The effect of applied water on water use in Tamarix ramosissima (saltcedar) was investigated along the Virgin River (SE Nevada) during a hot, dry summer period. Water was applied to 4 m2 (2 m x 2 m) basins surrounding Tamarix thickets once per week for 6 weeks. Irrigations were applied at 0, 50 or 100% of the previous weeks potential evapotranspiration estimate using a modified Penman equation. Transpiration was monitored with stem flow gauges, soil moisture with time domain reflectometry and plant water relations with a steady state porometer and pressure chamber. Results indicated that at least 4 weeks of irrigation were required before a significant increase in water use by Tamarix would occur. Sap flow measurements indicated that Tamarix growing in closed stands (canopy over-lapping on three or more sides) transpired higher amounts of water than Tamarix growing in open stands (canopy not over-lapping on any sides). Ratios of post- to pre-depth weighted salinity in the 0-60 cm depth indicated that water uptake was occurring in this soil region by Tamarix growing in closed stands to a greater extent than in open stands. A multiple regression equation accounted for 87% of the variability (P = 0.001) in Tamarix transpiration normalized on a stand volume basis when irrigation volume, leaf area density and average ratio of height to distance of nearest neighboring trees were included in the equation. We conclude that any attempt to characterize evapotranspiration of mature stands of Tamarix will require a detailed spatial assessment of stand density and an evaluation of water availability relative to atmospheric water demand over time and that it is doubtful under typical hot, dry summer conditions that Tamarix would effectively utilize water from most summer rainfall events. However, if alterations occurred along the river that increased the availability of water to Tamarix, increased transpiration rates would occur with time.