Shirokova Y.I
Morozov A.N.

Salinity of irrigated lands of Uzbekistan: causes and present state

Ajmal Khan et al (etc.) Sabkha ecosystems. Volume II: West and Central Asia. 249 -259. 2006. Springer series of book - Tasks for vegetation science - 42. ISBN: 1-4020-5071-2 â)

Abstract
In article is described existing situation with salinization of irrigated lands in Uzbekistan. The causes of this phenomenon from natural-climatic and anthropogenic factors are briefly explained. The role is showed of ground water in these processes.

Introduction
Most irrigated lands of the Republic of Uzbekistan are subjected to salinity. This is owing to arid climate, geological and hydrogeological conditions of irrigated areas.
Primary climate features, which form rate and tendency of salt accumulation in soils and underlying depositions, are abundance of warmth, moisture deficit, long hot and dry summer, and short and relatively warm winter. Radiation aridity index R [1] characterizing connection of between energetic, water and salt regimes, and also displaying set of environment-forming factors, varies within 2.5 to 12. Aridity coefficient [2] - Ka < 0.12 - 0.3, continentality coefficient [3] - Kk = 220-290. These indices are a cause of soil salinity under conditions hydromorphous regime [4].
Irrigated agriculture here till beginning of mass land-reclamation dating from the middle of the last century was developed at river valley, their first and second terraces and deltas. Technical capacity of off-taking water and hydro-geological peculiar properties of the area explained this. To surface salting only the outlying districts of so-called rivers debris cone and delta sites of ancient irrigation were undergone.
Plain Central Asia territory has, in a majority, soils naturally salted and potentially dangerous of secondary salinity development. The Aral Sea area's lowland is a region of ancient and contemporary salt-accumulation. Active mountain-forming processes on an area contiguous with the one are accompanied with continual carrying-out of soil-forming material inclusive salts and accumulation of that on the plains. Surface and underground flows have been formed at the Central Asia territory in the Cretaceous and tertiary periods still continue to act now. On their motion ways the flows carry salts out of strata being eroded and in course of their movement they are being enriched with salts of ancient deposits containing salts. On water motion way they change their chemical contents and mineralization. Streams running from mountains are partly discharged into rivers and dips and carry their water as far as zones of finite accumulation. As a result, thick layers of depositions composing lowlands during geological epochs were undergone to salt accumulation dependent on eluvial, transit and accumulative processes [4-8].
As the second source of salts in the region are deep-laid ascending brines. Though they have a rather local development at a number of deep dips, nevertheless, they play a great part in forming adverse alkali soils out off the surface of those salts are carried away to long distance through aeolian transfer.
At littoral zones of the Aral Sea the third source of salinization is seawater. Under drying coasts out alkali soils form along the seaboard, mainly chloride ones, but including sulphate-magnesium and natrium salts. Soils forming at sea depositions are originally salted. Ancient sea depositions are "suppliers" of salts, which are carried away by wind to surrounding plains.
Under the conditions of arid climate, the fourth source of soil salinization, particularly irrigated ones, are quick soluble salts in the Central Asia river water. As use of surface flow of the rivers to irrigation increases their accumulation part in soils and underlying depositions increases. Since expansion of irrigation to sub-mountain plains and steppes, area of lands (non-salted at upper, but having considerable relict salt reserve) potentially subjected to salting have abruptly increased [5].

Analysis of theoretical and monitoring materials connected by modern picture of salinity of irrigation soils.

As irrigated agriculture developed, conceptions of forming groundwater flows and transporting salts by them became deeper, for on those to a great extent depend success of reclamation on newly reclaimed lands.
Depending on hydrogeological features of a territory, hydrogeologists and specialists in land-reclamation marked out [6-10]:
- zones of submersion and transit of groundwater flow, generally, fresh one, hydrocarbonate chemism passing through well permeable depositions;
- zones of pinching out of fresh and slightly salted waters serving as a source of forming lime and gypseous layers, as well as alkali soils arising where permeability of strata transporting underground water slumps;
- zones of secondary submersion (or rather dispersion) generally high-minerailized groundwater having more prevalence of chlorides where their external inflow becomes negligibly small, and their level is determined only precipitation and evapotranspiration (fig.1).
As moving away from mountains natural and artificially formed when irrigating groundwater flows naturally change their chemism from hydrocarbonate through sulphate to chloride-sulphate and chloride those [5, 11, 12].
It is noted [5] a significant fact that in the region two kind of landscape are distinguished highly differing with soil salting processes:
- having relict salinity predominating on the territory with automorphic soils
- having contemporary salinization occupying without irrigated soils less than 10 % of the territory, along with hydromorphic soils.
Soils of river valleys within a mountain territory are generally not salted on the surface, for here falls precipitation amount sufficient to leaching regime, and groundwater have very low mineralization. However, even within mountain area valleys zones of forming thick gypsum and lime horizons in the subsoil layer occur, so called "shokh" and "arzyk" ridden on falling gypsum and lime out of relatively cold groundwater, getting into warm horizons thanks to reduction of these salts solubility as the temperature rises. Besides, here, though too seldom, zones of high-mineralized groundwater discharge are met with, salt source of those are relict fields.

Figure 1. Motion of groundwater under natural conditions and under irrigation

Main massifs of salted soils in Uzbekistan's natural conditions were situated just in regional zones of pinching groundwater, even having comparatively little mineralization of 2-5 g/l, as well at river delta sites and local lowlands as well. Just there forming alkali soils - "shory" or "sory" in local dialect, happened, having also a name as "sebkhy" in literature on salted soils.
As a typical example of zones of forming alkali soils in Uzbekistan at outskirts of submountain plains serve a sufficiently extensive area on intersection of the Djizak region and the Golodnaya Steppe.
Zones of spread of alkali soils at big and small rivers' delta sites exemplified by the delta part (the Karakul oasis) of the Zarafshan river and Amudarya river (the Khorezm oasis and Karakalpakstan).
Among the most typical lowlands on steppe and desert zones having big areas of alkali soils can be Shuruzyakskoe and Arnasayskoe lowlands at the Golodnaya Steppe, Charagylzsloe and Denghizkulskoe at the Karshy steppe, as well as Tudakulskoe, Shorsayskoe and Shorkulskoe at the Bukhara oasis.
On regularly irrigated lands as a place for salts accumulation can serve micro-highlands, which are consequence of bad leveling fields, and, also, adjacent non-irrigated areas and lowlands to those there is permanent inflow of groundwater from neighboring irrigated areas (fig.3). In literature this phenomenon is often referred as "dry drainage". "Dry drainage" on compactly irrigated lands ensures outflow of salts from the irrigated fields, if out of them 20 - 40 % and more areas are not irrigated. Most clearly this phenomenon became apparent at the Khorezm oasis and no the Karakalpakstan territory.

According to estimate good for irrigation and irrigated soils in Uzbekistan, more than half is salted to quite high extent.
Classical description of salt transportation in natural conditions under intensive influence of irrigation and drainage is harshly change both on local and regional level [4]. Irrigation essentially intensifies course of natural processes in soils. On fig.2 charts of groundwater motion under natural conditions and irrigation on various local relieves are shown.

Figure 2. Scheme of groundwater flows and salt transfer under irrigation conditions.

Under conditions of artificial irrigation soils salinity and trend of processes depends in the main upon economical activity, since irrigated agriculture radically changes hydrological regime of soils and hydrogeological processes on the irrigated areas.
Irrigation canals of reclamation systems form sources of concentrated supply to groundwater [13, 14].

Fig. 3

On fig.3 it is shown how under these conditions prevailing flows of groundwater form, and a character of forming their local pressure.

This all leads to localisation of sufficiently global (have been existing before irrigation) hydrogeological and hydrochemical processes, intensifies many times salt transfer processes and carrying them out to the sources, because pressure gradients between inflow and outflow elements on irrigated areas increase several times in comparison with pre-existing under natural conditions.
Irrespective of climate aridity, salt accumulation process in a soil is defined by the direction of net moisture flow in the soil layer for long period of time [15]. In an arid zone, where relatively big water amount is evaporated and transpired, these processes strongly accelerate, and to form water-salt regime of a soil it becomes very important by what means and how the one get there.
It is quite obvious that on the fact how far evenly over the field area agrotechnical requirements of a certain crop are kept depends its yield. In accordance with numerous data, a root-existing layer determining vital functions of plants of any sort (from herbs to wood growth) doesn't extend than 1 metre [16].
Field irrigation influences upon salt transfer in soils. Irrigation water in the Central Asia rivers having mineralization at mountains exits 0.2 to 0.3 g/l, and at lower situated areas - 0.1 g/l and higher is a strong source of salts to soils, since about 80 % of it is spent to evapotranspiration, and, on that how the irrigation is being carried out, how much it fills natural moisture deficiency in the arid zone, and not useless, by-passing the field surface, feeds groundwater, depends economical well-being of the irrigated lands and ecological state of the irrigated territories [4, 17].
On fig. 4 a scheme of forming soil water-salt regime of a field in identical vertical and horizontal scales is shown.

Figure 4. Scheme of movement of water flows and salts at subsoil horizons on irrigated lands.

On the figure one can see how thin the soil layer is and how accurate and evenly over the field area irrigation water should be allocated in order to make the required water regime and, especially, salt regime in the root existing layer. Poor consideration of this circumstance results, to a considerable extent, in the problems are being observed on irrigated lands subjected to salinity in the Aral Sea basin.
It historically has been developed that here only one link on the way of moisture towards plants in the soil layer has been missed by engineers and reclamation specialists, namely water distribution technique on fields which allows, according to reclamation experts, to transfer the water from free flow state into one of soil moisture.
Perfect irrigation technique is able to solve a set of problems. It can save up to 40 % of irrigation water on field, it forms water-salt regime increasing crop capacity as much as twice, enables to meet required agrotechnical demands when growing crops, prevents deep and surface water discharge, provides high evenness of water distribution over the field area, at the same time, thus, settling problems of lands reclamation state as well.
Current state of irrigated lands relating to salinity, according to data of the Reclamation Service of the Ministry of agriculture and water resources, is displayed on fig. 5.
The figure shows general comprehension of salted irrigated lands available in Uzbekistan.

Fig. 5.

On the fig. 6 allocation of irrigated lands in Uzbekistan's regions in comparison of 1991 and 2000 years is shown. On it one can see within that period areas of irrigated lands have considerably risen, especially in the regions where large land tracts potentially subjected to salinization were relatively recently reclaimed.

Fig. 6. Extension of salted lands on irrigated areas (according to autumn soil observations by the Monitoring Service of the Ministry of agriculture and water resources of the republic).

Even more clearly one can visualize the extent of anthropogenic impact by analyzing cosmic pictures. On the fig. 7 it has been shown a fragment of a cosmic picture of irrigated lands on the Golodnaya Steppe. On that one can see that area salinity depends on economic activity.

Fig. 7. Fragment of an irrigated area of the Golodnaya Steppe.

Materials of regime observations, brought below as examples (fig. 8), are indicative of how far dynamically salinity on individual fields within sufficiently short periods.

Fig. 8. Seasonal salt accumulation in a soil layer of thickness of 0-60 cm on the test site of the farm after Syddikov in the Syrdarya region.

Nowadays, under actual existing situation, seasonal salinization of irrigated lands happens rather owing to tightening of salts dissolved in groundwater taking place as a result of violation of irrigation regime than irrigation water quality. At evaporation to root-existing layer, more often from groundwater salts come in than when irrigating even with mineralized water.
Approximate balance calculations performed for individual fields (according to actual observations of the WUFMAS project of 1996-1999) have shown into the sol layer from the top and the bottom arrives about the same amount of salts, at irrigation water deliver - 7-8 thousand m3/ha with mineralization of 1.5 g/l, and depth of groundwater around 2 m with mineralization of 5-7 g/l. Under insufficient water supply to irrigation from the surface, even at irrigation with drainage water to 3-4 g/l, the main part of salts arrives from groundwater because of their high mineralization (18-20 g/l).
Data of last years' investigations on irrigation water composition and mineralization of the Aral Sea basin's main rivers testify their increasing mineralization 21 in comparison with previous years' data 22, 23 . As it was said above, these waters, playing an important part in salinization processes of irrigated lands, thus become more and more dangerous in salt accumulation processes.

CONCLUSION

In this article we have just touched upon principal issues of irrigated soils subjected to salinity. These consist in the fact that today, however strange it is, processes of salt transfer and their control just in soils have poorly been studied, a regional conception of their reclamation allowing for all economical and ecological consequences of application of some technical decisions. Under the conditions of the Aral Sea crisis connected mostly with exhaustion the basin water resources at the current technical state of hydro-reclamation systems these problems become essential to the region. To make day-to-day control it is necessary above all to strengthen the monitoring service of irrigated areas potentially dangerous with development of repeated salinization. Improvement of this service is regarded by application of remote mapping technology in conjunction with GIS methods. Besides, wide application have to find those of ground simplified operative monitoring over salinity extent to control soil salinity on certain fields during vegetation 24 .

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