To evaluate the furrow irrigation system, rate of advance and recession are observed as in case of border strips. However, the ring infiltrometer data will not be applicable to furrows as in case of furrows, only a part of the soil is exposed to the water.
The intake rate in the furrows is determined either by the ponding method or by the inflow- outflow method. In the ponding method a section of three consecutive furrows are selected and mild steel plates are driven to create buffer around the control portion of the furrow under observation. The results are plotted just like the ring infiltrometer test.
A correct assessment of the furrow infiltration is obtained by what is known as the inflow- outflow method. In this method, a 30 m reach of the furrows is selected. Water is allowed to flow in three consecutive furrows.
The inflow and outflow rates in the control furrow are measured. The outer furrows are known as guard furrows and they help in obtaining a representative value of the furrow infiltration in the central furrow. The field data may be tabulated as in the following example.
Example 1:
The following table gives data about the furrow infiltration test by the inflow and outflow method. The test length selected was 30 m and furrow spacing 1 m. Find the infiltration characteristics of the furrow.
The infiltration characteristics are plotted using log-log scales in Fig. 15.13.
Cutback Stream:
In furrow irrigation after the water has reached the end of the furrow, the stream size can be so adjusted as to meet the infiltration requirements. The reduction in the stream size is referred to as cutback and the reduced stream size as cutback stream.
The cutback stream will result in higher application efficiencies. In furrow irrigation the stream size can conveniently be reduced by reducing the number of siphons or reducing the opening in the gated pipe depending upon the particular appliance being used. The following example illustrates how the cutback stream can be calculated.
Example 2:
In irrigating a furrow of 120 m long, observations on advance time were taken as shown in the following table. The stream size used was 25 lpm. Assuming that the infiltration characteristics of the furrow are as given in Example 1, calculate the cutback stream size. It is assumed that after the water has reached the end of the furrow, irrigation with the original stream size was continued for 18 more minutes and then the stream size was cutback. If the crop needs 40 mm of water, calculate the time of irrigation and the average depth of water applied. Assume furrow spacing of 1 m.
Solution:
The infiltration characteristics are taken from Fig. 15.13 and using the information of furrow infiltration for different opportunity times, the following table is prepared.
From the above calculations it can be seen that at the end of 160 min, the infiltration rate for the entire furrow is 17.75 lpm and hence the stream size entering the furrow should be of this value. Time required to put 40 mm of water into the soil as per equation developed in Example 1.
This is calculated assuming that the stream size is 25 lpm for the first 160 min and 17.75 lpm for the remaining period of irrigation.
Surge Flow Irrigation:
In this system of irrigation a stream of water moves along the furrow at high advance rate and once the flow reaches the end of the furrow, it is stopped either manually or by using automatic devices.
Irrigation is completed in the second cycle using a reduced stream size. This system of irrigation pioneered by the scientists at Utah State University is gaining popularity in the furrow irrigated areas in USA.