Yu.I.Shirokova, A.N.Morozov

ON ALLOWING FOR DRAINAGE DEGREE AND MINERALIZATION
OF IRRIGATION WATER WHEN DESIGNING IRRIGATION REGIMES

Abstract:
In the article, the results of moisture-salt transfer processes modeling are brought made in order to study mechanisms of irrigation regime change depending on drainage degree of a territory and mineralization of irrigation water.

Introduction:
Modern methods of definition of crop irrigation regimes commonly do not take into account the factors indicated in the title of the article, although their effect is obvious and is corroborated by a number of theoretical investigations and experiments.
Drainage degree of agricultural lands has a substantial effect on moisture and salts dynamics in the zones of soils aeration and lower layers. This phenomenon is qualitatively considered under the natural conditions by distinguishing between zones of auto-morphic, semi-automorphic (semi-hydromorphic) and hydromorphic regimes of soil moistening. There is no sense in more separate division, since for even these, significantly different regimes, there are no data related to their effect on evapotranspiration and salt regime.
It is still more difficult to forecast the change of water-salt regime for artificially irrigated areas, as for normal growth of plants it is important not only moisture quantity in soil, but also its mineralization in individual stages of the plant growth. To assess quantitatively the effect of drainage rate (its ability to drain certain water amount a time unit by head unit) in the situation considered, let us apply the soil water-salt regime forecast model approved for Central Asia [1] that is based on the theory of moisture-salt transfer in soils. By drainage degree, in this case, is understood capability of drainage to carry certain water amount from one square meter area to one meter of head with the accepted in the model linear dependence of the drainage flow upon the head:

� = �� *(hd. - h),
here: � - drainage flow;
�� - drainage degree;
h - groundwater table;
hd - drainage location depth.

Strategy of investigations:
The particularity of the model [1] used in the forecasts is that in it moisture-salt transfer processes in an aeration zone are considered along with moisture transfer in the lower layers (making allowances for their hydro-geological features - conductivity, head). The model was adjusted to forecast moisture-salt transfer at irrigation regime during a vegetative period that is made automatically coming from agrotechnical demands for moisture and salt-contents of a root-inhabited layer of one or another crop. In order to estimate total effect of those two factors on the growth and development of plants, an existing hypothesis is applied in the model, according to that the action of osmotic component of the total moisture potential in soil is equivalent to the effect of a capillary-sorption component [2-5].
In the first forecast series, the influence of different drainage degree was studied at three groundwater table locations. At that, stable (in long-term context) water-salt regime of the aeration zone was kept automatically, which completely met existing agritechnical requirements.
Forecasts were made for two contrast cases - relatively fresh groundwater (5 g/l by solid residue) and highly mineralized (25 g/l). The second part of the investigations was dedicated to studying the influence of mineralization degree of irrigation water on irrigation rate size and their components.
One should specially note, that we forecasted values of water and salt net balances' components of, and this means that subject to irrigation technique applied, all the investigation results must be corrected by the values of surface and deep discharges characteristic of one or another irrigation method.

Investigation results:
The first part of the investigations was carried our for cotton crop on typical soils of middle-loamy granulometric structure for the central climatic zone (C-II-A according to the classification of the Central Asian National Design Institute of Water and Cotton Sredazghiprovodkhlopok). In picture 1, forecasted water losses to evapotranspiration (total evaporation a year, mm) are compared, with keeping given agritechnical modes in the conditions being considered at the drainage degree of 0.0005 to 0.1 m3/m2, that is virtually from hydromorphic to automorphic conditions.
The results achieved testify that with transfer from automrphic to semi-hydromorphic and hydromorphic conditions, keeping the set conditions range of water-salt regime in a root-inhabited layer makes for, with alternative variants for the plants, different evapotranspiration, the more the higher hydromorphic degree and lower drainage degree. Apparently, that should be expected, since at such a transfer the average moisture rises in a soil's root- inhabited layer and, correspondingly, the total evaporation does, too. Consequently, consumption of irrigation water increases to keep the set water-salt regime.

Figure 1. Forecasted variation of annual values of evapotranspiration depending on drainage degree and hydro-geological conditions in the case of high-mineralized water (25 g/l).

On figures 2 and 3, change of irrigation rate with irrigated area's drainage degree is shown.

Figure 2. Dependency of the annual irrigation rate (with moisture-charging irrigation) on the drainage degree of irrigated area for the conditions of the C-II-A climatic zone. Irrigation water mineralization �� = 1 g/l, one of groundwater - �gw = 5g/l.

Figure 3. Dependency of the annual irrigation rate (with moisture-charging irrigation) on the drainage degree of irrigated area for the conditions of the C-II-A climatic zone. Irrigation water mineralization �� = 1 g/l, one of groundwater - �gw = 25 g/l.

The second part of the investigations was carried our for cotton crop on typical soils of middle-loamy granulometric structure for a climatic zone (U-I-A according to the classification of the Central Asian National Design Institute of Water and Cotton Sredazghiprovodkhlopok), characteristic of the Karshi Steppe. Let us consider how irrigation rates of crops and their components vary depending on irrigation water mineralization. In that series of forecasts, in the above-mentioned model, a drainage degree level was accepted close to the design set for the conditions of the Golodnaya, Djizzak and Karshi steppes (0.001 m3/m2). During that series of forecasts, stable (in long-term context) water-salt regime of aeration zone was automatically kept, completely meeting existing agritechnical requirements despite different mineralization of irrigation water, thanks to automatic correction of irrigation terms according to the total soil moisture potential's value in root- inhabited layer of soil.

Figure 4. Variation of irrigation rate's components depending on irrigation water mineralization for the U-I-A climatic zone (Karshi steppe).

These forecasts unambiguously allow making a series of conclusions: maintenance of given water and salt regimes of soil when using high-mineralized water causes evapotranspiration to rise due to increase of the average moisture of root- inhabited layer and amount of drainage flow for keeping salt balance.

Conclusion: the forecast calculations performed allow valid correcting irrigation rates of crops subject to design drainage degree of agricultural lands and irrigation water mineralization.

References:
1. A.N.Morozov, V.A.Zlotnik. Estimation of usability of high-mineralized water for cotton irrigation. Collection of research works of SANIIRI (Central Asian Research Institute for Irrigation) and Sredazghiprovodkhlopok (Central Asian National Design Institute of Water and Cotton). Tashkent, 1983, pp.80-90.
2. �ampbell 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.Slatcher. Water regime of plants. Moscow, "Mir" publication house, 1970, p.365.
4. K.A.Black. Plant and soil. Moscow, "Kolos" publication house, 1973, p.503.
5. Y.I.Shirokova, A.N.Morozov. On the role of moisture-charging irrigation in increase of water supply to irrigated fields. Water resources of Central Asia. Alma-Ati, 2002. Pp.312-320.

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