In this article we will discuss about:- 1. Hydraulics of Borders 2. Unit Stream Approach 3. Evaluation of Border Irrigation.
Hydraulics of Borders:
The hydraulics of border irrigation consists in developing equations relating the flow phenomena. The fluid flow phenomenon of surface irrigation is a case of unsteady non-uniform and spatially varied open channel flow over a porous bed. It represents a complex problem in theoretical analysis, owing to the varying intake rate of soil both with respect to time and distance.
While several developments in hydraulics of surface irrigation are available in literature, some fundamental aspects are presented here:
1. Equation for Water Advance:
The equation for water advance using the volume balance method as proposed by Lewis and Milne (1938) is as follows –
The above equation represents the general solution of (15.4) in terms of Laplace transformation. Its usefulness depends upon the particular solution developed using the different expressions available for y.
Example:
In a border the infiltration characteristics are defined by the equation y = 0.8t0.5 + 0.6. Calculate the distance the water front would advance in 50 minutes for a discharge of 1.5 litres/min/cm of border and the mean depth of flow as 8 cm.
Solution:
2. Design of Borders using Volume-Balance Method:
As the name indicates, in this approach the volume of water introduced into the border is considered as the sum total of the water above the ground and the water infiltrated. Israelsen (1955) proposed the following equation based on volume-balance approach. Referring to Fig. 15.5 and considering unit width, his equation is –
The above equation does not represent the field conditions because of the assumption of constant infiltration rate. Singh and Chauhan (1974) assumed I = Iav, the average infiltration rate for the duration of irrigation. The approach presented by them for the design of ponded borders is outlined here.
Eq. 15.19 can be used to estimate x for known Iav, q, D and c or q can be determined if other values are known.
3. Average Infiltration Rate:
The infiltration characteristics of a soil can be expressed by the Kostiakov’s equation as –
4. Average Depth of Surface Flow:
Assuming normal depth of surface flow at upper end of the border as constant, the average depth of surface flow c, may be written as –
Eq. 15.24 gives the value of n with an accuracy of 96.8 per cent for q = 1050 cm3/min/cm to 1350 cm3/min/cm and S0 = 0.3 to 0.7 per cent.
5. Time of Application:
Knowing the Iav and D, (accumulated intake) time of water application is calculated from Eq. 15.20. This is the time for which irrigation is continued (i.e., duration of irrigation).
Unit Stream Approach:
The unit stream approach to the design of graded borders has been evolved with the help of field observations taken under a wide range of conditions. A unit stream is defined as the stream in cumecs (cu.m. per second) required for each 100 m length of border strip of 1 m wide. Unit streams are determined depending on the depth of water required to fill the rootzone, the infiltration rate of the soil and the border slope.
Fig. 15.6 shows the unit streams for 0.5 per cent land slope and for different basic intake rates. Unit stream values for other slopes are obtained using the slope factor given in Fig. 15.6. The unit stream values should provide sufficient depth of flow to ensure adequate surface coverage.
The unit stream value is multiplied by the length in hundreds of metres. This will give the unit width stream. In case the unit width stream provides less than an initial minimum flow depth of 1.5 cm,
the unit stream should be increased. Again the size of the stream should not be so large, to cause erosion. Fig. 15.7 gives the maximum stream sizes with different land slopes.
Once the unit stream, which is suitable for the soil and the slope is determined, the size of the stream, or the width, or the length of the border can be computed by the following equation –
Evaluation of Border Irrigation:
Field tests of the designed border strips should be carried out in order to evaluate the design. Such evaluation helps in suitably improving the system. The procedure consists in determining the intake rate by the ring infiltrometer for determining the time required for a particular depth to infiltrate into the soil.
A critical examination of the advance and recession curves indicates the necessary changes. The intake opportunity time for all points should be nearly equal and should be enough to infiltrate the required depth of water. For example – if the depth absorbed at the end is too much a cut-back stream is desirable.
If the intake opportunity time is not enough, the stream size may be reduced or the length of the field increased. To assess the soil moisture distribution in the field, soil sampling before and after irrigation are desirable.