freezing of the upper soil layer and the immobilization of the water during a considerable part of the year during which there is no leaching effect, with consequent very important effect on the formation and evolution of the soil in the Russian steppes.
Table 4 summarizes the results of the calculations of the hydric balance for the four components of the most typical rotation of the Ukrainian steppes. Values of the water accumulated in the soil in the fall-winter period were calculated, as well as the loss suffered in the spring-summer period each year. F r o m the precipitation values recorded during those same periods, the total values were obtained ( E „ f + R ) , b y subtraction or addition.
Available values for "black fallow" and fall wheat cover the period 1932-33 to 1938-39 (seven years), both as regards accumulation and loss of water. In the case of corn, only five years were available for accumulation and six for loss of water. O n the other hand, spring wheat was observed during six years for the estimate of accu-mulation of water and seven full years for loss of water.
In every case the calculation (E„t + R )
w a sm a d e taking into account the precipitation during the period under study (fall-winter or spring-summer) and this only for those years for which soil moisture data were available.
1. In accordance with recent bibliography ([5, 23] and others), wilting of the plants is equivalent to the wilting point of the soil only in the upper horizons where roots abound. In the lower horizons, moisture is maintained slightly above this limit (2-3 per cent higher).
T A B L E 4. Seasonal and annual water balance in a crop rotation cycle of four years (Record: 1932-39 inclusive; values in m m . )
Accumulated water
Spring and summer months (April-August)
Climatology and microclimatology / Climatologie et microclimatologie Furthermore, although, as already stated, the s u m of
both periods does not in every case correspond to averages for the same period of years, it does give some idea of the annual hydric balance.
The values of Table 4 show the magnitude of the conservation obtained by the "black fallow" practice, which is perfectly comparable to the case of the fall wheat, as observations for the same period were avai-lable for both. In the fall-winter period, this practice provides an accumulation of 144 m m . of water in the soil, of which only 50 per cent is lost in the spring-summer period. Fall wheat, which is the next crop, accumulates only 59 m m . during the cold period and consumes 136 m m . during the growing period. These figures, compared to those for precipitation—206 m m . during the cold period and 225 m m . during the w a r m period—
amount to an evaporation and run-off of 62 and 147 m m . during the fall-winter period and 302 and 361 m m . in the spring-summer respectively, for "black fallow" and fall wheat. In other words, whilst the year under
"black fallow" represents a loss of 364 m m . by run-off and evaporation, the lowest of all the rotation cycle, fall wheat, uses up 508 m m . by evaporation and run-off.
This figure is higher than the total precipitation by 77 cm., which is precisely the amount accumulated during the previous year by the "black fallow".
The corn and spring wheat figures cover 5 and 6 years of observations. Both crops allow for an accumulation of water in the soil of 159 and 150 m m . in the cold part of the year, since they are not sown until after the last
frosts. O n the other hand, during the w a r m part of the year, they show a loss in soil moisture of 125 and 184 m m . , which is quite considerable if it is taken into account that they are crops with a short vegetative period. These figures, compared with the precipitation figures, show a loss of water by (E„ f and R ) similar to the "black fallow" during fall-winter, i.e. 36 and 46 m m . , but as high as or higher than the fall wheat during the w a r m part of the year, i.e. 336 and 409 m m . for corn and spring wheat. The corresponding values for the year show that, whilst the corn crop produces higher losses than the "black fallow" (372 m m . ) , they are lower than those produced by spring wheat and, above all, by fall wheat.
Table 5 presents a comparison between the average monthly water consumption by normal evaporation and run-off in the region, and the corresponding figures when the field is subjected to the " black fallow" practice and fall wheat crops, which constitute the greatest contrast in the normal rotation cycle. It is assumed that the average monthly run-off is that registered in the Samara basin, and actual évapotranspiration is that registered by the Popov evaporimeter. Both these values are included in Table 3. In accordance with these data and those obtained by calculating soil moisture, the similarity between the regional figures and those for the
"black fallow" m a y be noted. The values for fall wheat are m u c h higher. These values eloquently show what agriculture means to the change in the regional water balance: an increase in the water-holding capacity of
T A B L E 5. Average of the monthly and annual evaporation of soil and run-off compared with that of "black fallow" and winter wheat (values in m m . )
Sept. Oct. N o v . D e o . Jan. Feb. M a r . April M a y June July A u g . Average Annua]
Average of the monthly run-off
(1931-35) 0.6 0.6 0.7 0.7 1.3 1.1 21.4 4.0 1.4 1.0 1.0 0.7 2.9 35 Average of the monthly
evapor-ation of soil (1940-42) 37.0 25.0 12.0 3.0 3.0 3.0 9.0 27.0 40.0 42.0 62.0 56.0 26.6 319 Run-off plus evaporation,
ac-cording to data of several
years 37.6 25.6 12.7 3.7 4.3 4.1 30.4 31.0 41.4 43.0 63.0 56.7 29.5 354 Run-off plus evaporation for
"black fallow" (mean values
for 1932-40) 28.0 15.0 8.0 2.0 3.0 1.0 32.0 68.0 46.0 72.0 46.0 43.0 30.3 364 Run-off plus
évapotranspira-tion for winter wheat (mean
values for 1932-40) 34.0 21.0 16.0 24.0 13.0 20.0 30.0 100.0 86.0 85.0 38.0 31.0 42.2 508
T A B L E 6. Annual water balance in a complete rotation Black fallow'
cycle (values in m m . ) Winter wheat1
Precipitation 431 431 Evaporation plus run-off 364 508 1. Record of 7 years. 2. Record of 5 years. 3. Record of 6 years.
Corn1
406 372
Spring wheat8
421 455
Complete cycle of rotation 1689 1699
Water storage in semi-arid soils the soil for cultivation, and an increase in the
consump-tion by local evaporaconsump-tion and run-off as a consequence of cultivation.
The figures for precipitation and losses by evaporation and run-off during one complete 4-year rotation cycle m a y be observed in Table 6. F r o m these it will be seen that, in spite of the difference in the period observed for corn and spring wheat, total values for the complete cycle are very similar. This shows the importance of considering the complete normal rotation cycle when studying the hydric balance of agricultural soils. Consi-deration of this problem is also most important when planning a rational rotation and in explaining the difficulties which sometimes arise from the application of a certain rotation.
A N A L Y S I S OF T H E H Y D R I C B A L A N C E O P T H E A R G E N T I N E P A M P A R E G I O N U N D E R N A T U R A L C O N D I T I O N S
In this section the analysis will be presented of the hydric balance of certain soils of the Argentine p a m p a region, which some authors have defined as steppes because of the characteristics of their natural vegetation [9, 22], The outstanding features of this vegetation are the absence of trees and the predominance of herbage, especially Gramineae, without the well-defined seasonal rhythm of vegetation of the typical steppes. Occasion-ally, owing to drought and, sometimes, to quite intense frosts, vegetation is interrupted both in summer and winter. However, this phenomenon is not very frequent, and the favourable topography of the land ensures the
development of one of the largest and most prosperous livestock and grain-producing regions of the world.
A study of the hydric regime under natural conditions, based on direct observation, shows the importance of different types of soil and climates in a given region—in the present case, the Argentine pampas—for the local variations of the water balance which determine its vegetation and natural grazing characteristics. In this case, it will demonstrate also one of the basic differences between this region and the steppes of the northern hemisphere. Subsequent investigations will show the change brought about in the natural balance by the agricultural process.
Furthermore, it is believed that the results obtained will show what m a y be expected from the application, on a geographical scale, of the method recommended by the World Meteorological Organization (1956) for the observation of soil moisture.
Method Followed
The observations on soil moisture used in this study of the Argentine p a m p a region were m a d e since 1945 to date, at the agrometeorological and meteorological stations of the National Meteorological Service located in that area, in accordance with a plan outlined b y Hirschhorn [11]. For the present analysis observations were used corresponding to the 6-year period 1945-51.
The method followed consists in extracting samples of soil every 5 days, at 7, 15, and 30 c m . ; every 10 days at 60 c m . , and every 15 days at 100 c m . depth, whenever the nature of the profile permits. The samples are then
T A B L E 7. Climatological and hydrological data of Argentine pampa
Static and months and months
Hydric indices
1. According to method of Thomthwaite [29].
Climatology and microclimatology j Climatologie et microclimatologie stove dried at 105-110° C , and the percentage of
mois-ture estimated in relation to the dry weight of the sample.
Samples are extracted from land plots representing the general characteristics of the region, which are main-tained with spontaneous vegetation. This is periodically cut at a height of 5-7 c m . The plots are kept free from animals, to avoid trampling. Thus, the eight soil profiles studied in this report involved the analysis of approxi-mately 12,300 samples.
Some normal climatological and hydrological data of the localities studied are given in Table 7. It will be seen from these values that precipitation is almost double that registered in the Russian steppes. It will be noted that in the Argentine pampas, similarly to the Russian steppes, owing to the thermal conditions, precipitation is almost balanced by évapotranspiration, which deter-mines the type of herbaceous vegetation. O n the other hand, évapotranspiration takes place all the year round and not only at one period of the year. In other words, in the pampas, as in similar regions of the southern hemisphere, winter is a season of consumption rather
than conservation of water. This explains w h y higher precipitation values are generally needed in the southern hemisphere in order to reach the same aridity values and to maintain types of vegetation comparable to those registered and observed in the northern hemi-sphere.
Hydric indices show that within the p a m p a region, or pampean steppes as some authors call them, sub-humid climates, as in Balcarce, Barrow, San Miguel, and Pergamino in the eastern part, gradually become quite semi-arid, as in Rafaela, Trenque Lauquen, Guatraché, and General Pico, towards the west. These last two localities, where precipitation values are still consider-ably higher than in the Russian steppes, are situated in what is considered a borderline region for agriculture.
This activity is only possible when certain crop rota-tion, fallow, and land management patterns prevent the aeolian erosion of the soil. Furthermore, natural vegetation here is definitely xerophilous.
Tables 8 and 9 show the hydrophysical, granulo-metric, and edaphological characteristics of the soils examined. Hydrophysical characteristics were
deter-T A B L B 8. Characteristics of pampean soils: leached and degraded soils on loess and loessoids
Localities