Balanço de água em capim elefante ( Pennisetum purpureum Schumach.) sob diferentes regimes de irrigação.

Abstract

The main objective of this study was to determine the water balance in a soil with irrigated Elephant Grass (Pennisetum purpureum Schumach). The works were carried out at the Experimental Station of Bebedouro, located in Petrolina, Pernambuco. The soil used in this experiment was an Oxisol. The water balance was carried out from the 22nd of January to the 30th of November. The parameters of the balance equation were computed daily and presented on a weekly basis. The unsaturated conductivity data, determined empirically for the soil under study, were used in conjunction with the Darcy equation to evaluate the drainage below the elephant grass root zone in the four water treatments. Irrigations were performed based on gravimetric determinations and were done when the available water content of the soil reached 60, 45, 30 and 15% in treatments A, B, C and D, respectively. Except for treatment D, the water flow in the soil was always descending, for the depth interval 90-120 cm. This flow below 120cm was considered as per per drainage. Treatment D showed an upward flow of water, reaching a maximum flow of 0.11 cm / day. During the 314 days of the study period, 32.8; 25, 1; 17.6 and 12.3 cm of water were lost by drainage for treatments A, B, C and D, respectively. Drainage was greater in treatment A in which the irrigations were on average every 16 days. The accumulated real evapotranspiration was estimated weekly, based on data of soil water content, obtained gravimetrically. Cumulative real evapotranspiration measurements for the study period were 131.62; 130.37; 131.61 and 125.21 cm, for treatments A, B, C and D, respectively. Actual cumulative evapotranspiration determinations were compared with cumulative vapotranspiration (Hargreaves), cumulative potential evapotranspiration (Garcia-Lopez) and with evaporation tank data. Correction coefficients (K), to transform tank evaporation into actual evapotranspiration, ranged from 0.39 to 0.88, depending on the stage of development of the crop. The highest yields of Capim Elefante were obtained with treatment A. This treatment was not the most efficient in the use of water due to the large drainage water losses.