The term water requirement (WR) of crops implies the total amount of water required at the field head regardless of its source, to mature a crop. It includes the evapotranspiration (ET) needs, application losses and any other special needs. It, however, does not include conveyance losses. The term consumptive use (CU) refers to the ET needs plus the water used in plant metabolism.
As the amount of water consumed in plant metabolism is very small CU and ET aDre more or less equal. In irrigation practice it is always not possible to exactly apply the required quantity of water to bring the rootzone to field capacity. Some losses are unavoidable and these are known as application losses. Water is also needed for special purposes like leaching of excess salts, puddling, presowing irrigation, etc.
Water requirement of crops may, therefore, can be expressed as –
Where, IR is the irrigation requirement of crops at field head, ER is the effective rainfall and S is the amount of moisture contributed from the soil profile either as stored moisture and/or that contributed from shallow groundwater table. Irrigation requirement is thus the gross amount of water applied through irrigation. Effective rainfall is the part of the rainfall that forms part of the consumptive use.
The irrigation requirement of a crop is only part of total water requirement. It may be written as –
The procedures for estimating the evapotranspiration values of the crops have been outlined. These values and the values of the other terms in Eq. 12.35, depend upon the particular situation that is being considered.
Irrigation Requirements of Some Crops:
The irrigation requirements of the crops depend upon the crop, climatic and soil conditions. Table 12.19 indicates the irrigation requirements of some important crops in India, determined at different locations.
Irrigation Requirements of Rice:
Rice is mostly grown under irrigated conditions with standing water except for some areas of upland rice where it is grown under rainfed conditions. Under irrigated conditions rice is transplanted on puddled soils and the fields are kept under submerged condition either through rain or irrigation.
Puddling and submergence, in general, reduce the percolation losses, check weed growth, increase the availability of plant nutrients, regulate soil temperature, favour the fixation of atmospheric nitrogen in soil through algal growth and improve photosynthesis in the lower leaves due to reflected light from the water surface.
In addition to the water requirements during the crop period, rice requires 300 to 500 mm of water per hectare for growing seedlings in nursery. The puddling and transplanting operations require nearly another 200 to 300 mm of water per hectare. Continuous submergence of about 50 ± 20 mm in general was found to be satisfactory for higher yields.
It is necessary to drain the soil once or twice during the growth period especially when water is ponded and not kept flowing from the field on poorly drained clayey soils. Drainage helps to remove toxic substances like sulphides and also regulate oxygen supply to the roots.
Drainage is effected just following tillering and flowering and the period for which the crop is left without submergence may last from 4 to 8 days. Irrigations are stopped about a fortnight prior to harvest and the standing water drained in order to allow the crop to mature and dry quickly to facilitate harvesting operations.
Water requirements of rice vary depending upon the soil and climate. A major portion of water could be lost through deep percolation depending upon the season and the soil type. Puddling to some extent reduces the percolation losses. Water and irrigation requirements of rice in some locations in India are given in Table 12.20.
Evapotranspiration and Crop Yields:
In general crops have maximum yield when the evapotranspiration rate is maintained at the potential rate. However, different crops have different requirements as to maximum soil water tension or degree of depletions between irrigations.
Most vegetable crops, potatoes and bananas require relatively wet soils. Yields from crops like deciduous fruits, barley and sugarcane normally do not respond when water is applied above 50 per cent depletion level.
In some cases, soil water deficit has a positive effect on the quality of the yield. For example – a slight soil moisture deficit improves the quality of the apples and in case of tobacco increases the aromatic quality. A reduction in ET (crop) before harvesting may increase the sugar content of the sugarcane. Different crops can tolerate soil moisture depletion to different extents without causing significant reduction in yields.
Again the crops have what are known as critical periods during which plants are sensitive to soil water stress. In order to sustain the desirable level of crop yields, the level of soil water depletion should not be exceeded, particularly during critical periods. Irrigations should be scheduled so as to take care of the specific soil water levels needed by individual crops.
Scheduling of Irrigations:
Irrigation Scheduling is the process of determining when to irrigate and how much water to apply per irrigation. Proper scheduling is essential for the efficient use of water, energy, and other production inputs. Among the benefits of proper irrigation scheduling are – improved crop yield and/or quality, water and energy conservation, and lower production costs. While discussing irrigation scheduling, the concepts of full irrigation and deficit irrigation need to be understood.
Full irrigation involves providing the entire irrigation requirement and results in maximum production. Exceeding full irrigation decreases crop yield by reducing soil aeration and restricting gas exchange between the soil and atmosphere.
Full irrigation is economically justified when water is readily available and irrigation costs are low. It is accomplished by irrigating to minimize the occurrence of plant stress (i.e., irrigating so that actual transpiration rates do not drop below potential rates).
Partially supplying the irrigation requirement, a practice that has been called deficit irrigation, reduces yield as small amounts of water, energy and other production inputs are used to irrigate the crop. Deficit irrigation is economically justified when reducing water applications below full irrigation causes production costs to decrease faster than revenues decline. Deficit irrigation is also used when the water supply or the irrigation stem limit water availability.
In these situations the level of irrigation, the amount of land to be irrigated, and the crop mix that maximize the benefits of irrigation must be determined. Deficit irrigation is accomplished by allowing planned plant stress during one or more periods of the growing season taking care that adequate water is supplied during critical growth stages.
Once the evapotranspiration values of the crops and their critical stages are understood, it is necessary to know when to apply the irrigation water to the crops.
Several methods can be used for scheduling of irrigations and more common methods are described below:
In this method, the moisture status of the soil is calculated at various times using the evapotranspiration values estimated. The crop is irrigated when the estimated soil moisture level attains a predetermined value.
For example – a crop has 200 mm of available water in the rootzone at field capacity and it has been decided to irrigate at a deficit of 100 mm. The following table can be constructed using the evapotranspiration values-
If average values of the ET(crop) are considered, an estimate of the irrigation interval (i) is obtained by –
Where, Sfc and Si, are the soil moisture contents (on volume basis) at field capacity and at the permissible depletion, respectively and D is the rooting depth.
Example 1:
For a crop with effective rooting depth of 140 cm calculate the irrigation interval given, field capacity = 14%; permissible depletion = 7%; and crop evapotranspiration = 280 mm/month.
The amount of rainfall contributing to the increase in soil moisture of the rootzone is known as effective rainfall. A part of the rainfall coming on the land is only effective as some of the rainfall goes as runoff and some as deep percolation.
In the above example, if during the period under consideration there is an effective rainfall of 30 mm, the irrigation interval will be –
2. Method Based on Soil Indicators:
In this method, the soil moisture content is monitored using any of the procedures and the field irrigated when the predetermined deficit is reached. It should be remembered that the availability of soil moisture is not same over the entire range from wilting point to field capacity and crop yields could be affected long before the soil moisture reaches the wilting percentage.
The intervals of irrigation for each crop are therefore to be fixed depending on the degree of depletion permissible for that crop. This technique, therefore, needs the soil moisture data obtained either by using instruments like tensiometer, resistance block etc., or by soil moisture sampling.
Soil-based irrigation scheduling involves determining the current water content of the soil, comparing it to predetermined minimum water content and irrigating to maintain soil water contents above the minimum level. The minimum water content is often varied according to growth stage, especially for deficit irrigation schedules.
Soil indicators of when to irrigate also provide data for estimating the amount of water to apply per irrigation.
The following example illustrates the use of soil indicators to determine when to irrigate-
Example 2:
Using soil indicators to determine when to irrigate.
Given-
i. The measured soil water contents in the table below
ii. Soil water content must be 16 per cent by volume or greater
Require-
Date of next irrigation
A summary of the soil-based indicators disadvantages is presented in Table 12.21.
3. Methods Based on Plant Factors:
The principle of these methods is to find some measure of the plant water stress which in turn should indicate the need for irrigation. The plant water stress could be used as a measure beyond which the growth or yield of the crop is reduced significantly.
Different plant characteristics like growth of certain parts of the plant, plant colour, leaf movement and growth have been used as indicators of plant water stress. Growth measurements like rate of growth of stem length in sugarcane and the fruit size in apples and oranges have been used to indicate plant water stress. The disadvantage of this method is that growth measurements may be influenced by other factors than soil water.
The plant parameters that can be used for irrigation scheduling and the advantages and-disadvantages associated are given in Table 12.22.
Irrigation Scheduling of Paddy:
Rice is grown both under rainfed and irrigated conditions. During rainy season, irrigation is scheduled whenever rainfall is inadequate. Irrigation may be required at the land preparation stage and subsequently to meet the water requirements including ponding depth.
Usually a ponding of 4.5 cm to 9 cm will be needed till the maturity of the crop depending on the crop variety and weed problems. At many places transplanting is being replaced by direct seeding of pre-germinated rice in order to save labour involved in transplanting. Direct seeding requires drainage of the rice fields at sowing time.
In Japan, intermittent irrigation and drainage is practised in paddy fields. This is shown schematically in Fig. 12.13. This practice requires appropriate field layout for irrigation and drainage and also considerably more irrigation water than the traditional method of continuous ponding. However, this practice is found to give higher crop yields.
Application of Water Balance Models:
Water balance models consist of determining the different components of inflows and outflows from a given field or a set of fields and for a chosen time period. The inflows to an irrigated field consist of irrigation and rainfall whereas the outflow could consist of evapotranspiration, drainage, seepage and percolation. Irrigation schedules to a given set of conditions can be planned knowing the components of the water balance equation.
The application of the water balance equation has to be considered separately for lowland paddy and upland irrigated crops as the dynamics of waterflows is somewhat different under these two conditions.
Considering a lowland paddy field situation, the water balance equation can be written as –
The above equation can be used taking any time period into consideration. In irrigation practice, a day is usually a convenient time period. Knowing the field properties in terms of seepage and percolation, embankment height or spillway level, optimum depth of water required in the field, rainfall during the period and crop evapotranspiration, the above equation can be conveniently used for calculating the irrigation requirements. Considering an upland irrigated crop situation, the water balance equation can be written as –
The application of water balance models involves several computations, and hence the use of computer becomes necessary. Simulation of overland flow or subsurface flow involves the computation of water balance.
Duty of Water:
This is a term commonly used in irrigation practice in India. The relationship between irrigation water and the area of crop that matures fully with the given amount of water is known as duty of water.
The duty of water can be expressed as:
(1) Area per unit rate of flow,
(2) Depth of water,
(3) Volume in terms of depth over unit area, or
(4) In terms of stored water.
In canal irrigation, duty is usually expressed as the area of the crop that can be irrigated with an amount of water flowing at a rate of one unit volume per second throughout the crop period. It is expressed as hectares per cubic metre per second.
As there are losses in water from the water source to its point of application on the field, the duty depends upon the point at which the discharge is measured. Unless specifically stated otherwise, duty for a channel means that the discharge is measured at the head of the channel and thus includes all losses occurring in the channel. Duty is also expressed in terms of depth of water and is referred to as delta (Δ). Delta is the total depth of water required for the entire crop period.
Unless otherwise stated, rainfall is excluded from this depth. It is usually expressed in cm. Duty as volume in terms of depth over unit areas represents the total quantity of water needed for a crop per unit area. It is expressed as hectare cm, or hectare m. In case of tank irrigation, duty may be expressed as the area that can be irrigated by a particular quantity of stored water. The unit generally adopted is the number of hectares per million cubic metres.
The relation between different expressions of duty of water can easily be calculated. Let d is the duty expressed as number of hectares per cubic metres per second, the duty expressed in cm over the area and B is the crop period in number of days.
Example 3:
A tank has a water spread area of 40 hectares with an average depth 3 m of water. Calculate the area of paddy crop (120 days duration) that can be irrigated, if the duty is expressed as –
(I) 960 hectares per m3/s
(ii) 110 hectares cm, and
(iii) 90 hectares/million m3 of water.
