Lowering irrigation costs

Picture showing an ET gage in the foreground established at the weather yard of Abhawa Bhaban at Agaorgaon in Dhaka.

The cost of irrigation at $51 per hectare (Hossain and Deb, 2003) in Bangladesh is the highest compared to neighbouring countries. Irrigation cost per hectare in India and Thailand is around $18. On the contrary, the human and animal labour costs at $231 per hectare is higher in West Bengal, India, as compared to $181 in Bangladesh, according to Hossain and Deb (2003). Eighty percent of dry-land irrigation in Bangladesh is from shallow pumps run by diesel. Diesel price in Bangladesh is also lower compared to West Bengal. These numbers tell us that irrigation cost should be lower in Bangladesh. So, we may conclude that the amount of water pumped by a Bangladeshi farmer is greater compared to a farmer in West Bengal and Thailand, which raises the cost. The increased pumping does not translate into an increased production. Rice yield of 4.83 tons per hectare in Bangladesh is slightly lower than 4.86 tons per hectare in India. We may see the effect of this in the open market. Rice from India finds its way into Bangladesh market. To remain sustainable, the Bangladeshi farmer has to reduce the cost of production. It is obvious from the information presented above that the cost of irrigation need lowering in Bangladesh. This is only possible by being efficient water managers.

Efficient irrigation managers need to know how much water to apply and when. To help in developing these skills the farmers need to acquire knowledge on crop water use and soil water holding characteristics of their field. This knowledge helps him or her in applying water according to the amount needed by crop and thereby reducing pumping cost. Timely application according to critical stage of crop growth will ensure profitable return. However, to implement proper irrigation scheduling, the water managers or farmers need tools. These tools fall in two categories, physical equipments and management techniques.

Physical tools like lining of canals or using PVC pipes reduce water loss from conveyance and distribution. Use of valves (automated or manual) help in better control of water. Tools that help in knowing how much water a crop has used are valuable in scheduling irrigation. Crop water use also known as evapo-transpiration (ET) is dependent on weather condition. Sun light causes the stomata (microscopic openings) of leaves to open through which water moves out of the plant to the atmosphere. Water from soil extracted by roots continuously move up through the stem and out through leaves. This process keeps physiologic processes to continue and maintain plant growth. This also helps keep plant cool during hot dry days, which is necessary for the metabolic processes to function. Scientists have developed mathematical equations to correlate weather condition to the actual crop water use. Temperature, dryness of air (relative humidity), sunlight, and wind movement affects the rate of escape of water from leaves. Weather stations provide temperature, sunlight, relative humidity, and wind data that are used in an empirical equation to calculate crop water use. Technologists have developed simplified devices based on scientific findings that may be used by farmers directly to determine crop water use without the complication of mathematics. One such device or tool is an atmometer or ET gage.

An ET gage measures the amount of water that evaporates to the atmosphere from a wet porous ceramic cup surface, which has been covered with a green fabric to provide a similar resistance and surface reflectance (albedo) of crop leaves. This device mimics a plant from which water escapes to atmosphere due to sunlight, heat, relative humidity or dryness of air, and wind. The amount of water loss from the gage may be observed on the glass tube that is attached to the ET gage (atmometer) cylinder to indicate the water level.

An ET gage has been set up at Agorgaon Abhawa Bhaban weather yard in Dhaka, Bangladesh, to calibrate the same with the crop water use (evapotranspiration) calculated by using weather data of the same weather station.

Bangladesh Agricultural Research Council and Abhawa Department are jointly conducting a study to correlate the observed water loss from ET gage with the calculated evapotranspiration under the supervision of the author. If found suitable these devices may be set up on the basis of region at research stations from where the information may be provided to farmers by agricultural extension scientists.

Each soil type has a water holding capacity. This capacity determines the size of the water reservoir from which crop plants are taking water through their roots. The amount of water stored in this soil reservoir will determine in how many days plants will run out of water much like a fuel tank in a car. Extension scientists have this information available according to soil type. By knowing the rate of daily crop water use by observing the ET gage a farmer may know when he or she needs to start irrigating again.

Irrigating according to plant need and adjusting the amount of water to apply each time according to water holding capacity of soil a farmer may reduce pumping. In this way the cost of irrigation may be reduced. Water managers and farmers need to be trained. Extension scientists also need proper training to equip them selves to be conversant on the use of water balance technique.

Dr. Mahbub Alam, Professor, Kansas State University, US, is a Senior Fulbright Scholar.