Impact of Macrophyte Evapotranspiration on Water Availability in Tropical Northeast Brazil

ABSTRACT

In the North‐eastern region of Brazil (1.56.10 ⁶  km ² ), surface reservoirs provide most of the water, which is significantly affected by evaporative processes: Up to 50% of river inflow is transferred to the atmosphere. Reservoirs are vulnerable to eutrophication, and this condition is responsible for the massive presence of macrophytes. We evaluated the impact of three types of macrophytes ( Eichhornia crassipes , Salvinia spp., and Typha spp.) on evaporative losses in reservoirs and compared the evapotranspiration of macrophyte‐covered surfaces with free‐water evaporation. Our experiments were carried out at two sites: in the State of Ceará ( Eichhornia crassipes : n  = 164 days) and in the State of Bahia ( Salvinia spp.: n  = 174 days; and Typha spp.: n  = 120 days). The evapotranspiration rates ( ET ) measured in reservoirs with Eichhornia crassipes , Salvinia spp., and Typha spp. surpassed the respective free‐water evaporation rates ( E ) by 32% (range from −9% to +166%), 24% (from +9% to +195%), and 40% (from −19% to +466%). Water‐balance simulation in the Jaguaribe River Basin (7.5.10 ⁴  km ² ; 4900 reservoirs, Ceara) indicates that macrophyte coverage is estimated to reduce basin water availability by up to 12% when a median ET/E ratio of 1.32 is assumed. Macrophyte impact on water availability is more severe (reduction up to 76%) in small reservoirs, which supply vulnerable rural communities.