Management practices for improving water use efficiency can be broadly grouped into two:
I. Efficient Water Management Practices for Water Use Efficiency:
Water management practices aimed at minimising irritation needs of crops, without significant reduction in economic yield, automatically improves water use efficiency (WUE).
i. Localised Irrigation Methods:
Localised irrigation is widely recognised as one of the most efficient methods of irrigation. Localised irrigation systems (trickle or drip irrigation, micro-sprayers) apply water to individual plants by means of plastic pipes, usually laid on the ground surface. With drip irrigation, water is slowly applied up to 12 l h-1.
With micro-sprayer (micro-sprinkler) irrigation water is sprayed over the part of the soil surface occupied by the plant with a discharge rate of 12 to 200 l h-1. The aims of localised irrigation are mainly application of water directly into the root system under conditions of high availability, avoidance of water losses during or after water application and reduction of water application cost (less labour).
The main characteristics are:
1. Low rate of water application (discharge rate < 200 l h-1 for mini-sprinklers, <12 l h-1 for drippers and an application rate 1-5 mm h-1)
2. Partial soil wetting- Wetted soil is a portion of the soil volume available to the roots, 30-40 per cent for tree crops and 50-80 per cent for vegetables
3. Low doses, high frequency, long duration of irrigation – Doses 1/3 to 1/10 of those used for surface methods. High frequency (usually one irrigation per 1-7 days)
4. High soil- water availability – Slow and frequent irrigations ensure that water content in the soil remains high and fairly constant and the soil-water tension remains low (1/3 of atm.) resulting in high water availability to the plant.
Drip irrigation’s combination of water savings and higher yields typically increases at least by 50 per cent the water use efficiency, yield per unit water and makes it a leading technology in the global challenge of boosting crop production in the face of serious water constraints.
TABLE 7.1: Irrigation water, yield and WUE in kiwi irrigated with different system
ii. Irrigation Scheduling:
Irrigation scheduling is the decision making process for determining when to irrigate the crops and how much water to apply. It forms the sole means for optimising agricultural production and for conserving water and it is the key to improving performance and sustainability of the irrigation systems.
It requires good knowledge of the crops’ water requirements and of the soil-water characteristics that determine when to irrigate, while the adequacy of the irrigation method determines the accuracy of how much water to apply. In most cases, the skill of the farmer determines the effectiveness of irrigation scheduling at field level.
With appropriate irrigation scheduling: deep percolation and transport of fertilisers and agrochemicals out of the root zone is controlled, waterlogging is avoided, less water is used (water and energy saving), optimum soil-water conditions are created for plant growth, higher yields and better quality are obtained and rising of saline water table is avoided.
In water scarce regions, irrigation scheduling is more important than under conditions of abundant water, since any excess in water use is a potential cause for deficit for other users or uses.
Irrigation scheduling can be established by using several approaches: based on soil- water measurements, soil-water balance estimates and plant stress indicators, in combination with simple rules or very sophisticated models. Many of them are still applicable in research or need further developments before they can be used in practice.
Most of them require technical support by extension officers, extension programmes and/ or technological expertise of the farmers. However, in most countries these programmes do not exist because they are expensive, trained extension officers are lacking, farmers awareness of water saving in irrigation is not enough and the institutional mechanisms developed for irrigation management give low priority to farm systems. Therefore, in general, large limitations occur for their use in the farmers practice.
Effective irrigation scheduling:
Farmer should be able to control the timing and the depth or volume of irrigation. However, practical application of techniques and methods has been far below expectations.
The main constrains are:
1. Lack of flexibility, either due to rigid schedules or the system limitations
2. Non-economic pricing of water (price covers less than 30 per cent of the total cost)
3. High cost of irrigation scheduling (either for technology and/or labour)
4. Lack of education and training of the framers
5. Institutional problems, the behavioural adaptation
6. Lack of interactive communication between research, extension and farmers
7. Lack of demonstration and technology transfer.
One of the major obstacles to effective implementation of crop-based and water saving irrigation scheduling is the inability of most conveyance and delivery systems to deliver water at the farm gates with the reliability and flexibility required. In surface irrigated areas supplied from collective irrigation canals, discharge and duration impose constraints to farm irrigation scheduling. In case that the time interval between successive irrigations is too long, farmers usually apply all water that is made available and over-irrigation is practiced.
iii. Deficit Irrigation Practices:
Present irrigation scheduling, in all the developing countries, is based on covering the full crop water requirements. In arid and semi-arid regions water availability is, usually, limited and certainly not enough to achieve maximum yields. Then, irrigation strategies not based on full crop water requirements should be adopted for more effective and rational use of water. Such management practices include regulated deficit irrigation, partial root drying and sub-surface irrigation.
iv. 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 if 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.
v. 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 level exceeding crop’s critical water requirements, must be established.
It will be the basis of promoting conservation, reducing losses and mobilizing 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.
II. Efficient Crop Management (Agronomic) Practices:
Agricultural practices, such as soil management, fertiliser application, weed management, mulching etc. are related with sustainable water management in agriculture and protection of the environment. Agriculture practice today is characterised by the abuse of fertilisers. Farmers very rarely carry out soil and leaf analyses in order to clarify proper quantity and type of fertiliser needed for each crop and they apply them empirically. This practice increases considerably the cost of crop production and is potentially critical for the deterioration of the groundwater quality and the environment.
i. 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. Average water use efficiency of different crops varies from 3.7 to 13.4 kg ha-1 mm-1 of water. (Tables 7.3 and 7.4)
Rainfed Crops:
Amount of rainfall converted into plant available soil-water is determined by the amount and intensity of rainfall, topography, infiltrability and water retentivity of soil, depth of root zone and soil depth. Depth of soil due to its effect on the available water storage capacity decides the type of cropping locality. On medium soil depth, monocropping or intercropping can be practiced whereas in deep soil with 200 mm available soil, moisture status double cropping can be practiced.
Limited irrigated crops: Selection of crops and cropping sequences under limited irrigation situation should be done as there should be minimum water stress during the growing season although some water stress to the crops and associated yield reduction is inevitable. Therefore, along with selection of crops special care should be taken for irrigation scheduling of these crops.
Fully irrigated crops:
Under fully irrigated condition, selection of crops is not constrained by water availability but by adoptability of the crops to prevailing climatic and soil condition. In general, water use efficiency of C4 plants is higher than C3 plants, particularly under semi- arid environment.
ii. Tillage:
Tillage practices mainly influence the physical properties of soil viz soil moisture content, soil aeration, soil temperature, mechanical impedance, porosity and bulk density of soil and also the biological and chemical properties of soil which in turn influence the edaphic needs of plants viz seedling emergence and establishment, root development and weed control. Tillage also influences the movement of water and nutrients in soil and hence their uptake by crops and their losses from soil-plant system.
Tillage affects the WUE by modifying the hydrological properties of the soil and influencing root growth and canopy development of crops. Tillage methods influence wettability, water extraction pattern and transport of water and solutes through its effect on soil structure, aggregation, total porosity and pore size distribution.
Tillage system suitable for a soil depends upon soil type, climate and cropping system practiced. Shallow inter-row tillage into growing crops reduces short term direct evaporation loss from soil even under weed-free condition by breaking the continuity of capillary pores and closing the cracks.
Deep tillage to a depth of 30-45 cm at 60-120 cm intervals helps in breaking sub-soil hard pans in Alfisols facilitating growth and extension of roots and improving grain yield of crops as well as increasing residual soil moisture. However, the benefit is absent in sub- optimal rainfall years and restricted to only deep-rooted crops in high rainfall years.
Conservation tillage practice normally stores more plant available moisture than the conventional inversion tillage practices when other factors remain same. High soil moisture content under conservation tillage is due to both improved soil structure and decrease in the evaporation loss under continuous crop residue mulch cover.
Increase in available water content under conservation tillage, particularly in the surface horizon, increases the consumptive use of water by crops and hence improves the water use efficiency. Off-season tillage or summer ploughing opens the soil and improves infiltration and soil moisture regimes.
ii. Fertilisers:
There is strong interaction between fertiliser rates and irrigation levels for crop yield and WUE. Application of nutrients facilitates root growth, which can extract soil moisture from deeper layers. Furthermore, application of fertilisers facilitates early development of canopy that covers the soil and intercepts more solar radiation and thereby reducing the evaporation.
iv. Weed Management:
Weed management is the most efficient and practical means of reducing transpiration. Weeds compete with crops for soil moisture, nutrients and light. Weeds transpire more amount of water compared to associated crops plants.
Timely sown crop results in good stand and vigour and thereby higher efficiency of the basally-applied N fertiliser. On the other hand, a crop sown late requires additional inputs like seed, fertiliser, irrigation etc. to compensate for the loss in crop stand and yield.
Inadequate crop stand is the major cause of low crop productivity under stress environment like rainfed, drought and flood prone conditions. Weed competes with crop plants for water, nutrients, sunlight and thereby reduces crop yields and consequently NUE.
v. Mulching:
Mulching influences WUE of crops by affecting the hydrothermal regime of soil, which may enhance root and shoot growth, besides it helps in reducing the evaporation component of the evapotranspiration. Under moisture stress conditions, when moisture can be carried over for a short time or can be conserved for a subsequent crop, mulching can be beneficial in realising better crop yield.
Organic mulches also reduce the rate of evaporation by reducing the amount of energy absorbed by the soil and air movement immediately above the soil surface. Organic mulches maintain high rate of infiltration as well as reduce the runoff and its velocity, thereby play a significant role in soil and water conservation.
vi. Antitranspirants:
Antitranspirants are materials which decrease water loss from leaves by reducing the size or number of stomatal openings leading to decreased rate of water vapour diffusion from leaf surfaces.
Two important points to be considered in using antitranspirants are:
1. They should restrict water loss from leaf surface without resisting entry of carbon dioxide for photosynthesis
2. Transpiration necessary for cooling of leaf surface should not be completely stopped by the application of antitranspirants leading to rise in leaf temperature.
vii. Amelioration of Problem Soils:
Soil related constraints affecting crop production influence the nutrient use efficiency crops. For example liming of acid soils with calcite, dolomite or paper mill sludge improves the phosphorus use efficiency. Similarly, amelioration of alkali and saline alkali soils with gypsum helps in improving nutrient use efficiency. Any other physical constraint like subsoil compaction should be ameliorated using appropriate tillage practices to improve the nutrient use efficiency.
viii. Interaction with Other Inputs:
Utilisation of nutrients can be improved by optimum and synergistic interaction with other inputs viz. water, tillage and mulches. These inputs modify the physical, chemical and biological environment of soil, which influence the nutrient recovery by crop plants.
Significant and positive interaction between applied N and water supply has been observed on several crops leading to improvement in WUE. With 80 kg N ha-1, N use efficiency increased up to 300 mm water supply on sandy loam soil. Interestingly, with 120 kg ha-1, it did not increase when water supply was increased from 50 mm to 125 mm, but increased markedly when water supply was further increased to 300 mm (Table 7.5). This implied that the balance between these two inputs influenced input use efficiency.
Application irrigation and nutrient in conjunction through pressure irrigation system result in efficient utilisation of both resources. This will save water as well as reduces nutrient leaching losses leading to increased WUE as well as NUE. This will increase the yield and quality of crops. There is saving of water and nutrient to the extent of 35 and 22 per cent, respectively. Fertigation is most commonly used for plantation crops like banana, sugarcane and orchards.