Submitted to: Ecohydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/3/2016
Publication Date: 11/10/2016
Citation: Carroll, R.W., Huntington, J.L., Snyder, K.A., Niswonger, R., Morton, C., Stringham, T.K. 2016. Evaluating mountain meadow groundwater response to pinyon-juniper and temperature in a great basin watershed. Ecohydrology. doi: 10.1002/eco.1792.

Interpretive Summary: This research highlights the combined use of integrated models and remote sensing products to explore inter-annual climate variability to determine all the components of the water budget in a watershed where the native tree species, pinyon and juniper have expanded into areas formerly dominated by sagebrush steppe vegetation. Several model simulations were run to determine the impact of tree removal on the water budget. Additional scenarios were run to determine what the effect on the water budget with a predicted 3°C rise in temperature. Changes in the water budget were small due increased temperature or tree removal and just redistributed water from plant water use to increased snow evaporation. However, even a small-scale removal of pinyon and juniper (0.5 km2) decreased the depth to groundwater in the meadow of the watershed, which would improve the vigor of groundwater-dependent vegetation. Results suggest the meadow groundwater levels operate on a threshold response to precipitation greater than 400 mm per year and are highly responsive to small changes in upland vegetation and climate. Use of remote sensing in conjunction with integrated hydrologic models provides insight on the complex interplay between climate, vegetation, and hydrology.

Technical Abstract: Expansion of deeply-rooted Pinyon-Juniper (PJ) has altered water partitioning and reduced water availability to discharging meadows. Research highlights the development and application of GSFLOW to a semi-arid, snow-dominated watershed in the Great Basin to evaluate PJ and temperature controls on mountain meadow shallow groundwater; while using Google Earth Engine Landsat satellite and gridded climate archives for model evaluation. Model simulations over three decades indicate that the watershed operates on a threshold response to precipitation > 400 mmy-1 to produce a positive yield (9%) resulting in stream discharge and a rebound in meadow groundwater levels. Observed and simulated meadow groundwater response to large precipitation correlates with above average predicted soil moisture and correlates with a NDVI threshold value > 0.3. A return to assumed pre-expansion PJ conditions or an increase in temperature estimated by mid-21st century shifts water yield by only ±1% over the multi-decade simulation period; but changes of approximately ±4% do occur during wet years with the greatest change in evapotranspiration (ET) occurring at hillslope toes. While changes in water yield are largely dampened by the spatial and temporal redistribution of ET, the impact of this redistribution produces change in water table depth in the meadow. Even a small-scale removal of PJ (0.5 km2) is predicted to promote a more stable, shallow groundwater system more resilient to droughts, while modest increases in temperature produce a meadow susceptible to declining water levels with meadow community structure likely to move toward dry and degraded conditions.