Criteria for Scheduling Irrigation or Approaches for Irrigation Scheduling
An ideal irrigation schedule must indicate when to apply irrigation water and how much quantity of water to be applied; several approaches for scheduling irrigation have been used by scientist and farmers. These are as under
1) Soil moisture depletion approach:
The available soil moisture in the root is a good criterion for scheduling irrigation. When the soil moisture in a specified root zone depth is depended to a particular level (which is different for different crops) it is too replenished by irrigation.
For practical purpose, irrigation should be started when about 50 percent of the available moisture in the soil root zone is depleted. The available water is the soil moisture, which lies between field capacity and wilting point. The relative availability of soil moisture is not same field capacity to wilting point stage and since the crop suffers before the soil moisture reaches wilting point, it is necessary to locate the optimum point within the available range of soil moisture, when irrigation must be scheduled to maintain crop yield at high level. Soil moisture deficit represents the difference in the moisture content at field capacity and that before irrigation. This is measured by taking into consideration the percentage, availability, tension, resistance etc.
2) Plant basis or plant indices:
As the plant is the user of water, it can be taken as a guide for scheduling irrigation. The deficit of water will be reflected by plants itself such as dropping, curling or rolling of leaves and change in foliage colour as indication for irrigation scheduling. However, these symptoms indicate the need for water. They do not permit quantitative estimation of moisture deficit.
Growth indicators such as cell elongation rates, plant water content and leaf water potential, plant temperature leaf diffusion resistance etc. are also used for deciding when to irrigate. Some indicator plants are also a basis for scheduling irrigation e.g. sunflower plant which is used for estimation of PWP of soil is used in Hawaii as an indicator plant for irrigation sugar cane.
3) Climatological approach:
Evapotranspiration mainly depends up on climate. The amount of water lost by evapotranspiration is estimated from Climatological data and when ET reaches a particular level, irrigation is scheduled. The amount of irrigation given is either equal to ET or fraction of ET. Different methods in Climatological approach are IW/CPE ratio method and pan evaporimeter method.
In IW/CPE approach, a known amount of irrigation water is applied when cumulative pan evaporation (CPE) reaches a predetermined level. The amount of water given at each irrigation ranges from 4 to 6 cm. The most common being 5 cm irrigation. Scheduling irrigation at an IW/CPE ratio of 1.0 with 5 cm. Generally, irrigation is given at 0.75 to 0.8 ratios with 5 cm of irrigation water.
Problem: Calculate cumulative evaporation required irrigation at 0.5 0.6 0.75 0.8 with 5 cm of irrigation water.
Cumulative pan evaporation at IW/CPE ratio of 0.5=IW/CPE=0.5
5 5 50
= ———- = 0.5, CPE X 0.5 = 5 CPE = —— = —— 10cm
CPE 0.5 5
Irrigation of 5 cm is given when CPE is 10 cm
CPE at 0.6 ratio = 5/0.6 = 8.33cm
CPE at 0.75 ratio = 5/0.75 = 6.66cm
CPE at 0.8 ratio = 5/0.8 = 6.25cm
In IW/CPE ratio approach, irrigation can also be scheduled at fixed level of CPE by varying amount of irrigation water.
Problem: Calculate the amount of water for each irrigation for scheduling irrigation at 0.5 and 0.8 IW/CPE with 10cm of CPE.
Amount of water to be given at IW/CPE ratio of 0.5=IW/10=0.5
IW=0.5 X 10= 5cm
Amount of water to be given at IW/CPE ratio of 0.8 =IW/10=0.8, IW=10 X 0.8=8cm
Estimating Evapo-Transpiration from Evaporation Data:
It is been observed that a close relationship exists between the rate of CU by crops and the rate of evaporation from a well-located evaporation pan. The standard United States weather bureaus class A pan evaporimeter or the sunken screen pan evaporimeter may be used for measurement of consumption use.
U.S class A evopometer:
It is most widely used evaporation pan. It is made of 20 gauge galvanized iron sheet 120 cm. in diameter by 25cm. in depth and is painted white and exposed on a wooden frame in order that air may circulate beneath the pan. It is filled with water to depth of about 20 cm. The water surface level is measured daily by means of hook gauge in a still well. Difference between two daily readings indicates the evaporation if there is no rainfall. When there is rainfall, record it separately with a rain gauge. Add that value to the initial water level in the still well. Difference between this reading and subsequent reading of the water would indicate evaporation. Water is added each day to bring the level to fixed point in the still well. A measuring cylinder can also be used for this purpose.
Sunken Screen Evapometer:
The sunken screen pan evaporimeter developed by Sharma and Dastane (1968) at the I.A.R.T., New Delhi provides a simple device to make reasonable estimate of CU. The ratio between evapo-transpiration and evaporation from U.S.W. class A pan (ET/E) is about 0.5 to 1.3 after establishment of the crop. the same ratio is the sunken screen pan evaporimeter was observed i.e. 0.95 to 1.05. in other words, it is claimed that the evaporation value obtained from it closely approximates the evapo-transpiration.
It consists of three parts, namely an evaporation pan, a stilling well and a connecting tube. The evaporation is 60 cm. in depth, is made of 20ngague galvanized iron sheet, and is painted white. it is fitted with a screen of 1/24 or 6/20 mesh, which is held tight over the pan by bending it at the rim and pressing hard. The stilling well is 15 cm. in diameter 45 cm. in depth and is fitted with a screen cover of the same mesh as that of the evaporation pan. It has a pointer to its side of the wall and bent upward in the center at right angle. The evaporimeter is installed by digging a pit of suitable size placing the pen and back filling the earth with due to compaction the top edge of the protrudes (sickout) 10 cm. over the soil surface. This is necessary to avoid run-off from the surrounding area entering the pan. The water level is maintained at same height as the soil level outside. Thus, the tip of the pointer free water surface in the pan and the pan and soil surface are at the same level.
The water level in the pan is brought in level with the pointed tip and pan is set at work. Observations of falling water level are recorded at suitable intervals say 24 hours. This is done by adding water with a measuring cylinder and recording the quantity of water added to bring the water level back to the pointer tip. The volume of water (ml) added is converted in to depth (mm) by dividing the area of pan plus that of stealing well.
The evaporimeter is installed in duplicate to enable leakage detection. The minimum distance between two evaporimeter is 3 meter. The pan is cleaned occasionally and painted white once in a year and cheeked scrupulously for leakages. The evaporimeter is located under natural conditions in a field, which does not provide obstruction to wind. It is aligned perpendicular to the main direction of wind to avoid mutual interference.
4) Critical growth approach:
In each crop, there are some growth stages at which moisture stress leads to irrevocable yield loss. These stages are known as critical periods or moisture sensitive periods. If irrigation water is available in sufficient quantities, irrigation is scheduled whenever soil moisture is depleted to critical moisture level. Say 25 or 50 percent of available soil moisture. Under limited water supply conditions, irrigation is scheduled at moisture sensitive stages and irrigation is skipped at non-sensitive stages. In cereals, panicle initiation, flowering, and pod development are the most important moisture sensitive stages.
Table: Moisture sensitive stages of important crops.
Important Moisture Sensitive Stages
Panicle Initiation, Flowering
Crown Root Initiation, Jointing, Milking
Flowering, Panicle Initiation
Panicle Initiation, Flowering
Rapid Flowering, Pegging, Early Pod Formation
Flowering & Pod Formation
Flowering & Pod Formation
Flowering & Pod Formation
Blooming stage to Maturity
Two weeks before & after flowering
From rosette to flowering
Blooming & seed formation
Flowering & Ball Formation
Transplanting to Full Bloom
Tuber Initiation to Tuber Maturity
Bulb Formation to Maturity
From the Commencement of Fruit Set
5) Plant water status it self:
This is the latest approach for scheduling of irrigation. Plant is a good indicator of a soil moisture and climate factors. The water content in the plant itself is considered for scheduling irrigation. It is however, not yet common use for want of standard and low cost technique to measure the plant water status or potential.