilizers, Crop Production | Crop Production Notes- Agricultural Engineering PDF Download

Crops need different amounts of water at different stages of their growth cycle. In addition, local climatic and soil conditions influence the availability of water to crops. It should be kept in mind that excessive water provision can also be counterproductive as crops cannot utilize excess water and may be stressed from reduced oxygen levels of saturated soil.

This practice will also waste not only water but also energy and pumping costs. Consequently, it is essential to plan for irrigation properly and match the amount of water provided to a crop’s water needs – both for yield optimization and for water efficiency. With proper irrigation scheduling, soil reservoir is managed such that optimum amount of water is available when the plants need it. Good irrigation scheduling requires knowledge of:

• Crop water demand at different growth cycles
• Moisture content of the soil and soil water capacity
• Weather conditions.

During the early season planting stage, the water requirement is usually about 50% less than what is required at the midseason stage, when the crop has fully developed and reached its peak water need. The late season demand, on the other hand, is as high as the peak demand for crops harvested fresh, and can be as much as 75% less for those plants harvested dry. It is essential for growers to be attentive to this irrigation schedule and for the irrigation system to be adaptable to such changing demands.

Although overall water needs of different crops can be approximated using the typical values given in Tables 5.1, 5.2, and 5.3 above, determination of these values at different growth stages is more complicated because water needs can show significant variations based on local climatic and soil conditions and crop variety. It is therefore important to consult competent authorities – e.g. Agricultural Ministries or local Irrigation Departments – to obtain relevant information.

Monitoring of soil moisture content provides a good assessment of the crop’s water needs. A wide range of methods offering varying accuracy levels is available for monitoring soil moisture, each having its respective strengths and shortcomings. Some of the common methods are summarized in Table 5.4.

Soil capacity, which is the ability of the soil to hold water between irrigation or precipitation events, is another important factor. Determinants of soil capacity include soil depth, ratios of different soil particles making up the soil, soil porosity, and soil water tension.6 These factors influence the amount of water available to the plants. Because soil properties change at various depths, it is important to know the soil capacity throughout the plant root zone. It should also be noted that during irrigation, or precipitation, water only reaches a zone at a lower depth once the preceding zone has become fully saturated. Soil capacity surveys are usually difficult to perform by individual farmers, but can be performed by competent authorities and the information can be made available for different regions.

The prevailing climatic conditions, such as average ambient temperature, intensity of solar radiation, humidity, and windspeed also affect both the moisture retained in the soil and the speed by which plants lose water through transpiration. The highest crop water needs are found in areas that are hot, sunny, dry, and windy. Thus, climatic conditions also need to be taken into consideration for proper irrigation scheduling.

Accurate monitoring of water used in irrigation is an essential part of irrigation scheduling and helps reach optimal performance, saving water while enhancing yields. Accurate readings can be obtained through different direct measurement methods available for pipes and closed conduits (propeller meters; orifice, venturi, or differential pressure meters; magnetic flux meters; ultrasonic meters) and for open channels (weirs and flumes; stage discharge rating tables; area/point velocity measurements; ultrasonic methods). Indirectly measuring irrigation water use can also provide sufficiently accurate approximations at lower costs. Common methods used include: