Cowpea crop coefficient in Teresina, Piaui state, Brazil.

Determining the cowpea crop coeficient (Kc) is fundamental in order to establish its water needs during the stages of its development. The objective of this research was to determine the cowpea (Vigna unguiculata L. Walp.) Kc values, BR-17 Gurguéia variety, in its diverse crop development stages, by using a weighing lysimeter. The test was carried out in the experimental area of Embrapa Middle-North, in Teresina, PI (05°05' S; 42°48' W and 74.4 m) aiming a rational planning and management of irrigation. The evapotranspiration reference was estimated by the Penman-Monteith method using climatic data obtained from an automatic weather station located in the experimental area. The Kc values were: 0.7, in the initial phase; 0.7 to 0.8, in the growing phase; 0.8 to 1.06 in the reproductive phase and 0.6 in the final phase.