Low spatial and inter-annual variability of evaporation from a year-round intensively grazed temperate pasture system
J. Pronger, D.I. Campbell, M.J. Clearwater, S. Rutledge, A.M. Wall, L.A. Schipper, Low spatial and inter-annual variability of evaporation from a year-round intensively grazed temperate pasture system, Agriculture, Ecosystems & Environment, Volume 232, 2016, Pages 46-58, ISSN 0167-8809,
Ecosystem scale measurements of evaporation (E) from intensively managed pasture systems are important for informing water resource decision making and validation of hydrologic models and remote sensing methods. We measured E from a year round intensively grazed temperate pasture system in New Zealand using the eddy covariance method for three years (2012-2014). Evaporation varied by less than 3% both spatially (770783mm) and temporally (759-776mm) at an annual scale. The low spatial and temporal variation largely occurred because E was strongly controlled by net radiation (r2=0.81, p<0.01, daytime, half-hourly), which did not vary much between sites and years. However, E was strongly limited when volumetric moisture content (VMC) declined below permanent wilting point causing a strong reduction in the decoupling coefficient and an increase in the Bowen ratio. Grazing events appeared to have no effect on E during autumn and winter but reduced E by up to 5% during summer and spring while complete removal of vegetation during autumn herbicide application reduced E by 30%. This implied that over the pasture regrowth period soil water evaporation (ES) could provide up to 70% of E relative to a vegetated site (during autumn) and, given that grazing events removed about 60% of leaf area, these findings suggest ES was likely able to compensate for decreased transpiration post-grazing. Agreement between measured E (EEC) and FAO-56 reference crop E (Eo) was good when soil moisture limitation was not occurring. However, during periods of soil moisture limitation Eo exceeded EEC and a correction factor was needed. We trialled the water stress coefficient (Ks) and a simple three bin VMC correction factor (KVMC) and found the KVMC approach worked better at a daily and monthly scale while both approaches worked well at an annual scale.
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