Surface energy fluxes of Phragmites australis in a prairie wetland

Abstract

Components of the surface energy balance were measured in three communities ( Phragmites australis, Scirpus acutus, and open water) in a wetland located in northcentral Nebraska, USA, during the growing season of 1994. This paper includes results from the area covered by Phragmites australis (reedgrass). The Bowen ratio – energy balance method was used to calculate sensible and latent heat fluxes. During daytime, with a water depth of about 0.5 m, the heat storage term ( G) consumed 20–30% of the net radiation ( R n). At night, G was a significant source of energy (on average, about – 40 W m −2). The magnitude of the daily (24 h) averaged G was small. Evapotranspiration (ET) was a major consumer of the incoming solar energy. During early and peak growth, the daily ET ranged between 2.5 and 6.5 mm per day. During senescence, evapotranspiration was between 0.5 and 3.1 mm per day. ET was partitioned into transpiration ( E v) and evaporation ( E s) using a dual-source modification of the Penman–Monteith equation. Results indicated that transpiration contributed 40–45% of the total ET in the beginning of the early growth stage. During the second half of the early growth stage and the entire peak growth stage, it contributed 53–62% of ET. The contribution decreased to 50% in the beginning of senescence, and to near zero in late senescence. The daytime variation of E s did not follow R n, and seemed to be controlled by thermal stability and air dryness. Before senescence, the ratio of the actual to equilibrium evapotranspiration (ET/ET eq) averaged 1.3. It decreased to about 0.5 during senescence. The McNaughton and Spriggs (1989, IAHS Publ. 177, 86–101) model, developed primarily for dryland vegetation, significantly overestimated the ET/ET eq ratio in Phragmites when the canopy stomatal resistance was larger than 150 s m −1. The model prediction improved significantly when the contribution of evaporation was eliminated by substituting the ET/ET eq ratio by E v/ E veq (transpiration/equilibrium transpiration).