Analysis of measured evapotranspiration shows multi-year regolith water storage can support evapotranspiration for years into a multi-year dry period. Measurements at 25 flux-tower sites in the semi-arid western United States, distributed across five primary land-cover types, show both resilience and vulnerability to multi-year dry periods. Average evapotranspiration ranged from about 700+200 mm per water year (October-September) in evergreen needleleaf forests to 350+150 mm per water year in grasslands and open shrublands. On average, in California’s Mediterranean climate almost half of the water-year evapotranspiration is supported by seasonal and/or multi-year regolith water storage, compared to a characteristic 20 to 30 percent value of energy-limited and inland sites. Below 35oN latitude, water-year evapotranspiration exceeded estimated precipitation in over half of the years on record. For non-energy-limited sites, water-year evapotranspiration increased with precipitation up to a maximum water-year evapotranspiration value of about 900, 750, 600, 425 and 300 mm per water year for evergreen needleleaf forests, mixed forests, woody savannas, grasslands and open shrublands, respectively. There were 15 multi-year dry periods on record that exhibited either an attenuation in evapotranspiration, defined as an annual value below 80% of the wet-year average, or withdrawal from multi-year storage. A multi-year dry period was defined as three or more consecutive water years in which all water-year precipitation values and the mean period value were in the lower 50 and 35 percent of the historical record, respectively. For sites exhibiting evapotranspiration attenuation, resistance to multi-year dry periods ranged from 9 to 49 months, drafting as much as 444 mm of regolith storage. At some mountain sites regolith storage provided up to 678 mm, almost the equivalent of the average water-year evapotranspiration for these sites, over the extent of the multi-year dry period.
Rungee, J. P.; Bales, R. (2017): Evapotranspiration response to multi-year dry periods in the semi-arid western United States. Fall Meeting, American Geophysical Union, December 2017. Abstract H11O-07..