3.1 Irrigation Timing and Application Depth
3.3 Water Supply Characteristics
3 What Factors are the Critical Determinants of Irrigation Performance?
Examination of performance measures describing the efficiency, adequacy and effectiveness of irrigation highlights four factors that are critical to achieving high levels of performance, for any of the measures. These factors are:
- timing;
- depth;
- uniformity;
- water supply characteristics.
If irrigation system design is to create the potential for high performance irrigation, it must result in an application system that farmers can use to irrigate uniformly, in the right amount, and at the right time.
3.1 IRRIGATION TIMING AND APPLICATION DEPTH
Getting the timing and application depth right requires information on the critical soil water content for the crop, the current soil water content of the soil, and how much water can usefully be stored at the time of irrigation. It is largely an operational management issue. However two outcomes of the design process significantly affect farmers ability to get irrigation timing and amount right. These are:
- irrigation rate (average area covered per day);
- application depth range.
If the irrigation rate is too low, it is probable that in long periods of dry conditions the effectiveness of irrigation will drop, regardless of depths applied. Soil water content will drop below critical levels and crops will come under sufficient stress to cause income reduction. If farmers increase application depths, rather than irrigation rate, in an attempt to maintain production, efficiency will fall because the additional water cannot be stored in the soil.
Application system design often concentrates on the maximum depth that is likely to be required. Application systems that can only apply a narrow range of application depths clearly have limited ability to match application depth to the soil water storage capacity at the time of irrigation. For annual crops or irrigation systems supplied on a rostered basis, the amount of water that can be usefully stored at the time of irrigation varies throughout the season. Application efficiency will be low if the application depth is unable to be adjusted downwards to match the minimum expected soil water storage capacity at the time of irrigation.
3.2 UNIFORMITY
There are two main determinants of the uniformity of the infiltrated depth of water. These are the spatial uniformity of water supply to the infiltrating surface (usually the soil surface) and the temporal uniformity of water supply to the infiltrating surface. The relative significance of each of these to the uniformity of infiltrated depth differs depending on the type of irrigation system.
3.2.1 Surface Irrigation
The spatial uniformity of water supply to the soil surface is essentially 100 percent for well-formed border strip irrigation systems of traditional design. However, the temporal uniformity of supply varies over the length of the wetted strip because the soil surface is used to distribute the water. Design parameters, such as flow rate per unit width, slope, and length, along with management parameters such as irrigation duration, surface roughness, and initial soil water content, determine the opportunity time for infiltration. This varies over the length of the wetted strip and thus the infiltrated depth of water varies.
3.2.2 Sprinkler Irrigation
The selection and placement of sprinklers significantly influences the potential for water to be applied in a spatially uniform manner. The availability of good design tools and a wide range of sprinklers means that sprinkler systems can be designed to apply water with uniformity coefficients in excess of 90 percent, under still-air conditions. Careful design can also yield sprinkler systems that maintain a high level of uniformity, relative to still-air performance, under reasonably windy conditions.
Application rate is a very important determinant of the uniformity of infiltrated depth for spray irrigation systems, and therefore of irrigation performance. If the application rate is less than the infiltrability of the soil there will be no surface ponding, and therefore no redistribution. The infiltrability decreases as the hydraulic gradient across the soil surface decreases. Generally, therefore, the larger the intended mean application depth, the lower the application rate should be. When the application rate exceeds the infiltrability, water will pond and redistribute from high spots to low spots. Provided the depth of ponding is less than the surface roughness the redistribution will be local (John et al., 1985) and probably have little practical effect on the uniformity of infiltrated depth. When the depth of ponding is larger than the local roughness, significant redistribution can occur, as is often evidenced by water running off paddocks or flooding hollows.
The temporal uniformity of water supply is usually very high for sprinkler irrigation systems. It is an issue for travelling irrigators. Sprinklers at the end of a centre pivot are travelling much faster than sprinklers near the pivot point. In order to achieve a uniform depth of water application it is necessary to adjust the application rate and the wetted footprint over the length of the pivot. Design techniques for achieving high levels of uniformity are well developed. Many travelling irrigators start moving as soon as water is supplied to them, and water application stops as soon as the irrigator stops moving. This means that the duration of water application is very small at the extreme ends of each irrigator run. At the beginning and end of an irrigator run, the application depth ramps up and down over a distance equal to half the wetted width of the irrigator. The effect of this variation on the uniformity of infiltrated depth becomes negligible for long irrigator runs.
3.3 WATER SUPPLY CHARACTERISTICS
Irrigation performance is significantly affected by interactions between application system characteristics and water supply characteristics. If, for example, water is supplied on a fixed roster basis, and the application system can only apply a fixed depth of water, the effectiveness and efficiency of irrigation is essentially determined by climatic conditions – a random outcome. Irrigation system design must take full account of the water supply characteristics to ensure that farmers have sufficient flexibility to irrigate at the right time and apply the right amount of water.
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