Gradient plane irrigation
by lengthwise circuit
The length of a delivery ditch L can be up to 2-2.5 km. It depends generally on maximum permissible elongation of the field brigade’s plot. It is desirable that the delivery ditch should serve one field brigade. This term and condition should be observed for all irrigation schemes.
The distance from the outlet from the delivery ditch is determined by the optimal length of irrigation furrows and equals to the quotient of the length L by the furrow length lf. The result is rounded off to the next integer. The interval between the delivery ditches with fall on one side depends on the following:
- standard discharge of the delivery ditch;
- conditions of command (fall) of the delivery ditch above the field surface, i.e. on maximum possible discharge in its outlets;
- rate of water flow received by the irrigation furrow;
- number of changeovers (cycles) of the watering units distributing water over the furrows.
Number of changeovers of watering units with fall on one side cannot be more than two. This makes the space between the delivery ditches narrower and sharply reduces the possibility of irrigation automation. In case of a ditch with fall on two sides, the space between them can be two times as much and thus the specific extent of the water conduits will be reduced considerably. However, on plain terrain with one-side slope along or crosswise the water conduits with only fall on one side is possible, because designing of double-side command (fall) would require large amount of leveling works. The requirement of selection of fall on one side or two sides of delivery ditches should be met when designing all schemes.
The length of run for any type of treatment before the first water application may be very large; after the first water application, it is limited by irrigated plot of land, i.e. by the length of the furrow. This may be due to that ok-aryks (irrigation ditches) are not levelled after water application and are kept for the full season. Certainly, they interfere with the operation of the aggregates. In old-arable zones, ok-aryks are levelled with mattock prior to each cultivation and restored prior to the following water application. These ok-aryks have small cross-section and consequently low discharge (Q = 15-20 l/s).
The experience of application of portable watering units (hoses) shows that it is necessary to have non-flooded borders in the field for mechanized assembly of the hoses after water application. This limits the length of run under after-irrigation cultivation.
At irrigation by relatively long furrows (350-400 m and longer) instead of ok-aryks and portable watering units (pipelines) it is reasonable to use stationary flumes with automatic water distribution between furrows.
The run can be very long when using closed (subsoil) irrigation units (suggestion of I.A. Sharov, G.Yu. Sheinkin, and I. Vyrlev), if irrigation is carried out simultaneously by all closed pipelines and at the same time the soil gets ready.
The longitudinal scheme is irreplaceable by any another one under higher soil permeability and steep slopes, when the length of the irrigation furrow is not to be longer than 250-300 m.
by crosscut circuit
The concept of an irrigated land plot on this scheme does not coincide with the concept of an irrigated land plot on the longitudinal scheme. Its area is not equal to L * lf or L * 2 * lf. The entire area attached to a delivery ditch includes several time-varying irrigated land plots. The length L of this area, i.e. of the delivery ditch, should not exceed 2-2.5 km, and the land plot width equals to the standard length of the irrigation furrow. At the same time, the irrigated land plot includes a few border strips or working plots.
The width of the border strip irrigated from outlets at the delivery ditch is equal to the quotient of the best suited (standard) discharge of outlet by the rated furrow discharge multiplied by the width of the space between rows. The standard discharge of outlet is determined by the following conditions.
Firstly, the land plot width should not to exceed 50 m, at most 70 m. In this case, it is easier to provide hydraulic automation of water distribution between furrows, because the horizontal level of the water that provides equality of the pressures over all irrigation outlets is maintained. If one water inlet is made in the middle of the watering unit or two water inlets on its both ends, the width of the irrigated land plot can be increased up to 100 m.
Secondly, too great discharge, e.g., equaling to a half of the delivery ditch discharge (200 : 2 = 100 l/s), should not come through the outlet to the watering unit. Otherwise, the command (fall) of the delivery ditch over field will have to be increased, which will entail additional construction expenses and create inconvenience in operation.
However, there are calculation methods and designs of watering units with longer irrigation front (up to 250-300 m) and greater discharge (up to 200 l/s).
The longitudinal scheme can be automated much easier than the crosswise one. To facilitate automation and exact water distribution with the longitudinal scheme with fall on one side, a stationary but not portable watering unit is better to be provided near to the delivery ditch to distribute water between the furrows. Under double-side command (fall) not one stationary unit, but now three ones are required along the route of the delivery ditch. That is why it is necessary to develop such a design that would enable distributing water to the both sides simultaneously and pass it down in transit. Probably, remote control of outlets by means of hydraulic or pneumatic automatic equipment will be needed.
The operating conditions of tractors are determined by the size lf, which is to be more than 350-400 m. This can be provided by making wide spaces between the rows and increasing discharges to the furrows, i.e. at medium and low slope gradients along the irrigation furrows. In this case, slope gradient of the delivery ditch does not matter.
Laktaev N.T. Cotton irrigation [in Russian], 1978
