Drainage system in agriculture: Agricultural drainage system drains water from the land to improve crop yields. It is done by removing water from poorly drained soils by means of surface ditches, subsurface permeable pipes, or both.

Drainage 

Agricultural drainage is the removal of excess water, known as free water or gravitational water, from the surface soil or below to create favorable soil conditions for plant growth. Drainage also deals with the lowering dove a of the groundwater table (GWT) below the root zone of the crops to improve the plant growth in some cases or reduce the accumulation of salt in the crop root zone.

Irrigation and drainage are equally important for successful crop production. Irrigation and drainage cannot be separated, and they should go hand in hand. Even for a short period, excess water in the soil may cause severe damage to crops and soil.

Irrigation provides sufficient moisture in the soil for the satisfactory growth of plants, whereas drainage is important for avoiding excess moisture in the root zone. The excess moisture may originate from excess rainfall, over-irrigation, seepage from canals or reservoirs, or ditches.

A land having a high water table or water stand on the land surface for a long period, excessive soil moisture content, humid or super humid regions with continuous or intermittent rainfall, or flat land with fine-textured soil may need drainage for high agricultural productivity. Following are the two types of drainage.

Surface drainage  

Removing the excess water from the land surface is known as surface drainage. Adequate arrangement for surface drainage in heavy rainfall areas is essential for speedy water disposal. Rainfall, snowmelt, waste runoff, seepage from adjoining higher land, overflow from stream channels, etc., are surface water sources. A surface drainage system may be required in humid or irrigated areas, which is usually an integrated part of the irrigation system on slowly permeable soils or in areas with high precipitation rates. Therefore, it is said that irrigation and drainage are like the two faces of a coin. 

Sub-surface or internal drainage 

Removing the water from the sub-surface or soil profile is known as sub-surface or internal drainage. There should be subsurface drainage in high water table areas. In either case, the system is conveniently divided into the following three functional parts: 

• Collection system: Budding, surface field ditches, row ditches, or divers; ditches are an integral part of the system that first picks up water from the land. 
• Disposal system: This is a part of the system that receives water from the collection system and conveys it, usually in an open channel to the outlet. 
• Outlet: This is the endpoint of the drainage system. The first step in the design of any drainage system is to locate the outlet because most the drainage systems fail due to the failure of the outlet. The most economic outlet is a natural waterway, such as rivers, streams, ditches, municipal drains, etc. Sub-surface drains are the underground artificial channel through which excess water of land may flow to a particular outlet. 

Objective of drainage 

Objectives of drainage are stated below: 

• Removal of excess water from the soil. 
• Arrangement for good aeration in the soil. 
• To control the loss of plant nutrients. 
• To enhance the activity of soil organisms. 
• To improve the soil structure. 
• To control soil erosion. 
• To prevent salt accumulation on the surface soil. 
• To develop favorable conditions for the growth of plant roots. 
• To lower the water level below the root zone.

Factors controlling method of drainage 

The drainage system/method depends on the following factors: 

• Topography and gradient of the land. 
• Type of the soil. 
• Permeability of the soil. 
• Crop and its water tolerance capacity. 
• The quantity of water to be disposed of and its speed.