Aspectos da seca de 1951, no Ceará


  • Hilgard O'Reilly Sternberg


Ceará, Climatologia, Geografia Física, Secas


  The recurring periods of drought which afflict Northeast Brazil are so severe that (1) a provision was included in the Constitution by which the Federal Government shall apply yearly not less than three percent of its income to relief and long-term development work in the region of intense drought distress, officially circumscribed by a polygonal boundary line, and (2) a permanent committee, the Comissão do Polígono das Secas, sits in Congress to deal with the problems of the area. The present article reproduces essentially an exposition made in the Chamber of Deputies at the request of the above-mentioned congressional committee and is based upon observations during recent (July 1951) field-work in Ceará, a state now experiencing a particularly disastrous drought.

            The fundamental problem of Ceará - i. e. water - is analyzed in its two distinct phases: (1) precipitation of air-borne moisture and (2) destination of rain water after it has reached the ground.

            Seasons in the Northeast are defined by precipitation, not temperature: "winter", the first six months of the year, is wet; "summer", the second semester, dry. According to data from the ten meteorological stations maintained in the state by the federal Serviço de Meteorologia, 91% of the aggregate annual precipitation normally falls in the "winter" months _ often in short-lived, violent downpours. At intervals, the rainy season sets in late and or shows a marked downward deviation from normal: acute moisture deficiencies bring about disruption of all economic activities. Attention is called to the influence of topography on rainfall distribution: ranges and tablelands, which rise - as a rule abruptly - above the surface of the gently undulating plain, . are favored by much more abundant precipitation; this fact is brought out m Table I, Which compares mean yearly rainfall in upland (serra) and ne1ghbouring plains (sertão) localities. Orographic rains are not limited strictly to the relief which produces them: areas at the foot of elevations often receive more rainfall than localities in entirely open country. The higher parts of the relief seem to derive additional benefit from the fact that they rise into and are probably dampened by the cloud cover which sweeps tantalizingly across the parched low country.

            The paper next considers the second phase of the hidrological problem: destination of rain water after it has reached the ground surface. In warm climates, a goodly part of the rain water is returned to the atmosphere; in cool climates, evaporate-transpiration being less pronounced, a given amount of a rain will prove more effective. Thus, for instance, London, Dublin Paris, Marseilles, Berlin, Warsaw or Moscow actually receive less precipitation than, say, Iguatti or Quixeramobim, in the heart of the semi-arid region of Ceará. After considering briefly the meteorological concept of rainfall effectiveness, primarily concerned with the return of moisture to the atmosphere, the paper proceeds to the idea that another, and very considerable, fraction of the rainwater is also ineffective, from the standpoint of plant growth, because it runs off over the land. This leads to the consideration of the storage capacity of the different geological formations (e. g. impervious crystalline basement, porous sandstone) and the various soils developed thereon. Since rate of weathering is partly controlled by moisture, the elevated tracts which receive more rainfall also possess a thicker mantle of weathered rock and soil and are, therefore, capable of storing more water.

            The paper next advances the question: is man, whose culture is strongly imprinted on the geographic landscape, not partly responsible for the calamitous effects of the recurrent droughts? Accumulated evidence suggests an affirmative answer: on the one hand, the floods which time and again inflict heavy losses on the riparian population and, on the other, evidence of accelerated soil depletion and erosion prove that a large fraction of the precipitated waters, not only is lost to agriculture, but, by removing precious topsoil, cause permanent damage. Through improper farming practices, man, at one stroke, (1) lessens the soil's water-storing capacity precisely in the areas where more rain is precipitated and (2), by inducing soil erosion, reduces the extent of farm lands in environments which, being less subject to rainfall deficiency, are just those most suited for cultivation.

            Having proceeded from the purely meteorological concept of rainfall effectiveness to the consideration of natural storage, the paper now introduces the significant concept of what might be termed "effectiveness in the use of rainfall". Whereas in areas of abundant precipitation the main hydrological problem is to evacuate excess water without damage to soil and crops, in the Northeast the essential thing is that every single drop of rain be turned to good account. There have been two traditional lines of approach to the problem of stabilizing the hydrologic regimen in the region. The tenet of the first school of thought, comprising mainly engineers, is that stabilization can best be accomplished by surface storage reservoirs. The second school holds the opinion that forests are the most indicated agents in the control of water: reforestation should be the keynote of watershed management. Although the writer does not dismiss or belittle the "hydraulic" or the silvicultural methods, he contends that neither one of them is capable of offering a full answer to the problem under consideration. The principal limitation of the solution based entirely upon large storage reservoirs is that, at best, when the impounded water is put to good use, it only benefits the irrigable lands downstream from the reservoir; with the exception of a narrow tract immediately contiguous to the water line, it is indifferent to the fate of the soils in the watershed - some of these may be counted among the most indicated for agriculture and they support a great portion of the rural population of Ceará. Another limitation lies in the fact that, on the average, irrigation of a given area, as now practiced, requires most all of the runoff furnished by a catchment basin one hundred times larger. It follows that, under such conditions, only a small part of the dry lands in the region can ever be irrigated.

            As to forests, they cannot be used over the entire area, as some people would seem to think, forgetting that man needs fields for his crops and pastures for his cattle. Besides, forest trees are great consumers of water: their beneficial effect in stabilizing the hydrologic regimen are accompanied by an appreciable reduction in total soil moisture. In those areas which are best suited to agriculture, it is possible to do without forests, providing adequate measures are taken to retain rainwater and prevent soil erosion. In some cases, it may be desirable to associate forests and crops, as, for instance, when it is desired to provide shade for coffee plantations. Areas of very steep slopes and poor and shallow soil should, of course, be entirely forested.

            Attention is now drawn to a third, hitherto neglected, solution for water conservation: conservation farming. In the writer's opinion, it should be the core for any long-range program in the region. The detailed planning which it presupposes does not exclude surface reservoirs or forestry projects, but integrates them in an organic system established after a careful regional analysis.

            In order to review some principles of conservation farming in relation to the present drought, rainfall for each of the 1951 "winter" months is expressed as a percentage of normal (Table II). Precipitation during this year's considerably retarded rainy season shows an average reduction of 45%; in some stations, deviation from normal reached about 70%.

            If one did not proceed beyond the above comparison, the drought would appear as an act of Providence; yet if one considers, in absolute numbers, the amount of rain which fell this year, it is surprising to discover that there are peoples in other parts of the world whose agricultural pursuits are carried out under average precipitation conditions not superior to those obtaining in Ceará during this year of deficit.

            It is not necessary, however, to draw comparisons with remote corners of the world. During the field trip, green patches were often observed in the midst of wilted cornfields, even when these occupied steep slopes; they are due to the fortuitous presence of boulders and rocks scattered on the surface of the soil. The fact that this discontinuous mantle or mulch of rock helps to preserve moisture in the soil is recognized by many of the farmers. Some operators in the Serra of Baturité dig holes on the slopes, in order to trap rainwater and promote infiltration. The principle, of course, is the same one which underlies a number of advanced agricultural practices. In view of the torrential character of the rains in Ceará, on the one hand, and the nature and degraded state of the soil, on the other, it is evident that only a small fraction of rain water infiltrates in the soil. The possibility of increasing infiltration is strikingly brought out by a fortunate piece of luck: a farm in the município of Quixeramobim had been equipped (with a tractor-drawn plough) to begin mechanized agriculture this year. But the rains did not come: by April, according to the rain gauge set up on the farm, the aggregate rainfall for 1951 was still under 30 mm. When the rainy season eventuality did set in, the plowed field was able to absorb a much greater amount of moisture. At the time, however, people did not fully grasp this fact: when - only too soon - rains stopped, no one believed the crops could resist without the benefit of at least one more rain; yet, although this did not materialize, the amount of water stored in the soil was sufficient to ensure a good harvest on this farm. Other operators, in the area, still yoked to the old methods of hoe-agriculture, lost out. As an example of large scale conservation farming carried out by private enterprise, the case of the Peixe farms in Pernambuco is mentioned.

            Conservation farming not only settles (entirely or in part) the problems of moisture deficiency, but simultaneously handles the problem of soil conservation. In this, it differs most significantly from surface storage in reservoirs. Incidentally, the number of dams washed out during each rainy season indicate that the reservoirs themselves are endangered by lack of water conservation in the catchment area.