Variable rate irrigation (VRI) can be defined as site-specific management of water. VRI encompasses a combination of innovative precision ag tools that enables a center pivot irrigation system to optimize irrigation application. VRI technology allows growers to easily apply varying rates of irrigation water based on individual management zones within fields. Management zones are field areas possessing homogenous features for landscape and soil properties. These features lead to similar crop yield potential and input-use efficiency for seed, nutrients and water (Doerge, 1998) and can be created via processes described by Zhang et al. (2002). Control systems on center pivots allow the right amount of water to be applied to these management zones within the field.
Water is applied to specific management zones via a prescription (Rx) using a sector or zone control VRI pivot system as shown in Figure 1 and Figure 2, respectively. A sector control system varies the speed of the pivot in each sector or pie wedge, with faster speeds reducing and slower speeds increasing application rate (Perry and Pocknee, 2003). Pivot speed typically changes every 6 degrees, allowing 60 slices in a pivot; however, some systems are capable of changing every 2 degrees. The zone control irrigation system (Figure 2) allows the pie wedges to be further broken into smaller zones, allowing for the creation of over 5,000 management zones within a field. Water is variably applied by varying the sprinkler control valve pulse rate in each management zone according to the VRI Rx. Thus, water application rates are matched with spatial variation in soil properties for each management zone.
Multiple Variable Rate Irrigation Prescriptions? - Some growers may question whether there is a need for multiple VRI prescriptions for different application rates as crop water demands change. Generally VRI prescriptions are built using a base application rate of 1 inch. During the growing season, irrigation scheduling may call for a different application rate than 1 inch, depending on anticipated crop water needs. If 1 inch is too much water, the base application could be reduced to 0.75 inches by increasing the speed with sector control systems. Conversely, irrigation scheduling may call for more water to be applied - for example, 1.5 inches. Thus, the base increases to 1.5 inches and slows down the pivot speed. The VRI prescription is adjusted without building a new prescription. Table 1 shows adjustment of the 0 to 30 degrees of a prescription based on 6 degree increments.
Table 1. The changes in the variable rate irrigation prescription due to varying the base application rate. The speed of the pivot in this example alters the inches applied.
|Angle (°)||0.75" Base
NDVIG (Normalized Difference Vegetation Index-Green) - The NDVIG (Figure 5) is an index that is well-suited to identify characteristics in the crop canopy when there is significant green tissue present past the V9 crop growth stage (Gitelson et al., 1996). The green and NIR light bands are used to calculate NDVIG. Typically, NDVIG is correlated to water stress, where low NDVIG values indicate that the plants are experiencing water stress.
The use of remote sensing imagery to monitor your fields during the growing season does not mean that you should stop scouting your fields. However, remote sensing can be used as tool to detect and direct scouting efforts in a field.
1Pioneer Encirca Services - Business Unit Lead, Northern BU, Mankato, Minn.
2Pioneer Regional Agronomist - Emerging Leader, NA Production, Dysart, Iowa.
3Pioneer Agronomy Research Manager, Johnston, Iowa.
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The foregoing is provided for informational use only. Please contact your Pioneer sales professional for information and suggestions specific to your operation. Product performance is variable and depends on many factors such as moisture and heat stress, soil type, management practices and environmental stress as well as disease and pest pressures. Individual results may vary.