Hydrology and water quality of a underground dam in a semiarid watershed

In the Brazilian semiarid regions, underground dams can efficiently store water under possible scenarios of climate change. However, the annual humidity vector in these regions is vertical, and annual evapotranspiration exceeds annual rainfall, resulting in the accumulation of salts in groundwater reservoirs. This study investigated over the course of two agricultural years the hydrologic behavior, soil moisture and the seasonal behavior of electrical conductivity (EC) of the irrigation water from an underground dam constructed in the Jacu watershed in a Brazilian semiarid region. The underground dam retained more soil moisture than other nearby areas during the rainy season; however, during dry periods, its storage capacity was reduced by evapotranspiration occurring inside and outside of the groundwater dam and after rainfall the same level of evaporation occurred from soil as from the dam. During the dry season the underground dam raised the concentration of salts in the irrigation water which was categorized as C4, corresponding to the far too saline irrigation water; however, in subsequent rainy seasons, the electrical conductivity of irrigation water decreased to be included in a group of low salinity of 0.95 dS m-1 (C1). The irrigation water sodicity changed from the risk of sodium accumulation to without sodium risk category.   Key words: Evapotranspiration, soil moisture, electrical conductivity, sustainability of semiarid.