ANRA:Textbook:Irrigation

Irrigation

by the Academy of Natural Resources & Agriculture
contributions by Thorfinn Hausselkiff, Rika Vandor

Introduction

The goal of irrigation is to provide an efficient, regular, and controlled source of water for plants and crops where these conditions do not exist. Too little water leads to non-productivity, withering, and in most cases, plant death. On the other hand, too much water can lead to disease and is a waste of water efficiency in general. Also, the amount of time watering and the number of times affects plants greatly. Longer, more thorough waterings are preferred as short frequent ones lead to shallow roots. In hot, dry weather, plants with shallow roots dry out, wilt and die faster.

Irrigation Efficiency

Before one can talk about irrigation and various methods, you need to step back and look at a few things that affect the efficiency of irrigation. Firstly, soil composition. Sandy soil retains little water, straining watering efforts as a constant supply of water is needed. That and there are a limited number of plants that grow well in sandy soils. Mulching helps retain water in the soil. Bright, full sunlight as well as wind increase the loss of water. Using shading techniques and blocking the wind can help reduce the loss of water. Removing unwanted plants (weeding), reduces the competition for water. All of these methods, when used, can increase the efficiency of irrigation.

***Note: See ANRA texts on soil and water conservation, and mulching.***

Water Sources

Rain is the most common source of water. Rain is also unpredictable and irregular. Therefore, additional sources of water are needed. Storing rainfall in wood and stone containers offers a supplemental source of water. Unfortunately, this still relies on rainfall. In times of drought, these containers will easily and quickly dry out. Freshwater lakes, streams, rivers, and wells can all provide a more constant source of water. Water from these sources can be directed to fields or containers using ditches or constructed channels.These channels can be controlled with simple wooden or stone gates, to allow water in, or restrict its flow. Water can also be poured into the fields using a simple mechanical device we will call a river dipper. This consists of a long pole attached to an upright post or tripod. A counterweight hangs from one end of the pole, and a bucket is hung from the other end. A person dips the bucket in water (river, stream), lifting the bucket using the counterweight, and swivels the pole to move the bucket which could then be emptied into an irrigation channel (((shadouf))). More complicated methods and systems could be constructed with gnomish help.

Irrigation Methods

Choosing the type of method of irrigation depends on the amount of rainfall expected for an area, and the type of crop planted. Some crops are more "thirsty" than others, and require more water (notably fruits and vegetables require more moisture than grains or root crops). Typical rainfall for an area should be measured before implementing one of the methods of irrigation below. Rainfall can be measured on a daily, weekly and monthly basis, using a large funnel, and a standard measure (for example, a funnel affixed to a glass tube 1 inch in diameter, and about 12 inches high, and marked in one-inch intervals).

• "Flooding" method - The idea is to more or less "flood" the area with water. Good for covering large areas but chaotic, no control over what area gets watered or how much.

• "Bowl" method - Mounds are constructed on the borders of the field. Water is allowed to flow freely into the created "bowl". This method allows for more control over which field get watered. However, there is potential for overwatering, especially if the soil does not drain well. In the case of poor drainage, subtle sloping can help move water across a field, draining at one end.

• "Furrowing" or "Channeling" method - Uses small trenches, channels or ditches within a field. Water flows in these channels and then seeps into the surrounding soil. This method allows the flow of water to be controlled and directed more. However, it takes more work and effort to sustain, as these ditches need to be kept clear of weeds, debris and kept to sufficient depth.

***Note: Too much water and excessive sloping can lead to soil erosion.

There have also been some rumors about some mechanical means of carrying water over long distances in raised stone or wooden trenches, but stories of these have not been confirmed nor tested. For more information or illustrations, please contact the Academy.

Related Links

• Academy of Natural Resources and Agriculture
• Crop Rotation
• Soil and Water Conservation
• Problems associated with Farming
• Soil Creation
  
• Mulch
  
• Forestry
• Basic Carpentry