Y.I.Shirokova, A.N.Morozov

Central Research Institute of Irrigation (SANIIRI) after V.D.Jurin

ON MOISTURE-CHARGING IRRIGATION ROLES IN INCREASING WATER SUPPLY TO IRRIGATED LANDS

(The article was published in the full form in the Water Resources of Central Asia collection. Almaty, 2002, pp.312-320)

Operation mode change of the cascade of over-year-regulation-reservoirs in the Syrdarya river basin and, partly, in the Amudarya river one taking place last decades owing to their transition from the irrigation to energetic discharge schedule causes some tension when carrying out vegetative irrigation, especially in mean and low water years. Agricultural producers experience considerable lack of irrigation water during most dry and hot vegetation months - June and July. How to plot an optimal scheme of crop irrigation in order to mitigate irrigation water deficit in these months (it is desirable without crop loss)?

The article does not touch upon the water economy question applying antifiltration measures in canals, efficient methods of irrigation technique and technology those are big, very important, and independent issues; but this is completely devoted to ways of forming soil water-salt regimes using techniques available at the farmer's disposal: regulation of irrigation terms and rates.

Farmers' experience in irrigation canals taking water from small rivers, flood of which happens in early spring and ends in the very beginning of summer, indicates necessity in making moisture-charging and on salted lands preventive moisture-charging irrigation. When is it more efficient and economical to conduct such arrangements in republic's diverse climatic zones under conditions of altered (to energetic) reservoirs operation schedule? What is effectiveness of these arrangements for soils different by hydromorphic properties? How to gain the most efficient use of limited scarce water resources?

Accessibility of soil moisture is formed both of its availability in a soil root-inhabited layer when it is needed for plants, and its quality, i.e. mineralization degree.

Let us consider, how to use most efficiently excess autumn-winter-spring water resources under the conditions of automorphic, semiautomorphic, and hydromorphic soils, when groundwater mineralization either affects not at all upon irrigation regime (automorphic conditions); or does feebly (semiautomorphic conditions) and it is difficult, without forecasting, to define expediency degree of this agrotechnical method; or does highly (hydromorphic conditions).

As it was mentioned in the beginning of the article, for plants to grow normally, it is significant not only moisture quantity in the soil, and its mineralization as well. To assess quantitatively the effect of prophylactic irrigation in the case in question, we will apply forecasting soil water-salt regime on the model approved for Central Asia conditions [1]. The model forecasts moisture-salt transfer under irrigation regime during a vegetation period; this is formed automatically according to agrotechnical requirements for moisture and salt concentration in root-inhabited layer of some crop. For correct estimation of total effect of these two factors on plant growth and development, the existing hypothesis was used in the model, according to which effect of total soil moisture potential's osmotic component is equivalent to the that of the capillary-sorption component [2-4]. This condition is written as follows:

Pc = P + Po (1)
here, Pc -
P - capillary-sorption potential in the soil:
Po - osmotic water potential in the soil.

Osmotic pressure of the soil solution containing mixture of various salts is impossible to be exactly calculated, and direct experimental determination of that is rather difficult. For engineering calculations, one can apply indirect way of osmotic pressure determination through solution's specific electric conductivity using empiric dependence suggested in work [2] that was tested on experimental materials [5]:

Po = 0.365& (2)

here, Po - osmotic potential of the soil solution, Bar;
& - specific electric conductivity of the solution, mmo/cm.

We determined critical values (allowable limits) of the total water potential in the soil (total moisture pressures in the soil) for definition of irrigation term in the model of soil root-inhabited layer control and control of irrigation regime by processing materials of experimental data [6]. For that, values of capillary-sorption moisture potential in the soil, which had not been studied in the tests [6], were defined by pF = f(O) curves, determined by us, for soils of diverse granulometric composition: sand, loamy sand, light loamy soil, mean loamy soil, heavy loamy soil, and light clay. Further, these values were summed with the actual ones of osmotic potential (pressure), which had been gained in these experiments at measuring under the least moisture capacity and corrected to pre-irrigation moisture value equal to 0.7 HB.

According to comparison results of the calculation data with the relative crop capacity in the experiments (as 100 % was taken the crop capacity on non-salted background), values of critical moisture potential in the soil were defined, those are within 4-6 atm for cotton and does not actually depend on salts composition and fertilizer background. Under poor availability of feeding elements, surplus concentration of soil solutions even brings to some comparative yield rise. That criterion was taken when fixing irrigation in the model experiment. That is, regular irrigation was set upon reaching critical pressure of soil moisture in the root-inhabited layer regardless of that on what the pressure rides - matrix or osmotic potential of moisture.

As an example, we'll compare forecasts steady in the long-term context of cotton irrigation regimes on soils of mean loamy granulometric composition for the central climatic zone, which keep water-salt regime of the aeration zone stable during many years completely meeting agrotechnical requirements, for five cases:
- without prophylactic irrigation;
- at spring prophylactic irrigation;
- at autumn prophylactic irrigation;
- at autumn prophylactic irrigation by higher rates;
- at spring and autumn prophylactic irrigation.

Forecasts conducted for the central climatic zone (C-II-A according to Central Asian State Design Institute of Cotton (Sredazgiprovodkhlopok) classification) for typical, by granulometric composition, soils under automorphic, semihydromorphic, and hydromorphic conditions, subjected and non-subjected to salinity, allow concluding that prophylactic irrigation can conduce to reduction of vegetation water consumption of 11-77 %, depending on natural conditions.

On figures 1,2, and 3, one compares moisture content and salt content regimes in a soil meter-layer in the cases forecasted. One should note that these two indices, taken separately, insufficiently indicate if the agrotechnical requirements were violated in the root-inhabited layer or not. For a more detailed analysis, in addition, indices of soil moisture's total potential are compared with the given agrotechnical requirements on the picture.

Figure 1. Predicted regimes of moisture and salts without spring and autumn moisture-charging

Figure 2. Predicted regimes of moisture and salts at spring moisture-charging

Figure 3. Predicted regimes of moisture and salts at spring and autumn moisture-charging
(Graphical symbols on the graphs: MR1, SR1 - moisture reserve and salt reserve of the soil dead layer; DP - design pressure and NP - normative pressure of soil moisture).

As one can see on figures 1-3 and tables 1 and 3, both spring and autumn prophylactic irrigation enable reducing number of vegetation irrigation without violating agrotechnical requirements recommended for a vegetation period thanks, mostly, to water supplied during non-vegetation period and decrease of soil solution concentration as a result of this. Application of both spring and autumn prophylactic irrigation simultaneously results in even higher effect in reducing vegetation irrigation rates.

Table 1. Forecast results on water need to ensure agrotechnical requirements for loamy soils at diverse terms of prophylactic irrigation.

Table 2. Forecast results on water need to ensure agrotechnical requirements for loamy soils at diverse rates and terms of prophylactic irrigation for hydromorphic conditions at groundwater mineralization of C = 20 g/l.

Table 3. Forecast results on water need to ensure agrotechnical requirements for loamy-sand soils at diverse terms of prophylactic irrigation.

 

These forecasts allow unambiguously drawing a number of conclusions:
- there are several possible alternative scenarios of water allocation over a year, at which moisture and salts regime does not exceed the limits of agrotechnical requirements;
- conduction of prophylactic irrigation in all considered cases proved to be efficient in respect to water saving in a vegetation period;
- on lands with close fresh groundwater, this method results in somewhat lower effect than on hydromorphic soils with mineralized groundwater;
- increase of prophylactic irrigation rates to some reasonable limit (table 2) gives certain positive effect in water saving in vegetation period, but one should remember that, in practice, in order to supply net-field rate of 3.0-4.0 thousand m3/ha, nearly twice water amount is required (gross-field), not to mention the rate in system head that will redouble owing to losses to filtration from canals.
In addition to the methods have been considered, they certainly should use everywhere known-to-farmers methods of closing moisture by tallage of soil upper layer, and it is advisable after every irrigation and even heavy rain, right up to closure of the plant cover down on soil surface, when evaporation part from the surface becomes inessential.
Terms of prophylactic irrigation conduction, depending on climatic conditions of a certain region, have to vary. So, in the south zones, where moisture of autumn irrigation keeps badly to vegetation beginning, early spring irrigation is more effective. In the north zones, where winter atmospheric precipitation strengthens leaching effect, late autumn prophylactic irrigation is more efficient. As to the central zone, time of carrying out of prophylactic irrigation (spring or autumn) is of little importance.
Thus, the suggested methods can substantially help in optimal use of the water resources discharged during a non-vegetation period from reservoirs for power generation purposes.

References
1. Morozov A.N., Zlotnik V.A. Assessment of high-mineralized water usability for cotton irrigation. - Scientific works collection of the SANIIRI (Central Asian Research Institute of Irrigation) and Sredazgiprovodkhlopok (Central Asian State Design Institute of Cotton) Institutes, Tashkent, 1983, pp.80-90.
2. яampbell R. B., Bower C. A., Richards L. A., Change of electrical conductivity with temperature and the relation of osmotic pressure to electrical conductivity and ion concentration for soil extracts, Proc. Soil Sci. Am., 13, 1949 p.66-69.
3. R,Sleycher. Plants water regime. Moscow: "Mir", 1970, pp.365.
4. K.A.Black. Plant and soil. Moscow: "Kolos", 1973, pp.503.
5. Theory of soil salinization and desalinization processes. Alma-ata: "Nauka", 1981, 296 p.
6. Ryjkov S.N. Development and crop capacity of cotton under various soil solution concentration. - Collection of mineralized water use for irrigation. Moscow: "Kolos", 1973, pp.26-45.

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