A.N.Morozov

IMITATION MODELS OF SOIL WATER-SALT REGIME

We have imitation models of several complexity levels with assistance of achievements of scientists from the former All-Union Research Institute of, and Central Asian Research Institute of Irrigation (SANIIRI) developed and adapted to the Central Asia region's conditions.

Balance models
As a basis of the imitation model for optimization of an irrigation regime a common balance model of a root-inhabited layer is taken, in which the soil moisture state is taken into account when defining a stress situation in the crop model. Prototypes of this model are the model worked out by G.A.Khorst in 50s of the last century and the Yaron D. at al model.
More upgraded models of this sort offered by FAO thereto do not make an allowance for salinity, that is absolutely inadmissible for lands subjected to salinity, because this disfigures the required irrigation regime. With the help of a few modifications of this imitation model the following problems can be solved:
* water and salt regime of a given soil layer, crop capacity, and a profit with an assigned irrigation regime are forecasted;
* irrigation regime is found, which provides the maximum yield;
* irrigation terms and rates are optimized in accordance with the yield gained and water consumption, and conducting irrigation (in money terms) are done, too, depending on a soil type, its original salinity, productive capacity, irrigation water mineralization, kind of crop grown, and given climatic conditions.
These three modifications allow sufficiently exact forecasting moisture transfer phenomena in automrphic soils taking into consideration initial soil salinity and irrigation water mineralization and, thereby, rational (by water consumption or money cost to a crop unit obtained) using irrigation water and finding optimal irrigation regime variants under the conditions of lands prone to salinity.
The model will enable to improve the methods suggested by FAO for calculation of irrigation regimes of various crops for soils not subjected to salinity to apply for soils subjected to salinity, and those in Uzbekistan make up about 50 % of irrigated ones. The problem for this model class still is taking into account groundwater influence, or rather moisture and salts inflow from lower horizons, though, formally, possibility for their consideration is provided in the models.

Imitation models
The subsequent improvement of the moisture-salt transfer model in order to develop crop irrigation regimes under the conditions of dynamic state of groundwater table that is dependent on irrigation and a sub-pressure groundwater flow is an imitation model based on solution of hydraulic conductivity equation and convective salt transfer in a soil; it is designed for long-term forecasts of moisture and salts dynamics over the soil horizons, groundwater level of an irrigation regime, and crop capacity for wide spectrum of tasks:
* forecasting water and salt regimes, crop capacity at the set irrigation regime in daily details;
* forecasting an irrigation regime by a critical state of soil moisture in diverse plant growth phases at steady-state, from long-term aspect, salts regime;
* all the above-mentioned forecasts at change of irrigation water mineralization, groundwater inflow rate and its mineralization, drainage degree of the area;
* forecast, in essence, of a drainage effluent.
The model allows quite more precise forecasting crop irrigation regimes compared to the balance model, taking, at the same time, into consideration irrigation water quality; as well it allows complex solving strategy issues of the inter-seasonal water resources distribution, which is especially important under the conditions of transition of over-year regulation reservoirs from the irrigation mode operation to the energetic one (see the section "On moisture-charging irrigation roles in increasing water supply to irrigated lands"). In this section, basing on a series of forecasts, it is shown by which way one can efficiently use reservoir water discharges in a non-vegetation period in order to reduce irrigation water deficit in a vegetation period.
Who takes an interest in details of the water-salt regime forecasting methods, he can refer to the section "Water-salt regime forecast methods".

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