# Different Furrow irrigation design consideration

Efficient irrigation by furrow method is obtained by selection of proper combination of spacing, length and slope furrows.

(i) Furrow spacing – Furrows can be spaced to fit the crops grown and types of machines used for planting and cultivation. Crops like potato, maize, cotton, etc., are planted 60–90 cm apart and have furrow between all rows. Carrot, lettuce and onion are spaced 30–40 cm and often have two rows between furrows. Furrows should be close enough to ensure that water spreads to the sides of the ridge and the root zone of crop to replenish soil moisture immediately.

(ii) Furrow length – Optimum length furrow is usually the longest furrow that can be efficiently and safely irrigated. Long furrows are an advantage in inter cultivation. Proper furrow length depends largely on hydraulic conductivity of soil. It should be shorter in porous sandy soil than clayey soil. If only a small area is to be irrigated, the length of field may determine the length of furrow. In large area it may be desirable to have furrow length equal to an even fraction of the total length of the field.

(iii) Furrow slope – The slope or grade of furrow is important because it controls the speed at which water flows down the furrow. A minimum furrow gradient of 0.05% is needed to ensure Surface drainage.

(iv) Furrow stream – The size of the furrow stream is one factor which can be varied after furrow irrigation system can be installed. The size of furrow stream usually varies from 0.5-2.5 lit/sec. The max nonerosive low rate in furrow is estimated by following equation,

qm = 0.6/S where,

qm = maximum no-erosion stream (lit/sec)

S = Slope of furrow (%)

Average depth of irrigation water applied during irrigation can be calculated by the following relationship.

D = (q × 360 × t)/(w × l)

Where,

D = Average depth of water applied (cm)

q = Stream size (lit/sec)

t = duration of irrigation (hrs)

l = Furrow length (m)

w = Furrow width (m)

Surge irrigation – Surge irrigation is a method of surface irrigation through furrows or border strips wherein water is applied intermittently in a series of relatively short on and off time periods during the irrigation (Humphrey, 1989). Water is let into a long furrows or border strips in an intermittent flow instead of conventional continuous flow. Each flow is termed as a surge. Surge irrigation practiced under favourable conditions can improve the performance of surface irrigation system compared to the other methods of surface irrigation. Irrigation is given in an on-off cycle or by cut back method.

The cycle time means the time from the beginning of one surge to the beginning of next surge. Cycle ratio is the ratio of flow time (continue) to the cycle time. Assuming the cycle time as 20 minutes and cycle ratio as 1:2 (0.5), the on-time is 10 minutes and off time is 10 minutes. This cycle ratio can also be the ratio of on-time and off-time as 1:1, if the on time is 10 minutes. Water is allowed for 10 minutes and stopped for 10 minutes. This 20 minutes is the surge time or cycle time. This surge is repeated until the water reaches the whole furrow or strip.

The first surge of water over a portion of dry furrow wets the soil surface at a slow advance rate and high infiltration rate. When the next surge is allowed to flow along the first surge length, water makes faster to the second surge length. Thus in surge flow, the advancing water along the furrow is faster resulting in uniform wetting from the head to the tail end of furrow. Under the conventional continuous flow, wetting is more in head end than at tail end. When more water is allowed to increase the wetting depth in the tail end, it leads to loss of water through tail end run off. This loss and the rate of infiltration along the whole length of flow distance are reduced in surge irrigation, in addition to saving time of irrigation.

• Reduction in infiltration rate

• Rapid advance of wetting front

• Less difference in intake opportunity between upper and lower ends of furrow

• More uniform distribution of water along the length

• Improvement in application uniformity and irrigation efficiency

• Reduces water requirement

• Water reaches the furrow end much earlier than under continuous stream

• It is a non erosive method, suitable for erodible soils

• Useful for light textured soils with high infiltration rate

• Saves irrigation time and the energy cost for lifting water

• About 20% of land area is saved in cross channels with shorter furrow lengths

• It offers scope for automation of surface irrigation.

Limitations

• Little or no advantage in clay or silty soils

• Tail end water loss may increase if not managed properly

• Lengthy furrows of more than 100 m are required

• Ensuring proper gradient to such lengthy furrows is difficult

• With progress in surge cycles and number of irrigations, the bulk density is increased due to soil consolidation

• More suited to shallow rooted crops only.