Stulina G.V.,
Shirokova Yu.I.,
Morozov A.N.

CURRENT WATER FORMATION PATTERNS IN CAR AND PECULIARITIES OF HIGH-SALINE WATER USE IN IRRIGATION UNDER CONDITIONS OF UZBEKISTAN

Abstract:
The paper describes current formation of water resources in the region and the peculiarities of irrigation under abnormal seasonal and long-term river flow regulation. It also grounds the main principles of using the high-saline water in Central Asian plains, based on possible negative effects in the soil. The paper gives theoretical approaches and analyses results of the past research and new experiments conducted by the authors. Moreover, it demonstrates the results of field studies and simulations of moisture and salt transport in the soil irrigated with high-saline water, and explains the irrigation regimes, depending on a degree of area drainability and irrigation water salinity.

Introduction:
High-saline water is widespread in Central Asia, particularly in the lower reaches due to inflow of drainage water from the upstream areas. River water salinity comes to 1,5 - 2,0 g/l in the lower reaches of the rivers Syrdarya and the Amudarya.
The problem is that historically the water sources have served also as intakes for drainage and waste waters in the valleys of Central Asian rivers. It is not possible to solve the problem completely in foreseeable future; therefore, re-use of high-saline water in the area of its formation is considered as forced.

Water formation
Primary water resources in the Central Asian Republics (CAR) are formed by the surface flow from the rivers Amudarya and Syrdarya and their tributaries.
The river flow is strongly variable within a year and in long-term. Long-term flow variations depend on climate cycling.
The fluctuations of river flow are considerable from year to year: during low-water year (90% flow probability) river flow is 20 km3 less than that of normal year.
A series of high-water years occurs in 6-10 years, its duration is 2-3 years. Low-water periods occur in 4-7 years and last for a long time (up to 6 years).
Coefficient of river flow variation is 0,12-0,32.
The fluctuation of annual flow is wider which is explained by river feeding. Mountain snow, snowfields, and glaciers mainly contribute to formation of flow in larger rivers. Contribution of snow and rain prevail in tributaries of those rivers.
The cyclicity of flow fluctuations in given rivers and the prolonged low-water periods make it difficult to use water sources for economic purposes, and this calls for flow regulation.
Technical capabilities of the reservoirs constructed in the Amudaray and Syrdarya river basins allow for seasonal (Amudarya river) and long-term (Surdarya river) regulation for the purposes of water and power usage; however, since 1991 a part of mountain reservoirs has been operating in power mode, thus increasing abruptly water releases in winter period and reducing releases during growing season. Such mode of operation reduces the chances to meet demand of upstream and midstream irrigation water users and leads to irrigation water deficiencies in low-water years.
In addition to above-mentioned factors, diversions for irrigation needs and inflow of return waters affect the basins' river flow and its hydrochemical regime. As a result, the flow underwent considerable changes. The increase of withdrawals from the rivers to irrigation canals and the channel losses cause the reduction of flow, while discharge of drainage waters leads to disturbance of natural flow regime and deterioration of quality.
Water use trends in irrigated agriculture and other water-consuming sectors may be observed by the annual trend of consumptive water withdrawals from the rivers, as shown in Figure 1. The figure demonstrates the reduction in flow in the main nodes of the rivers Syrdarya and Amudarya caused by anthropogenic factors from 1932 to 1998. According to the data, consumptive use of river flow has increased twice since early sixties, and current water use increased four times as compared to 1930-40's.

Figure 1. Dynamics of annual flow as observed in river nodes over 1932-1998

Salinity dynamics as observed in the main river nodes for 1932-1999 is shown in Figure 2.

Figure 2. Water salinity dynamics by the main river nodes

The above-mentioned data indicate to growing crisis in water use practices in CAR caused by the increase of consumptive use and pollution of water sources by drainage and return waters, as well as by reduced seasonal and long-term regulability of river flow. This violation of regulation in the sources and their anthropogenic pollution create serious difficulties for irrigated agriculture.
During growing season, drainage and waste waters having salinity of 3-5 g/l and more are used for irrigation in areas where irrigation water of good quality is scarce (practically everywhere in low-water years). The acceptability of saline water for irrigation or leaching depends on both physical-chemical soil features and water chemistry (Table 1).

Analysis of the acceptability of high-saline water for irrigation
Acceptability of water for irrigation is determined by: salinity and chemistry of irrigation water; hydraulic-physical features, grading, and salinity of soil; degree of natural and artificial drainability; general climatic conditions; groundwater level and chemistry; irrigation rates, regime, and methods; farming techniques and crop characteristics.
Specific characteristics of soils in southern part of Central Asian region, where most lands are irrigated, are prevalent fractions of silt and low content of clay. As a result, cation exchange capacity is low (to 10 - 15 mg-eqv/100 g). The soils contain a lot of calcium and therefore the degree of soil buffering is high and, respectively, alkalinity practically is not observed.
Within the framework of regional projects, such as WUFMAS, IWMI, Copernicus, OIMP, etc., the chemical analyses were made of more than 3000 water samples taken from different areas of Central Asia and a comprehensive estimation of their quality was made, involving tests, according to FAO's classification [4], for risk of soil alkalinization when using these waters for irrigation. The obtained data may serve as a basis for the development of database on water chemistry and salinity.

Table 1 Volumes and quality of return waters in Uzbekistan*

Note: * "Vodproyekt's" data for 1988-1998 (taking into account availability of drainage flow in the main collectors during the period of low-water years).
** Ministry's for Agriculture and Water Resources data (inter-farm collectors).
*** SANIIRI's (Central Asian Irrigation Research Institute) laboratory data (primarily on-farm collectors).
**** SAR* - indicator of potential alkalinization.

The analysis of 357 water samples having salinity between 0,215 to 70 g/l (samples taken from more than ten points within the Amudarya and Syrdarya river basins) showed that where water salinity varied within 0,3-6,0 g/l, SAR* was not more than 9 (i.e. half-dangerous level in terms of alkalinization). It is established that as to risk of alkalinization, waters with salinity not exceeding 2 g/l and the soil water solutions with salinity reaching 4 g/l show SAR*=3 and are not hazardous, while waters having salinity of up to 6 g/l and the soil water solutions having salinity of 9 g/l are half-hazardous (SAR*=9). In case of higher salt concentrations in water and the soil water solutions, there is a risk of alkalinization since SAR*, as a rule, is more than 9. As to effect on soil percolation properties, all analyzed drainage waters are not hazardous as salinity of waters used for irrigation is usually not more than 0,5-2 g/l.

The theoretical ground for using high-saline waters for irrigation purposes is that salt concentration in these waters is much lower than that in the soil water solutions. In case of irrigated soils, the optimal salt concentration of the soil water solutions is 3-5 g/l; at salinity levels reaching 6 g/l we observe the slight reduction in plant growth; 10-12 g/l lead to heavy suppression of growth, while salinity of 25 g/l causes death of the plant [2].
Thus, theoretically water having the salt concentration of up to 3-5 g/l may be used (given free gravity flow and continuous water delivery) without damage to the plants.

However, in practice one should bear in mind the following for irrigation plots: the salt tolerance of crops and of particular crop development phases; high evaporation; inadequate control of salinization or soil osmotic potential; untimely irrigation and low level of irrigation technologies; lack of water outflow (including, due to shallow saline groundwater).

In this context, if water salinity is more than 3 - 5 g/l, this water should be used carefully. One should necessarily take into account both crop species and varieties that can be more salt-sensitive. Use of drainage waters in order to compensate shortage of irrigation water is more promising for cultivation of salt-tolerant crops, such as cotton and winter wheat.

In case of using high-saline water for irrigation, calcium is substituted by sodium and magnesium (by 5-6% of the total) in absorbing complex. It is established that the increase of adsorbed sodium in the soil is related to increased soil salinity and is reversible, i.e. leaching and irrigation with ordinary river water reduces the ratio of sodium and magnesium exchange cations and raises calcium exchange cations [3]. Whereas the use of saline water practically does not represent the risk of alkalinization to given area, the risk of secondary salinization is high. Given the low drainability of given area and the good preventive measures, there exists a tendency to salt accumulation due to irrigation with saline water, as was observed in the experiment carried out in Hunger Steppe during six years [6]. During irrigation of cotton and alfalfa with water having salinity of up to 5,3 - 5,9 g/l (at SAR* of up to 9,9 and irrigation norm of 5 - 5,5 thousand m3/ha) in combination with leaching during non-growing season with water at salinity level of not more than 1,5 g/l and at a norm of 2,4 thousand m3/ha the percentage of salts and adsorbed cations in the soil changed seasonally. It was observed that adsorbed sodium increased in the soil (in the sixth year of irrigation - 15-25% of initial percentage), this in turn impacted the intake rate of soil during first few hours of the observation. Other changes in soil properties (capillary, field capacity, liquid and plastic limits) were not observed. However, irrigation with water, salinity of which is more than 1,5 g/l led to the reduction of both cotton and alfalfa yields [5].

A possibility to leach soil with saline water originates from the fact that salt concentration of the soil water solutions in salinized land is higher as compared to that of water. In order to evaluate a degree of water salinity, which is acceptable for leaching, one can suppose that the soil water solution could be desalinated to a level of salt concentration corresponding to that of water.

A model developed together with A.N.Morozov was used in elaborating approaches to soil water-salt regime control under the conditions of long-term application of saline waters. Model computations were made under the provision that during irrigation with saline water of light soils (light loam, sandy loam, sand) the salt concentration of soil water solution should be maintained at a level that do not damage crop yield. As a result, they showed that (Figure 3): at water salinity level of 2 g/l irrigation norm must be increased by 5-7%; at 3 g/l, by 20%; and, at 4 g/l, by 30-50%. In case of medium loam, even salinity of 2 g/l requires that application of water be increased by 10%. Such increase in irrigation norm depends on many aspects, and, first of all, on groundwater level and drainability to ensure outflow of the additional water volumes.

Figure 3. Predicted increase in water consumption and drainage flow when using high-saline water and maintaining good conditions for cotton growth (Karshi Steppe).

The simulation of water-salt regime and change in annual irrigation norm in time under conditions of Surkhandarya, using actual soil and climate parameters and different degrees (slight, medium, high) of initial soil salinization in the experimental plot and of irrigation water salinity (1,5 and 3 g/l), while maintaining optimal for plants conditions to achieve 100% of cotton yield, resulted in (Figure 4) that even at slight initial salinization and irrigation water salinity of 1,5 g/l it is necessary to increase irrigation norms (against practiced norms) at expense of leaching fraction in order to maintain mentioned conditions in the long-term. By the same reason, at high level of initial salinity at the beginning of irrigation the higher norms of water application are required and then as salinity decreases, the required norm is reduced too. The required additional leaching fraction depends on the salinity of water used for irrigation; consequently, irrigation norm is raised when salinity increases. If irrigation water salinity is 1,5 g/l, in time, the irrigation norm needed to maintain an optimal regime asymptotically approaches 8000 m3/ha and comes to 10000 m3/ha at water salinity level of 3 g/l. If initially the soils were not saline, then given irrigation water salinity of 3 g/l, crops may be irrigated by the same irrigation norms during four years.
Thus, long-term use of high-saline water requires that soil salinity be controlled through leaching. The intensity of leaching directly depends on the initial soil salinity and the salinity of water used for irrigation.

Figure 4. Predicted changes in irrigation norms required to maintain acceptable for cotton degree of salinity when using irrigation water at salinity level of 1,5 g/l. (Surkhandarya, medium loam)

To prevent accumulation of salts in the aeration zone, it is desirable to use high-saline water mainly for irrigation of light-graded and high permeable soils.
Currently, in fact, as outflow of groundwater is complicated due to poor drainage, seasonal salinization of irrigated lands is mainly caused by salts dissolved in groundwater and, to a lesser degree, by quality of irrigation water. During evaporation often more salts are brought to the rooting zone rather than during irrigation with saline water.

Approximate balance calculations made for some fields (using data of field observations under WUFMAS Project, 1996-1999) showed that roughly the same amount of salts is brought to the soil layer both from above and from below when we irrigate with 7-8 thousand m3/ha of water, salinity of which is 1,5 g/l and the groundwater depth is about 2 m and its salinity is 5-7 g/l. In case of shortage of irrigation water, and even when using drainage water with salinity of 3-4 g/l, major share of salts comes from groundwater due to its high salinity - up to 18-20 g/l (fields of Surkhandarya province). Thus, sometimes under-irrigation, to a higher degree, creates a danger to crop yields and soil quality than single irrigation with saline water.

Conclusion
Thus, the soil characteristics, water quality, and cropping patterns in Central Asian republics enable us to relatively safely use collector-drainage waters, taking into account that salt-tolerable crops (cotton, winter wheat) are primarily cultivated in the region. The main negative effect of such use is salt accumulation. Because of low soil sorption properties and high ratio of calcium salts in water and soil, alkalinization processes are practically excluded. Salt accumulation, as a rule, leads to increase in percentage of exchangeable sodium and magnesium in the absorbing complex. The experiments show that these processes are reversible under desalination. Analysis of salts in local waters and the soil water solutions allows for conclusion that water with salinity of up to 9 g/l theoretically could be used at a certain risk of toxic effects in plants (and, respectively, of yield losses) and provided that concentration of salts is controlled and regulated by timely irrigation. However, even water having salinity of more than 3-5 g/l should not be used. But in case of need, it is necessary to take into consideration the type of irrigation crops since they have different degrees of salt-tolerance which for some types changes by development phases, as well as the soil permeability and grading. At the same time, it is important to prevent salinization of soil by regulating application of additional water volumes. Given the available water and good outflow from the field, the above-mentioned is feasible during growing season by performing more frequent irrigation or increasing net norms. If there is lack of water during growing season and drainability is poor, it is advised to leach the soil during non-growing season.
For the adequate planning of high-saline water use, the zoning of irrigated lands is required, depending on available data on soil grading, chemical composition and salinity of drainage waters, and actual drainability of the area.

REFERENCES
1. Hillel D. Salinity Management for Sustainable Irrigation, 2000
2. Babayev A.Kh. Evaluation of water quality for the purposes of irrigation or watering// Transactions-M., 1973 - p.12-23.
3. Glukhova T.P., Strelnikova G.A. Saline water in Uzbekistan as an irrigation reserve - Tashkent: Faan, 1983.
4. Booker Tropical Soil Manual, edited JR Landon
5. Morozov A.N., Ignatikov V.N. Changes in soil properties and crop yields under prolonged irrigation with saline waters// SANIIRI and Sredazgiprovodkhlopok transactions.- Tashkent, 1986. p.53-62.
6. Shirokova Yu.I. The use of collector-drainage waters for leaching of saline soils in the modern zone of Hunger Steppe: Author's abstract - Tashkent, 1985.

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