Scaling sap flux measurements of grazed and ungrazed shrub communities with fine and coarse-resolution remote sensing

We measured transpiration by black greasewood (Sarcobatus vermiculatus) (SAVE) and fourwing saltbush (Atriplex canescens) (ATCA) over a nitrate-contaminated aquifer in Monument Valley, Arizona, on the Colorado Plateau. Heat balance sap flow sensors were used to measure transpiration by shrubs in 2006 and 2007 and results were scaled to larger landscape units and longer time scales using leaf area index (LAI), fractional vegetation cover, meteorological data, and the enhanced vegetation index (EVI) from MODIS sensors on the Terra satellite. Transpiration was high depending on leaf area (2·95–6·72 kg m⁻² d⁻¹) and was controlled by vapour pressure deficit (D) in the atmosphere. SAVE tended to have higher transpiration rates than ATCA and had a steeper response to D, but both exhibited midday depression of leaf conductance. Over most of the site, fractional vegetation cover (f_c) and area-wide LAI were low (0·10 and 0·37, respectively) due to heavy grazing by cattle and sheep. However, a portion of the plume that had been protected from grazing for 10 years had f_c = 0·75, LAI = 2·88. Transpiration rates on a ground-area basis varied with LAI, with midsummer daily values ranging from 1·44 mm d⁻¹ (LAI = 0·36) to 13·1 mm d⁻¹ (LAI = 2·88 mm) over the site, corresponding to projected annual values of 159–1447 mm year⁻¹. Controlling grazing could, theoretically, slow or halt the movement of the contamination plume by allowing the shrub community to extract more water than is recharged in the aquifer.