Agriculture notes prepared by IAS Topper for all agriculture based competitive exams-Agronomy-Scheduling of Irrigation

Scheduling of Irrigation

Soil Moisture Depletion approach

Availability of moisture in root zone is a good criterion for scheduling irrigation when a particular amount of moisture in root zone is removed. It can be replenished based on amount of water removed. Therefore, irrigation water can be applied in predetermined soil water tension at a specified depth as it is considered that response to irrigation is better correlated with soil water suction than the soil water content. This approach is more reliable but cannot be recommended to farmers because measurements are difficult. But approximately can be estimated by farmers by feel and appearance method.

Climatological approach

Amount of water lost by evapo-transpiraion is estimated by climatological data and when evapo-transpiraion reaches a particular level, irrigation is scheduled. There are different methods of climatological approach

a). OIW/CPE approach

Known amount of Irrigation water is applied when cumulative pan evaporation (CPE) reaches a pre-determined level. Every time irrigation given amounts to 4-6 cm, commonly 5cm.If IW/CPE = 1 means, it’s the ratio of 5cm of irrigation water to 5cm loss of water in cumulative pan evaporation. Normally irrigation is scheduled at 0.75 to 0.8 ratio with 5cm of irrigation water. It is a good method to adopt, but such a technical measurement and availability of evaporimeter with farmers to be ensured,

b). Can evaporatin approach:

Small canes of one liter capacity is taken with a fitted pointer 1.5cm below and covered with 6/20 mesh. When field is irrigated and brought to field capacity level, the can is filled. Then predetermined level can be fixed. As the evaporation preoceed which amy be equal to evpotranspiration from crops. Theefore, when it reaches predetermined level, irrigatin is given. Normally level is fixed in such a way that amount of water to give equal it.

c). Combined approach: It is the combination of moisture approach and climatological approach. …………………

d). Critical stage approach

Some stages of crop cannot tolerate the water stress. So based on water avail and moisture content of soil, irrigation is schedule, non-sensitive stages are skipped.

Ex. For cereal-Panicle initiation and flowering stage

Simple techniques for Irrigation scheduling

·        Soil-cum-sand-miniplot technique

·        Sowing high seed rate among the crop

·        Feel and appearance method

·        Irrometers or tensiometer approach

·        Plant indices like wilting of leaf, leaf rolling, plant water content, water potential etc

·        Remote sensing methods for larger cropped area

·        Infrared thermometer: Calculate the stress degree days and schedule irrigation

SDD= E (Tc-Ta)

Tc-canopy temperature

Ta-air temperature

If Tc-Ta is negative, it means there is no moisture stress

Tc-Ta is zero or positive, then moisture stress is present.

Yield reduction is maximum when the total number f cumulative SDDs exceeds 10-15.

Irrigation methods……………