On groundwater fluctuations, evapotranspiration, and understory removal in riparian corridors

This study utilizes 7 years of continuously monitored groundwater-level data from four sites along the Río Grande riparian corridor in central New Mexico to calculate evapotranspiration from groundwater and assess impacts of understory vegetation removal during a restoration project. Diurnal groundwater fluctuation measurements were used to compare the well-known White method for estimating evapotranspiration from groundwater (ETg) to colocated measurements of total riparian evapotranspiration (ET) measured using the eddy covariance method. On average, the two methods were linearly correlated and had similar variability, but groundwater hydrograph estimates of ET g tended to be larger than tower ET estimates. Average ETg estimates for two wells at one site ranged from 91.45% to 164.77% of measured tower ET estimates, but were also shown to range from 57.35% to 254.34% at another site. Comparisons between the methods improved with deeper water tables, reduced groundwater and river connectivity, and where soil profiles were dominated by coarse-sized particles. Using a range of texture-based estimates of specific yield (Sy) with water table position improves the field application of the White method. River-induced fluctuations in groundwater increased the variability of ETg measurements. Removal of understory vegetation at one site resulted in a small but significant reduction in diel groundwater fluctuation amplitude of 19-21%. Caution is required when understory vegetation removal is used as a means to decrease overall riparian ET. Diel groundwater fluctuation amplitudes can be useful in gauging the hydrological effects of vegetation removal. Riparian groundwater hydrographs are critical to investigating the hydrologic connectivity between river and shallow groundwater, the temporal patterns of vegetative consumption, and monitoring changes to the vegetation community. Copyright 2009 by the American Geophysical Union

Tree effects on urban microclimate: diurnal, seasonal, and climatic temperature differences explained by separating radiation, evapotranspiration, and roughness effects

Increasing urban tree cover is an often proposed mitigation strategy against urban heat as trees are expected to cool cities through evapotranspiration and shade provision. However, trees also modify wind flow and urban aerodynamic roughness, which can potentially limit heat dissipation. Existing studies show a varying cooling potential of urban trees in different climates and times of the day. These differences are so far not systematically explained as partitioning the individual tree effects is challenging and impossible through observations alone. Here, we conduct numerical experiments removing and adding radiation, evapotranspiration, and aerodynamic roughness effects caused by urban trees using a mechanistic urban ecohydrological model. Simulations are presented for four cities in different climates (Phoenix, Singapore, Melbourne, Zurich) considering the seasonal and diurnal cycles of air and surface temperatures. Results show that evapotranspiration of well-watered trees alone can decrease local 2 m air temperature at maximum by 3.1 – 5.8 °C in the four climates during summer. Further cooling is prevented by stomatal closure at peak temperatures as high vapour pressure deficits limit transpiration. While shading reduces surface temperatures, the interaction of a non-transpiring tree with radiation can increase 2 m air temperature by up to 1.6 – 2.1 °C in certain hours of the day at local scale, thus partially counteracting the evapotranspirative cooling effect. Furthermore, in the analysed scenarios, which do not account for tree wind blockage effects, trees lead to a decrease in urban roughness, which inhibits turbulent energy exchange and increases air temperature during daytime. At night, single tree effects are variable likely due to differences in atmospheric stability within the urban canyon. These results explain reported diurnal, seasonal and climatic differences in the cooling effects of urban trees, and can guide future field campaigns, planning strategies, and species selection aimed at improving local microclimate using urban greenery