FEATURES
OF SOIL WATER-SALT REGIME IN AN ARID ZONE
Arid
zones are characterized of high dryness of the climate, and irrigation
is economically justified actually for all crops. Demands of the crops
to regime of moisture and salts in the root-existing layer (which
changes as well with plants' growth) considerably vary over a vegetative
period. These demands can be assigned in the form of a certain range
of permissible values of moisture and salt concentration in the soil's
active layer which increases with growth of plants, at those crops
can normally develop.
Since water and salt regimes of soils are formed through a series
of factors, it is very complicated to forecast these regimes for long
periods without specific computations. Nevertheless, available materials
of investigations, analyses, modeling of the phenomenon enable qualitative
and quantitative forecasting; this question will be considered in
the next sections.
As to an arid zone, we can say with all evidence that to meet agro-technical
demands of plants, one have to solve contradictory, per se, task:
how to feed a field with required water amount with minimum introduction
of salts, and how to withdraw maximum salts out off the field with
minimum water drainage. This is the main feature of soil water-salt
regime in an arid zone, solution of which lies in the range of irrigation
regime optimization and improvement of the irrigation systems in the
whole. Availability of relatively simple models allowing carrying
out of numerical experiments offers interesting challenges for perception
of regularities of forming irrigated soil's water-salt regime. Appearance
in the calculations, in addition to soil moisture assessment, of the
second factor - salinity (more truly, concentration of soil solution)
allows designating a new concept - soil moisture quality, making specified
at that the wide-used concept - accessibility of soil moisture. This,
as shown in the "Examples of
using water-salt regime models" section, open up
possibilities, at first, for rational planning of water resources
use in a specific water situation; at second, allows making forecasts
and analyzing consequences of one or other design decisions; and,
at third, enable targeted planning of expensive field, and especially,
long-term experiments for confirming or elaborating items of the forecast
and, thus, of the models.
Probably, the most interesting, relatively "unexpected"
conclusion is drawn from a series of numerical experiments that should
be verified by an experimental way; this consists in possibility of
actually equivalent (by efficiency) using irrigation water in spring-autumn
instead of that in summer, that is conduction of a part of vegetative
irrigation during a non-vegetation period without crop yield losses
owing to improvement of soil moisture quality by the vegetative period.
This, taking into consideration the current situation with energetic
operation mode of the Toctogul reservoir, the major regulator of the
tandem reservoir system for long-term regulation of the Syrdarya river
flow, is one of the alternatives for way-out of the critical situation
have been developed last years. See the "
On moisture-charging irrigation's roles in increasing water supply
to irrigated lands ".
As well forecasts of irrigation water expenditures under different
conditions of draining a territory are interesting. On the picture,
the results of forecasting the net irrigation rate value for different
drainage degree are shown. If one considers those without taking onto
account water losses on fields because of an imperfect irrigation
technique, then it may seem that they contradict the practice; and
if one analyzes the situation more carefully, considering the data
of independent experiments, then so interesting ideas occur that they,
in the author's opinion, may be crucial when selecting means and directions
for settlement of the critical situation in CAR.
 Perfect
irrigation technique can solve problems concerning lack of water resources
and, at the same time, answer the question - "what to do with
the salts". By the way, the American water specialists for many
years have been professed and put into effect similar ideas in their
country (T.V.Belyaeva).
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