Does transpiration from invasive stream side willows dominate low‐flow conditions? An investigation using hydrometric and isotopic methods in a headwater catchment

Abstract

Understanding seasonal partitioning of water in riparian areas is important for assessing how vegetation affects water resources. A combined hydrological‐isotopic field study was conducted within a headwater catchment to explore the dynamics of stream discharge and the effect of riparian evapotranspiration on summer low‐flow conditions. In addition to collection of meteorological data and depth to unconfined groundwater, streamflows were measured at three locations along the length of the river. Isotope ratios of local precipitation, stream water, groundwater, and willow xylem water were used to estimate pathways and sources of water used by vegetation. Using meteorological variables, leaf area index and stand area measurements, willow transpiration was estimated using the Penman–Monteith method. Combining the data from hydrometric, isotope, and vegetation evapotranspiration analysis revealed that water abstraction by stream‐side willows peaked to 5.6 mm/day and had a distinct impact on summer low‐flow conditions and patterns of stream discharge at the daily time scale. Average annual willow transpiration was 270 mm, whereas average annual precipitation during the study period was 1067 mm. However, willow transpiration reduced streamflow and altered water budgets most strongly during critical summer low‐flow periods. Our analysis of transit times, young water fraction, and depth to groundwater water data showed Waipara headwater areas have limited water storage capacity, making them vulnerable to annual variations in precipitation and any other changes in water usage. Removal of streamside willows could potentially influence water balance during summer months when flows tend to be the lowest.

Water sources for woody shrubs on hillslopes: An investigation using isotopic and sapflow methods

“…(a) δ 18 O and δ 2 H values of groundwater and streamwater (data from Marttila et al, ) and (b) and (c), plant stem water and soil water from within the Lang's Gully catchment, plotted against the local and global meteoric water line (LMWL and GMWL, respectively). Groundwater and streamwater showed close adherence to meteoric water lines, whereas plant stem and soil water showed substantial deviation, indicating partial evaporation after precipitation…”

“…(a) Annual patterns of δ 18 O in precipitation (data from Marttila et al, ). (b–e) Annual patterns of δ 18 O‐intercepts, indicating precipitation source δ 18 O for soil water and matagouri stem water.…”

“…The δ 2 H and δ 18 O values for precipitation, streamwater, and groundwater were analysed using similar wavelength‐scanned, cavity ring‐down spectrometer equipment at the National Institute of Water and Atmospheric Research in Christchurch, New Zealand, using three further isotopic standards calibrated to Vienna Standard Mean Ocean Water. Isotope values for soil and plant water and for streamwater collected within Lang's Gully were compared with isotope values in groundwater collected at the outflow point of Lang's Gully, and in daily precipitation collected within the greater catchment as part of a concurrent study examining riparian vegetation effects on river flows (Marttila et al, ). Sampling locations for these data are indicated in Figure .…”

“…Precipitation offsets of water samples from within the catchment (soil water, streamwater, groundwater, and stem water) were calculated by their deviation from the local meteoric water line (LMWL) established in Marttila et al () according to Landwehr and Coplen (), who referred to his method as line‐conditioned (lc) excess: where a and b are the slope and y intercept of the LMWL, respectively, calculated from weighted values of precipitation collected daily at each study site, and S is one standard deviation of the measurement uncertainty for both δ 2 H and δ 18 O.…”

“…Precipitation in the study area (and in many temperate areas) shows a pronounced annual cycle of isotope values, which can be used to shed light on the timing of precipitation supplying various stores of water in the catchment (Gat, ). We calculated the isotopic composition of the precipitation from which soil water and matagouri stem water originated, for comparison with the precipitation sources supplying groundwater and streamwater, characterized for this catchment by Marttila et al (). We calculated the y coordinate (δ 2 H value) and x coordinate (δ 18 O value) of the intersection points in isotope space of evaporation lines with LMWLs, to give the isotope values of these precipitation sources using a method similar to Javaux, Rothfuss, Vanderborght, Vereecken, and Brüggemann (): where a , b , m , and n are the slope of the LMWL, the LMWL intercept, the evaporation line slope, and the evaporation line intercept, respectively, and z is the distance in δ 2 H space of individual sample δ 2 H values above or below the evaporation line, calculated as where, as above, m is the slope of the evaporation line and n is the intercept of the evaporation line.…”