In this article we will discuss about:- 1. Meaning of Regulated Deficit Irrigation (RDI) 2. Steps for Implementing Regulated Deficit Irrigation (RDI).
Meaning of Regulated Deficit Irrigation (RDI):
Regulated deficit irrigation (RDI) is an optimising strategy under which crops are allowed to sustain some degree of water deficit and yield reduction. During regulated deficit irrigation, the crop is exposed to certain level of water stress either during a particular period or throughout the growing season.
Main objective of RDI is to increase water use efficiency (WUE) of the crop by eliminating irrigations that have little impact on yield (Tab 7.2) and to improve control of vegetative growth (improve fruit size and quality.) Resulting yield reduction may be small compared with the benefits gained through diverting the saved water to irrigate other crops for which water would normally be insufficient under conventional irrigation practices.
RDI is a sustainable issue to cope with water scarcity since the allowed water deficits favour water saving, control of percolation and runoff return flows and the reduction of losses of fertilisers and agrochemicals; it provides for leaching requirements to cope with salinity and the optimisation approach leads to economical viability.
Adoption of deficit irrigation implies appropriate knowledge of crop ET, of crop response to water deficits including the identification of critical crop growth stages and of the economic impact of yield reduction strategies. Therefore, appropriate deficit irrigation requires some degree of technological development to support the application of irrigation scheduling techniques.
Before implementing RDI, it is necessary to know the crop yield response to water (growth stage or whole period). Crop yield response for deficit irrigation is described by the equation Y/Ym = 1-Ky [1-ETa/ETm], where Y and Ym are the expected and maximum crop yield, ETa and ETm the actual and maximum ET and Ky the crop response factor. Ky gives an indication of whether the crop is tolerant to water stress and depends on crop species, cultivar, irrigation method and growth stage.
High yielding varieties are more sensitive to water stress. Crops or varieties with a short growing season are more suitable for RDI. Furthermore, in order to ensure successful RDI, it is necessary to consider the water retention capacity of the soil.
In sandy soils, plants undergo water stress quickly under RDI, while in deep, fine textured soils, plants have ample time to adjust to low soil- water potential and may be unaffected by low soil-water content. Under RDI, agronomic practices may require modifications, e.g. decrease plant population, apply less fertiliser, adopt flexible planting dates and select shorter season varieties.
Development of RDI is not possible without first understanding patterns of tree and fruit growth. RDI must be applied during the period that shoot growth is rapid while fruit growth is slow (Fig 7.2).
Steps for Implementing Regulated Deficit Irrigation (RDI):
Necessary steps for implementing RDI successfully are:
1. Measure shoot and fruit growth to determine the RDI period for fruit species/varieties
2. Determine the root zone distribution (width and depth)
3. Determine the wetting pattern of the irrigation system and estimate the wetted root zone
4. Develop a season irrigation plan (amount and interval) based on soil type and Epan or ETo
5. Install soil moisture sensors (depth and number depends on soil).
During the RDI period:
Measure and record soil suction and irrigate at -200 kPa, and irrigate to wet the top 0.3 m depth of the root zone.
During full irrigation period:
Irrigate to wet when soil suction at 0.3 m depth dries out to -30 or -50 kPa and irrigate to wet at least the top 0.6 m of the root zone depth.
Salt content of irrigation water:
Where RDI is applied and irrigation water contains moderate to high salt content (Na or Cl) careful monitoring of soil salinity during the RDI period is necessary and strategic leaching irrigations (every 5-7 weeks) should be applied.
The RDI has been applied successfully for row crops like maize, soybean, sugar beet, sunflower, potato, wheat and tree crops like citrus, olives, peaches, grapevines etc.
Scheduling RDI — current recommendation:
Following is the list of necessary steps for implementing RDI successfully:
1. Measure fruit and shoot growth to determine the RDI period for fruit species/varieties in an orchard
2. Dig up a tree to determine the root zone distribution — width and depth (80 per cent of total)
3. Determine the wetting pattern of the irrigation system and estimate wetted root zone
4. Develop a season irrigation plan for run time and interval based on soil type and average Epan or reference crop evapotranspiration (ETo)
5. Install soil moisture sensors (preferred measure is soil suction using gypsum blocks) at 0.3 m and bottom of root zone in shallow soil and at 0.3 m, 0.6 m and bottom of root zone in deep soil.
During RDI period:
1. Measure and record soil suction and irrigate when the entire root zone dries out to a minimum of 200 kPa
2. Irrigate to wet the top 0.3 m of the root zone
3. Measure and record soil moisture 6-12 h after irrigation and, where necessary, adjust the amount applied in previous irrigations
4. Irrigate when the wetted root zone soil at 0.3 m depth dries out to 200 kPa
5. Measure evaporation (or ETo) interval between irrigations — irrigate in future years based on this evaporation interval
During rapid fruit growth:
1. Irrigate to wet at least the top 0.6 m of root zone
2. Measure and record soil suction 6-12 h after irrigation, and, if the soil is dryer than 30 kPa (sandy soil) or 50 kPa (clay soil) at 0.6 m, apply more irrigation
3. Irrigate when the wetted root zone soil suction at 0.3 m depth dries out to 30 or 50 kPa
4. Measure evaporation (or ETo) interval between irrigations — irrigate in future years based on this evaporation interval
5. Repeat steps 2-4.
Measuring shoot and fruit growth:
An understanding of the changes in fruit and shoot growth for different varieties is critical for timing of RDI. Water stress should be applied only during the vegetative growth period when fruit is growing slowly. Water stress must be avoided or minimised (where water is limited) during rapid fruit growth.
Stages of fruit growth for a given variety can be determined by tagging several fruit and shoots and weekly measuring their circumference and length with a tape measure. Converting fruit circumference to volume [volume = 0.02 x (circumference)3] gives a true indication of fruit weight. This technique is simple and the measurements are useful for adjusting irrigations, especially where shoot growth continues despite high soil-water deficits.
Root distribution:
Root distribution is an important component for RDI scheduling because of the potential store of available moisture in the soil. Best method for determining root distribution is to dig a pit next to an orchard tree and estimate the amount of roots in 0.2 m depth increments until the bottom of the root zone (80 per cent of roots). Root depth is important for determining the volume of water in the root zone when the profile is wet from rainfall and for deciding where to site soil moisture sensors.
Wetted root zone:
It is critical to determine volume of wetted root zone. This can be estimated from root distribution and wetted volume of the soil. To determine the wetting volume, it is necessary to observe the wetted surface area and depth following an irrigation event.
A hole is dug to observe wetting at depth. Wetted root zone is then estimated from the volume of roots that are wet following irrigation. Calculation in the following irrigation plan assumes that the wetting pattern is a continuous strip of soil with a wetting depth of 0.3 m. This wetted strip pattern will occur with closely spaced micro-jets or drippers where the wetting pattern overlaps. For other irrigation systems where the wetting patterns are separate, wetted root zone is calculated assuming the shape of a cylinder.
Irrigation plan:
The aim of setting out a season irrigation plan for approximate interval and run time is to provide a theoretical basis for irrigation scheduling and water budgeting.
For each month of a growing season, interval between irrigations is calculated based on the equation:
At the start of season, interval between irrigations is equivalent to the withholding irrigation period where the volume of water in the root zone (i.e. stored soil moisture) can be calculated by substituting the wetted volume with the root volume:
Volume of water in root zone (liters per tree) = Lateral root distribution width (m) × Tree spacing (m) × Root depth (m) × Deficit available water ranging from 9 per cent (sandy soils) to 13 per cent (loams and clays) × 1000.
Once irrigation commences, volume of water in the root zone is equivalent to the irrigation amount to be applied:
Volume of water in root zone (i.e. irrigation amount) (liters per tree) = Width of wetted strip (m) × Tree spacing (m) × 0.3 m wetting depth (m) × Deficit available water ranging from 9 per cent (sandy soils) to 13 per cent (loams and clays) × 1000.
Run time calculations use the emitter rate per tree and the system irrigation efficiency:
To estimate average daily water use, the plan uses local long-term average USWB Class A Pan evaporation data and appropriate crop factors for RDI. Alternatively, it is possible to use ETo and crop coefficients (Kc) and appropriate per cent replacements for RDI to estimate daily water use.
Conjunctive Use of Water Resources:
Conjunctive use of multi quality waters such as use of saline water or drainage water to meet the crop water requirements at times of irrigation water scarcity aids in increasing the water use efficiency of irrigation water. Water from these sources can be applied either separately or mixed.
Mixing of waters to acceptable quality for crops also results in improving the stream size and thus uniformity of irrigation, especially on sandy soils. Allocation of the two water sources separately can be done either to different fields, seasons or crop growth stages such that higher salinity water is not applied to sensitive crops or at sensitive growth stages.
Irrigation Water Pricing:
For proper water pricing, volumetric water metering and accounting procedures are recommended. Progressive, seasonal and over-consumption water tariffs as well as temporary drought surcharges rates contribute to water savings and should be promoted. Furthermore, an increasing block tariff charging system, that discourages water use levels exceeding crops critical water requirements, must me established.
It will be the basis for promoting conservation, reducing losses and mobilising resources. Furthermore, it could affect cropping patterns, income distribution, efficiency of water management and generation of additional revenue, which could be used to operate and maintain water projects.