Influence of variability of climatic indicators on water consumption of cotton

. Processing and analysing average annual climatic parameters over 89 years revealed some trends in air temperature, relative air humidity, and precipitation in the typical grey soils (sierozem) region. These trends demonstrate sustained growth of the climatic indices. However, evaporation rates, deficit indices, and their movements, calculated and performed based on the above climatic parameters, have been decreasing with time. Over the recent years, such a downward tendency in climate elements change has reduced irrigation water requirements of early-ripening cotton plants. The article performs experimental data on evaporation rates measured using small-diameter evaporation pans about water temperature and wind speed. In the region with typical grey soils over quite a long observation period, significant changes in climatic elements have occurred, which have had a substantial impact on irrigation schedules and total water consumption of crops; these changes are the following:


Introduction
According to the Intergovernmental Panel on Climate Change (IPCC), the world's air temperature increased by 1 degree Celsius over a short period [1].As the temperature rises, the concentrations of such gases as carbon dioxide, methane and dinitrogen monoxide in the air grow as well, which results in the escalation of the greenhouse effect.In turn, this effect provokes many factors influencing, in particular, the water abundance of rivers, precipitation amount, radiation balance, etc.Eventually, climate element change can significantly affect crop management practices in the arid zone.*Corresponding author: ubezborodov@rgau-msha.ruTo consider the consequences of climate parameters change, it is reasonable to perform their preliminary analysis, where the long-term weather sequences (series of observations) are taken as an example.Thus, the research object was the hydrological and meteorological station "Akkavak", located on the territory of the Central research and trial facilities (Tashkent district, Kibray region).

Results and Discussions
Climatic indices at the "Akkavak" meteorological station have been recorded since 1925.The average values of daily air temperature, rainfall, wind velocity and relative air humidity were calculated each month and over vegetation periodsfrom April to September and from May to Augustfrom 1925 up to 2013, in other words, over the 89 years.For each month, they were calculated.
Evaporation rates and evaporation deficit indices, which afterwards, among the other parameters, were used to estimate the annual average values.The average yearly climatic indices allowed for plotting the chart's trends and determining analytic dependences (Fig. 1).
It is clear from the chart that the rise in air temperature by 1 degree for 89 years was accompanied by significant growth of relative air humidity values and rainfall depth.Besides, evaporation rates and deficit indices substantially reduced throughout observations due to the increase in relative air humidity, as long as its growth rate was considerably superior to the speed of air temperature rise.There is one climatic peculiarity that draws attentionthe rainfall depth trend is steadily going up.Moreover, it is of tremendous importance that such a tendency is typical of the vegetation period and the year-round.
The performed analysis of climatic elements reveals the possibility of reducing irrigation water volumes applied for crop growing due to increased precipitation and declined evaporation deficits in a region with the same climatic conditions.1-air temperature о С, 2relative air humidity, %, 3rainfall amount, mm, 4evaporation rates, mm, 5evaporation deficits, mm The results of the recent investigations allowed researchers to develop the optimal irrigation schedule for early-ripening cotton varieties.Irrigation rates varied significantly despite the same cotton practices and crop density irrigation time thresholds being appropriately followed.So, irrigation rates were only.
Diverse weather conditions influenced it in the vegetative period and out-ofseason.The unique matrix was developed to determine the interrelation of irrigation rates with air temperature and relative air humidity (table 1).Using this matrix data and software application DIASTA, the following dispersion equation was obtained, which adequately confirms experimental results - = 787 + 0.3 × 10 −3  1 − 6.43 2 ,  2 = 1 (1) It is known that to calculate cotton water demands, it is necessary to have information on soil moisture and rainfall consumption by the crop.In simplified form, these two parameters can be substituted by the depth of precipitation falling from January to May.For this purpose, the following matrix was compiled, including three factors: air temperature, humidity and rainfall depth (table 2).The parameters from this matrix were processed using the software application mentioned above.As a result, the linear multiple-factor dispersion equation was obtained, which adequately describes experimental data,  = 628 + 0.42 1 − 0.68 2 − 0.57 2 ,  2 = 0.934.
(2) In areas with typical grey soils (sierozems), the ground waters are known to be found at more than 3 m depth; thus, cotton total water requirements are made up of three components: moisture consumed from the soil over the vegetative period, rainfall accumulated over the out-of-season period, and irrigation water volumes applied for wetting the root zone of cotton plants.
To calculate the dependence of total cotton water demand on the three components listed above, it is possible to use available data on rainfall over the December-May period instead of soil moisture content at the beginning of the vegetation period; in this case, the rainfall amount is called "effective" or net rainfall amount.Such an approach to rainfall is quite applicable because of their partial loss through surface runoff when heavy rains occur and through evaporation.Some methods of crop irrigation schedules development and water requirements calculation are based on such climatic parameters as evaporation rate.

Considering their
High-significance evaporation rates are recorded at many regional hydrometeorological stations.
Method of crop evapotranspiration calculation based on water evaporation intensity from special tank evaporimeters is included in FAO-24 and FAO-56 recommendations as tank evaporation measurement method [2].
At large hydrometeorological stations of Uzbekistan Hydrometcenter for measurement of evaporation from the free water surface, special evaporation tanks are 4 E3S Web of Conferences 454, 02002 (2023) ICAS 2023 https://doi.org/10.1051/e3sconf/202345402002adopted: the Russian GGI-3000 pans with a surface area of 0.3 m2 and the Russian 20m2 tanks.However, these evaporimeters are installed outside the irrigated territories, and their constructive features contribute to the error in evaporation measurements (additional radiant energy intercepted by high sidesside-wall effects; moreover, appreciable heat exchange occurs between the pan and the soil).
On farms, one can use small-diameter evaporation pans installed directly on fields to study the possibility of small-diameter evaporation pans application for predicting the timing of cotton irrigation.The two polyethene microevaporimeters were produced and had a diameter of 16.2 cm, 12 cm height, and a surface area of 20 cm 2 .Evaporimeters wrapped in a thick layer of polyethene film and filled with water of different temperatures were exposed so that the tank's main body was sunken below ground level, the evaporating surface at the surrounding soil surface level.Evaporation stations were located on an unvegetated plot on a cotton field at 300 m distance from the "Akkavak" hydrometeorological station.
The chosen method of evaporation measurement provided an approach to recording water evaporation (water temperature was different in two evaporimeters) using weighing evaporimeters on electronic scales right in the field.Thus, evaporation was measured for daylight hours and daily average.Along with evaporation rates, air temperature and wind speed data were also considered (table 3).The data obtained allowed the calculation of evaporation rates by the Ivanov formula [3]; the results are presented below in Table 4.
Calculated evaporation rates were obtained using the formula: NB: water temperature (Tw)here is given the average water temperature between two evaporimeters It is seen that evaporation significantly depends on water temperature; over 15 days, the average difference of 3.1 0 С in water temperature between two evaporimeters resulted in 16.8 m 3 /ha or 23.7% evaporation variance.
It is presumed in hydrology that evaporation is a function of only atmospheric factors, such as temperature, relative air humidity (Ivanov formula [3]), wind speed, vapour-pressure deficit, and water vapour pressure.However, in the case of waterunlike airthe temperature usually is not necessary to be known.That is why the field experiment included studies on water's evaporation characterised by different temperatures.
The averaged data on evaporation rates obtained from both microevaporimeters were used for developing dependency comparable to the Ivanov equation but supplemented with wind velocity values.For this purpose, the relevant matrix was compiled and presented in Table 5; on its basis, with the use of DIASTA software, the multiple regression equation was developed - = 2.1 1 − 0.23 2 − 1.13 3 + 96,  2 = 0.77, (4) where, Yevaporation rate, X1-air temperature; X2wind speed; X3relative air humidity.According to the given equation, the evaporation rate in evaporator 1 calculated for August reaches 2311 m 3 /ha.The evaporation value computed using the Ivanov formula is 1304 m 3 /ha.So, the ratio between these two values is 2311:1304= 1.77.
Evaporation was proven by hydrologists' research to be influenced by water body size.Thus, the evaporation rate from a 100 km2 water area is 7% higher compared to 1000 km 2 water surface, from 1 km 2 surfaceby 15%, from 1 haby 22%, from 20 m 2 evaporation tanks, which are commonly installed on large hydrometeorological stations,by 30-35%.Therefore, the less evaporation surface, the stronger the influence of advective exogenous warmth transport on the evaporation process.
Suppose the evaporation rate from a standard evaporation tank with a 20m 2 water area is accepted as one unit.In that case, for the GGI-3000 pans with a surface area of 0.3 m2, the evaporation coefficient amounts to 0.8, and for micorevaporimeters with 0,021m 2 water surface area, this coefficient is 0.56 [4].
The developed equation is valid, provided that the following condition is fulfilled: 0.021 2 ≤  ≤ 20 2 , and allows calculating the evaporation coefficient for any evaporimeter, the water surface area of which lies within the mentioned range.

Conclusion
In the region with typical grey soils over quite a long observation period, significant changes in climatic elements have occurred, which have a great impact on irrigation schedules and total water consumption of crops; these changes are the following: the rise of annual average air temperature by one degree, relative air humidity by 25%, precipitationby 40%.
Changes in climatic indicators, primarily the relative humidity of the air and the precipitation layer, contribute to reducing the need for cotton in irrigation water.This conclusion is essential for this region, where water use is carried out according to a limited schedule.
To maintain an optimal regime of pre-watering soil moisture and forecasting pl ant s' water needs, it is advisable to install small evaporators directly on irrigated fields and correlate their indicators with soil moisture [5].

Fig 1 .
Fig 1. Dynamics of the average climatic indices over the vegetation period.

Table 1 .
Matrix of dependency coefficients of cotton irrigation net demands on air temperature and relative air humidity

Table 2 .
A matrix containing dependency coefficients of cotton irrigation net demands on air temperature, relative air humidity and rainfall depth over the period from January to May

Table 5 .
Matrix parameters for calculation of daily evaporationCalculated evaporation values are performed in the last column of Table5.The average evaporation rate measured is 83.25 m3/ha, average calculated -86.5 m3/ha; the difference amounts to 3.9%.6