Hydrodynamic changes of the soil-cactus interface, effective actual evapotranspiration and its water efficiency under irrigation/Alteracoes hidrodinamicas da interface solo-palma, evapotranspiracao real efetiva e sua eficiencia hidrica sob irrigacao

The knowledge on soil water dynamics is the basis of crop water management. The soil water balance (SWB) method is used for this purpose. However, its application in cactus may lead to misinterpretation in water efficiency analysis, since it does not consider the amount of water retained in the plant (WRP). This study aimed to evaluate SWB applicability, hydrodynamic changes and water efficiency of forage cactus clones under irrigation. The clones 'Orelha de Elefante Mexicana' (OEM), 'IPA Sertania' (IPA) and 'Miuda' (MIU) were submitted to irrigation depths (2.5, 5.0 and 7.5 mm) and frequencies (7, 14 and 28 days), in Serra Talhada, PE, Brazil, between March 2012 and August 2013. The SWB was applied, by adding the WRP in the estimate of the effective actual evapotranspiration ([ETr.sub.EF]). The water efficiency indicators were calculated. The actual evapotranspiration on SWB ([ETr.sub.SWB]) overestimated [ETr.sub.EF] and, like other SWB components, it was affected by the factors irrigation depth, frequency and clone. The clone OEM is the most efficient, due to the use of the WRP, while MIU leads to highest gross economic returns for sale of cladodes as seed. As conclusion, the application of the soil water balance method in areas cultivated with cactus species must be accompanied by WRP. Key words: water retained in the plant soil water balance cactus modelling O conhecimento da dinamica de agua no solo subsidia o manejo hidrico na lavoura. O metodo do balanco de agua no solo (BAS) e usado para este fim, porem sua aplicacao em cultivos de cactaceas pode promover interpretacao erronea na analise da eficiencia hidrica uma vez que nao considera a quantidade de agua retida na planta (ARP). Objetivou-se avaliar a aplicabilidade do balanco de agua no solo, as alteracoes hidrodinamicas e eficiencia hidrica em cultivos de palma forrageira irrigada. Os clones Orelha de Elefante Mexicana (OEM), IPA Sertania e Miuda (MIU) foram submetidos as laminas (2,5; 5,0 e 7,5 mm) e frequencias de irrigacao (7, 14 e 28 dias), em Serra Talhada, PE, entre marco de 2012 e agosto de 2013. O BAS foi aplicado adicionando-se a ARP na estimativa da evapotranspiracao real efetiva ([ETr.sub.EF]). Indicadores de eficiencia hidrica foram calculados. A evapotranspiracao real baseada no BAS ([ETr.sub.BAS]) superestimou a [ETr.sub.EF] e semelhante a outros componentes do BAS, foi afetada pelos fatores lamina-frequencia-done. O clone OEM e o mais eficiente em razao do uso da ARP enquanto o MIU