Resistance and reilience of ecohydrological function to an episodic drought cycle in constructed sagebrush steppe communities

Evapotranspiration (ET) caps are vegetative communities constructed to prevent percolation of precipitation through buried wastes into groundwater.  ET caps need to function over long time scales that include variable precipitation and drought cycles in semiarid regions.  Vegetative cover, soil water, and ET were measured on experimental ET caps during and following drought in 2002-2006.  The experiment consisted of a split-split plot layout of three factors; cap design, vegetation community (crested wheatgrass or a 12 species native mix), and receiving summer, fall, or no supplemental irrigation.  Water accumulation in excess of field capacity at the bottom of caps was considered potential “breakthrough” of water into interred wastes.  Caps receiving fall irrigation (200 mm pulse) had frequent breakthroughs, particularly those planted with crested wheatgrass.  Caps receiving no- or summer irrigation resisted breakthroughs, except for 2006 when breakthrough occurred for three cap types planted with crested wheatgrass.  Summer irrigated caps were capable of depleting soil water more rapidly following breakthrough events, demonstrating greater resilience.  Contrary to expectation, variations in cover were rarely correlated positively with ET and could not explain cap performance, even though a significant correlation between precipitation and cover was found.  Cap performance, specifically resistance and resilience, is clearly enhanced by plant diversity.  However, the lack of association between community cover and ET makes it difficult to make specific recommendations for cap plantings.  Thus, basic information needs for designing effective ET caps include how abundance of vegetation cover and ET are related, and how antecedent soil moisture affects community rain use.