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Climate and vegetation can dramatically alter the water cycle on local to regional scales. A change in the surface energy and water balance, especially in arid regions, can have significant impacts on local water availability and, therefore, water resource management. The purpose of this study is to understand the role of climate and vegetation in the energy and water balance of a riparian wetland in the central Great Plains. The site is located near the Republican River in southwestern Nebraska, where decreases in streamflow have been observed in recent decades. In an effort to reduce consumptive use from evapotranspiration (ET), and thereby reclaim surface water, invasive species such as Phragmites australis are being removed throughout the riparian corridor of the river basin. In this study, we have installed two energy/water balance monitoring stations, a Large Aperture Scintillometer (LAS), and various water and soil temperature probes in the P. australis and native (Typha latifolia) portions of the wetland to determine the energy and water balance during the 2009 growing season (April 11−October 3). Sensible heat flux is measured using the LAS, while ET is calculated as a residual of the energy balance (i.e., net radiation minus the sensible heat flux and heat storage in the canopy, water, and soil). Comparisons are also made with ET rates calculated from the simpler Priestley-Taylor method. Additional calculations of ET rates for both P. australis and T. latifolia (Cattail) were made using Landsat remote sensing imagery during five days from April 19 to September 26. Lastly, an open water evaporation model was developed to simulate the potential rate of evaporation from an open water body, in the absence of vegetation. The model assumes a shallow, well-mixed water layer that is similar in area and depth to the existing wetland, and the results are used to infer the potential amount of “water savings” that might be achieved by the removal of P. australis vegetation from the wetland.
The results of the energy budget analysis show that the average ET rate for the wetland during the growing season is 4.4 mm day-1, with a maximum rate of 8.2 mm day-1 occurring around June 29. This measured daily ET is higher than some values found in previous studies (e.g., 6.9, 6.5, 6.3, 5.0, and 5.8 mm day-1) and is attributed to differences in plant structure/biology, environment, and regional climate. The vegetation phenology and net radiation are the two largest meteorological/vegetation drivers for the seasonal variability in ET. Remote sensing-based estimates of ET are similar than those of the in-situ energy balance during full vegetation, and they also reveal that the ET rates from P. australis are, on average, about 28% (1.18 mm day-1) greater than those for T. latifolia. We concluded that the Priestley-Taylor equation provides a good estimate of ET, particularly during the height of the growing season, when most of the net radiation is balanced by latent heat flux. Results of the water balance analysis reveal a reasonable correspondence between water level fluctuations and the energy budget-derived estimates of ET. Periods when the two curves do not agree imply influx (outflux) of groundwater early (late) in the growing season. Results suggest that the removal of P. australis from wetlands within the Republican River basin could potentially result in a growing season “water savings” of up to 28% if the native species of T. latifolia replaces the non-native P. australis. If a free water surface replaces P. australis, depending on wind sheltering, a “water loss” or a “water savings” could occur. We conclude that due to the general presence of wind sheltering throughout the riparian corridor of the Republican River basin, this would more likely lead to a small, but tangible amount of “water savings” if replaced by open water.