Importance of soil-water to the Caatinga biome, Brazil

Northeastern Brazil is hydrologically characterized by recurrent droughts leading to a highly vulnerable natural water resource system. The region contains the Caatinga biome, covering an area of approximately 800000km². To increase insight in water balance components for this sparsely studied ecosystem, hydrology simulations were performed with the SWAP (Soil Water Atmosphere Plant) model for a Caatinga basin of 12km². SWAP model was developed to simulate hydrology under short-cycle crops, and its parameterization and validation to a diverse ecosystem is a novelty. The validation of the simulations was performed using a dataset of daily soil water content measurements taken at 0.2m depth in three sites in the basin in the period from 2004 to 2012. Average Nash-Sutcliffe efficiency coefficient for these simulations was 0.57 and root mean square error of prediction was 4.3%. The results of the simulations suggest that water components do not diverge statistically among different sites of the biome. The Caatinga biome returns 75% (±17%) of the annual precipitation to the atmosphere, whereas the partitioning of total evapotranspiration into its components (transpiration, evaporation and interception) on annual basis accounts for 41% (±7%), 40% (±6%) and 19% (±3%) respectively. Regarding water availability, the surface soil layer (0.0-0.2m) is the most important layer in the rooted profile, supplying up to 90% of atmospheric water demand. According to our analysis performed on daily basis, evapotranspiration and air temperature are most sensitive to soil moisture during the periods June-September and December-January.