Perdas de água por evaporação de um solo cultivado com milho nos sistemas de plantio direto e convencional

Abstract

Worldwide, the agricultural sector is the largest consumer of water, reaching about 69% of total water derived from rivers, lakes and underground aquifers. Because of this huge consumption of water by agriculture and the scarcity of water in various parts of the world there is a social and environmental pressure very big, mainly on irrigated agriculture in order to improve the efficiency of the use of water. This study aimed to quantify the loss of water from the soil by evaporation, in the early stages of development of the corn crop sown in cultivation from no-tillage system and conventional tillage. The work was developed in Experimental Area of the Department of Rural Engineering of the Federal University of Santa Maria (UFSM), RS. The experiment was conducted in the period from October 28, 2006 to December 10, 2006 and consisted of four treatments: (i) direct maize sowing in bare soil (PD0), (ii) direct sowing of corn with 3,0 Mg ha-1 of straw on the soil surface (PD3), (iii) direct sowing of corn with 6,0 Mg ha-1 of straw on the soil surface (PD6), and (iv) conventional tillage of the soil without coverage of straw on the surface (PC). The loss of water from the soil were determined in four cycles of dry soil during the first 38 days after the emergence of culture, holding up the water balance in soil layers from 0 to 10 cm and 10 to 30 cm, from data of volumetric soil moisture determined with a TDR at intervals of 15 minutes. It was also determined, for the first cycle of drying soil coefficients of evaporation(K) and reducing of water evaporation from the soil (Kr), as the methodology of Kc Dual. For the short duration of the cycles of drying of the soil, growth and development of plants were not affected by the system of cultivation of the soil in the early stages of development of the corn crop. In layer from 0 to 10 cm depth for the first cycle of drying of the soil, 3,0 Mg ha-1 of straw on the surface provided a reduction in the loss of water by evaporation of 24,6% and 19,9% in relation to the PC and PD0, respectively. With 6.0 Mg ha-1 of straw, the reduction was 50,7% and 47,6%, respectively. In the first 30 cm of the soil profile, the reduction in losses of water was 18.2% and 23% respectively to 3.0 Mg ha-1 of straw. Increasing to 6.0 Mg ha-1 reduction was 38,3% and 42,0% respectively. With the growth of culture, the increase in the loss of water in the soil was due to the bigger development of plants with increasing participation of transpiration in water consumption by culture, by shading the surface at the higher leaf area index of the plants. Treatments with PC and PD0 show Ke higher, reaching 1,32 and 1,17, respectively. With the presence of straw, the maximum value of K was reduced to approximately 50% compared with the treatments without straw on the surface reaching values of 0,70 and 0,44, respectively, when EToac was 10,0 mm. The treatments of PC and PD0 showed a fast reduction in the coefficient of reduction of water evaporation from soil, after 6,5 mm of EToac. The lowest value of Kr at the end of the cycle of drying, 0,47, was observed for the treatment PD0. The PD6 has remained longer in the first stage of evaporation, reducing the Kr when EToac exceeded 20,0 mm.