Basin flooding (flooding irrigation systems) is the most ancient irrigation method; it is used for cultivation of crops that are able to withstand short-time flooding. Moreover, such an irrigation method is used for charge watering, soil salinity control, rice irrigation. Flooding irrigation is applied for rice cultivation, inundative (or so-called liman) irrigation.
The advantage of the flooding method is its simplicity, high productivity and the possibility of uniform submerging the soil surface (except the soil submergence at liman irrigation). At flooding irrigation the salts are uniformly driven down into deep layers of soil crossover.
However, the drawbacks of this method are apparent too. At flooding irrigation, high water application rates are applied to ensure the possibility of quick rise of ground water table. Lastly, submergence causes anaerobic conditions to arise in the profile’s surface horizons (epipedons), increases the mobility of the organic substance, damage of the soil structure, crustification, slitization (hardsetting), and other adverse phenomena.
Basin flooding is applied for irrigation of maize, alfalfa, rice, winter and spring wheat, barley, oats, sorghum, etc. These crops can withstand short-time flooding.
At flooding irrigation, water comes to so-called checks with an area from 4 to 50 ha and more. In rice-growing farms, checks usually have horizontal surface. For regular field crops they can have slope grade of 0.0005-0.001. Coming to the check, water fills it by a sheet from 10 to 25 cm; some part of the water is absorbed in the check, and excess water is dumped during irrigation of field and feed crops. At alternating flooding, water is delivered to the check every 7-8 days. In the case of continuous flooding, for example for growing rice crop, a water sheet of 10-15 cm is kept from seedling or tillering period to the beginning of waxy ripeness. At flooding irrigation of regular vegetable and field crops deep furrows or strips (ditches) are cut in checks, that is combined irrigation method is applied here.
In some regions with low slope grades and pitted slopes of the landscape, the flooding method is applied for maize and other feed crops. In some farms, if there is network of checks, flooding irrigation is used for watering orchards and vegetable crops. If deep furrows are made and excess water is promptly dumped, vegetables can withstand flooding irrigation. The attempt to apply the flooding method for many crops under production conditions is explained by: high labour productivity at water application (15-20 ha per shift for one irrigator); possibility to implement control and automation of the water application process (the water application quality is determined by the presence of water sheet from 10 to 15 cm in the check); simple structure of small regulating irrigation network (the whole network consists of irrigation ditches-drains and fixed earth borders).
A check should have properly leveled surface to ensure uniform water covering and required depth of flooding of the check. A great drop in the check elevation will result in non-uniformity of check flooding, water and salt stagnation and, as a consequence, in damping-off of not only regular field crops, but also of rice. Therefore, a drop in the check elevation should not to be more than 0.2-0.3 m.
Especially good surface leveling should be made for field crops (grain drops, cultivated crops, alfalfa and other), because water stagnation in microdepressions may not only lower completeness of young crops, but may also lead to crop damping-off.
Irrigation of field crops in checks, which are leveled at a single elevation, is carried out by flooding, because water movement occurs only under pressure. For the purpose of lowering water application rates and ensure soil overwetting, irrigation of field crops at flooding should be carried out by so-called checks (check plots) or irrigation strips or by furrows. Checks or irrigation ditches narrowed by temporary earth ditches 0.3-0.4 m high are made inside cultivation checks. Checks or irrigation strips divide cultivation checks of 4-6 ha and more into plots 12.5-25 m wide and with an area from 0.25 to 0.5 ha.
Arrangement of checks and irrigation strips and, for intertilled crops, cutting deep irrigation furrows with a width on top of 0.35-0.4 m and depth of 0.18-0.2 m will ensure fast water movement and consequently more uniform and optimal moistening of the soil, lowering of water application rates in large checks with a discharge from 4000-4500 to 1200-1500 m3/ha, protection from water stagnation in microdepressions and fast diversion of excess water from the check.
The length of checks or irrigation strips as well as of deep irrigation furrows corresponds with the width of the cultivation check, i.e. 200-250 m. At basin flooding of regular crops, water is supplied to checks or border strips from a temporary irrigation ditch to a water-receiving or distribution furrow that unites a section of irrigation furrows or flooding strips arranged in an irrigating check. Each check is served by one water-receiving or distribution furrow.
Irrigation of regular field crops by flooding is carried out at concentrated and increased discharges. After flooding of an irrigation check all the excess water is diverted to a wasteway (discharge) canal to avoid death of plants, then to a field drain. In the period of early growth and development of crops, long-time submergence is particularly dangerous; therefore, the acceptable duration of water stand in the check even with a water sheet of 0.12-0.15 m should not exceed 0.25-0.5 days in this period. During the period of full-blast growth of plants, the acceptable duration of submersion is not to exceed 1 day.
According to the data from A.N. Kostyakov, the acceptable duration of submergence of crops during the vegetation period should not exceed 0.2-0.25 days for vegetables, 0.2-0.5 days for cereals, 0.7-0.8 days for pastures, and 1.0-1.5 days for hayfields.
When carrying out water application by check flooding, it is very important to properly establish water discharge by the elements of the irrigation and check network. The calculation is based on the check area and irrigation depth. The more the check area and the less the estimated water application rate, the faster it gets filled with water, and consequently the discharge of the water supplied to the check is to be higher.
Estimated discharge increases with expanding check area and reducing water application rate. With a check area of 4-5 ha, the estimated discharge comes to 20-40 l/(s*ha); with a check area of 15-20 ha, the estimated discharge rises up to 60-80 l/(s*ha). With further expansion of the check area up to 40-50 ha the specific water discharge rises up to 100-150 l/(s*ha) and over.
The less slope grade of the check surface, the faster it is filled up. If the check surface is 5-10 ha, specific discharge should be about 20-40 l/(s*ha) in order to use a water application rate m=2000 m3/ha. With the same depth and the check area of 15-30 ha, the specific discharge should be raised to 60-120 l/(s*ha).