A simple, efficient, and economic way to provide localized sub-surface irrigation known as “pitcher irrigation” was developed at the Central Soil Salinity Research Institute, Karnal, India (Mandal, 1974).  In this technique, a baked earthen pot called as pitcher is used to provide and distribute water in the rootzone.  

Fertilisers and insecticides are also added in the pitcher itself for uniform application.  Various aspects of pitcher irrigation are discussed below.  
 Functioning of Pitcher Irrigation

i)       The waterhead at a point: This is maximum at the bottom and thus move water flows through the bottom surface.

ii)       The apertures and intensity of pores in the earthen wall: Normally, the pores are quite small and thus the discharge is slow but enough to meet the needs of the plants.  Within a reasonable limit, one can increase or decrease the size of apertures by varying the ratio of clay and sand, which are the basic materials used for making the pitchers.

iii)       Evapotranspiration need of the plant: It is greater during the day than at night.  The water flow is affected by the suction created by the evapotranspiration of the plant.

iv)      Texture of the soil: The discharge is usually greater in a light-textured soil than in a heavy textured soil.  
 

When the pitcher is filled, water starts coming out through the pores and wetting front advances both horizontally and vertically.  However, the horizontal movement is restricted and can be predicted by the following power curve, expressed as:

L = at ^b

Where L = length of advance from the centre of the pitcher,

            t  = time in days, and

         a,b = are the empirical constants, the value of which depends upon soil type, evaporative demand, discharge of the distributor, and the salinity of the irrigation water.  During the winter, value of a was 16.0 and for b for a 0.25. sandy loam soil. Generally, for a pitcher of 30 cm diameter and 10 litres water holding capcity, wetted surface area is around 0.7 m ²   It is sufficient to grow at least four plants of any vegetable crop.  
 

The moisture content of the soil around the pitcher varies from a maximum of 20 per cent near the pitcher wall to normal levels of dry soil, i.e. 5 to 10 per cent, at the end of the wetting front.  The maximum moisture is nearly equal to field capacity of the soil and thus is most favourable for the growth of plants.  Due to limited hydraulic conductivity of the pitcher wall, moisture levels above these are rarely obtained, except at night when, because of low evapotranspiration demand, the moisture percentage may slightly increase and encourage deep percolation losses.  However, these losses can be minimized by compacting the soil or putting a plastic sheet just below the pitcher.  
 

The pitcher irrigation method being essentially a sub-surface irrigation system, the salts accumulate at the soil surface and at the boundaries of the wetted front, leaving the rootzone in equilibrium with the salinity of the irrigation water used in the pitcher.
 

Baked clay pitchers can be continuously used for 3 to 6 years.  Clogging of pores due to preceipitation of salts, as with the dripper, is a remote possibility when the pitcher is kept wet continuously.  Clogging of pores from outside due to salts from the soil is limited because the flux is always continuous and occurs outwards, i.e. away from the wall of the pitcher.  The plant roots can neither enter the pitcher by penetrating through the pores nor seal the surface from outside.  They also do not exert enough pressure to break the wall of the pitcher.

To prolong the life of the pitcher, the following precautions should be taken:  

i)        While not irrigating, keep the mouth of the pitcher covered.  This will minimize the loss of water as well as prevent sunlight from entering the pitcher and minimize algal information and growth.

ii)       Only clean water should be used for filing the pitcher.  Muddy runoff from rainfall should be used after it is passed it through a sand filter.

iii)       Before storing the pitchers, wash them with good quality water so as to remove the salts and to prevent their precipitation and subsequent clogging of the pores on drying.  
 

Pitcher irrigation is a most effective and low-cost technology for using  a limited supply of good quality water and also for using saline water resources, it is most suited to the following situations:

i)       Where availability of good quality water is scarce.

ii)       Undulating areas where soils are difficult to level for uniform application of irrigation waters.

iii)       Light soils with high infiltration rate, which cannot be efficiently irrigated by surface method.

iv)      Areas with highly saline water sources, which cannot normally be used through surface method of irrigation.  

      Though this technique is costlier than surface irrigation system, it is worth the additional cost in view of the easy labour availability in many developing countries and the saving of water it provides.  
 
Source : Summarised from the topic "Irrigation & Salinity Control" by Ranbir Chhabra, Author, Soil Salinity & Water Quality