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Accounting of groundwater and precipitation when setting irrigation rates

Irrigation rate is the total quantity of water that needs to be supplied to an irrigated field to get target crop yield:

M = E P +ΔW Wgw

Where:

is irrigation rate;

is water consumption;

is probable maximum precipitation used during the growing season;

ΔW is soil moisture storage being used;

Wgw is water volume coming from groundwater.

All the values are measured in m3/ha.

Used soil moisture storage +ΔW is the difference between the moisture storage at the beginning and the end of the growing season. The volume of water coming from groundwater table Wgw depends on its level, physical properties of the soil, climatic conditions, cultivated crops, and degree of development of plant rootage. If groundwater lies at a depth of 2-2.5 m, its contribution to the root layer water regime is low. If groundwater lies at a depth of > 3 m, its contribution to the root layer can be neglected at practical computations.

Source: Zaydelman, F.R. Soil reclamation. 3rd edition, revised and enlarged.
Moscow, Moscow State Universitys Publiching House, 2003, p. 448,
(General academician textbook) (in Russian)

Design values of crop irrigation rates in Syrdarya and Amudarya river basins (V.R. Shreder et al, 1969) were provided for justifying the projects of irrigation of new lands. In turn, this work served as the basis for separation of irrigation rate components for the purpose of specifying irrigation rates by the Central Asian State Design and Research Institute of Cotton Sredazgiprovodhlopok (T.A. Trunova, 1985). This enabled carrying out feasibility study of the effectiveness of adopted engineering decisions in the field of land reclamation.

The principle of independent estimation of irrigation rate components underlied the method of irrigation rate computation by T.A. Trunova:

M = 10 * C1 * C2 * C3 * E + Sd K P

Where:

is total evaporation; = *51.58 / 31.62, for the growing season, estimated on a monthly basis;

C1 is the correction factor for effect of water and physical properties of soil;

C2 is the correction factor for deep seepage under automorphic conditions or drainage flow under shallow groundwater;

C3 is the correction factor for the species of the crop cultivated and durability of the growing season;

Sd is the value of surface discharge;

is the value of groundwater contribution ( = * mx /100; mx is the share of groundwater contribution);

P is precipitation.

Source: Morozov, A.N. Key concepts amd peculiarities of the local procedures
used for estimation of plant water requirement and irrigation schedule (in Russian)

Irrigation with groundwater

Groundwater is broken down into perched groundwater, shallow groundwater, and artesian water.

Perched groundwater is represented by temporal aquifers formed in lenses and interlayers of low-permeable rocks owing to seepage of precipitation, surface and irrigation waters. The rate of water inflow to such lenses or interlayers is higher than that of percolation through those.

Shallow groundwater lies on the impermeable or low-permeable layer first from the ground surface. The surface of this layer is free, that is the pressure on it is equal to atmospheric pressure.

Artesian waters are located in the permeable soils between impermeable layers. They fill all the holes in the stratum and are under pressure; this is why the water level in artesian wells rises over the elevation of completion and often above the ground surface (the wells flow). Mostly ground and artesian water is used for irrigation. The quality of groundwater is characterized by its physical properties, chemical and bacteriological composition, and content of organic matters. Applicable temperature of water for irrigation is at least 1416 C.

Irrigation with groundwater. Water for irrigation must have such chemical composition that is physiologically permissible for plants and will not cause salinization and alkalinization of soil. Its chemical composition is determined by analysis. Water suitability is estimated by solid residue, sodium concentration and its ratio to calcium and magnesium, and by alkalinity. Water with solid residue of up to 11.5 g/l is suitable for irrigation. In permeable soils, salt concentration of up to 6 g/l is permitted under small irrigation rates and good agrotechnology.

Withdrawal of groundwater can be carried out by bore and shaft wells, horizontal catchment area and horizontal wells, as well as by captation of sources and springs.

Wells can be of shaft, tubular and combined types. The depth of shaft wells is up to 3040 m, the well walls are strengthened by reinforced concrete rings, timber blocking, and stone. The tubular well is a well with walls strengthened by pipes 30100 cm in diameter; its intake portion is equipped by a filter. If water does not blow out, it is lifted by piston or centrifugal pumps as well as by airlifts.

Karezes/karizes/kahrezes (drainage galleries) consist of water-collecting adits located in aquifer, aqueduct for water tapping-out to ground surface, vertical wells for ventilation and excavation from adits, drain well and drainage channel. Galleries are often made rectangular in section with a size of about 0.7 x 1.4 m, with walls strengthened with stone, concrete, or reinforced concrete.

Captation of springs and sources is used in most cases for irrigation of downstream slopes. The output of particular sources and springs is usually low, but in the aggregate they allow watering large areas.

Use of groundwater. As opposed to irrigation from rivers, irrigation with groundwater has the following pecularities: the output of springs is little; the water has no suspended sediments; water temperature is usually below 12 C; the water source is located near to the irrigated plot of small area.

To expand the irrigated area, it is necessary to install regulating basins (water storages).

Experience of irrigation of vegetables with groundwater shows that the cost of construction of an irrigated plot, including installation of closed irrigation network, water storage with watertight concrete blanket and boring of wells, is compensated for one or two years of operation.

Source: Mse-Online.Ru

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