Duty of water (or water duty), or hydromodulus (the term coined from the Greek hydro “water” and the Latin modulus “measure”), means the average water consumption by a cropped hectare for a certain period, i.e. specific water consumption. Water duty (q) is expressed in l/s per hectare. There are duty of water consumption (q'), water consumption per hectare without including water losses in the irrigation network, and duty of water supply (q''), water consumption with account for the water losses in the irrigation network. At a water application rate T (m³/ha), water application period t (days), and twenty-four-hour water application, we obtain as follows:

, l/s per hectare

If the irrigation system efficiency during the period t is equal to h, then

, l/s per hectare

If the area of the irrigated plot w (ha) and water duty are known, the one can determine quantity of water consumption by the plot (net Q') and the quantity of water supply (gross Q") to the head part of the irrigation system for the period t as follows:

Q' = w · q' l/s; Q" = w · q" l/s.

When sowing a few types of crop on an irrigated plot, occupying, respectively, a1, a2, ..., ai percent of the plot area, we obtain the following:

In the similar manner we determine the values q"1, Q'1, Q"1, i.e. the values q", Q', Q" are multiplied by . When irrigating several crop types, their water duties are summarized.

Having found out water application and irrigation rates for each crop, water application time and water duties, a graphic plan is drawn up for the water use for the irrigated plot throughout the growing season, or water duty schedule. To this end, time t is plotted along the horizontal (X) axis, and water duty q is plotted against the vertical (Y) axis. If the Y-axis values sharply differ from each other and indicate the time interval between water delivery processes, the schedule gets regulated, viz. water application time and duration are changed (within a range admissible for each crop) and water application rates, keeping irrigation rates unchanged. Approximate value of water duty for cotton rotation in Central Asia is 1.05-0.80 l/s per hectare; for grain-fodder and wheat-grain crop rotation in the southern areas of Ukraine and trans-Volga region is the 0.50-0.40 l/s per hectare; for vegetable and fodder crops in the Central Black Earth region is 0.5-0.3 l/s per hectare. The water duty of rice growing irrigation system is higher: 2.5-2 l/s per hectare at initial flooding; 2.0-1.0 l/s per hectare at kept flooding.

Source: Great Soviet Encyclopedia

Hydromodule zoning implies division of a territory into taxonomic units with the view of high usage of land and water resources and establishing scientifically grounded, differentiated irrigation regimes that ensure gaining stably high crop yield.

Four taxonomic units are marked out at hydromodule zoning: edaphic-climatic district or oasis; edaphic-climatic zone; soil amelioration area; and hydromodule zone.

Edaphic-climatic district is a part of a republic territory with characteristic geomorphological, climatic, hydrogeological soil conditions and vegetation.

Edaphic-climatic zone is a part of a soil and climatic district with homogeneous meteorological conditions and a single type of soil formation.

Soil amelioration area is a part of a soil and climatic zone with homogeneous hydrogeological and meliorative conditions and genetic proximity of the soil-formation process.

A hydromodule zone is a part of a soil amelioration area, which is characterizes by relatively equal values of the thickness of soil cover, texture, soil configuration and consistency in the aeration zone, hydrophysical properties, groundwater table, which govern the size and regime of crop irrigation, and hydromodule ordinate. Duty of irrigation water is the water quantity supplied per second per unit of irrigated area.

Given the peculiarities of the natural and economic conditions that determine hydrographic zoning are dynamic, hydromodule zones cannot be regarded as invariable, they should be revised from time to time. Analysis of the variability of particular factors is used as the basis for such revision.

Textbooks and tutorials

Voznyuk, S.T. Kuzmich, P.K., Kryshtof, V.G., Kirienko, T.N., Olinevich, V.A. Reclamative pedology with hydrology principles (in Russian) (1984) 

Regulatory and procedural guidelines and reference information

Stulina, G.V. Guidelines on hydromodule zoning and crop irrigation (in Russian) (2010)