As it is known, areas with an arid climate with groundwater runoff determine the main conditions of crop production [1] . In establishing mode of an irrigation plants, especially cotton, the state of groundwater determines the amount of water-supply via irrigation canals. Soil, affected by salt and in the presence of groundwater toxic salts can give condition of groundwater, which determines the main conditions of plant life, for example: a small distance from the ground surface leads to salinization and soil degradation, and causes a deep state of increased demand for irrigation water [2] . Prediction of ground water regime is a scientific challenge. The challenge is caused by the presence of a variety of factors that affect the groundwater regime; the most influential of them are precipitation and runoff [3] . At the same time, the groundwater regime has a significant impact to the area. In this regard, the object of research was established in the Chirchik river basin area in the Tashkent region of Uzbekistan (Figure 1). As a subject research, primarily influence of precipitation was determined in conjunction with other components of the water balance of a river basin in groundwater areas. The objectives of this paper are to predict the ground water regime in the region with less groundwater observation wells. Also the results of the prediction are applicable to protect soil from erosion and soil salinization.

The target area of research is the area of Chirchik river basin, where crop production is carried out exclusively by irrigation [4] . Therefore, prediction of the state of groundwater enhances the effectiveness of water management in the production of crops and especially of cotton. Cotton for the mentioned region is the main plant and occupies more than 60% of the irrigated area of the Syrdarya river basin [4] . As the research methods defined the water-balance method of the river basin, the method of unspecified filtering and interpolation method of climatic factors, developed by the authors.

The water balance equation in the Chirchik river basin:

W: water increment of groundwater;

Q_Char+Ugam: water inflow to the Chirchik river basin from the Ugam river and the Charvak reservoir;

Q_ohan: water inflow to the Chirchik river basin from the Akhangaran river;

P: precipitation;

Q_chinaz: water abstraction in the Chinaz station;

Q_use: water use in the Chirchik river basin;

ET: total amount of transpiration;

E: total amount of soil evaporation;

Q_gw: ground water discharge and expenditure;

Q_syr-darya: effluent of groundwater to Syr-Darya river;

Q_return: return water from collectors.

The Richard’s equation of unsteady filtration of water:

where―volumetric soil moisture; t―time (day); S(h)―the rate of extraction of water by plant roots (evapotranspiration), m³/m³ day; h―pressure of soil moisture (cm of water); z―vertical coordinate pointing upwards; K―the coefficient of hydraulic conductivity (hydraulic conductivity), cm/day [5] .

Interpolation method of the climatic data is shown in Figure 2.

The values of the coordinates of hydro-meteorological stations are based on the relative origin to the Tashkent weather station in meters (Figure 3).

Using the parameters of the water balance Chirchik river basin, topographic maps using the program MIKESHE simulation model to get a map of seasonal changes in groundwater levels throughout the basin of Chirchik river (Figure 5).

To establish reliability of the data on changes in groundwater levels were the measured values of the groundwater level according to data on observation wells in the basin with the results obtained by the simulation model (Table 1, Figure 3). Calibration and comparison of the results obtained by natural observation and calculation method of feature points in the basin are shown in Figures 4-6.

In Figures 7-9 the dark lines show the land surface and the red and blue lines- groundwater levels, determined by different methods [7] . As the graphs show similar values to predicted (Table 2) and actual (Table 3) values of groundwater level fluctuations within acceptable RMSE for W06 point is 0.580954, RMSE for W03 point is 0.65943 and RMSE for W09 point is 0.645235.

As a result, the model developed made it possible to reliably estimate ground water level by using climatic data in the river basin. Taking into consideration that groundwater together with irrigation water is sources of water

use plants, prediction method can effectively manage water resources for irrigation of crops, especially cotton. Forecast for the groundwater level contributes to the adoption of scientific and technical measures to protect the irrigated land from waterlogging and salinity.