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Grand Island, NE, United States

Kabenge I.,Makerere University | Irmak S.,University of Nebraska - Lincoln | Meyer G.E.,University of Nebraska - Lincoln | Gilley J.E.,University of Nebraska - Lincoln | And 4 more authors.
Transactions of the ASABE | Year: 2013

Quantifying actual evapotranspiration (ETa) of riparian zones is important for more robust water balance analyses that will enable better planning, managing, and allocating of water resources as well as developing strategies to protect delicate riparian ecosystem functions. The ETa, sensible heat flux (H), net radiation (Rn), soil heat flux (G), meteorological variables (air temperature, Ta; incoming shortwave radiation, R s; wind speed, u3; relative humidity, RH; vapor pressure deficit, VPD; precipitation, etc.), and albedo were measured on an hourly time step, and leaf area index (LAI) and plant height were measured on a weekly basis for a common reed (Phragmites australis) dominated cottonwood (Populus deltoides) and peach-leaf willow (Salix amygdaloides) riparian plant community in 2009 and 2010 through extensive field campaigns conducted in the Platte River basin in central Nebraska. The two growing seasons were contrasted by warmer air temperatures, higher precipitation, and presence of flood water on the surface during the 2010 season. The seasonal variations of daily average ET a were mainly controlled by Rn and air temperature. In 2009, total ETa and precipitation were 679 mm and 280 mm, respectively, and the values were substantially greater in 2010 (982 mm and 508 mm, respectively). The seasonal daily ETa for the mixed plant community ranged from 0.5 to 8.5 mm d-1 with a seasonal average of 3.7 mm d-1 in 2009 and from 0.5 to 11 mm d-1 with a seasonal average of 5.5 mm d-1 in 2010. In 2010, ETa varied widely with meteorological conditions and in response to variations in phenology of the vegetation to flooding. In 2009, on a seasonal average basis, a total of 77% and 14% of the available energy was partitioned into ET a and H, respectively. In 2010, over 90% and -12% (negative due to flooding) of the available energy was partitioned into ETa and H, respectively. The research results presented here provide valuable ETa data and information for enhancing the understanding of the interactions between the surface/vegetation conditions and the surrounding microclimate and surface energy balance for mixed riparian vegetation. The results of this research should aid water managers and decision/policy makers in accounting for water use rates of phragmites-dominated cottonwood and peach-leaf willow riparian plant communities in water balance analyses to make better-informed water resources planning and management decisions. © 2013 American Society of Agricultural and Biological Engineers ISSN 2151-0032. Source

Irmak S.,University of Nebraska - Lincoln | Kabenge I.,Makerere University | Rudnick D.,University of Nebraska - Lincoln | Knezevic S.,University of Nebraska - Lincoln | And 2 more authors.
Journal of Hydrology | Year: 2013

Application of two-step approach of evapotranspiration (ET) crop coefficients (Kc) to "approximate" a very complex process of actual evapotranspiration (ETa) for field crops has been practiced by water management community. However, the use of Kc, and in particular the concept of growing degree days (GDD) to estimate Kc, have not been sufficiently studied for estimation of evaporative losses from riparian vegetation. Our study is one of the first to develop evapotranspiration crop coefficient (KcET) curves for mixed riparian vegetation and transpiration (TRP) crop coefficients (KcTRP) for individual riparian species as a function GDD through extensive field campaigns conducted in 2009 and 2010 in the Platte River Basin in central Nebraska, USA. KcTRP values for individual riparian vegetation species [Common reed (Phragmites australis), Cottonwood (Populus deltoids) and Peach-leaf willow (Salix amygdaloides)] were quantified from the TRP rates obtained using scaled-up canopy resistance from measured leaf-level stomatal resistance and reference evapotranspiration. The KcET and KcTRP curves were developed for alfalfa-reference (KcrET and KcrTRP) surface. The seasonal average mixed riparian plant community KcrET was 0.89 in 2009 and 1.27 in 2010. In 2009, the seasonal average KcrTRP values for Common reed, Cottonwood and Peach-leaf willow were 0.57, 0.51 and 0.62, respectively. In 2010, the seasonal average KcrTRP were 0.69, 0.62 and 0.83 for the same species, respectively. In general, TRP crop coefficients had less interannual variability than the KcrET. Response of the vegetation to flooding in 2010 played an important role on the interannual variability of KcrET values. We demonstrated good performance and reliability of developed GDD-based KcrTRP curves by using the curves developed for 2009 to predict TRP rates of individual species in 2010. Using the KcrTRP curves developed during the 2009 season, we were able to predict the TRP rates for Common reed, Cottonwood and Peach-leaf willow in 2010 within 7%, 8% and 13% accuracy, indicating a good performance of the two-step approach proposed in this study for estimating TRP for riparian vegetation. The surface conditions of the riparian ecosystem need to be considered when using the two-step approach to estimate ETa or TRP rates of riparian plant communities. The results of this study provide important water use information and data for riparian vegetation that can be used for more robust hydrologic/water balance analyses. © 2012 Elsevier B.V. Source

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