Ground water discharge by evapotranspiration in wetlands of an arid intermountain basin

To improve basin-scale modeling of ground water discharge by evapotranspiration (ET) in relation to water table depth, daily ET was measured using the Bowen ratio energy balance method during 1999–2005 in five herbaceous plant dominated wetlands in an arid intermountain basin in Colorado, USA. Three wetlands were wet meadows supplied primarily by regional ground water flow and two were playas supplied primarily by local stream flow. In wet meadows, mean daily water table depth (WTD) ranged from 0.00 m (ground surface) to 1.2 m, with low inter-annual variability. In wet meadows, annual actual ET (ETa) was 751–994 mm, and ground water discharge from the shallow aquifer (ETg) was 75–88% of ETa. In playas, mean daily WTD ranged from −0.65 to 1.89 m, with high inter-annual variability. In playas, annual ETa was 352–892 mm, and ETg was 0–77% of ETa. The relationship of annual ETg to WTD was compared to existing ETg–WTD models. For wet meadows, ETg decreased exponentially as WTD increased from 0.13 to 0.95 m (r2 = 0.83, CV = 5%, p < 0.001). In comparison with our findings, existing models under- and over-estimate ETg by −30% to 47% at WTD of 0.13 m, and they under-estimate ETg by −12% to −42% at WTD of 0.95 m. This study found that as the water table declined from near the soil surface to 0.95 m, ETg decreased only ∼26% versus 39–55% estimated by existing models. The magnitude of ETg decrease was 220 mm, whereas existing models predicted decreases up to 700 mm (218% greater). In playas, there was no clear ETg–WTD relationship. Instead, ETg was strongly dependent on the surface water supply. When sufficient surface water inputs occurred to meet ET demand, ETg was ≈0 mm/yr and independent of WTD. When inputs did not meet ET demand, ETg was positive though highly variable at WTD up to 1.68 m.