In this article we will discuss about the strategies adopted for dealing with limited irrigation water.
1. Reduce irrigation during non-critical growth stages.
2. Use soil moisture and evapotranspiration (ET) measurements to schedule irrigations. Do not rely on crop appearance. Understand ET and how seasonal water use requirements vary by crop type, elevation, short term weather and length of growing season.
3. Increase residue, reduce tillage and manage weeds with herbicides. These measures help capture and store precipitation, reduce runoff and evaporation and maximise water use efficiency.
4. Conservation tillage improves soil moisture storage and reduces irrigation needs.
5. Make equipment upgrades to improve irrigation system efficiency and uniformity of application through an incentive cost share program, where available.
6. Under furrow irrigation, optimise row lengths and slope to shorten surface irrigation set times and increase uniformity. Use polyacrylamide (PAM) and surge valves to increase application rates. This increases uniformity and decreases set times.
7. Manage soil-water depletion carefully. Allow soil to reach its maximum allowable depletion (MAD) before completing the next irrigation.
8. In situations where good quality water is unavailable, consider using marginal quality water for irrigation. It is a short term solution requiring constant monitoring and depends on crop type and electrical conductivity (EC) of soil and water.
9. Reduce irrigated acreage. Revert some land to dryland crops or grass.
10. Forage crops are a good way to take advantage of precipitation when it occurs and accommodate drought conditions, while high value or quality driven crops are not good choices for limited irrigation. Consider reducing acreage and applying full irrigations to ensure leaching of salts through the root zone.
Strategies for managing with limited irrigation water supplies can be grouped as indicated in Table 7.6.
The above options for managing with limited irrigation options are briefly presented.
Agronomic Options:
Agronomic strategies for managing with limited water supplies are, generally, within the reach of farmers as they are only marginal adjustments for normal agronomy of crop production.
Reduce Area Under Irrigation:
Reducing irrigated acreage is one response to limited water supplies. When the irrigated area is reduced, the amount of irrigation per ha more closely matches full irrigation requirements and it’s corresponding per ha yield. Ideally, the land that reverts to dryland production should still produce some level of profitable returns. This is the simplest management option for managing with limited irrigation water supplies.
Selection of Crops and Cropping Systems:
Selection of crops and cropping systems for high water use efficiency should be done on the basis of availability of water under rainfed crops, limited irrigated crops and fully irrigated crops.
Enhance Precipitation Capture and Reduce Evaporation:
Increasing precipitation storage in the crop root zone and its reduced loss through evaporation appears to be ideal under limited irrigation conditions.
Residue Management:
The goal when working with limited water is to capture, store and preserve every possible source of water in the production system. These sources include rainfall and irrigation water. Residue management can have a significant impact on increasing the availability of water to crops. In the recent past, no-till for dryland production is in avocation even in India.
No-till increases the amount of water stored in the soil due to reduced evaporation from tillage operations, improved infiltration and reduced runoff. Surface residue during the growing season can also reduce the amount of evaporation from the soil during the growing season for irrigated crops compared to bare soil.
Changes in tillage management can be successfully used in irrigated production for moisture conservation. In spite of its beneficial effects, no-till system is not common in India due to several constraints. In the recent past, deep ploughing has been advocated for improving the water retentive capacity of shallow Alfisols.
Runoff from precipitation is also reduced when surface residue is present. Residue reduces the impact of rainfall and irrigation on surface sealing, which increases infiltration rates. As droplets impact the soil surface, they destroy the surface structure which will seal the soil surface and reduce infiltration rates. Residue protects the soil surface from the impact of these droplets. Residue also acts as small dams that slow water movement and allow for more time for the water to infiltrate into the soil.
Plant Population:
Plant spacing and distribution can be adjusted in a way that influences the time when stored moisture is used. Choosing the optimum plant population and width of row spacing continues to be one of the most difficult challenges for dryland producers. At too high densities, crop yields are reduced because too much of the soil-water is used up for vegetative growth early in the season; too low densities do not effectively exploit available moisture.
Recommendations are frequently made that do not differentiate between crops grown on stored moisture (September-October to January-February) and those grown during rainy season (June-July to October-November).
For crops grown during rainy season, the usual recommendation is to increase the distance between plants within the row, to adjust to a low moisture supply. The dynamics of competition for moisture are evidently different for a rainfed crop than for a crop growing on stored soil moisture.
For sorghum grown on stored soil moisture, it was found that the accepted procedure of increasing the distance between the plants within the row, in order to adjust to low moisture levels, generally defeats its own purpose: the young plants, with little or no intra- row competition, show excessive vegetative development; soil moisture is rapidly depleted and the plants are unable to form satisfactory ears or to mature their grain normally.
The alternative is to space plants more closely within the row and to increase the distance between rows in order to compensate for low moisture levels. In the wide spaced rows, the soil moisture supply is not exhausted as rapidly as in narrow rows. Intra-row competition prevents excessive vegetative growth and the laterally developing roots have to grow farther to reach moisture.
They, therefore, continue to find available moisture between the rows later in the season, when it can be used for grain production, provided the distance between the rows has been well adjusted to the available soil moisture. The heads are small but relatively numerous under these conditions, and maturity is more uniform than with plantings in close spaced rows, with wide, intra-row spacing.
Crop Rotations:
Crop rotations can have major impact on total water needs through irrigation. Crop rotations that have lower water use crops such as sorghum, pearlmillet, fingermillet, groundnut, pulse crops or soybean can reduce irrigation needs considerably.
With low capacity wells, planting the area with multiple crops with different peak water need periods allows for water to be applied at amounts and times when each crop needs the water. Net effect of irrigating less area at any one point in time is that ET demand of that crop can be better met.
Irrigation management can be as needed, rather than in anticipation of crop ET. With low capacity systems, farmers, generally, begin to irrigate early to keep the soil moisture as close to field capacity as possible in anticipation that their system cannot meet crop water needs later during peak water needs.
Allocating Limited Water Supplies:
When water is unlimited, management strategy is to add inputs such as water until the return from that input is equal in value to the added crop production. However, when water is limited, management strategy should look at maximum return from each unit of input of water.
When farmers are limited in the amount of water they can either pump or are allocated and that amount of water is less than what is needed for maximum economic production, farmers must look at management options that will provide the greatest possible returns to the operation.
A Single Irrigated Crop and a Dryland Crop:
The easiest production option would be to look at a single irrigated crop with the remainder of production in either a dryland crop or fallow. When the amount of water is less than adequate for maximum production, farmers must ask themselves whether the yield increase from increasing the amount of irrigation to each acre will offset the reduction in irrigated acres and increased dryland production. Increasing the amount of irrigation to a crop reduces the total number of irrigated acres.
An example of this would be if you have 10 inches per acre available for irrigation. One option is to irrigate all acres at 10 inches. A second option would be to irrigate 2/3 of the acres at 15 inches and have the remainder at dryland production. The question to answer is “Does the yield increase offset the reduction in irrigated acres and having 1/3 of the potential irrigated acres in dryland production?”
With a 130 acre irrigation system, a change in strategy such as this would reduce the irrigated acres from 130 to 87 acres and increase the dryland acres from 0 to 43 acres. If maize is the primary irrigated crop, several crops could be used as dryland crops in this scenario including winter wheat, soybeans or sunflowers.
Two or More Irrigated Crops:
Use of two or more irrigated crops in a rotation may increase the number of irrigated acres as compared to a single irrigated crop and a dryland crop. The philosophy of this strategy is to use a high water use and response crop such as maize and a low water use and response crop such as winter wheat, soybean, groundnut or sunflower.
This strategy uses the yield Vs irrigation to its maximum advantage. The first amounts of irrigation that are applied are used efficiently resulting in a yield response similar to that of the yield Vs ET response.
Strategy to find the most economical split of water and acres is similar to that of the one irrigated crop strategy. Farmers must look at the yield increase of adding water to one crop and the effect upon the irrigated acres and yield of the other irrigated crop.
The potential options become more numerous because now producers need to look at increasing the irrigation amount for one crop Vs reducing irrigation amount to other crop or increasing the number of irrigated acres for the other crop to compensate for the additional water to that crop.
An example of this would be if you again had a water supply of 10 inches per acre available and are irrigating two crops such as maize and winter wheat. If a farmer irrigates maize at 15 inches per acre and wheat at 5 inches per acre, the irrigated acres would be even at 65 acres per crop to match your water supply. If this farmer decides to irrigate wheat at 6 inches per acre, a first option would be irrigating maize at 14 inches per acre to keep the irrigated acres of each crop similar.
A second option to keep maize at the 15 inch per acre of applied water would be to reduce the irrigated acres of maize and increase the irrigated acres of wheat. Using the second option, the final acres would be irrigating 58 acres of maize and 72 acres of wheat. When using three potentially irrigated crops, the options become even more numerous.
Engineering Options:
Engineering options include improvements to reduce irrigation water losses. Most obvious way for increasing irrigation efficiency for improving water use efficiency at times of limited water supplies is to reduce losses in irrigation water conveyance, distribution and application to as small as possible.
A list of possibilities is given below:
1. Significant conveyance losses in an open channel can be reduced by channel lining, channel realignment or installing a closed pipeline.
2. Improve application uniformity to reduce deep percolation.
For surface systems, quicker flow advance to reduce the differences in infiltration opportunity time along a furrow. Options include land leveling, surge irrigation, furrow firming etc.
For sprinkler systems, options include changing sprinkler types, re-nozzling the system or changing nozzle spacing to improve the overlap between heads.
3. Modify the timing and amount of an irrigation to match the WHC of the soil profile better, thereby reducing percolation and runoff losses.
4. Convert to a more efficient irrigation system (e.g. flooding to furrow to sprinkler) to reduce application losses. If the new system is well-designed and managed, applications are more uniform reducing deep percolation and runoff.
The implicit assumption is that if a physical change is made in the irrigation system, management also changes appropriately. For example, converting from surface to sprinkler irrigation can greatly reduce water application depths, but if irrigation management does not change as well, then it is still possible to apply as much water as with a surface system.
Irrigation Management Options:
At times of deficit water supplies, limited irrigation water should be used as efficiently as possible to maximise water use efficiency. Localised irrigation methods, irrigation scheduling, deficit irrigation practices etc. assume importance in this regard.
Institutional Problems:
Unless volumetric water metering and accounting procedures are adopted for pricing irrigation water there in no other way to stop misuse of irrigation water, irrigation scheme operation and maintenance without farmer’s involvement is a failure at several situations.
User Participation in an Irrigation Scheme Operation and Maintenance:
Limited irrigation water management calls for well-planned long term strategy for sustainable irrigation water management. Participatory irrigation water management (PIM) could play effective role in limited water management.
Participatory irrigation water management is an overarching concept giving roles to farmers to collectively decide in a manner that affects their lives. It provides opportunities for collective action, dialogue between users, agencies and government. Community based and community driven approaches have come to be norm in most rural development strategies.
Effective participation gives opportunities for equity, better management and improved collection of water charges. Studies of farmer managed systems indicate that the active participation of farmers in water deliveries and allocation of water, improved design and construction, reduced conflicts over water, improved maintenance of irrigation system, accessibility to government and system personnel and increased agricultural productivity especially at the times of limited water availability.
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 crop’s critical water requirements, must be 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.
Training and Educational Opportunities for Learning Newer, Advanced Techniques:
Sophisticated technology and its high cost, lack of interactive communication between research, extension and farmers and lack of demonstration and technology transfer are the major problems associated with implementing recommended crop irrigation schedules for optimising crop production under scarce irrigation water.