Complex seasonal cycle of ecohydrology in the Southwest United States

This study investigates the causes for, and distribution of, unimodal versus bimodal seasonal cycle of vegetation greenness in the Southwest United States using extensive site observations, climate data, satellite data, and the Lund-Potsdam-Jena (LPJ) vegetation model. Peak vegetation greenness is achieved in a clockwise manner across the Southwest, beginning in spring in the Sonoran Desert following winter rains, then in Utah-Colorado with snowmelt/summer rains, and finally in New Mexico-eastern Arizona with late summer monsoon rains. At high elevations, spring-summer snowmelt is critical for supplying the necessary soil moisture to trigger vegetation growth. A bimodal seasonal cycle of vegetation greenness is evident in satellite data and LPJ simulations across eastern Arizona and western New Mexico, characterized by peaks during late spring-early summer and late summer-early autumn. This bimodal green-up remains a pressing paradox for which many competing hypotheses exist. The mechanism for this seasonal pattern is demonstrated using LPJ and observational data and is found to deviate from the traditional pulse-reserve paradigm. This paradigm states that rainfall events in arid lands produce nearly immediate pulses of vegetation growth and accumulation of reserves but does not consider cold dormancy, time-lagged vegetation responses, or rainfall seasonality. The following soil moisture based mechanism for bimodal greening is proposed. The initial peak in vegetation greenness during late spring-early summer results from a break in cold dormancy and benefits from the gradual winter-long accumulation of deep soil moisture from weak synoptic rain events and snowmelt in colder regions. Limited precipitation and ongoing transpiration, from the initial vegetation greening, trigger a midsummer drying of the soil and a consequential minimum in vegetation activity. Later, pulses of monsoon rainfall in late summer-early autumn support the secondary greening, although significant runoff of brief, intense rainstorms and substantial soil evaporation limit moisture to the upper soil layers.