Methodology for planning environmentally safe irrigation regimes for agricultural land

Calculation of irrigation regimes based on long-term hydrometeorological observation data

Periodic soil moisture due to natural precipitation is the most characteristic variant of the regime formation for irrigated lands in the steppe and dry-steppe zones. The level of soil moisture reserves changes over time from the moisture of capillary rupture to field capacity. However, the moisture indices for irrigated and non-irrigated plots differ. In rainfed areas, the water regime type is non-leaching, while irrigated areas are characterized by spatial-temporal fluctuations in soil water regime types ranging from non-leaching to leaching.

In irrigated agriculture, irrigation must regulate soil moisture so as to ensure maximum productivity of agricultural lands under specific natural and agrotechnical conditions, while economically using water-energy resources and minimizing the negative impact on soil fertility and the environment. With sufficient resources, the main task of irrigation is to maintain the moisture reserve in the active layer within the boundaries of the optimal range for plant growth and development. Maximum crop yields are observed when soil moisture varies within 0.8-1 of field capacity.

Therefore, the main criterion used for irrigation management is the dynamics of soil moisture reserves in the calculated soil layer. Change in moisture reserves over the calculation period (mm):

where soil moisture reserves at the end and beginning of the calculation period;

ET - total evaporation, which is a function of potential evaporation E and the type and development stage of the agrobiocenosis;

M - irrigation rate;

G, J - values of groundwater recharge and infiltration.

Thus, a prerequisite for accurate irrigation management (maintaining soil moisture within the optimal range) is the availability of reliable information on the dynamics of soil moisture reserves.

Total evaporation is the main expenditure element in the water balance of an irrigated field, and its intensity largely determines the dynamics of the regime of the root-inhabiting soil layer. Therefore, the accuracy of determining the value of total evaporation is a decisive factor influencing the accuracy of assessing the dynamics of moisture reserves, and consequently, the accuracy of calculating irrigation rates. Total evaporation can be determined with high precision using instrumental methods (heat balance, water balance), but in practice, the application of these methods is difficult.

To optimize irrigation management, information-advisory systems for operational irrigation planning are used, based on the application of mathematical models that provide the ability to calculate and forecast the dynamics of the water regime of crops and total evaporation, and on this basis, manage operational irrigation regimes.

The realized efficiency of management depends both on the level of users' access to communication and computing equipment and on the quality of the models themselves. The quality of mathematical models depends on the accuracy of determining total evaporation, the values of which are calculated using its correlative links with meteorological factors and biological characteristics of plants, taking into account bioclimatic coefficients.

One of the indicators of heat and moisture supply, reflecting the complex characteristics of the external environment and its energy resources, is potential evaporation. Potential evaporation is understood as the maximum possible evaporation with an unlimited supply of moisture to the evaporating surface.

Of the many methods for determining potential evaporation, theoretically and experimentally substantiated, the method of the Federal State Budgetary Scientific Institution VNII "Raduga" is recommended, in which a formula derived from N.N. Ivanov's model with the addition of a dynamic component is used:

where E - potential evaporation, mm;

Kt - energy factor of evaporation, mm/mb; d - air humidity deficit, mb; wind function taking into account the influence of wind speed on evaporation intensity.

The components included in the formula are determined by the following relationships:

Kt = 0.0061 (25 + t)2, where t - average daily air temperature for the calculation interval, °C;

Ea - saturated vapor pressure at this temperature, mb; f = (1 - 0.01 A), where A - Relative humidity of the air">relative humidity of the air; 0.64 (1 + 0.19 V2), where V2 - wind speed at a height of 2 m from the ground surface, m/s.

Calculations of potential evaporation based on weather stations are performed for the warm period of the year with air temperature above 5°C. For each weather station, a chronological series is established, the statistical processing of which allows determining probabilistic (ensured) values of potential evaporation, i.e., potential evaporation in years of varying humidity: wet year - 5% probability, medium-wet year - 25, average year - 50, medium-dry year - 75, dry year - 95% probability.

Natural moisture coefficient and agroclimatic zoning of the territory

The zoning of evaporability by natural zones is carried out using the ratio of the inflow and outflow parts of the water and heat balance of a territory in its current physical-geographical form. This indicator, called the natural moisture coefficient (Ku), meets the principles of general physical-geographical and specialized (applied) zoning.

For an objective assessment of the heat and moisture availability of a territory, a method is recommended according to which heat and moisture availability is determined by the relationship:

' Е where Ku is the natural moisture coefficient for the period with t > 5°C;

Wa is the active soil moisture reserves in the one-meter soil layer at the beginning of the calculation period (the date when the air temperature passes +5°C), mm;

P is the atmospheric precipitation for the calculation period, mm;

E is the evaporability (potential evapotranspiration) for the same period, mm.

The duration of the calculation period in the equation is accepted taking into account that it encompasses the growing seasons of all agricultural crops.

Active soil moisture reserves Wa are determined by the formula

K = KLi - RL where Wm is the soil moisture reserves in the one-meter soil layer corresponding to its field capacity (water-holding capacity), mm;

/l is the coefficient characterizing the degree of actual saturation of the soil layer with moisture at the beginning of the calculation period, in fractions of WH6;

Po is the coefficient corresponding to the pre-irrigation soil moisture threshold (permissible soil drying threshold), in fractions of Wm.

Field capacity, or the water-holding capacity of a specific soil, depends on its mechanical composition and water physical properties.

The coefficient jj varies from 0.8 to 1 depending on the nature and amount of atmospheric precipitation during the winter-spring period.

The lower threshold for determining the calculated soil moisture reserves is defined by the formula

D = 0.5 $ nv + D), where Dv is the soil moisture corresponding to field capacity, % by mass;

D is the wilting moisture, % by mass.

In the absence of specific data, according to the methodology of the FSBSI VNIIR "Raduga", D can be taken in fractions of Dv: for sandy and sandy loam soils - D = (0.50-0.65) Dv; for loamy soils - D = (0.65-0.75) Dv; for clay soils - D = (0.75-0.8) Db.

As can be seen from the formulas, data on precipitation, air temperature and humidity, wind speed, and soil moisture reserves are used to calculate evaporability (potential evapotranspiration) and the moisture coefficient Du, i.e., a multi-factor assessment of the calculated parameter is performed.

The moisture coefficient Du can be considered a generalizing indicator of the lack or excess of atmospheric moisture in the territory under consideration, objectively reflecting both its climatic and geomorphological features.

Ku calculations were made on the basis of source data including meteorological data from representative weather stations for a 40-60-year retrospective period (from April to October): average decadal air temperature, C; Relative humidity of air">relative humidity of air, %; average decadal wind speed, m/s; sum of decadal atmospheric precipitation, mm.

Information on the mechanical composition, water-physical, and chemical properties of the soil for the one-meter layer and the aeration zone is taken from reference literature.

Agroclimatic zoning of a territory is carried out by the moisture coefficient Ku by mapping its long-term average value.

Ku gradations and their corresponding natural zones:

Ku < 0.2 - desert zone;

Ku = 0.21-0.3 - semi-desert;

Ku = 0.31-0.4 - dry steppe;

Ku = 0.41-0.5 - moderately dry steppe;

Ku = 0.51-0.8 - forest-steppe;

Ku > 0.8 - forest.

Equations of relationship are established between the moisture coefficient Ku and evaporability E.

The analysis of spatial-temporal relationships between evaporability E, which characterizes the energy resources of the climate, and the moisture coefficient Ku, which characterizes the ratio of heat and moisture resources in the territory, allows for the evaluation of practices and methods of agricultural production in general and irrigated agriculture in particular.

Total water consumption of agricultural crops

Total water consumption Ev - plant transpiration plus evaporation from the soil surface - is the main element of the outflow part of the water balance of an agricultural field and one of the main parameters of irrigation.

According to the methodology developed at FSBSI VNIIR "Raduga", total water consumption is established on the basis of a bioclimatic model that includes evaporability, as well as biological and microclimatic coefficients reflecting the role of plants and weather conditions in the expenditure of moisture from irrigated fields. The relationship recommended for determining total water consumption is as follows:

Ka, where Ev is the total water consumption, mm;

Kb is the biological coefficient characterizing the role of plants in the expenditure of moisture by an agricultural field;

Ko is the microclimatic coefficient that takes into account the change in the microclimate of an agricultural field under the influence of irrigation;

E — evaporability (potential evapotranspiration), mm,

The microclimatic coefficient Ko, reflecting a possible change in the microclimate of an agricultural field under the influence of irrigation (as a result of a decrease in air temperature and wind speed, and an increase in air humidity in the surface layer of the atmosphere), quantitatively depends on the size of the irrigated area and the coefficient of natural moisture (heat and moisture availability) Ku.

The biological coefficient Kb represents the ratio of actual water consumption (total water evaporation by an agricultural field) to evaporability. The Kb coefficient varies territorially (by physical-geographical zones), i.e., in the same development phase of a crop, Kb can differ quantitatively by 10-20% in different zones. In addition, the biological coefficient can change in real time, i.e., in years with different moisture levels, phase-based coefficients can be different during the growing season of the crop.

The values of Kb are shown in Fig. 1 using the forest-steppe zone of the Russian Federation as an example.

Forest-steppe zone of the European part of Russia (humid)

X phX g o

1000 1200 1400 1600 1800 2000 2200

Sum of temperatures from the beginning of the growing season

— winter cereals; -ж-— maize for silage;

cereal spring crops late cabbage;

—А- • - potato late cabbage early;

—х-----early potato tomatoes;

Fig. 1. Long-term average Kb coefficients for major agricultural crops

When calculating the total water consumption in specific years that differ in weather and climatic conditions from the long-term average values, the so-called current biological coefficient Kbi is used, which is determined by the relationship:

Kbi = Kbo (0.21 Ei/Eo + 0.79), where Kbo is the long-term average biological coefficient for the calculation period (decade);

Eo is the long-term average evaporability for the calculation decade, mm;

Ei is the actual evaporability for the same period in a real year, mm.

In the adopted calculation model, the change in biological coefficients throughout the growing season is tied to the cumulative (total) air temperature curve, i.e., to the curve of Σ °C every 100 or 200°C. Such a gradation of Kb is quite sufficient for accounting for the role of plants in water consumption by an agricultural field.

To exclude the influence of organizational and economic conditions, all indicators are tied to air temperature, which determines the timing and duration of the growing season, as well as the rates of plant growth and development. The time of sowing or the resumption of the growing season is taken as the beginning of the water consumption period, and the following phases as the end: for spring wheat — waxy ripeness 1450°C; maize for silage — temperature sum 1950°C; alfalfa of previous years — end of the growing season; late potato — wilting of the haulm; late cabbage — harvest ripeness.

For zoning the total water consumption of agricultural crops by territory and its mapping, spatio-temporal links with components of the natural environment are used, as well as linear interpolation of values with subsequent adjustment of the position of isolines Ev based on the physical-geographical features of the territory.

The equations of the relationship between total water consumption Ev and the moisture coefficient Ku can be used to determine the total water consumption at any selected point of the territory in years with different moisture (availability) levels.

Long-term series of total water consumption for the crops under consideration are statistically processed, and as a result, statistical characteristics are established: accuracy of the experiment (accuracy of calculated values), coefficient of variation, etc.

The reliability and sufficiency of the initial data for calculating the total water consumption and establishing its probabilistic (attained) values are confirmed by the values of the mean relative error of the coefficient of variation Cv. 2.1.4. Irrigation rates (water consumption deficits)

Plant water consumption under conditions of insufficient moisture availability differs from the optimal one, which determines the productivity of agricultural lands. The difference between optimal plant water consumption and water consumption under conditions of insufficient water availability creates a water consumption deficit, which, in total for the growing season, is numerically equal to the irrigation rate.

Fig. 2. Calculation of irrigation rates according to the VNII "Raduga" methodology

The water consumption deficit (irrigation rate) for the growing season of a crop is recommended to be determined by the following relationship: n where AEV is the total water consumption deficit for the growing season of the crop, mm (m3/ha);

Mnt is the net irrigation rate, mm (m3/ha);

Aevi is the water consumption deficit for the selected calculation period (month, decade).

The decadal water consumption deficit Aevd is determined by the water balance equation:

where Evd is the optimal total water consumption for a decade, mm;

P is the sum of atmospheric precipitation for a decade, mm;

Wa is the active soil moisture reserves in the calculated soil layer at the beginning of the decade, mm;

G is the capillary recharge from groundwater at their shallow depth, mm.

The total atmospheric precipitation is taken from stationary meteorological observations at representative weather stations.

The calculation of active soil moisture reserves Wa at the beginning of the ten-day period is shown in the section defining the moisture coefficient Kuk.

The use of groundwater in the case of its shallow occurrence (G) is determined by the relationship:

where Ev is the total water consumption for the ten-day period, mm; gz is the capillary recharge coefficient as a fraction of Ev, depending on the depth of the groundwater table, the mechanical composition of the soil, the thickness of the aeration zone, and the depth of penetration of the root system of the plants.

In the absence of experimental data on the dynamics of groundwater at an irrigation site, data from Table 3 are recommended, obtained based on the generalization and analysis of domestic and foreign experience.

To construct a probability curve (frequency) of irrigation norms, it is recommended to use the statistical testing method, according to which the maximum possible variations of natural (empirical) series of natural factors are covered. To construct a theoretical distribution curve, a model corresponding to specific conditions is selected. Links between irrigation norms and Ku are established for years of varying moisture levels (probability).

The spatiotemporal variability of irrigation norms (water consumption deficits) is displayed on maps of isolines of their long-term average value.

Calculated values of irrigation norms are recommended when developing project and operational irrigation regimes.

Statistical processing of long-term series of irrigation norms shows that their coefficient of variation CV is higher than that for series of total water consumption and evaporation, and varies from 0.3-0.45 in the steppe zone to 0.55-1 in the forest-steppe and forest zones. High CV values indicate significant fluctuations in the magnitude of irrigation norms over a long-term period and the need to take them into account when designing irrigation systems, reconstructing and operating existing irrigation systems. Calculations of the relative root-mean-square error of the coefficient of variation Cv show that with series lasting 40-60 years or more, the Cv error is within acceptable limits - 11-15%. With a shorter series length, for example 35 years, the Cv error goes beyond the acceptable range (up to 20%).

According to the established equations of the relationship between irrigation norms and Ku, it is recommended to calculate the irrigation norm for any object selected in the region and any year in terms of humidity (probability).

The chronological series of irrigation norms (sums per season) is ranked in ascending order, then in the ranked series, the averaging of ten-day water consumption deficits is performed according to a moving schedule for every 4 years, i.e., in each ten-day period, the water consumption deficit is determined as the average for the 1st, 2nd, 3rd, and 4th years, then for the 2nd, 3rd, 4th, and 5th years, for the 3rd, 4th, 5th, and 6th years, etc., according to the ordinal numbers of the sums per season of each year. Based on the 4-year average ten-day water consumption deficits, their sum for the season is determined, i.e., the irrigation norm. Based on the sums per season, the empirical and theoretical probability of each member of the series is established, and values of a given probability (5, 25, 50, 75, 85, and 95%) are identified.

The distribution of ten-day water consumption deficits in a year of a given calculated irrigation norm probability is accepted as a basis and is recommended for the development of irrigation regimes.

The intra-seasonal distribution of irrigation norms, the establishment of which is necessitated by the need to identify critical periods of plant water supply during the growing season and to develop irrigation regimes, is characterized by great variability by year, especially for areas of unstable moisture, and is calculated according to the given probability of the irrigation norm.

The obtained variants of the intra-seasonal distribution of irrigation norms for forage and vegetable crops for average and dry years (50, 75, and 95% probability) by natural zones of the Central Federal District are given in Table 4.

Growing season Natural zone Design year

Steppe zone Alfalfa for hay (Ku = 0.4-0.5) Average 10 28 25 21 16 100

Moderately dry 12 27 25 21 15 100

Dry 13 26 24 23 14 100

Cabbage late

Average 11 35 30 20 4 100

Moderately dry 15 32 28 20 5 100

Dry 15 30 28 20 7 100

Cereal grains

Average 14 59 27 - - 100

Moderately dry 15 58 27 - - 100

Dry 21 54 25 - - 100

Subsequent calculation of irrigation regimes (irrigation schedules) using calculated irrigation norms is recommended based on a graphical-analytical method or automated calculation in the 2.1.4 environment. Irrigation norm

The irrigation regime of agricultural crops is the basis for the rational use of irrigation water and the preservation of the quality and soil fertility in irrigated agriculture. In general, the timing, norms, and frequency of irrigation should ensure an optimal water regime for plant growth and development in the root zone of the soil. A single irrigation norm represents the volume of water applied per 1 ha of irrigated area per single irrigation and is measured in m3/ha or mm. It depends on the hydro-physical properties of the soil, the degree of its drying by the time of irrigation, the state of the agro-background, the relief of the irrigated surface, as well as the method and technology of irrigation. It is the irrigation norm and the regime of its implementation that are the main components of operational irrigation management.

The calculated irrigation rate, corresponding to the water-holding capacity of the soil, is determined by the following relationship:

10 U 50), where Wm - soil moisture reserves in the calculated soil layer, corresponding to FC (field capacity), mm;

W0 - permissible or actual pre-irrigation reserves in the same soil layer, mm; y - bulk density of the soil in the calculated layer, t/m3; hnp - calculated depth of soil wetting, m;

Д e - soil humidity at FC, % of mass;

Д - pre-irrigation (permissible) soil humidity, % of mass.

The calculated layer hnp depends on the type of irrigated crop, the condition of the agro-background (development phases of the crop and the depth of the root system distribution), as well as the irrigation method.

The greatest difficulty in calculating the irrigation rate is determining the permissible (critical) threshold of soil drying before irrigation. It is recommended to determine the critical soil humidity using the formula

Д = 0.5 ■(Д6+ ДА where Д - wilting point humidity, %.

According to reference and regulatory literature, the following values in fractions of FC are recommended as critical humidity: for sandy and sandy-loam soils Д, = (0.5-0.65) Д т; for loamy soils Д, = (0.65-0.75) Д нв; for clay soils Д, = (0.75-0.85) Д нв.

To determine pre-irrigation moisture reserves Vo (in fractions of Wm) as applied to a particular type of soil (given a known WHe value) for a one-meter layer, we recommend using the following equation:

Vo = 0.36 + 1.4810(-3) ■Wm-9.5 2 ■10('7) ■WJ.

When using sprinkler irrigation, the maximum irrigation rate depends not only on the water-holding capacity of the soil in the range from Wa to WH69, but mainly on its infiltration capacity, taking into account the relief and slopes of the surface of the irrigated field, the agro-background, and the intensity and structure of the rain. In this case, the implemented irrigation rate must not exceed the maximum (erosion-permissible) rate, which can be established by the dependence:

т =— Ъ — д where тд - non-runoff irrigation rate, mm;

K v _ indicator of free non-pressure water infiltration into the soil, mm; p 0 - average rain intensity characteristic of a given sprinkler machine (unit), mm/min; dk - average drop diameter of the rain cloud, mm; e - base of the natural logarithm, equal to 2.75.

According to N.S. Erkhov, for light-loamy and sandy-loam soils, the Kv indicator is 61-90 mm, for medium-loamy - 31-60, for heavy-loamy - 21-30 mm.

The denominator in the formula represents the energy characteristic of the rain 0.5dK, which reflects the technical and operational parameters of a specific sprinkler machine or unit (rain intensity and structure).

The non-runoff irrigation rate for various types of soil and rain parameters (p0 and dk) is shown in Table 5.

Average Rain intensity p0, mm/min Rain drop diameter dk, mm od 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Soils of medium water permeability (Kv —60 mm)

1.0 145 81 66 58 51 47 43 41 38 36

1.5 90 63 52 45 40 37 34 32 30 29

2.0 70 49 40 35 31 28 26 25 23 22

2.5 55 39 31 28 25 22 21 19 18 17

3.0 44 31 25 21 19 17 16 15 14 13

Soils of high water permeability (Kv = 90 mm)

1.0 172 122 100 86 77 70 65 6i 58 55

1.5 135 95 78 67 60 55 51 47 45 43

2.0 104 74 64 52 47 43 40 37 35 33

2.5 81 58 47 41 37 33 30 29 28 26

3.0 65 45 36 32 32 27 24 22 21 20

The non-runoff irrigation rates presented in Table 6 are approximate and averaged. In relation to specific soil and relief conditions, they must be adjusted taking into account the slope of the irrigated field surface, development phases of the irrigated crop, and the condition of the agro-background.

Cereal crops Alfalfa (for hay) Late cabbage Water Soil permeability, Kv emergence - tillering tillering heading - wax ripeness start of budding - flowering planting transplants - head formation

Forest-steppe zone Ku =0.51-0.8 Medium (0.31-0.6) 250-300 350-400 300-400 400-500 200-300 250-300 Increased (0.61-0.9) 300-400 400-500 400-500 500-600 250-350 350-400 Weak (< 0.3) 200-250 300-350 250-300 300-400 150-200 200-300

Steppe zone < 0.5 Medium (0.31-0.6) 300-350 350-450 350-400 400-500 200-300 300-400 Increased (0.61-0.9) 350-400 400-500 400-500 500-600 200-300 300-500 Weak (< 0.3) 200-300 300-400 300-350 350-400 150-200 200-300

Grapho-analytical calculation of irrigation regimes

Operational irrigation schedules are built for crop rotation crops and the available fleet of sprinkler equipment, taking into account their placement in the fields. The preparation of schedules is preceded by the development of planned irrigation regimes for the forecast year, the main parameter of which is the irrigation rate and its intra-seasonal distribution.

Irrigation parameters and regimes are the basis for the rational use of irrigation water and the preservation of quality and soil fertility. In general, the timing, rates, and frequency of irrigation should ensure an optimal water regime for plant growth and development in the root-inhabited soil layer.

Irrigation regimes for agricultural crops for specific soil and climatic conditions are established both experimentally and using calculation methods.

The irrigation regimes for agricultural crops developed using the grapho-analytical method are, to a certain extent, statistical (averaged). They can be adjusted for the conditions of a specific site. At the same time, lower irrigation rates should be applied during the initial phases of plant development and in wet years, while higher rates should be used during critical crop development phases and in dry years with persistent soil moisture deficits.

In all cases, the maximum possible approximation of the applied irrigation rate to the calculated (technological) one should be ensured by optimizing irrigation technological schemes.

An example of irrigation regimes for alfalfa grown for hay, developed using the graph-analytical method and adapted to the soil and climatic conditions of the Central Federal District for years with different levels of moisture supply, is presented in Table 7 and in Fig. 4.

Yearly rate start consumption, irrigation period, days rate, mm, m3/ha irrigation m3/ha m3/ha Average 1 300 21.05 dry, P 75%

6 500 22.08 2500 Average, 1 300 24.05 P 50% 18 16.7

4 500 07.08 1600

Fig. 4. Graph-analytical calculation of the irrigation regime for alfalfa (forest-steppe zone) for 50%, 75%, and 95% probability of moisture supply