An ideal irrigation schedule for optimum yield must indicate correct timing (when to irrigate) and amount of irrigation water at each irrigation. Scheduling irrigation is the process of determining when to irrigate and how much water to apply. There are several approaches for determining when to irrigate the crop.
The approaches can be broadly classified into three groups:
(1) Soil moisture regime approaches.
(2) Climatological approaches.
(3) Plant indices.
(1) Soil Moisture Regime Approaches:
In these methods, soil moisture content is estimated to know the deficit in available soil moisture at which it is proposed to irrigate based on predetermined soil moisture content to bring the soil to field capacity. Soil moisture content is estimated either by direct gravimetric method or indirect measurements such as tensiometers. resistance blocks, neutron probe etc.
Soil moisture content can be judged by feel and appearance of the soil, with experience. Soil samples from root zone depth are formed into balls, tossed gently into the air and caught in hand. While accuracy of judgement improves with experience.
Table 7.39 may serve as a guide for estimating the available soil moisture. Based on the allowable depletion in available soil moisture (DASM) irrigations can be scheduled at 25, 50 or 75 per cent DASM.
(2) Climatological Approach:
An integrated approach to SPAC is essential for an ideal irrigation schedule. Potential rate of water loss from a crop is primarily related to evaporative demand of the atmosphere. Irrigation can be scheduled if allowable depletion of moisture in the root zone and evapotranspiration during the crop period is known.
Pan evaporation (EPan) is used to determine the amount of irrigation water to be applied in the ratio of irrigation water (IW) and cumulative pan evaporation (CPE) from USWB Class A pan, usually known as IW/CPE ratio method.
IW/CPE ratio of 1.0 indicates scheduling irrigation with quantity of irrigation water equal to that lost in evaporation. If 5 cm water is applied when the cumulative pan evaporation is 10 cm, the IW/CPE ratio will be 0.5 (5/10 cm).
The ratio is usually fixed anywhere between 0.5 and 1.0. Smaller ratio indicates irrigation at longer intervals and larger ratio indicate frequent irrigations. This method appears to be simple, provided evaporation pan is available. In the absence of any evaporation pan, simple one litre can with a pointer to measure evaporation rate (can evaporimeter) can be used for scheduling irrigation.
(3) Plant Indices:
Any plant character, related directly or indirectly to plant water deficit which responds ready to integrated influence of soil, water, plant and evaporative demand of the atmosphere may serve as a criterion for timing of irrigation to crops.
a. Visual Plant Symptoms:
Visual signs of plants wilting can be used to schedule crop irrigation. Farmers frequently use dropping, curling and rolling of sensitive plant parts as an indication for plant water deficit. Distinct change in foliage colour is also used to time irrigation, especially to bean. In these methods, the crop has already suffered before exhibiting wilting or change in foliage colour reflecting the influence of water deficits on final yield.
b. Profile Modification:
This method also known as soil-cum-sand mini plot technique is used for timing irrigation to crops. The principle involved in this method is to reduce artificially, the available water holding capacity of soil in root zone depth in the mini plot by mixing sand with it.
Plants on sand mixed mini plot show moisture stress symptoms earlier than plants in rest of the area. Usually, an area of 1.0 m2 is selected in the field and a pit of 1.0 m depth is excavated in layers of about 15 cm depth.
Each layer of soil is mixed with 5 per cent by volume of sand and the pit refilled in the same sequence of layers as excavated by compacting each layer to bring bulk density of soil in the mini plot as that of the surrounding area. Symptoms of plants in the mini plot indicate time for irrigating the crop.
c. Increased Plant Stand:
An area of about 1.0 m2, preferably in a high spot, is sown with the same crop to maintain about four times the plant population compared with that in the surrounding area. Crop with high stand establishment wilts earlier than the crop in the rest of the field indicating timing of irrigation.
Reduction in growth rate of sensitive plant organs may indicate irrigation need of crops. In the case of orange plants, irrigation is given when the growth rate of fruit circumference falls below 0.2 to 0.3 mm day-1.
Since stem elongation of crops like sugarcane, tomato and cotton is highly correlated with water stress, it is possible to use this character as an indication of irrigation needs. Relative leaf water content, plant water potential, stomatal resistance and plant temperature which can adequately reflect the internal water balance of the plant may be used as potential indicators for scheduling crop irrigation.
After fixing timing of irrigation, the next step is to determine how much water to apply. Irrigation water applied should bring the soil to field capacity.
As such the quantity of irrigation water to be applied to the soil at each irrigation depends on:
1. The amount of available soil moisture (ASM) in the effective root depth at the start of irrigation or the level of available soil moisture depletion (DASM) considered for irrigation.
2. Effective rainfall and/or ground water contribution during the interval between two irrigations.
3. Additional quantity of irrigation water if any, required to leach the soils beyond the root zone depth.
4. The application losses.
The objective should be to realise highest production per unit of water use than to produce highest yield per unit of water use, especially under the conditions of limited irrigation water availability.
Net Irrigation Requirement:
It is the depth of irrigation water, exclusive of effective rainfall, ground water contribution and available soil moisture level considered for timing the irrigation that is required to meet the consumptive use requirement for crop production.
It is the amount of irrigation water required just to bring the soil moisture content in the root zone depth to field capacity. Thus, the net irrigation requirement is the difference in soil moisture content between field capacity and the soil moisture content in the root zone before application of irrigation water.
NIR = (M fc – M bi)/100 x BD x Ds
where, NIR = net irrigation requirement (cm)
Mfc = soil moisture content (%) at field capacity
Mbi = soil moisture content (%) before irrigation
BD = soil bulk density
DS = depth of soil (cm) considered.
Gross Irrigation Requirement:
It is the amount of irrigation water that must be applied to the soil surface to meet the net irrigation requirement for each irrigation. Hence, gross irrigation requirement (GIR) is the net irrigation requirement plus water application losses or the total amount of irrigation water to be applied through irrigation.
Gross irrigation requirement = Net irrigation requirement/ Efficiency of irrigation system
For a given irrigation method, field efficiency varies with skill used in planning, laying out and operating the system besides soil physical properties and climate. To be sure that the net amount of moisture to be replaced at each irrigation enters and retained in root zone, it may be necessary to apply a larger amount of water to the soil surface to offset any losses.
Irrigation Interval:
Number of days between two successive irrigations during the peak period of consumptive use of the crop is known as irrigation interval. It depends on consumptive use rate and amount of available soil moisture in the root zone depth between field capacity and starting moisture level for irrigation.
It is a function of both the crop and soil. For a given crop, light shallow soils require more frequent irrigations than fine textured heavy soils. Moisture use rate increases with increase in leaf area and as the days become hotter.
Irrigation interval to be used for design is the number of days between irrigations during the period of highest consumptive use of the crop. The average moisture use rate during this period is the rate to be considered in designing and planning irrigation systems and this is known as design interval.
Design interval = Net amount of moisture between field capacity and starting level/Peak period moisture use rate
= Net depth of application/Peak period use rate
Irrigation Period:
Irrigation period refers to the number of days that can be allowed for applying one irrigation to a given design area during the peak period consumptive use of the crop under consideration. It is the Oasis for capacity and equipment design. Irrigation systems are to be designed in such a way that the irrigation period is not greater than the irrigation interval.
Irrigation period = Net amount of moisture between starting of irrigation and lower limit of moisture depletion / Peak period moisture use rate of the crop
In most cases, irrigation system is designed in such a way that the irrigation interval is roughly equal to irrigation period. This may cause problem in completing an irrigation in rainy season, if start of irrigation is delayed too long following rainfall. When the soil is at its field capacity following the rain, the entire crop area will be uniformly reduced to the same moisture level.
If the crop has to be irrigated at 50 per cent DASM, it may not be possible to irrigate the entire area before the crop experience soil moisture stress. Under conditions such as cyclic irrigations with well water, if the field has been brought to field capacity by rainfall, irrigation should be started at about 25 per cent depletion in available moisture to complete irrigation to the entire crop area before any part becomes dry enough to seriously affect the yield.
Example 1:
Given the following soil moisture status and irrigation efficiency 60 per cent, find out the net and gross amounts of irrigation water requirements?
Solution:
Soil moisture deficit in the four soil layers
First layer = [(20 – 10)/100] x 1.4 x 15 = 2.10 cm
Second layer = [(18-12)/100] x 1.5 x 15 = 1.35 cm
Third layer = [(18 – 14)/100] x 1.6 x 15 = 0.96 cm
Fourth layer = [(19 – 16)/100] x 1.6 x 15 = 0.72 cm
Net quantity to be applied = 2.10 + 1.35 + 0.96 + 0.72 = 5.13 cm
Gross water application at 60 per cent irrigation efficiency is 5.13/60 x 100 = 8.55 cm
Example 2:
Given, oil moisture content at FC = 30 per cent
Soil moisture content at PWP = 15 per cent
Soil bulk density = 1.5 g cm-3
Effective root zone depth = 75 cm
Rate of evapotranspiration = 0.6 cm day-1
Available soil moisture depletion level for irrigation = 75 cent
Irrigation efficiency = 80
Answer the following:
1. Net depth of irrigation application
2. Gross depth of water application
3. Irrigation period.
Solution:
Available soil moisture in the root zone= {[(30-15) x 1.5]/100} x 75 = 16.88 cm
Net depth of irrigation to be applied = [(75 x 16.88)/100] = 12.66 cm
Gross depth of irrigation to be applied = (12.66/80) x 100 = 15.83 cm
Irrigation period = 12.66/0.6 = 21 days
Example 3:
Soil-water content in the root zone depth at start of irrigation is 10 cm for a soil capacity of 20 cm. Peak period consumptive use rate is 8.0 mm day-1. If the irrigation is 60 per cent, find out the irrigation period?
Solution:
Net irrigation requirement = 20 – 10 = 10.0 cm
Gross irrigation requirement = (10/60) x 100 = 16.67 cm
Irrigation period = 10.0/0.8 = 12.5 or 13 days
Example 4:
It is proposed to test the relative efficiency of scheduling irrigation to groundnut crop at 25, 50 and 75 per cent depletion in available soil moisture. Field capacity of soil in the effective root zone depth of 60 cm is 16 per cent with a permanent wilting point of 6 per cent. At which soil moisture content irrigations are to be scheduled under the three treatments?
Solution:
At 25 per cent depletion (75% available soil moisture)
Available soil moisture = 16 – 6 = 10 per cent
10 x (75/100) + 6 = 13.5 per cent
At 50 per cent depletion (50% available soil moisture)
10 x (50/100) + 6 = 11.0 per cent
At 75 per cent depletion (25% available soil moisture)
10 x (25/100) + 6 = 8.5 per cent
Example 5:
If the bulk density of the soil in the example 4 is 1.4 g cm-3 and daily consumptive use is 8 mm, calculate the irrigation interval for the three levels of irrigation schedules?
Solution:
Moisture to be replaced to bring the soil to field capacity
At 25 per cent DASM
In terms of percentage = 10 x (25/100) = 2.5 per cent
In terms of depth = [(2.5 x 1.4 x 60)/100] = 2.1 cm
At 50 per cent DASM
In terms of percentage = 10 x (50/100) =5.0 per cent
In terms of depth = [(5 x 1.4 x 60)/100] = 4.2 cm
At 75 per cent DASM
In terms of percentage = 10 x (75/100) = 7.5 per cent
In terms of depth = [(7.5 x 1.4 x 60)/100] = 6.3 cm
Irrigation intervals for the three schedules
For 25 per cent DASM = 2.1/0.8 = 2.6 or 3 days
For 50 per cent DASM = 4.2/0.8 = 5.25 or 5 days
For 75 per cent DASM = 6.3/0.8 = 7.88 or 8 days
Example 6:
It is proposed to schedule irrigations to groundnut crop at IW/CPE ratios of 0.5, 0.75 and 1.0 with 6 cm depth of irrigation water. What should be the CPEs for each of the three irrigation schedules?
Solution:
CPE for 0.5 IW/CPE ratio = 6/CPE = 0.5 = CPE x 0.5 = 6
CPE = 6/0.5 = 12cm
CPE for 0.75 IW/CPE ratio = 6/0.75 = 8 cm
CPE for 1.0 IW/CPE ratio = 6/1.0 = 6 cm
Example 7:
It is proposed to schedule irrigation to groundnut crop at CPE of 8 cm at IW/CPE ratios of 0.6, 0.7 and 0.8. What should be the depth of irrigation for each of the three irrigation schedules?
Solution:
IW for 0.6 IW/CPE ratio = IW/6 = 0.6
IW = 0.6 x 8 = 4.8 cm
IW for 0.7 IW/CPE ratio = 0.7 x 8 = 5.6 cm
IW for 0.8 IW/CPE ratio = 0.8 x 8 = 6.4 cm