Agricul tural Me teoro logy - Elsevier Publ i sh ing C ompany , A m s t e r d a m - Pr inted in The Ne the r l ands

DROUGHT AND SOIL MO1STURE

M. S. KULIK

Chief Administration of the Hydrometeorological Service of the U.S.S.R., Moscow (U.S.S.R.)

(Received February 24, 1964)

A considerable proportion of the cultivated areas in the Soviet Union is concentrated in the zones supplied only inadequately and irregularly with moisture. The conti- nental character of the climate in these districts indicates the possibility of frequent droughts. The nature of these droughts and the agronomic techniques for controlling them in steppe districts is widely studied. Analysis of the data of experimental and agrometeorological stations as well as investigations carried out by research institutes have led to the conclusion that plants subjected to the same amount of precipitation may or may not undergo growth depression using high temperatures and dry winds, depending on the level of agronomic techniques applied. Drought occurs where there is a combination of inadequate rainfall and high evaporation, and, in the absence of necessary measures, leads to a serious disparity between the plants' water requirements and their water intake from the soil; this results in a marked reduction in the yield of agricultural crops.

Drought is a phenomenon taking many forms, but its specific and most essential feature is a disparity between the plants' requirements of moisture and the latter's supply from the soil.

The soil moisture reserves are an integral index of the meteorological conditions and of the extent of agronomic measures applied.

In the non-irrigated farms of the steppe districts, the moisture accumulating during spring in the root layer of the soil is the plants' main source of water during the entire growth period. In years of normal evaporation in the steppe districts the lowest field- water capacity of this layer, i.e., the amount of water which the soil is able to retain after deep settling of the ground waters, provides only 60-70% of the optimum moisture requirements of modern winter wheat varieties.

In steppe and forest-steppe districts, the lowest field-water capacity in a metre-deep layer of soil corresponds to 170-190 mm of productive water. 1he water capacity of sandy soils is significantly lower.

One of the important indice~ of the water supply for all crops in these districts is the degree to which the spring reserves of productive water in the rhizosphere cor- respond to the minimal field-water capacity.

Data collected over many years indicate that the soil reserves of spring water for

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different geographical districts of the steppe and forest-steppe zones may be represen ted by the figures listed in Table 1.

TABLE 1 SOIL RESERVES OF SPRING WATER

WaIcF FeSCI'VCS Stations (ram)

Krasnokutsk (Zavolzh's dry steppe) 99 Kamyshinsk (lower Volga steppe) 97 Saratov (central Volga steppe) 120 Bezenchuk (central Volga steppe) 13 I Kamennaya steppe (central chernozem zone) 135 Sinel'nikov (northern steppe of the Ukrainel 141 Poltava (forest-steppe of the Ukraine) 167 Odessa (southern steppe of the Ukraine) 104

The above data indicate that both in the steppe and in forest-steppe zones, the spring reserves of soil water do not reach the minimal field-water capacity, and in some districts they are only slightly in excess of 50~0 of that value. It is thus clear that, in steppe and forest-steppe districts, the accumulation and retention of soil moisture is one of the most important problems ol agriculture. It has been shown that the spring reserves of soil moisture may be considerably increased by timely and well-executed skimming, ploughing, skilful harrowing, retention of snow, flood waters, and spring moisture, and by treatment of the spring fallow in accordance with soil and climatic conditions.

The effectiveness of measures increasing soil moisture varies according to the district, the weather, and the combination of the agronomic techniques applied Some examples are given below.

In the steppe zone., the effective moisture in a metre-deep layer of soil is not renewed in late autumn after the harvesting of cereals, but if the soil is skimmed immediately after harvesting, about 25 mm of effective moisture may be accumulated in the soil by the end of the first 10 days of October. Autumn ploughing rather than spring ploughing increases the spring reserves by an average of 20 mm. Snow retention aJso supplies about 20-25 mm of water to the field, and this significantly increases crop yields. At the Saratov experimental station the average yield of spring wheat on fields with a snow cover was found to be 14.6 c/ha (centners per hectare), as against 10..9 c/ha on fields without a snow layer, over an 18-year period.

Autumn levelling of the ploughed field reduces the loss of water by evaporation, and in districts with little snow this increases the spring-moisture reserves by 15--20 mm. In the steppe districts of the European part of the U.S.S.R., there is 80-85 mm more moisture, on average, in a metre-deep layer of soil on pure fallows than on non-fallows soils.

Effects of the various combinations of agricultural techniques on yields may be illustrated by the results obtained at the Kuznetsk experimental station (Table 11)

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D R O U G H T AND SOIL MOISTURE 8 [

TABLE II

INCREASES IN YIELDS EFFECTED BY VARIOUS AGRONOMIC MEASURES

Agronomic measures Grain yields of oats (c/ha)

Ploughing of fallow across the slope 16.5 Ploughing of fallow plus snow retention 19.8 Ploughing of fallow plus snow retention plus retention of thawed waters 24.5 Ploughing of fallow plus snow retention plus retention of thawed waters plus

mineral fertilizers 26.7

q'he above data show that retention of snow and the thawed water increase crop yields by 8 c/ha. Correct positioning of ideally constructed forest shelter belts gives the same retention of snow and thawed waters as that achieved at the Kuznetsk experimental station (KuLIK, 1956).

The significance of soil moisture varies according to the crop and the stage o f its development.

Drying up of the plough horizon of the soil results in a lowering of the yield at all periods of growth, but its effects are particularly marked in cereals just prior to and immediately after booting, in maize before the emergence of the tassel, and in tuberous- root crops during the period ot active tuber and root growth.

Drying up of the upper layer of the soil delays development and complicates the functioning of the roots, hampers utilization of the nutrient substances within the richest part of the soil, and interrupts the activity of useful microorganisms.

If rainfall provides 20 mm or more moisture to the pIough horizon, and if more than 60 mm of water is present in a 20-100 cm layer during the entire period between sowing and flowering, this is usually an indication of completely satisfied moisture requirements. Under these circumstances, yields of 20 c/ha may be obtained from spring cereals in the steppe and forest-steppe zones, even without fertilizers.

When the moisture reserves in the 0-20 cm layer are less than 20 mm, the produc- tivity of crops is adversely afflected. This effect is markedly increased when the moisture reserves fall below 10 mm. The periods in which the moisture reserves in the 0-20 cm soil layer consist of 10-20 mm are regarded as drought-affected (dryish), whilst those in which the reserves are below 10 mm are arid. Both arid and dryish periods cause a lowering of the yield of grain crops; this drop may reach 20-30 ~ if the drought occurs at the booting stage (KULIK, 1938, 1952, 1955).

In steppe districts, booting in winter wheat occurs a month earlier than in spring wheat, and, where there is adequate moisture in the plough layer, earing is almost always concluded in autumn; on productive soils the initial root system reaches a depth of 70-100 cm with the onset of winter, while the secondary system reaches 60 cm, and waxy ripeness is obviously terminated before the commencement of the dry winds period. Where there are large moisture reserves in spring, high yields of winter wheat are obtained irrespective of the weather conditions in April, May or June, and

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82 M. S. KUI,IK

vice versa. The dependence of winter-wheat yields on spring-moisture reserves is particularly pronounced in years of atmospheric drought. For the same moisture reserves, yields show marked differences depending on the condition of the wheat and on the quantitative yield components, one of the most important of the latter being the number of ear-bearing stalks per mL On the productive soils of steppe and forest-steppe districts of the Ukraine and northern Caucasus, where the density is more than 1,000 shoots/m 2 (of course, a proportion of these dies in due course), the yield, in c/ha, of modern winter-wheat varieties may be expressed as y =~- 0.24x - - 10.Z where x represents the productive water reserves in a metre-deep soil layer in the spring.

The correlation coefficient of this relationship is I' ~- 0.86 ± 0.017, the error of the regression equation being iSy := ~ 3.4 c/ha.

]he limiting equation for the lowest yields under any weather conditions in April, May and June is of the form I' = 0.24x ~- 16.0, the limiting equation for the maximum yields under favourable weather conditions in April, May and June is)' ..... 0.24x .... 4.4 (KUL1K, 1938; ULANOVA, 1963).

Application of these equations to the forecasting of the estimated minimal yield of winter wheat in the Ukraine and northern Caucasus requires merely the knowledge of the productive moisture in the top metre of the soil and of the number of viabie shoots per m 2 in spring. Accurate correspondence of the obtained average and high yields of winter crops depends in large measure on the accuracy of seasonal and monthly weather forecasting.

Analysis of multi-year observations at agrometeorological stations showed (KuLIK, 1963) that, in steppe and forest-steppe districts in the European part of the U.S.S.R., sowing of winter crops on a large scale begins in the 10-day period in which the average air temperature is 16-1TC. During this period, the soil temperature in the area of seed placement is usually 2-3 ° higher than the air temperature. As a result. germinating seedlings of winter crops sown on time are practically never retarded by inadequate warmth.

The condition of the winter crops in autumn is determined by the moisture con- tent of the plough layer, from the date of sowing up to the time when the average daily temperature changes by 10°C (25-30 days). 3he state of sown crops depends on numerous factors, but soil moisture is the major decisive influence. Effective moisture reserves lower than 10 mm during the first 10,day period of growth in a 10 cm soil layer, and lower than 20 mm in a 20 cm layer during the second and third 10-day periods, do not provide for normal autumnal development of winter-wheat plants. The winter crops do not germinate if the moisture supplies are below 5 mm in a 20 cm layer. If there is slightly more moisture, germinated seedlings may appear, but they subsequently die off. To avoid such occurrences, we must know the moisture reserves in the upper layers of the soil in fields reserved for sowing winter crops. Measurement of the soil moisture allows the forecasting of the degree of danger to seedlings, which may eventually die owing to a shortage of water. The autumn condition of winter crops may be predicted on the basis of the moisture supplies in the soil.

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DROUGHT AND SOIL MOISTURE 83

Equations used for predicting the yield of winter crops are unsuitable for the evaluation of agrometeorological conditions adapted to spring crops. Thus, in the steppe districts of the northern parts of Kazakhstan and Kulunda, the relationship of the yield of spring wheat to the amount of water it utilises during the whole growth period may be expressed by the following equation.

)' ~ 0.104x - - 8.0 )'~- 0.78 ~ 0.33

where y is the grain yield of spring wheat in c/ha, and x is the total consumption of moisture (in mm) in the period between the time of sowing and the attainment of waxy ripeness. The limits between which this relation may be applied are 0.5-20 for y, and 75-250 for x. The probable error of the equation is ~ 2.0.

Within the complex of agronomic measures for controlling drought in non-irri- gated farms, decisive role is played by the systems which provide for the accumulation, retention, and economic utilization of soil moisture. Agrometeorologists participate in the research on the establishment of drought-control measures, taking into account the agroclimatic features of difl-erent geographical districts. In these investigations, particular emphasis is laid on the determination of quantitative indices for assessing soil moisture, its significance as related to meteorological conditions, and the develop- mental phases and conditions of plants.

CONCLUSION

The degree of disparity between the plants' water requirements and their water intake from the soil is the fundamental criterion of the severity of drought on non-irrigated land. The introduction of modern drought-resistant varieties, high level of cultural measures, and the curtailment of unproductive water expenditure by wasteful evapo- ration and drainage, can significantly reduce the harmful effects of drought conditions on crop yields.

REFERENCES

KULIK, M. S., 1938. Moisture supplies in Ukranian soils. Vesm. Meteorol. Gidrol., (1938) 6-7 : 24-33. KULIK, M. S., 1952. Evaluation of drought phenomena. Meteorol. Gidrol., (1952) 1 : 35~40, KULIK, M. S., 1955. The Agroclimatic and Water Resources in Virgin and Unused Land Districts

Undergoing Development. Gidrometeoizdat, Moscow, 463 pp. KuuK, M. S., 1956. Climatic Resources of the Central Provinces of the European Part of the U.S.S.R.

and their Utilization in Agricultural Production. Gidrometeoizdat, Moscow, 309 pp. KULIK, M. S., 1962. Expansion and modernisation of the agrometeorological service. Meteorol.

Gidrol., (1962) 6 : 3-9. ULANOVA, E. S., 1963. Methods of long-range forecasting of agrometeorological conditions in yield

production of winter wheat. Meteorol. Gidrol., (1963) 11 : 12-20.

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