Technical Abstract: Sap flow measurements can be an important tool for unraveling the complex web of ecosystem fluxes, especially when it is combined with other measurements like eddy covariance, isotopes, remote sensing, etc. In this talk, we will demonstrate how sap flow measurements have improved our process-level understanding of fluxes in semiarid regions by reviewing results from several experiments that were carried out in variety of ecosystems in the Southwest U.S. The examples include: quantifying the transpiration of linear riparian forests, partitioning of evapotranspiration in desert shrublands, and determining the ecohydrologic significance of hydraulic redistribution of mesquite trees in riparian and upland woodlands. We will also discuss methods to scale up branch- or plant-level measurements to ecosystem scale.